HomeMy WebLinkAboutDRC-2009-006470 - 0901a0688014f6a8JON M h ."SMAN. JR
GARY HERBERT
Lieutenant Coventor
State of Utah
Department of
Environmental Quality
Richard W. Sprott
Ejieciitive Director
DIVISION OF RADIATION
CONTROL
Dant L. Finerfrock
Director
MEMORANDUM
TO: Loren Morton, P.G., Manager
FROM: Tom Rushing, P.G., Hydrogeologist
DATE: January 13,2009
SUBJECT; Infiltration and Contaminant Transport Modeling Report, White Mesa Mill Site,
Blanding, UT, Received November 21, 2007: DRC Review Memo
Requirements
Ground Water Permit No. UGW370004, Parts I.H.l J (Infiltration and Contaminant Transport
Modeling Work Plan and Report), Part I.D.6 (Closed Cell Perfonnance Requirements) and Part
I.C.I (Pemiit Limits), require Denison Mines U.S.A. to prepare an Infiltration and Contaminant
Transport Model (ICTM.J
Partl.H.U
"11. Infiltration and Contaminant Transport Modeling Work. Plan and Report - the Permittee
shall submit for Execulive Secretary approval an infiltration and contaminant transport
modeling report that demonstrates the long-term ability of the tailings cells cover system
to adequately contain and control tailings contaminants and protect nearby groundwater
quality of the uppermost aquifer. Said report shall demonstrate how the tailings cell
engineering design and specifications will comply with the minimum performance
requirements of Part I.D.6 of this Permit. Within 180 days of Permit issuance, the
Permittee shall submit a work plan for Executive Secretary approval, that;
a) Identifies all applicable and pertinent historic studies and modeling reports relevant to
tailings cell cover design and tailings cell system performance.
b) Determines all information necessary for infiltration and contaminant transport
modeling, including but not limited to representative input values for vadose zone and
aquifer soil-water partitioning (Kd) coefficients, tailings source term concentrations,
tailings waste leach rates, vadose zone and aquifer groundwater velocities, vadose zone
and aquifer dispersivity, contaminant half-life or other rates of decay, etc. In the event
168 Nonh ! 050 West • PO Box 144850-Sail LjikeCit>. UT 84114-48.S0 • phone (801) 536-4250 • fax (801) 533 4097
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Denison Mines
White Mesa Uranium Mill
Infiltration and Contaminant Transport Modehng Report
Review Memo
Page 2
that any required information is not currently available, the Permittee may select
conservative assumptions for use in the required infiltration and contaminant transport
models. Otherwise, the Permittee shall identify how information will be collected that
is representative of actual field conditions, and a timetable by which said information
will be submitted for Executive Secretary approval.
c) Identifies all computer models that will be used to simulate long-term performance of
the tailings cells cover system. All predictive models used shall be publicly available
computer codes that adequately represent field characteristics at the tailings disposal
site.
d) Determines the conceptual model to be used and justifies why it is representative or
conservative of actual field conditions at the site. Said concepmal model will identify
the physical domain(s) and geometries to be simulated including the tailings cell
design and construction, al) boundary and initial conditions to be assigned in the
model(s), and the shallow aquifer locations where future potential contaminant
concentrations will be predicted.
e) Justifies how the infiltration and contaminant transport problem has been adequately
conceptualized and planned to demonstrate compliance with the requirements of Part
LD.6 of this Permit.
Within 180 days after approval of the modehng work plan, the Permittee shall complete all
modeling in accordance with the approved work plan and submit a final report for
Executive Secretary approval. In the final report, the Permittee may include supplemental
information to justify modification of certain Permit requirements, including, but not
limited to: the ntmiber and types of groundwater compliance monitoring parameters,
tailings cell cover system engineering design and construction specifications, tailings cell
operational requirements, etc. Upon Executive Secretary approval of the fmal infiltration
and contaminant Iransport report, the Reclamation Plan may be modified to accommodate
necessary changes to protect public health and the environment."
Pan I.D.6.
*'6. Closed Cell Perfonnance Requirements - before reclamation and closure ofany tailings
disposal cell, the Permittee shall ensure that the fmal design, construction, and operation of
the cover system at each tailings cell will comply with all requirements of an approved
Reclamation Plan, and will for a period of not less than 200 years meet the following
minimum performance requirements:
a) Minimize infiltration of precipitation or other surface water into the tailings, including,
but not limited to the radon barrier, and
b) Prevent the accumulation of leachate head within the tailings waste layer that could
rise above or over-lop the maximiun FML hner elevation intemal to any disposal cell,
i.e. create a "bathtub" effect.
c) Ensure that groundwater quality at the compliance monitoring wells does not exceed
the Ground Water Quality Standards or Oound Water Compliance Limits specified in
Denison Mines
White Mesa Uranium Mill
Infiltration and Contaminant Transport Modeling Report
Review Memo
Page 3
Part LCI and Table 2 of this Pennit.
The Denison Mines USA ICTM for the White Mesa Mill Site in Blanding Utah was received by
DRC on November 21, 2007 (Preparation Date November, 2007.) This memo summarizes the .
review the plan."
CeDl .
White Mesa cell 1 is not included in the ICTM by reasoning that the cell will not contain tailings
upon closure. The ICTM briefly summarizes some portions of the closure phasing of this cell
which will include removal of evaporate crystals from current use as a dewatering cell and
placement and cover of some demolition materials at the mill.
Per Utah Administrative Code R313 and R317-6, Cell 1 falls under the requirements for modeling
since wastes placed there will be contaminated with byproduct material. Alternatively, White
Mesa needs to show that the wastes will have a de-minimus effect on ground water quality.
Therefore, there has been a large oversight by Denison Mines U.S.A. in not including cell 1 in the
modehng effort or alternatively showing cause for its omission.
Overview
The ICTM uses two flow modeling programs; HYDRUS ID for cover infiltration and unsaturated'
flow, and MODFLOW for dewatering sinks in the^saturated tailings and flow modeling in the
underlying aquifer.
The ICTM additionally uses the contaminant transport model, MT3DMS (associated with
MODFLOW), for chemical transport through saturated zones. PHREEQC is used as an
associated model with HYDRUS 1-D to characterize geochemical process in the vadose zone (e.g.
chemical reactions of leachate with the underlying soil matrix to estimate the Kd input values for
Hydrus.)
The tailing cell model design is highly dependent on large evapotranspiration (ET) sinks in the
cover infiltrarion model to calculate the water balance and eventually achieve steady state for cells
2, 3 4A and 4B. Additionally, the model attempts to simulate long term dewatering of the tailings
(14 year transient state) in cells 2 and 3 and associated pressure head drops within the cells and a
shorter duration dewatering phase in cells 4A and 4B.
Table 1. below summarizes the uses of MODFLOW and PTfDRUS-lD through the vertical
domain ofthe model. Additionally, an accompanying flowchart is provided to better visualize the
inter-dependency ofthe independent models. Per the flowchart, 5 distinct models have been used
to characterize the flow and contaminant transport.
Denison Mines
White Mesa Uranium MiJi
Infiltration and Contaminant Transport Modeling Report
Review Memo
Page 4
Table I. - Modeling Platforms Used throug
Model
No.
1 '
2
3
4
5
Descriptive
Depth in
Domain
Encompasses
7.5 ft thick
cover system
Encompasses
saturated
tailings zone
to the top of
the PVC or
HDPE
membrane.
Geomembrane
Liner Flux
Encompasses
the 3 Vadose
Zones of the
Dakota Fm.
SS and Burro
Canyon Fm.
Encompasses
the zone from
flux out of the
cover system
to flux into
the saturated
Burro Canyon
Fm.
Model Platform
*HYDRUS ID-Cover
System
MODFLOW-Tailings
Dewatering Model,
Calculates time to reach
pseudo steady state head
in the tailings cells
Giroud & Bonaparte
Anaiytical Equations
PHRFRQC-Chemical
Speciation and
Complexation Model
Used to Develop Kd and
Retardation Coefficients
in the Vadose Zone
*HYDRUSID-
Comprehensive Model
Incorporates Inputs from
Models 1, 2, and 3 to
calculate flow and •
chemical transport to the
water table
1 the Vertical Domain
Model Type
PET Water Balance
Dewatering
Models Flux
through the HDPE.
(Cells 4A and 4B)
and PVC (CeUs 2
and 3)
Geomembrane
Liners
Ion Balance/
Chemical
Speciation
(geochemical
. "beaker*' model)
Flow and Chemical
Transport—Solves
for the Richards
Equadon using
modified
VanGenuchten
Fitting Parameters
and numerical
solutions for heat
and solute transport
Major Model Inputs
Precipitation
Evapotranspiration
Infiltration
Water Storage
Loam and Clay Inputs
(Cover Materials
Specs.)
Elevation Head
Dependent Sink Term
(Slimes Drain)
Based on
Manufacturing and
Installation Defects
and Decomposition of
the Geomembrane
Material.
PIFO and Acid
Neutralizing Data from
Core MW-23 and
MW-30 Analysis
Dzombek and More!
Diffuse Layer Ion
Exchange Parameters
-Model Layer
Thicknesses
-Residual Water
Contents by Layer
-Inverse Air Entry
-Pore Size Distribution
-Saturated K values
-PVC and HDPE
Membrane Inputs as
Saturated K Values
Parameters
Predicted by
Model
Flux through cover
into tailings zone
(infiltration rate
through the cover)
Time lo reach
pseudo steady state
head in tailings
cells after cover
consfruction
Flux through the
bottom of the
tailings cells.
Vadose Zone K^
and Retardation
Coefficients***
Water Balance
Flux and Moisture
Distribution**
Breakthrough
Curve ~ Uranium
Denison Mines
White Mesa Uranium Mill
Infiltration and Contaminant Transport Modeling Report
Review Memo
Page 5
6
Saturated
ButTO Canyon
Fm.
(Horizontal
aquifer flow
&. transport)
MODFLOW - Perched
water table flow
modeling. Burro Canyon
Aquifer
MT3DMS - Chemica]
transport within the
BuiTO Canyon saturated
zone
MODFLOW -
Modular three-
dimensional finite
difference flow
model
MT3DMS -
Multiple
Contaminant
Transport
-Finite Difference Grid
constant 50 ft spacing
-Parameters for
Saturated Flow Single
Layer based on the
perched aquifer
-Hydrauhc Boundary
Conditions
-Hydraulic Head
Distribution
-Advective and
Dispersive
Processes
-Breakthrough
Curves - Chloride
and Sulfate
*Hydrus is not incorporated until pseudo steady stale conditions have been obiained Ci-c. - 3' head in cells 2 and 3 and 1' head in
cells 4A and4B)
**Flux calculaiions (HYDRUSJ arc based on pseudo steady state heads on the liner, 3" head in cells 2 and 3 and 1' head in cells
4A and 4B
•"'Reiaidation coefficiem calculated using PHREEQC predicted Kd and Hydrus (Model 5) predicted moisture content.
Flowchart Description of Models Used
Model 1 — Conceptual Model for the
Vegetated Cover Utilizing Hydrus ]-D,
Conceptual Model Calculates Flux
through the Cover
Model 2 - Time to reach pseudo-steady state
conditions in the tailings which equates to 3 ft
of saturated conditions in cells 2 and 3 and 1
ft of saturated condidons in ceUs 4A and 4B
using MODFLOW, Once Pseudo Steady
State is achieved time 0 is set for further
modeling
Model 3 - Giroud & Bonapane analytical equations (predicts steady state infiltration
through the geomembrane liners)
Model 4 - PHREEQC Chemical Complexation (Beaker Chemistry) is modeled for 3 vadose
zones using core data (inputs for neutralization potential and total HFO quantities) and
existing tailings data. Model is limited to single ion reactions. Kd and Retardation
coefficients are calculated using chemical speciation data in the PHREEQC Program for
input into HYDRUS Model 5
Model 5 - Hydrus 1-D model is used for the modeling domain frora flux through the cover
to fiux into the samrated Burro Canyon Formation, Values.for Model Layer Thicknesses,
Residual Water Contents, Inverse Air Entry, Pore Size Distribution and Saturated
Hydraulic Conductivity are entered into Hydrus
Model 6 - MODFLOW and MT3DMS are used to model horizontal flow and transport
through the saturated zone (advection and dispersion.) Chloride and Sulfate are modeled
based on the findings of Model 3, no other ions are considered in the saturated zone model
Denison Mines
While Mesa Uranium Mil!
Infiltration and Contaminant Transport Modeling Report
Review Memo
Page 6
Cover Design Discussioii
The cover design presented in the plan is described as a shght modification to the NRC approved
cover as detailed in the White Mesa Mill Reclamation Plan, Docket No. 40-8681, Revision 3, July
2000. Both top slope and side slope designs are detailed on Figure A-5.1-2 of the Reclamation
Plan, "Reclamation Cover and Cross Sections."
The ICTM discusses a change in philosophy at the White Mesa Mill. The current top-slope
design standard, as outlined in the White Mesa Reclamation Plan includes a cobble layer cover
system underlain by a frost barrier/water storage layer of 2 ft. thickness. The conceptual idea of
DUSA is to alter this previously approved (NRC) cover design to a vegetated cover which will
provide a larger ET sink term in the model. Note; the side slope design has not been altered and is
not considered significant due to the relatively flat top-slope (0.2 %.) DRC did note that the side-
slope is not considered in the HYDRUS 1-D model and based on the assumption of no runoff or
intemal filter drainage from the top-slope area due to the relatively flat top-slope. The side-slope
geometry is basically the same as the top slope design in the Reclamation Plan and has not been
considered or redesigned in associarion with the ICTM. As discussed below, the assumption of no
runoff or intemal filter drainage from the top-slope area may not be reasonable and may reflect a
weakness in the selection of a one-dimensional model to simulate cover infiltiration performance.
The top-slope cover designs listed in the Reclamarion Plan and ICTTM are summarized by layers in
Table 2. below:
Table 2. NRC Approved Tailings Cell Top-Slope Cover Design and ICTM Proposed Design by
Layer
Current Tailings Cell Cover, Reclamation Plan,
Revision 3 Detail, Top to Bottom
Tailings Cell Cover Proposed TCTM
6" Thick Riprap Layer (consisting of rock with a
D50 minimum of 0.3 inches)
Top Soil with Gravel (Vegetated with
Grasses) 6", Saturated K = 6.3 cm/day (Per
Table 3-1 ICTM)
Frost Barrier, 2 ft 95 % compacrion, Average
-7 Hydraulic Conductivity = 8.87 x 10 cm/sec =
0.077 cm/day (Per Page 3-19, Reclamation Plan)
Frost Barrier 3 ft Sandy Silt and Silty Sand,
Saturated K = 6.3 cm/day (Per Table 3-1
ICTM)
Clay Layer (Radon Barrier), 1 ft 95 %
compaction. Average Hydraulic Conductivity =
3.7 X 10"^ cm/sec = 0.003 cm/day (Per Page 3-19,
Reclamation Plan)
Clay Layer (Radon Barrier), 1ft, Saturated K
= 0.041 cm/day (4.75 x 10"'' cm/sec) (Per
Table 3-1 ICTM)
Piatform Fill, 1 ft 90% compaction
3 ft Bridging Lift 80% compaction
Average Hydraulic Conductivity = 8,87 x 10"^
cm/sec = 0.077 cm/day (Per Page 3-19,
Reclamation Plan)
Platform Fill, 3 ft., Saturated K = 63 cm/day
(7.29 X 10"^ cm/sec) (Per Table 3-1 ICTM)
Denison Mines
White Mesa Uranium Mill
Infiltration and Contaminant Transport Modeling Report
Review Memo
Pase 7 *&*
The alteration to the proposed cell cover has removed the 6" rip rap erosion protecrion layer and
replaced it with a tops oil/gravel admixture surface layer vegetated with grasses. The redesigned
top soil layer is proposed in order to greatly enhance the ET sink. However, the modified cover
may also potentially cause penetration of deeper rooted species of plants onto the cover system
and may increase long term permeability of the radon barrier.
It was noted that the modified cover does not provide for any prevenrion of bio-intrusion into the
radon barrier by plant roots or animals. The 3.5 ft total thickness of the top soil and frost barrier
layers is questionable both from the perspective of bio-intrusion, as well as adequacy in protecting
the radon barrier from frost/freeze damage. Per review of the ICTM, the issue frost depth analysis
has not been adequately addressed, in that no frost penetration mode] or calculations were
provided. Based on potenrial soil property differences and design differences between the
Reclamation Plan and the ICTM, these analyses need to be re-evaluated for any approved changes
of cover design.
DRC reviewed the ET inputs into the Hydrus 1-D cover infiltration model (Partitioned into
Potential Evaporation (PE) and Potential Transpiration (PT) for the modified cover design. It was
noted that the ET calcularion was based on daily precipitation totals using the 3 "wettest" years in
record from Blanding. However, it appears that this approach is not representative of actual site
conditions in that the HYDRUS model assumes the daily precipitation is distributed evenly over
24 hours. In contrast, storm intensity is important for infiltration models for arid sites where the
daily precipitation can arrive in a matter of minutes. Consequently, DUSA needs to explain and
justify why the daily precipitation values used in the HYDRUS model are representative or
conserv'ative of the storm intensities expected at the White Mesa site. This has imphcations for
the DUSA use of a l-dimensional model to simulate embankment infiltration perfonnance, in that
consideration of storm intensity will likely make mnoff and intemal filter drainage more
important in the overall water budget. This may result in the need for a 2-dimensional infiltration
model for the facility.
It was noted that ET was calculated based on standard assumptions for grass cover and was not
calculated based on field studies (e.g. lysimetry) or indigenous plant species. The ICTM states
that the Feddes el al. (1978) water uptake model with water response funcrions for grass was
selected in HYDRUS. The selection of this parameter is not well jusrified and no literature or
local reference guides seem to have been used in choosing this parameter. DUSA needs to
explain and jusrify why the grass water response functions from Feddes, et al. are representarive
or conservative of the properties and characteristics of the actual White Mesa site.
The cunent infiltration model assumptions are likely not calculating a representative water storage
capacity since they don't account for short storm precipitation volume intensities (hourly
precipitation runs), indigenous species transpiration properties, leaf-area indices, root volumes,
growing season, rainfall season, or root structure development timeframes. It is suspected that the
HYDRUS grass input is simulating a species such as Kentucky Blue Grass which is not at all
representative of local narive vegetation and would not be sustainable on the tailings covers. The
use of a standard grass input for rooting and canopy ET processes is not adequate given the large
Denison Mines
White Mesa Uranium Mill
Infiltration and Contaminant Transport Modeling Report
Review Memo
Pages
dependence on this parameter to achieve water balance and the eventual establishment of steady
slate flow to the perched aquifer.
The ICTM also points to the construcrion of tailings cell covers per a study by Albright, and more
specifically to the Monricello tailings cell cover system, and seems to infer that the White Mesa
tailings cell cover proposed in the ICTM is a close analog of these other cover systems. Per DRC
review of the Albright study and Monticello tailings cover it was noted that the White Mesa cover
does not bear a close resemblance to these other cover system in location, design or in the amount
of field data collected. The Monticello study utilized extensive site characteristic study and field
lysimetry data prior to development of engineering design and also incorporates mnoff controls, a
capillary barrier, thicker frost protection layer and bio-intmsion barrier and FML above the clay
radon barrier. The Monticello site is also located at an elevation of about 7,000 ft. amsl, whereas,
the White Mesa site is found at a lower elevation of about 5,500 ft amsl. DRC will request
information from Denison USA regarding the omission of capillary barrier and filter layers (above
the radon barrier) which appear to be fairly siandard in the ET cover designs per Albright.
Justification also needs to be provided on why the higher Monticello sile is representative of
White Mesa.
Water Balance
Per the ICTM summary of infiltration model results, section 4, flux rates at the water table below
the taihngs cells (2, 3,4A, and 4B) are expected to reach steady state sometime after 200 years, as
evidenced by steadily declining flux rate predictions over years 0 to 200 in the model (ICTM,
figure 4-4). These declining flux rates at the water table appear to be a product of post dewatering
flux rates through the bottom liner. DRC noted that the model was based on several simplifying
assumptions given the 1-D aspect of the HYDRUS model and sparse site data. The model appears
not to calculate steady state water contents in unsaturated zones of the tailings or geologic
formations below the cells. Additional information needs to be provided, so as to verify steady-
state moisture content profiles across the entire vadose zone.
The ICTM does not present a consolidated calculation of water balance because of the use of
various sequential and compartmentalized models (see Table 1. above). Per DRC review the
values of average infiltration and flux to the water table was included in various sections. The
model used to simulate infiltration conditions in the cover system and upper tailings reported
duration of 228 years based pn a 57 year period daily rainfall average record which was run 4
consecutive times. It is questionable whether this is an adequate duration of modeling, even
though the permit only requires the 200 year duration, given that the first mode] does not
adequately display the achievement of hydrologic or chemical equilibrium. However, the
infiltration model may be adequate and conservative ifthe FTYDRUS flux calculations consider
the following:
1. The HYDRUS model (Model 5) uses early lime, higher vadose flux/velocity through
the liner, based on Giroud and Bonaparte calculations (Model 3) and head on the liner
Denison Mines
White Mesa Uranium Mill
Infiltration and Contaminant Transport Modeling Report
Review Memo
Page 9
system which would cause more contaminant mass to enter the vadose zone (in the
model) and provide a conservative 200 year contaminant mass estimate, or:
2. The HYDRUS flux prediction (Model 5) is directly linked to the transportation code,
year by year. In this case the 200 years of prediction is based on declining flux rates.
This would portray progressively less contaminant mass in future years.
The ICTM does not clearly present the input criteria or methodology to insure that the 200 year
modeling uses either of these 2 conditions, or is conservative in predicting contaminant mass at
the groundwater monitoring wells.
An overview ofthe water balance components is below:
Infiltration Model Inputs:
Precipitation (Model 1) - Precipitation is the only input into the water balance for the tailings
cells (2,3,4A, 4B). The ICTM uses daily precipitation and air temperature measurements which
were obtained from the Blanding Weather Station. Based on these precipitation volumes the
model assumes direct infiltration into the cover based on a nearly flat top-slope cover (0.2%.) The
modeling assumes, a unifonn infiltration through the cover. It does not address mnoff or intemal
filter drainage from up gradient areas. DUSA needs to explain and justify why this is
representative or conservative in light ofthe storm intensity issue described above.
The ICTM is based on a 100 year, annual precipitation histogram which was created from the
daily weather records for Blanding. Based on holes in the local precipitation record which were
culled when the annual precipitation record was incomplete, White Mesa formulated a 57 year
daily average precipitation input to the model. This approach may be conser\'ative in that it
avoids bias that could be caused by including years with incomplete weather records which would
represent under-reporting of actual precipitation (would amount to omission of precipitation due
to no weather records.) No substantiation is given in the ISTM as to the suitability of a daily
precipitation rate and its association with storm intensity impacts, nor is any explanation given to
soil freeze and snow accumulation impacts.
Infiltration Model Outputs:
ET (Model 1) - As discussed in the cover section above, the water balance elements and
conceptual model of the HY'DRUS simulation depend on a very large ET sink term. The model
does not use site specific information to determine vegetation stmcture, leaf area index, growing
season vs. wet season duration, planl mortality or revegetation with indigenous species. The
model in effect uses a siandard input for grass which calculates these parameters according to the
Feddes el al. (1978) water uptake model with water response functions for grass. Given the
magnitude of this input to the water balance the development of the ET sink term needs to be
based on local weather, soil, native plant data and related studies. At the very least, additional
justification is needed.
Denison Mines
White Mesa Uranium Mill
Infiltration and Contaminant Transport Modeling Report
Review Memo
Page 10
Surface Runoff (Model 1) - No surface mnoff is considered in the model. All precipitation is
assumed to be transferred through ET or to infiltrate (soil water storage and recharge.) Model 1
does not account for surface runoff or differential infiltration from surface ponding etc. In general
the conceptual model does not adequately justify the omission of overland flow potentials or
adequately describe hypothetical infiltration events orqtiantify the moisture capacity ofthe 6"
topsoil layer in preventing surface runoff This would include an evaluation of catastrophic storm
events (10 yr, 25 yr, 100 yr) and potential exceedences of the hydrauhc conductivity of the cover
soil. Preferably this type of evaluation should be done in standard units (e.g. m/hr) over a short
time duration to evaluate the effects of strong storm intensities. The HYDRUS cover model
(Model 1) is not conservative in the respect that it can not consider overland fiow and/or
differences in seepage due to potential pond areas on the cover. In tum, this omission ignores the
effect of top-slope runoff and intemal filter drainage on the side-slope areas.
Infiltration (Model 1): The model utihzes infiltration rates based on standard soil descriptions
(loam) in the HYDRUS program. It is unclear Whether the input considers a measure of moisture
storage capacity with respect to the cover materials or whether this parameter has been adequately
characterized. A description of moisture capacity is necessary in order to completely evaluate
runoff potential from the cell surface or ponding. The rainfall data needs to reficct hourly
averages in order to simulate variances due to storm intensity as summarized in the Surface
Runoff section above.
Storage (Model 1) - Storage is not well specified in Model 1, as summarized in the Infiltration
section above. The ICTM does not include a narrative of temporal changes in water storage and
also does not explain whether these impacts are standard input assumptions in Model 1. Per
review of the input decks, specific information conceming local climate in respect to seasonal
variations, freeze/thaw effects, and potentia] snow accumulation was not identified in Model 1.
RechargelFlux Rates Through the Cover (Model 1) - The tailings cover in Model 1 was run for
228 years, and repeats the same 57 year climatic record four times. The ICTM uses one ofthe 57
year periods to summarize the cover flux rate (Input as MAX precipitation.) It is reported that the
minimum rate of flux ihrough the cover is 7.4 X 10"^ cm/day to a maximum rale of 2.0 X 10'^
cm/day, with an average flux rate through the cover system of 1.0 X 10^ cm/day.
The ICTM additionally contains a curve depicting water flux rate cm/day plotted against time
(years) encompassing the full 228 year model duration. For each of the 57 year precipitation
cycles it was noted that the flux rates in the beginning of the plot is relatively low with a large
spike of higher flux in the tail end of the record (from about year 42 until.year 57.) It was noted
that the spike is evident in all plots depicting flux rates through the tailings cell and vadose zone,
clearly this is an artifact of the rate of apphed precipitation in Model 1.
Flux Rate Tiirough the Tailings (Model 2) - The hydraulic properties of the tailings were
estimated based on comparison with the Canon City Mill where the properties were measured by
aquifer testing. Based on the assumption that tailings hydraulic properties are.the same for Canon
Denison Mines
White Mesa Uranium Mill
Infiltration and Contaminant Transport Modeling Report
Review Memo
Page 11
City and White Mesa; MODFLOW simulations in Model 2 indicate that water levels in the
tailings will rise on occasion during the transient phase (per the 57 year precipitation data plot) but
will not overtop the liner. Again, this storage behavior predicted in Model 2 is strongly influenced
by the ET predicted by Model I. For use in steady state modeling (Model 5), once transient
dewatering of the tailings is done, the ICTM has made an assumption that the flux through the
cover will match flux through the tailings which is reasonable since the infiltration flux is
conserved in the model (assuming the predictions from Models 1, 2, and 3 are representative or
conservative).
The cmrent MODFLOW calculations (Model 2) for transient dewatering of the tailings is
assumed to be gravity driven (head dependent.) As a result, the model sink term discharge rate
differs based on the amount of head on the bottom liner for all four cells. In contrast, the actual
slimes drain dewatering system is a pump out system where the pump is operated on an episodic
basis and leachate is allowed to accumulate. Consequently, the elevation head in Model 2 is
greater than the conesponding field condition because these fluids in the field are not removed
instantiy bul allowed to build up as the pump cycles on and off. As a result, the head dependent
boundary used to represent taihngs de-watering produces an artificially high sink term.
Additional evaluation is needed to justify how the head dependent boundary and sink term iS:or
can be representauve of field conditions. Since DUSA has never metered flows from slimes drain
pumping it is unknown if flows have been continuous or have declined over the years. It is also
unknown if cunent pumping rates can be sustained in the future.
Model 5 does not begin simulation of infiltration and contaminant transport until the completion
of: 1) the dewatering phase MODFLOW Model (Model 2) which is used solely to calculate a time
duration until pseudo steady state water content in the tailings is achieved, and, 2) analytical
infiltration flux calculations via model 3. Thus, all operational and post remediation phase water
and contaminant fluxex on the system are not accounted for in Model 5. The model language
indicates that, according to Denison Mines reasoning, the quantity of flow is minimal enough to
discount these periods of discharges into the vadose zone, however, by DRC reasoning flows
during the operational and transient conditions could potentially add a significant amount of
unaccounted time (> 30 years) of liner seepage at a greater velocity due to higher heads in the
tailings. This would affect the initial water content calculations within the unsaturated zone
(cunentiy entered as 0% moisture) and could have a significant impact on chemical transport
calculations over the required 200 year model period. Al the least, it is Denison Mines
responsibility to show the claimed de-minimus impact through sensitivity testing or by other
means such as comparisons of calculated seepage.
Flux Rate Through the Liner (Models 1, 2 and 3) - Flux rates through the liner for all cells were
calculated by use of the Giroud and Bonaparte equations separately for two of the models in the
ICTM. As was mentioned above the MODFLOW model 2 and HYDRUS model 5 are
independent. As such separate inputs (Model 2) for liner flux were used for the two models and
are summarized in the table below:
MODFLOW Liner Flux Inputs, Model 2 HTDRUS Liner Flux Inputs, Model 3 (Refers
Denison Mines
While Mesa Uranium Mill
Infiltration and Contaminant Transport Modeling Report
Review Memo
Page 12
tailings dewatering to Flux inputs into Model 5)
Assuming no-flow boundaries around the
domain, the average flux through the liners of
cells 2 and 3 calculated by MODFLOW (as a
negative recharge rate) was *6.9 X 10"^ cm/day.
As calculated by Geosyntec Consultants the
potential flux rate through the liners of cells 4A
and 4B per Giroud and Bonaparte analytic
equations in Model 3 was 2.6 X 10"* cm/day.
Per DRC review of the ICTM and cited
document (Geosyntec 2007), it is unclear how
this flux rate was derived. Therefore, DRC will
request a full explanation as well as calculations
used to derive the used flux rate.
Per the ICTM the saturated hydraulic conductivity
of the liner was selected as a fitting parameter, and
was calibrated to match potential flux rates
predicted by Knight-Piesold, based on this
methodology a calibration target flux rate of *4.6 X
10"* cm/day was used for cells 2 and 3, and a
calibration target flux rate of 2.6 X 10"^ cm/day was
used for cells 4A and 4B.
Per the saturated K method. Model 2 uses a flux rate
analog input for pseudo steady state hydraulic
conductivity of the liners (PVC and HDPE), this
assumes a 4 ft saturated thickness on cells 2 and 3
and a 1 ft saturated thickness on cells 4A and 4B.
Based on this, the actual HYDRUS inputs (saturated
hych-aulic conductivity) are 7.3 X 10"^ cm/day for
cells 2 and 3, and 9.2 X 10'^ cm/day for cells 4A and
4B.
Per DRC review of Model 3, it appears that the
values used could be appropriate for HDPE and
PVC liners based on estimated flux calculations
conducted by DRC engineering stafi". It was noted,
however, that the range of hydraulic conductivity
values is several orders of magnitude and the
developmenl of a similar flux value, as that used in
the ICTM, required the use of an assumed K value
within a large range. DRC conducted a review of
the GeoSyntec Computations (5/10/07) which were
cited in the ICTM as the source ofthe used values,
however, DRC was unable to locate the calculations
corresponding to the used values for flux.
Therefore, DUSA needs to provide the calculations
for Model 3 to justify these values.
*It is unclear why the flux values for cells 2 & 3 and 4A & 4B vary between the two models
Flux to the Ground Water Table (Model 5) - Flux through the vadose zones of the Burro Canyon
and Dakota sandstone formations in Model 5 was assumed to match the flux through the taihngs
cell flexible membrane liners (Model 3.) This flux was then applied to the vadose zone in the
Model 5, and moisture distribution was calculated. No information regarding the in sim residual
water contents ofthe vadose sandstones was provided, hence model calibration was not possible.
The use of'a 0% initial moisture content is not reasonable or conservative since the initial wetting
front will take a long time to reach the water table based on low advective velocity in the vadose
zone.
Unsaturated Flow (Models 1 and 5)
Denison Mines
While Mesa Uranium Mill
Infiltration and Contaminant Transport Modeling Report
Review Memo
Page 13
Goveming Equation — The HYDRUS-ID program solves for the Richards Equation using the
VanGenuchten/Mualem equations and inputs for water retention parameters. The ICTTvI states
that HYDRUS was selected because it is capable of simulating the dominant processes affecting
infiltration and contaminant transport in semi-arid conditions and with multiple layer covers.
The ICTM refers first to the Richards equation applicable to 1 dimensional (single dimension)
vertical flow through unsaturated media:
dQ/di=d/dz[K.(h)idh/dz+i)]~S
Where z is the vertical coordinate positive upward [L], t is time fT], h is the pressure head [L], is
the water content PL3 L-3], 5 is a sink term representing root water uptake or some other source
or sink fT-ll, and K(h) is the unsaturated hydrauhc conductivity function, often given as the
product ofthe relative hydraulic conductivity, Kr (dimensionless), and the saturated hydraulic
conductivity, Ks [LT-1].
In solving the Richards equation the ICTM states that the Van Genuchten is used to specify the
relationship of unsaturated hydraulic conductivity K(ev) vs water saturation (Sw) and of pressure
head (h) vs water saturation (6).
The ICTM lists the following equation (van Genuchten 1980) to describe the relationship of h vs
e(h)=er + e„ 8^/(1+ |ah|")"'
Or = residual water content L^L"^
Gs = saturated water content L^L'^
h = pressure head L
a = inverse ofthe air-entry value (bubbling pressure) L"^
n = pore size distribution index (Dimensionless)
m = 1-1/n, n>l (Mualem)
The ICTM lists the following equation (Van Genuchten, 1980, Mualem 1976) to describe the
relationship of K(h) vs. S*:
As written in the ICTM:
K(h) = KsS,'[l-(l-Se^'T]^
K(h,z) = unsaturated hydraulic conductivity function LT"'
Ks= saturated hydraulic conductivity LT"^
Se = effective saturation (dimensionless)
L = pore connectivity parameter (dimensionless)
Denison Mines
White Mesa Uranium Mill
Infiltration and Contaminant Transport Modeling Report
Review Memo
Page 14
M= 1-1/n, where n>l
Tailings Cell Cover (Model 1) - Hydraulic properties for the tailings cell cover materials were
estimated and taken from the soil-properties database in HYDRUS. The tailings cell cover was
assumed in the model to be unaffected by hysteresis due to uncertainty ofthe soil properties.
Tailings Model (Model 1) — Unsaturated hydrauhc properties for the tailings were estimated using
grain size data for these materials and were taken from the soil-properties database in HYDRUS.
The cover materials were simulated as loam for the top layer (topsoil and frost barrier), compacted
clay for the radon barrier and loam for the platform fill. The ICTM states that the clay type for the
radon barrier selected from the standard model inputs based on the saturated hydraulic
conductivity ofthe clay (Table 1.) The loam types were predicted using an average grain size
comprised of 43% sand, 41.5 % silt and 15,5% clay and a dry-bulk density of 1.71 g/cm^. The
clay layer input seems reasonable and representative, however, the loam input used did not
differentiate between the topsoil layer or compaction differences between the frost barrier and
platform fill.
The tailings were assmned to be unaffected by hysteresis due to uncertainty of the soil properties
and as mentioned above were assumed to have the same hydraulic properties as the tailings ctW
cover during pseudo-steady state and steady state flow. These states apply to all HYDRUS inputs
into the model.
Dakota Sandstone and Burro Canyon Unsaturated Zone (Model 5) — The characterization of the
unsaturated zone of the Dakota Sandstone and Buno Canyon Formation was achieved for
HYDRUS modeling in the ICTM by using field sample testing results from two locations. The
cores were collected during the drilling for monitoring wells MW-23 and MW-30. The cores
were taken from 4 different elevations for the MW-23 site and 2 different elevations for the MW-
30 site. The sample identification numbers include depth ranges for core samples. Per the
modeling input table in the ICTM, the sample labeled MW-30 35.5-36.0 result was used to define
the vadose zone top, the sample labeled MW-23 55.5-56 was used to define the vadose zone
middle and MW-23 74.3-74.6 was used to define the vadose zone bottom. No additional
explanation or foomole was included in the ICTM to justify the use of these particular samples or
reasoning that there are three distinct layers within the vadose zone other than differences in
hydraulic properties. The model layer thicknesses for each of these three zones are approximately
14 feet or 1/3 of the modeled total vadose domain. This seems to be an arbitrary thickness.
The document titied "Perched Monitoring Well Installation and Testing at the White Mesa
Uranium Mill, April through June, 2005" prepared by Hydro Geo Chem, Inc. for Intemational
Uranium Corporation was reviewed by DRC to determine stratigraphy encountered in the drill
holes as well as collection procedures and preservation of the core samples. The sample
stratigraphy was reviewed to detennine whether the assumption of three distinct layers is
appropriate. The stratigraphic summary for zones containing each ofthe three core samples is
included in Table 3 below.
Denison Mines
White Mesa Uranium Mill
Infiltration and Contaminant Transport Modeling Report
Review Memo
Page 15
Table 3. - Stratigraphic Summary of Core Samples used to define Hydraulic Properties for the
Dakota and Burro Canyon Unsaturated Zone (Used in Model 5)
Sample No., Depth (ft) Geologic
Formation
Description of Core per the Drilhng Log (Hydro Geo Chem.
2005)
MVv^-30, 35.5 - 36 Kd Core Log for depths 30.0-40.0 ft - Core recovery 100%, quartz
sandstone, cross-bedded, medium - grit sized grains, tan, non
calcareous, grit zone from 31.3 - 31.7, dark gray clay galls from
32.1 - 33.0 ft, no mineralized partings
Per DRC review of the core photo, it appears to be well intact, the
''clay galls" are prevalent in the photo.
Core Log for depths 49.0 - 59.5 ft - Core recovery 100%, 49.0-59.5
ft., quartz sandstone, fine — medium grained, tan, non calcareous
cement, cross-bedded, very uniform, most partings occm: along
cross beds and are mechanical (broken during drUling, no
mineralized or weathered surfaces.
Per DRC review of the core photo, the core was well intact and
appears to be uniform sandstone in the 55.5 -56 ft range.
MW-23 55.5-56 Kd
BmroK^angiim'-iBca^taftlQa^JEbctop^
MW-23 74.3-74.6 Kbc Core Log for deptiis 70.0 - 80.0 ft - Core recovery 90 % 70.0 -70.5
ft no core recovered, 70.5 -73.5 ft, siltstone, ver)' light gray-green,
soft core, low angle parting with limonite at 73.0 ft, 73.5-80 ft.,
quartz sandstone, light gray-tan to light pink-tan, limonite stained
low angle parting at 73.7 ft, grit zone at 75.0 ft and from 75.5 - 76.5
ft, small limonite blebs after sulfides'at 77.5 -78.0 ft., some
manganese dendrites from 78.5 - 79.5 ft., calcareous zone from
78.5 to 79.5 ft.
Bnjsliy^asiii'J&iation. Jinb top contact is at^apprbxiinately: 105 ^ - '126*- Ijgsr^^f^T^'^ •:::!
•Source - Titan Environmental, July 1994, Umeico Geophysical Log Data
Per DRC review of the core logs it was determined that the unsaturated zone stratigraphy .
represents a fairly uniform quartz sandstone layer, ranging from non calcareous to calcareous
cement, with minor interbeds of silt stone and non-uniform clay blebs. It was noted that the two
upper cores were within the Dakota SS formation which is a Paleozoic shoreline deposit, and* the
deepest core was frorn the Burro Canyon SS which is a primarily a Paleozoic channel deposit.
The Burro Canyon SS show a higher cementation with silica instead of kaolin, this varies from the
Dakota where most of the cementation is kaolinitic.
The core samples were analyzed by Daniel B. Stephens & Associates, Inc. According to the
summary of tests performed, all core samples were analyzed for; inilial water content, dry bulk
density, calculated porosity, saturated hydraulic conductivity (falling head), moisture
characteristics (hanging column, pressure plate, water activity meter, relative humidity box), and
unsaturated hydraulic conductivity (calculated.) A list of the laboratory methods used is included
as Appendix A to this memo.
Denison Mines
White Mesa Uranium Mill
Infiltration and Contaminant Transport Modeling Report
Review Memo
Page 16
The validity of the laboralory residual water content used for model input in developing water
saturation and unsaturated hydraulic conductivity is of concem. Table 2-1 in the ICTM
summarizes the unsaturated hydraulic properties (Van Genuchten Parameters) for cores from
While Mesa. It was noted that the residual water contents Hsted for all samples is 0.00 % vol.
The values were cross checked with plots of Moisture Content vs. Pressure Head, using the lab
data, included in the ICTM. Per review it was noted that the moisture content at a pressure head
of 1 E+06 (-cm water) was approaching 0 but did not meet the 0 axis. It was noted that the head
values at very low moisture contents contained a plot point detennined by Relative Humidity
(RH) Box analysis. The methodology for this test was not included in the ICTM and it is
unknown if the RH test provided a reliable result. Tables of data indicate tiiat a maximum of
851,293 -cm pressure was applied to the sample during analysis. Per Table 3-1 of the ICTM,
residual water contents used in the HYDRUS model were shghtiy above 0.
Per the JCTM language it is stated that residual water content in the vadose zone has not been
determined. This would also indicate that the Van Genuchten parameters for matching of
laboratory data curves and characteristic curves could not be achieved. The assumption, therefore,
appears to have been made that the residual water content in the sandstones was 0% and that flux
through this zone will initially match the hner fluxes. The ICTM does not clarify vadose zone
fiow beyond this assumption. Although Daniel B. Stephens & Associates reported the residual
water contents as 0, which DRC assumes to be an attempt on the part of the laboratory to be
conservative, Denison Mines used non 0 values. The use of the non 0 values will be slightly less
conservative due to an increased-K through the sandstone vadose layer and more pore dilution
capabihty of the sandstone matrix.
Layers and Nodes ~ The unsaturated zones are broken up inlo slightly different layers for cells 2
and 3, and 4A and 4B based on different liner materials and configurations. The HYDRUS Model
5 also depicts saturated layers in order to more accurately determine total flux ihrough overlying
layers to the water table. Table 4. depicts the layers in order to display differences between the
Cell 3 & 4 and 4A & 4B Model 5 inputs. These layers are depicted for Model 5 only, other
models use individualized inputs and layers.
Table 4. Model 5 (HYDRUS) Layers, Comparison Table for Cells 2 & 3 and 4A & 4B
K used for
Cells 2 and 3, Thickness ft \^^^]^ ,
(cm/day)
HYDRUS Loam, 3.5*
Compacted Clay, 1
Loam, 3
Tailings, 30
Sandy Loam, 1, Slimes Drain
Loam, ]
Geomembrane Liner, I*'
Loam, 0.5
6.3
0.041
6.3
146.3
106.1
6.3
7.3 X 10"'
24.96
K used for
Cells4A and4B,Thickness ft 1^^^!, ,
(cm/day)
. HYDRUS
Loam, 3.5''
Compacted Clay, 1
Loam, 3
Tailings, 30
Loamy Sand, 0.1, Slimes Drain
Geomembrane Liner, 1 "
Geosynthetic Clay Liner, 0.03
Loam, 0.5
6.3
0.041
6.3
146.3
350.2
9.2x10"'
0.024
24.96
Denison Mines
White Mesa Uranium Mill
Infiltration and Contaminant Transport Modeling Report
Review Memo
Page 17
Vadose Zone Top, 13.8
Vadose Zone Middle. 14.1
Vadose Zone Bottom, 14.1
69.81
9.33
2.47
Vadose Zone Top, 13.2
Vadose Zone Middle, 13.5
Vadose Zone Bottom, 13.3
69.81
9.33
2.47
model assumes that the top two cover layers are combined into one equivalent layer (topsoil with gravel [0.5 ft] +
frost barrier [3.0 ft]).
^ note, the geomembrane iiner layer is reflected as a 1 ft. layer and seems to correspond to the composite thickness of
the liner protective blanket and 30 ML PVC Flexible Membrane Liner in cells 2 and 3 and a composite of the 2 (60
ML) HDPE GeoTTiembrafies and Geonct for cells 4A and 4B. The model inputs are based on saturated hydraulic K as
an analog ofthe seepage flux.
Several intermediate boundary conditions were also included in the various models (Table 1): 1.
An atmospheric upper boundary condition was applied across the top ofthe model representing
the tailings cell cover to simulate meteorological conditions and as a function of precipitation and
potential evapotranspiration in Model 1, 2. Free drainage was assumed for the lower boundary
condition of the model representing the tailings cell cover in Model 1, 3. Specified fiuxes were
applied to the upper boundary at the top of the tailings in Model 4, and, 4. The lower boundary at
the base of the domain was assumed to be fully saturated.
It is unclear why FTYDRUS Model 5 uses layers above the flux into the vadose zone, given that at
this point, through evaluation of Model 2, the head in the tailings should be pseudo steady state; ^
DUSA will be asked to explain and justify the need forthe top 7 layers listed in Table 4.
Model 5 does not include a cross section spacing of nodes within the layers or at layer boundaries.
The plan does state that the nodes are most concentrated within the ceil cover systems, varied
spacing between 0.1 and 1 cm. The plan also states that node spacing is tighter in the syslem
where changes in hydraulic properties are expected. The layers and nodes used in each Model
need to be presented graphically, preferably showing nodes as applied to water contents in the
layered systems where interface reactions will occur.
Saturated Flow -MODFLOW (Model 6)
Burro Canyon - Per geologic descriptions in the Reclamation Plan, the perched aquifer is bound
on the bottom by the Brushy Basin Shale Member of the Jurassic Morrison Fonnation. Overlying
strata includes the Cretaceous Dakota Sandstone, the Buno Canyon Formation, and includes
"random discontinuous shale layers." T^e perched aquifer is commonly referred to as the Burro
Canyon Formation Saturated Zone.
Evaluation of saturated hydraulic conductivity refers to aquifer pump test, slug tests, packer tests
and laboratory analysis of core samples. The summary refers to the Reclamation Plan, Revision
3.0 (Inlemational Uranium (USA) Corporation, 2000, for detailed information regarding hydraulic
conducfivity measurements. It was noted that the sources of information and field tests used to
calculate hydraulic conductivity were taken from tests at multiple times and from other report
data, which includes; 1. Dames cS: Moore, "Environmental Report, White Mesa Uranium Project,"
January, 1978 (Laboratory Tests & Injection), 2. Peel Environmental Services, UMETCO
Denison Mines
White Mesa Uranium Mill
Infiltration and Contaminant Transport Modeling Report
Review Memo
Page 18
Minerals Corp., "Ground Water Study, White Mesa Facility," June 1994 (Single Well Drawdown
& Injection Tests), 3. Hydro-Engineering "Ground Water Hydrology at the White Mesa Tailings
Facility," July, 1991 (Single Well Drawdown and Recovery Tests), 4. D'Appolonia, "Assessment
of the Water Supply System", White Mesa Project, Feb. 1981 (Multi-Well Drawdown &
Recovery Tests.)
The summary states that horizontal hydraulic gradient in the perched aquifer ranges from
approximately 0.01 to 0.04 ft/ft and generally moves south and southwest with local variations in
magnitude and direction. DRC findings of review of the water table contour maps included in the
December 1, 2004 Statement of Basis (Attachment 1) confirm the direction of groundwater flow.
According to the Statement of Basis pp 23 "recenl water table contour maps of the shallow aquifer
have identified a significant westerly component to groundwater flow at the White Mesa
facility... this change appears to be the result of wildlife pond seepage and groundwater
mounding." Also, per DRC review of the July, 1994 Titan Environmental Corp. Documenl titied
"Hydrogeologic Evaluation of White Mesa Uranium Mill" the investigation of the perched aqiiifer
indicated an average hydraulic gradient from the center of the site to the edge of the site as 0.015
ft/ft. Although gradient calculations and potential ground water travel times seem consistent with
past studies and documentation, DRC did note that according to recent investigations the highest
gradients beneath the tailings cells (between wells MW-4 and TW4-8) are approx. 0.071,
Therefore, DUSA will be asked to verify the sotu-ces of their current gradient inputs and provide a
review of data which may be more current.
The summary slates that discharge from the perched aquifer "is believed" to be springs and seeps
along Westwater Creek Canyon and Cottonwood Wash to the west-southwest and along Corral
Canyon to the east of the site but does not attempt to confirm the hydraulic connection. The
summary does include a known surface discharge point to be Ruin Spring in Westwater Creek
Canyon located 2 miles from the taihngs cells.
The ICTM includes a summary of the volumetric flow budget used for the Model 6 inputs. The
ICTM states that water fluxes into the groundwater flow model, include vertical areal recharge
from precipitation, vertical fluxes from wildlife ponds, vertical potential fluxes from the tailings
cells and water entering through upgradieni specified head boundaries (upgradient groundwater
domain.) Additionally, water leaves the ground water flow model through specified head
boundaries, which includes simulation of horizontal groundwater flow to the aquifer beyond the
model domain and discharge to seeps and springs where the aquifer outcrops (e.g. Ruin Spring.)
The overall mass balance error for the flow Model 6 was 0. ] % which amounts to 2 ft^/day with a
total flow through the model of 2,244 ft^/day. Areal recharge from precipitation accounted for
26% of water entering the model, while fluxes from the wildlife ponds and potential fluxes from
the tailings cells (2, 3, 4A and 4B) accounted for 67% and 6% respectively. The specified head
boundaries represented less than 2% of the flow entering the model, however, specified head
boundaries accounted for 100% of the flow leaving the model. Per DRC review of geologic and
hydrogeologic descriptions and data of the area, these boundary conditions seem appropriate
given the isolated conditions of the perched aquifer.
Denison Mines
White Mesa Uranium Mill
Infiltration and Contaminant Transport Modeling Report
Review Memo
Page 19
The ICTM MODFLOW stmcture uses predicted piezometric surface contours for the perched
aquifer to test the model calibration and assumptions.
Hydraulic investigation of the saturated zones has been fairly comprehensive in relation to the
developmenl of the monitoring well network and subsurface investigation associated with
engineering approvals.
Contaminant Transport Modeling and Breakthrough Curves (Models 4. 5 and 6)
Evaluation of Tailings Wastewater Quality ~
According to the ICTM, tailings wastewater chemistry is based on data collected by Denison
Mines between September 1980 and March 2003. The ICTM states that the wastewater analysis
used is summarized and presented in the DRC Statement of Basis for the Groundwater Permit.
The ICTM states thai average values for the data are used. Table B-5 of the ICTM summarizes the
concentrations used for the wastewater quality. Per DRC review it was noted that the
concentrations used were taken directiy from the December 1, 2004 Statement of Basis for the
White Mesa Facility (jround Water Permit. The table below summarizes the tailings wastewater
concentrafions used in the model. -
Table 5 - Summary of Tailings Wastewater Concentrations Used in Models 4
Concentrations as per the December 1, 2004 DRC While Mesa Ground Water
of Basis")
, 5, and 6 (Average
Permit "Statement
Analyte
Aluminum
Ammonia
Arsenic
Barium
Beryllium
Boron
Cadmium
Calcium
Chloride
Chromium
Cobalt
Copper
Fluoride
Concentration
mg/L
1827
3131
149
0.048
0.5
6.9
3.4
368
4608
6.2
60.7
234.4
• 1695
Analyte
Iron
Lead
Magnesium
Manganese
Mereur>'
Molybdenum
Nickel
Nitrate
Phosphorous
Potassium
Selenium
Sihcon
Silver
Concentration
mg/L
2212
3
4774
146
3.5
52.8
82.6
24
273
433
1.4
210
0.1
Analyte
Sodium
Strontium
Sulfate
Thallium
TOC
Uranium
Vanadium
Zinc
Analvte
pH
pe
Temp.
Concentration
mg/L
5809
7
64914
16
78.5
94
263.1
641
Concentration
1.83 S.U.
20.2
9.9 "C
The use of average concentrations is not conservative (max. concentrations should be used) and
also the use of a non-weighted data sel as found in the Statement of Basis table is not valid since
the slimes drain samples are consistently much lower in concentration than samples laken from
the tailings cells.
Denison Mines
While Mesa Uranium Mill
Infiltration and Contaminant Transport Modeling Report
Review Memo
Page 20
Furthermore, DRC has noted, per comparison of the Statement of Basis data with data results
found in the University ofUtah Study, ''Summary of work completed, data results, interpretations
and recommendations For the July 2007 Sampling Event At the Denison Mines, USA, White
Mesa Uranium Mill Near Blanding, Utah," submitted to DRC May 2008, that the Statement of
Basis tailings maximum concentration data result is significantiy lower than the University of
Utah data.
The table below summarizes the analysis result for the 2007 University of Utah SampUng Event.
The study included a limited number of results since the primary goal ofthe study was isotopic
ground water age dating.
Table 6 - Summary of Results for the University of Utah July 2007 Sampling Evenl per the May
2008 Study Document
Site
Tailings CeU 1
Tailings Cell 2 Slimes Drain
Tailings Cell 3
Site
Tailings Cell 1
Tailings Cell 2 Slimes Drain
Tailings Cell 3
Site
Tailings Cell 1
Taihngs Cell 2 Slimes Drain
Tailings Cell 3
N02+N03, N (mg/L)
113
5.19
19.6
Selenium (pg/L)
16.200
<400.0
1,550
""U(Mg/L)
581.000
. 23,700
68.100
Sulfate (mg/L)
2,500,000
666,000
107,000.
Manganese (ug/L)
869.000
139,000
248.000
DRC expects that Denison Mines will justify the use of current average tailings wastewater
concentration data, amend the ctnrent data to include more recent data, or use maximum
concentration values.
The 2004 ''Statement of Basis" data set has an autocorrelation problem since 6 of the 17 samples
in the set were collected in March 2003 by DUSA. It was also noted that the sampling procediu-es
for these grab samples are unknown. These issues will be included in correspondence to Denison
Mines. The use of the 2004 "Statement of Basis" data will need to be justified as valid
statistically and as conservative.
Model 3 PHREEQC Development ofKd Coefficients and Rj Factors used in Model 4 -
Model 4 is a precursor lo the development of Hydrus Model 5 and is used solely for the
development of Kd Coefficients and R Factors.
Core samples from the 2005 drilling of wells MW-23 and MW-30, the same cores described in
Table 3 were used for analysis of the mass of hydrous-ferric oxide (HFO) present in bedrock
underlying the White Mesa Mill. The samples included additional depths as summarized Table 7
Denison Mines
White Mesa Uranium Mill
Infiltration and Contaminant Transport Modeling Report
Review Memo
Page 21
below. The analysis was conducted by ACZ Laboratories, Inc., report date August 27, 2007.
Results were submitted for the following parameters: Dissolved Aluminum, Dissolved Calcium,
Dissolved Iron, Dissolved Magnesium, Dissolved Manganese, and Dissolved Uranium. It was
noted that the reports from ACZ Laboratories list the concentrations in units of mass/volume,
mg/L. Foomote a) of Table B-5 in the ICTM states that "PHREEQC adjusts tiie initial
concentrations according to the total mass of solutes; as a result, all input values were converted
from mg/l to mol/kgw (moles per kg of water.)" Denison Mines U.S.A. will need to explain the
conversion from liquid concentrarion (ACZ report) to the inputs into PHREEQC as moles per kg
of water. Per review it is not clear exactiy when or how this conversion is being made.
Table 7 - Core Depths used for HFO Analysis
Core Sample ID
lMW-30
MW-30
MW-30
MW-23 •
MW-23
MW-23
MW-23
MW.23
Formation
Kd*
Kd*
Kd*
Kd*
Kbc*
Kbc*
Kbc*
Kbc*
Depth bgs (feet)
37.5-38
43^3.2
43.2-43.5
53-53.5
74-74.3
82.5-82.7
99.8-100
103-103.3
'Source - Titan Environniental, July 1994, Umctco Geophysical Log Data
Samples were also analyzed (ACZ Laboratories, Inc., report date August 10, 2007) to detenhme
the acid-neutralization potential of the bedrock. The ACZ reports include results for; Acid
Neutralization Potential (calc) and Neutralization Potential as CaC03. Per DRC review ofthe core
samples used for acid-neutralization tests, the samples were not labeled according to well # and
depth. Denison needs to clarify the source and applicability of the data.
Dz-ombak and Morel Surface Complexation Model (PHREEQC Model 4)- Appendix B of the
ICTM is titied "Speciation and Surface Complexation Modeling ofthe Tailings Porewater." This
appendix presents the methodology used to develop Kd Coefficients and Rf Factors for the Vadose
Zone Model.
Appendix B of the ICTM references the book by Dzombak and Morel "Surface Complexation
Modeling, Hydrous Ferric Oxide" (1990) which discusses surface chemistry of hydrous oxides
(e.g. Fe, AL Mg, Silica.) This chemical modeling is strongly dependent on solution pH, ionic
strength and competing ion effects. Dzombak and Morel ascertained that acid base chemistry and
oxides would lend themselves to ion sorption models (which could then be translated into Kd and
Rf variables.) Specifically the following bullets were noted in the introduction ofthe Dzombak
and Morel book which have been utilized to develop a thermodynamic database for sorption
reactions:
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L Sorption on oxides lakes place at specified coordinate sites,
2. Sorption reaction on oxides can be described quantitatively via mass law equations,
3. Surface charge results from the sorption reactions themselves, and,
4. The effect of surface charge on sorption can be taken into account by applying a
conection factor derived from the "Electronic Double Layer (EDL)" theory to mass
law constants for surface reactions.
One of the objectives of this type of modeling was to develop thermodynamic sorption databases
for ion reactions based on laboratory tests and statistical evaluation. It appears that multiple
databases which have been developed are part of the PHREEQC modeling code.
Core analysis results as described above were used as inputs to the PHREEQC Model 4 in order to
mimic tbe vadose zone mineralogy. Basically, it appears that the obtained data was used to
quantify the number of hydrous oxide ion sorption sites available. The vadose zone was broken
into 3 layers within Model 4 and as consistent with HYDRUS Model 5. Each layer represents a
different receiving mineralogy (for the tailings water) and PHREEQC has computed different Kd
and Rf values based on the Offering inputs from the ion sorption data. The PHREEQC model
looks at available chemical species present at different pH and oxidative states thus the
neutrahzation polential ofthe receiving matrix was also entered into Model 4 to mimic the
chemical species evolution present in solution at these differing states.
It appears that the ICTM is calculating an almost immediate neutralization upon tailings water
release to the vadose zone, based on the evaluation of the core samples for acid neutralization
potential due to CaC03 measurement. This is particularly tme for vadose zone layer 2. However,
based on DRC review of the DUSA acid neutrahzation potential data, 2 of the 5 samples showed
no neutralization capability, 2 other samples showed very low neutralization capability with only
0.1 % CaC03 and 1 of 5 samples showed a potentially low neutralization capabihty as 0.4 %
CaC03 (See Appendix C "Core Neutrahzation Data" for laboratory data sheets.) The ICTM
additionally refers to water quality data of the infiltrating porewaters which indicates high
buffering capacity, however, there is no data supplied with the ICTM tp support these claims.
Although zones of high neutrahzation are claimed, Denison needs to supply more information to
support these claims.
The vadose zone mineralogy has not been well characterized in the ICTM. The ICTM includes
copies of laboratory reports for core analysis (recovered from well bores — the same bores
summarized in Table 3 - for wells MW-30 & MW-23), per review^ the analysis included 6
dissolved metals: Aluminum, Calcium, Iron, Magnesium, Manganese, and Uranium; and at depths
for MW-30 of 37.5-38 ft bgs, 43-43.2 ft bgs, and 43.2^3.5 ft bgs; and at depths for MW-23 of
53.0-53.5 ft bgs, 74.0-74.3 ft bgs, 82.5-82.7 ft bgs, 99.8-100.0 ft bgs, and, 103-103.3 ft bgs. The
laboratory analysis results were then separated for each of the three layers in this zone in Model 5
(the zones were discussed above.) It is not clear how this information was used to provide a clear
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understanding ofthe mineralogy ofthe vadose zone, however, it was noted that the concentrations
have been converted to mass of HFO in the receiving mineral matrix. The ICTM states that model
5 is conservative given that it only calculates ion sorption based on a single iron-oxyhydroxide
phase and is not considering the sorption of inorganic ions to specific mineralogical matrices, e.g.
goethite.. From DRC perspective, to show that the model is truly conservative, the outputs of
several different model assimiptions should have been included and compared. This is a particular
concem since the downstream modeling (MODFLOW model 6) is dependent on the assumptions
of model 5.
Table 8 below presents the Kd Coefficients and Retardation Factors that were generated by
PHREEQC for use in the Hydrus Model 5. Note that based on the core analysis, the Kd and Rf
factors differ for the 3 vadose zone lavers.
Table 8 - PHREEQC Model 3 Generated Kd Coefficients and Rf Factors
T"te3ose3 :m^-sS
I IMS
"*:--.Li .-•'.. • '
••i ,:*
w^^ -.rr-.'.:.*
Top 7.19 82.1 0.001 0.557 0.000 4.13 0.000 0.000 9.48 0.001
Middle 7094 72140 1.033 4.90 0.115 1220 0.0003 0.000 2197 0.901
Bottom 0.119 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000-0-000
^s^tv-?i
m^
.t>-.'i
•Se^iJ. M. ms .4.
-_. •*
Top 0.000 0.014 0.005 0.015 0.000 0.002 0.000 8.47 0.000 0.009
Middle 0.000 0.663 1.380 0.015 0.000 0.003 0.0001 0.4 559 11.3
Bottom 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.000 0.000 0.000
fVadose
.Zone.
;Layer
As-.
.Rf
(l/kg)
Cd
Rf
(l/kg)
R-f •.-
-(l/kgji
'^ gGii/:.
(l/kg)-
?5ie';^H-Pb
Top 213. 2428 1.02 17 LOO 123 1.00 LOO 281 1.02
Middle 161804 1645434 25 113 3.63 27822 LOI LOO 50105 22
Botlom 3.30 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00
Vadose
Zoiie
Layer
Hg ..
Rf "
(l/kg)
Mo
Rf
Ni •
Rf'
(l/kg)
Se .
R:f :
(l/kg)
•Ag
Rf
S04
Ri:'-
--(l/kg).
Ti
Rf
•:(-l/kg):
u .
•Rf'.-
.(•l/kg)
Rf .
(I/kg).
Zn-
Top 1.00 1.41 1.14 1.46 LOO 1.07 1.00 251 LOO 1.26
Middle LOO 16 32 1.34 1.00 1.07 1.00 239 12744 260
Bottom 1.00 1.00 1.00 1.01 1.00 1.00 LOO 1.00 1.00 1.00
Model 5 Scenarios — The model assumed that for the analytes chloride and sulfate there was no
retardafion of movement through the vadose zone. The transport for these two analytes calculates
transport from the tailings liner through these zones based on average calculated hydraulic
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conductivity assumptions. /Uthough there are no state water quality standards associated with
sulfate and chloride these analytes are modeled more extensively since they arc considered
completely mobile and conservative. The natural background concentrations in the receiving
rnedia for chloride and sulfate were set at zero and not considered in the transport calculation.
Model 5 Predicted Breakthrough Curves ~ Four plotted breakthrough curves were submitted with
the ICTM. It was noted that the plotted breakthrough curves only relate to predicted
concentrations in the vadose zone from tailings pore waier within 200 years and only included
single ion analysis for Cl, S04 and U. The plots are included in Appendix B. of this memo.
Table 5 below summarizes the infonnation provided on the plots.
Table 5 - Summary of submitted breakthrough curve plots, HYDRUS (Model 5) - Note: Plots
included in Appendix B
Plot Figure No. & Titie
Figure 4-5 - Model 5 Predicted
Chloride Concentrations in
Vadose Zone Pore Water
Immediately Above the Perched
Aquifer Beneath Cells 2 and 3
Figure 4-6 - Model 5 Predicted
Chloride Concentrations in
Vadose Zone Pore Water
Immediately Above the Perched
Aquifer Beneath Cells 4A and 4B
Figure 4-7 - Model 5 Predicted
Sulfate Concentrations in Vadose
Zone Pore Water Immediately
Above the Perched Aquifer
Beneath Cells 2 and 3 (Note: S04
plot was not included in the ICTM
for concentrations below cells 4A
and4B)
Figure 4-8 - Model 5 Predicted
Dissolved Uranium
Concentrations in Vadose Zone
Pore Water at 200 Years
Nanative
Per the ICTM description the chloride concentration in the
HYDRUS model 4 was initially set to 0 mg/L at the lower
model boundary. The plot shows that (with cunent
inputs), chloride will not arrive at the top of the perched
aquifer until 130 years after model start time below cells 2
and 3, The chloride concentration then rises from 0 mg/L
lo 0.38 mg/L in 70 years. The modeled flux rate beneath
cells 2 and 3 upon steady state is 2.5 X 10"^ cm/day.
The plot shows that chloride concentrations will not arrive
at the top of the perched aquifer until 145 years after the
model start time below cells 4A and 4B. The chloride
concentration at 200 years is 0.012 mg/L. The modeled
flux rate beneath cells 4A and 4B upon steady state
(approx. 200 years) is 1.4 X 10*^ cm/day.
As with chloride, the sulfate concentration in the
HYDRUS model was initially set to 0 mg/l at the lower
model boundary: The plot show that sulfate will not
migrate to the top of the perched aquifer beneath cells 2
and 3 for approx 155 years. The concentration in this
zone in 200 years is 0.08 mg/L.
The plot represents Dissolved Uranium Concentration
(mg/L) against Depth Below Tailings.Cell Liner (cm.)
The plot depicts that (with current modeling inputs)
uranium concentration at 200 years will not be deeper than
approx 23 feet in cells 4A, 4B, 2 and 3, however, it was
noted by DRC that the scale on the figure is not adequate
to show U concentrations at the Utah Water Quality
Standard (0.030 mg/L). The figure needs to be rescaled to
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show the depth ofthe 0.030 mg/L concentration. The plot
also depicts approx -10 mg/l concentration differential
under cells 4A and 4B from cells 2 and 3 to a depth of 10
ft then a narrowing of concentration differentia] to 0. It
was noted that the dissolved U concentrations in cells 4A
and 4B showed a lower concentration curve (raised) down
to a depth of approximately 23 feet. (See Appendix B
Uranium Plot, Figure 4-8)
Model 5 Kd Coefficient and R Factor Assumption Weaknesses - After DRC review of the model
5 inputs and assumptions to derive contaminant transport through the vadose zone, it was
recognized that the modehng platform chosen is not adequate. Specifically, the development of
Kd Coefficients and R Factors is all contrived through modeling assumptions which may not be
reflective of tme site conditions.
DRC delermination is that the development of Kd Coefficients and Rf Factors through empirical
procedures is completely feasible and will provide a much more realistic determination ofthe
mineral matrix within the vadose zone. Additional field data needs to be collected and analyzed.
Sensitivity Analysis - The issue of sensitivity analysis was briefly mentioned above in terms of
Model 2 inputs and claims that the modeling inputs are conservative. DRC noted that the actual
output files should be included with the ICTM to show that this is the case, especially in terms of
derived mode] inputs and assumptions. Per DRC review, and as per the section above, the model
5 assumptions are not appropriate since empirical data will supersede the current model 5
assumptions.
The redevelopment/refining of Model 5 will also require that Model 6 be reconstmcted based on
the new Model 5 output. It is DRC's recommendation that DUSA prepare a robust discussion of
sensitivity analysis for Model 5 and Model 6. Ultimately, from DRC perspective, sensitivity
analysis should determine whether sufficient data has been collected to deem the model output as
credible. If the choice is to not collect additional data, then the sensitivity analysis needs to
detennine that, based on what has been collected, the model output is the most conservative (e.g.
is based on conservative flow and transport inputs.) In many cases, it is expected that
conservative modeling will guide the need for additional empirical data. As noted, this is the case
for Models 5 and 6.
Conclusion
Generally, DRC review concludes that empirical data is lacking to develop a representative model
of infiltration, flow and chemical transport. This is primarily due to a lack of site specific data or
local data reference regarding long term vegetation establishment inputs (e.g. plant density, leaf
index, and rooting structure, etc.), long term degradation of the system (e.g.' cover flow impacts
from frost heaving, erosion, intrusion, etc.), tailings wastewater pollutant concentrations based on
sound statistical evaluation and representative sampling, initial vadose zone water content (based
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on empirical core evaluation), vadose zone mineralogy, and geochemical Kd and R characteristics
(based on empirical data,)
RFI Requirements
The following additiona] information will be required and will be hsted on an RFI letter to
Denison Mines U.S.A.
White Mesa Cell 1 RFI
Per the ICTM, White Mesa states that Cell 1 will not contain mill taihngs upon decommissioning
of the sile. It appears that Cell 1 has.not been included in the ICTM based on the detennination
by Denison Mines U.S.A. that it does not meet the permit language requirement of a closed
tailings cell.
DRC does not agree with this interpretation for the following reasons:
1. The demolition and decommissioning wastes in question will be contaminated witii by-
product material (See Utah Administrative Code (UAC) R313-12-3.)
2. Denison Mines U.S.A. has not made a demonstration that the wastes in cell 1 meet the
definition of "de-minimus" effect on local ground water quality, pursuant to UAC
R317-6-6.2(A)(25), nor has the Executive Secretary approved such a demonsti-ation.
As a result, Cell 1 does not qualify for Permit-by Rule status.
Therefore, because contaminated materials from mill site decommissioning, including by-
product materia] will be disposed in Cell 1, Cell 1 is required to be modeled, and included in
the ICTM. Alternatively, Denison Mines U.S.A. may submit and secure Executive Secretary
approval of an apphcation that demonstrates how Cell 1 will have a "de-minunus" effect on
local ground water quality pursuant to UAC R-317-6-6.2(A)(25.)
Model Platforms RFI
DUSA needs to justify the use of independent sequenced modeling platforms. DRC noted that
input errors from one model to the next could result in trickle down errors of great magnitude.
Table 1 and the accompanying flowchart display the weakness ofthis approach.
1. Denison Mines U.S.A. has not provided adequate justification that the conceptual
approach of using several sequenced, compartmentalized models (HYDRUS,
PHREEQC, and MODFLOW) provides a representative or conservative simulation of
tailings cell performance.
2. Provide verification that the use of a variable saturated coupled flow and transport
model (VSCF model) was considered in the conceptualized modeling phase and
• justification why the VSCF modeling platform was not appropriate or necessary.
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Cover Design RFI (Model 1)
1. The modified cover design eliminates the cobble surface layer. Please justify that long
term surface erosion will not occur with the new surface layer. Specify topsoil
gradations (e.g. admixture components) which will be used to replace the rip-rap layer.
2. The modified cover includes a top vegetated layer. The ICTM only describes the
vegetation type as grasses. Please specify vegetation grass species and justify long
term growth on the cover including reasoning for not including long term sustainability
and potential for in growth of other natural species or plant succession (e.g. indigenous
brush.) Also, provide additional information regarding root structures and plant type
inputs entered into the HYDRUS 1-D surface layer model regarding long term
establishment of vegetation. Provide the basis for any assumptions made, including
documented studies of local vegetation/soil models (e.g. lysimetry) to support claims.
3. The HYDRUS inputs for ET from plant surfaces are unclear. Per appendix C, a grass
layer was entered into the HYDRUS model. Please provide information regarding the
details of this entry. Examples of needed information include the specific vegetation
type or species, cover density and rooting depths and density, leaf area index, andra
description of calculations performed in the FTYDRUS model and self-sustainability of
the vegetation at the White Mesa Facility. Provide specific reference to studies used to
support the long term vegetation estabhshment and assumed ET component.
4. The modified cover does not appear lo include design to avoid biointrusion into
underlying layers (e.g. the compacted clay, radon barrier.) Please justify why this is
not a concem for long term degradafion, including justification that the 3 ft frost
barrier layer provides adequate protection from intmsion by roots, animals, etc. into
the underlying layers. Alternatively, modify the design to include a biointrusion layer
at an appropriate location above the radon barrier.
5. Provide documentation of frost depth analysis and maximum projected cover frost
penetration depth in the ICTM to justify that the 3 ft frost barrier thickness is adequate.
Provide reference to local and/or regional studies to support all claims.
6. Provide justification why the daily average precipitation rates used for the modeling
assumption are representative (or conservative) of true field condirions and storm
intensity effects. Specifically, the HYDRUS model distribution of a daily average
precipitation over a 24 hour period negates storm intensity effects which are pertinent
to the semi-arid environment at White Mesa. This assumpfion substantially changes
soil saturation and hydrologic effects due to surface infiltration variations. Explain
how the effects of these processes may change hydrologic properties of the cover and
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justify how these 2 and 3-dimensional phenomena can be modeled with a 1-
dimensional model (HYDRUS 1-D.)
7. It was noted that the ICTM references ET cover design studies and articles in which
the majority of the top cover designs include a capillary barrier and intemal drainage
layer. Please provide discussion and justification in the ICTM regarding the omission
of a capillary barrier/drain layer and surface runoff layer in the top cover design at
White Mesa.
8. Include discussion regarding how the HYDRUS 1-D modeling inputs account for any
long term changes of saturated hydrauhc conductivity in the vegetated suiface layer or
increases in saturated hydraulic conductivity within the compacted radon-barrier clay
layer due to long term degradation (e.g. root penetration, freeze-thaw damage, etc.)
9. Provide additional infonnation ifegarding seasonal variations of ET and water storage
in the cover and how the HYDRUS model compensates for such seasonal differences
(e.g. frozen soil and snow accumulation, spring snow-melt, etc.)
10. Explain and justify why the HYDRUS model assumes die FML to be 1 fool thick.
Water Balance RFI (Models 1,2,5, and 6)
1. Provide a cross section(s) depicting: I) The model layers and node geometry used in •
each model, and, 2) A plot of steady state water saturation with depUi from the top of
the cell cover surfaces (cell's 2, 3, 4,4A) vertically downward lo the top of die water
table. Identify on the cross section(s) which models were used to simulate the
hydrauhc performance of each specific layer.
2. Provide graphs to demonstrate steady state water content at representative depth
intervals for each layer in the model to represent flux through the Uner system
(recharge component), flux through bottom hner for each cell, and flux through vadose
zone to the perched aquifer for each cell (based on steady state saturation) and the
modeled time (years) needed to reach steady state. The ICTM includes average flux
through the cover system and anticipated fluxes through the vadose zone, however,
these flux rates are not clearly associated with steady state predictions of water content.
3. For the unsaturated tailings and vadose zone please justify why the initial saturation in
the PTYDRUS model (Model 5) was entered as 0%,
4. Please provide and justify the Van Genuchten/Mualem fitting parameters used in the
HYDRUS model for both the unsaturated tailings (Model 1) and vadose zone materials
(Model 5). Please ensure this justification accounts for grain size distribution and
corrections for differential deposition within the tailings cell (e.g. shore deposits vs.
mid ceU deposition.)
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5. The slimes drain layer shown in cross section for cells 2 and 3 depicts a uiuform sand
layer, per cell design and constmction specificafions this layer is not uniform across
the bottom of each cell. Explain potential input differences based on anon-uniform
sand layer. Emphasis is needed in explaining how and why a gravity driven, head
dependent sink term in the MODFLOW model (Model 6)is representative of actual
slimes drain construction and operation, that includes but is not limited to episodic
operation of the slimes drain pump.
6. Provide calculations and plots of data reflecting expected/estimated leakage rates from
the single FML (via factory defects) into tailings cells 2 and 3 foundations (e.g.
specific calculations of Giroud and Bonaparte (Model 3) as representative of White
Mesa materials and installation, and/or leakage rales estimated by field study,) Also,
provide details of Giroud and Bonaparte Model for cells 4A and 4B (Model 3) as
approved by DRC and appropriately applied to the ICTM modeling.-
Flow Modeling RFI
1. Provide additional clarification regarding the 3 vadose zone intervals used in the .-.
HYDRUS model (Model 5), including specific descriptions of each to discuss the
different hydrauhc characteristics of each zone. Provide additional justification
regarding the thickness ofthe zones. Provide a stratigraphic cross section ofthe
vadose zone intervals in the ICTM, based on core log mineralogical evaluation and/or
other data. For ease of review, provide the vadose zone interval descriptions, summary
of data, and stratigraphic representation in a single section within the report.
2. Provide information regarding enor flags on the flow modehng runs (HYDRUS
Models 1 and 5, and MODFLOW Models 2 and 6), including error messages for
potential random data based on statistic values and the impact of these errors on the
model output. DRC noted that several different types of error messages were logged
on the HYDRUS output decks. It would be useful for Denison to include a table in the
ICTM summarizing the location of these enor flags and explaining why the messages
occurred, their impact on model output results, and potential enors which may be
present in the output data.
3. Provide clarification regarding HYDRUS (Vadose Zone Model 5) and MODFLOW
(Dewatering Phase Model 6) interface during transient dewatering and corresponding
flux rates through the bottom liner. DRC needs to understand how the flux through the
bottom liner during operations.and during slimes drain dewatering (transient flow
period) is accounted for. According to language in the ICTM, it appears that
wastewater entering the vadose zone during this time period is not included based on
the assumption that it will have a minimal effect on long term model outcomes. Please
jusrify this position arid explain why contaminant mass released to the foundation
during the operational phase ofthe tailings cells can be ignored in the transport
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predictions. This is particularly important in light of the fact that the highest driving
heads are likely found during the operation of the taihngs cells.
4. Provide a graph depicting model layers associated with the HYDRUS Vadose Zone
flow model (Model 5) to depict steady state water contents in each layer.
5. Provide a specific discussion regarding differential flow characteristics which define
the layers in tiie model (e.g. differences in K,. flow direction, head distributions, etc.)
This could be achieved through graphical representation and additional explanation of
Table 3-1 in tiie ICTM.
6. Justify the use of a head dependent sink term in the MODFLOW model for tailings
dewatering (Model 2). Explain and quantitatively justify how the simulation is
representative of pmnp dependent slimes dewatering systems at each of the tailings
cells.
7. . Explain and Justify why HYDRUS Model 5 breakthrough curves were based on a
lower flux rate than predicted for other upstream models (Model 3.)
8. Provide specific calculations used to determine the geomembrane flux rates for cells 2
and 3.
Contaminant Transport Modeling RFI
DRC has significant concems with the current contaminant transport modehng, models 5 and 6
per the flow chart above.
Two primary issues were evident through DRC review; 1. Tailings wastewater chemistry
calculations are not conservative, and 2. The PHREEQC ion sorption model is based largely on
assumption and sketchy field data which does not adequately represent the receiving
mineralogical matrix or geochemical environment.
The ICTM HYDRUS model 5 fails to provide a reasonably conservative evaluation of the
physical system. This is primarily due to a lack of empirical data to represent chemical
partitioning and retardation through the vadose zone. The failure of HYDRUS model 5 is further
exasperated downstream in MODFUDW Model 6 where non-conservative HYDRUS inputs are
used for the development of Cl and S04 breakthrough curves.
Reguest for Information
1. Regarding the evaluation ofthe tailings cell wastewater chemistry, the use of average
concentrations is not conservative (max. concentrations should be used) and also the
use of a non-weighted data set as found in the Statement of Basis table is not valid
since the slimes drain samples are consistently much lower in concentration than recent
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University of Utah samples taken from the tailings cells. DRC also noted that no
tailings cell wastewater data newer than the 2004 Statement of Basis (such as data
included in the University of Utah Study document, "Summary of work completed,
data results, interpretations and recommendations For the July 2007 Sampling Event
At the Denison Mines, USA, White Mesa Uranium Mill Near Blanding, Utah,"
submitted to DRC May 2008) has been incorporated into the data evaluation. DRC is
also concemed that auto-conelation is present in the data set used for the ICTM.
Please revise the evaluation of the taihngs wastewater and include conservative
estimates of the wastewater contaminants along with a full statistical review and
justification of the data set used.
2. Clarify whether flux rates from the tailings cell liners, used to evaluate the 200 year
contaminant iransport breakthrough curves include differentiating flux during transient
phases of dewatering heads on the bottom liner. Include references to specific
files/input decks showing the differential heads if applicable. If the transient
dewatering phase is not included in transport modeling provide justification why this
would not impact the result of the 200 year breakthrough curve modeling.
3- Per the ICTM, p. B-6, "water quality data for the White Mesa Mill tailings porewaters
and leach extraction data for the underlying bedrock was examined to calculate -
adsorption of dissolved species under varying geochemical conditions." Provide
spreadsheets summarizing the data used as well as a copy of the laboratory data sheets
including QA/QC verification of results and statistical analysis ofthe data set used.'
Provide concentration data which was used for adsorption calculations and an
explanation of the calculations made. Please demonstrate how these laboralory tests
and calculations were performed using standardized methods recognized by the
regulatory and relevant technical communities.
4. Provide additional justification that the laboralory results of digested core data are
representative of the mineralogy of the vadose zone. Also, please prepare spreadsheets
summarizing laboralory data results for the core samples encompassing all results
characterizing the mineralogy.
5. Addifional clarification is needed regarding the modehng of tailings leachates and bulk
reactions with the receiving aquifer matrix. Please perform laboratory tests for Kd
using representarive soils/rock in the presence of multiple tailings leachate sampies
with a range of contaminant concentrations to better characterize the geochemical
relarionships. DRC anricipates that the development of empirical Kd coefficients and
Retardafion Factors will supersede the use ofthe PFCREEQC ion sorption model.
6. Figure 4-8 of the ICTM, "Model-Predicted Dissolved Uranium Concentrations in
Vadose Zone Pore Water at 200 Years" does not allow review of the migration of
dissolved U below the tailings impoundments to the State Ground Water Quality
Standard (GWQS) concentration, 0.030 mg/L. The figure needs to be magnified (re-
Denison Mines
White Mesa Uranium Mill
Infiltration and Contaminant Transport Modeling Report
Review Memo
Page 32
scaled) to allow review of U migration to tiie GWQS of 0.030 mg/L. Also, please
insure that all breakthrough curves which depict parameter (pollutant) concentrations
(based on revised modehng) which are subject lo a State Water Quality Standard, are
drawn to scale lo show the State Standard.
7. The current geochemical model does not appear to anticipate the effects of changes in
neutrahzation and HFO capabihty as minerals are consumed. Provide explanation of
how this factor was considered in the model.
8. Per DRC review it was noted that 11 potential contaminants are hsted with very low
Kd values predicted by PHREEQC. Although the breakthrough curves for CL and
S04 are considered conservative by the model justification that they are modeled as
un-retarded, they also do not have associated water quality standards for comparison of
predicted concentrations with time. In association with the additional breakthrough
curves please justify why the Kd and Rf used for Uranium is conservative in relation to
the geochemical environment.
References
Albright, Wilham H., Benson, Craig H., Gee, Glendon W., Roesler, Arthur C, Abichon, Tarek,
Apiwantragoon, Preecha, Lyles, Bradley F., and. Rock Steven, A., 2004 "Field Water Balance of
Landfill Final Covers", Journal of Environmental Quality 33:2317-2332.
Denison Mines (U.S.A.) Corp., Modeling Report, While Mesa Mill Site, Blanding, Utah,
November 2007 .
Dzombak, David A. and Morel, Francois M. M., 1990, "Surface Complexation Modeling,
Hydrous Ferric Oxide," John Wiley & Sons Inc.
Environmental Protection Agency, Understanding Variation in Partition Coefficient, K<j Values,
EPA 402-R-99-004B. August 1999
R. Allan Freeze & John A. Cherry, "Groundwater". 1979
Hydro Geo Chem Inc., prepared for Intemational Uranium Corporation, August 3,2005; "Perched
Monitoring Well Installation and Testing at the White Mesa Uranium Mill April through June
2005
Intemational Uranium U.S.A. Corp., White Mesa Mill Reclamation Plan, Revision 3.0, July 2000
Meyer, P.D., Rockhold, M.L., Nichols, W.K, Gee, G.W., Pacific Northwest laboratory (Battelle
Memorial Institute), 1996, "Hydrologic Evaluation Methodology for Estimating Water Movement
Through the Unsaturated Zone and Commercial Low-Level Radioactive Waste Disposal Sites"
prepared for the U.S. Nuclear Regulatory Commission, NUREG/CR-6346 PNL-10843, 1996
Denison Mines
White Mesa Uranium Mill
Infiltration and Contaminant Transport Modeling Report
Review Memo
Page 33
Smith, Gregory M., Weston, Roy F., U.S. Department of Energy Grand Junction Office, 1999,
"Evolution of Disposal Cell Cover Design Used for Uranium Mill Tailings Long-Term
Containment"
Van Genuchten, M. Th, 1980. A Closed-form Equation for Predicting the Hydraulic Conductivity
of Unsaturated Soils
Waugh, W.J., 2002 for U.S. Department of Energy Grand Junction Office, "Monticello Field
Lysimetry: Design and Monitoring of an Altemative Cover"
APPENDIX A - Core Sample Analysis
Denison Mines
While Mesa Uranium Mill
Infiltration and Contaminant Transport Modeling Report
Review Memo Appendix A. ASTM Core Analysis Methods Summary
Page 1
Core Samples for MW-23, and MW-30 (White Mesa Uranium Mill) were analyzed by
Daniel B. Stephens & Associates, March 21, 2007
ASTM Methods Used and summary:'
DRY BULK DENSITY
ASTM D453] - "Standard Test Methods for Bulk Density of Peat and Peat Products",
The method covers the measure of bulk density of samples taken with a piston sample or
appropriate core sampler to obtain an undisturbed core, or by the paraffin wax method
using undisturbed irregular pieces of clods of wet peat and compressed peal. The
methods correspond to direct wet mass and mass by water displacement respectively.
DRY BULK DENSITY AND MOISTURE CONTENT
ASTM D6836 - "Standard Test Metiiods for Determination of the Soil Water
Characteristic Curve for Desorption Using a Hanging Column, Pressure Extractor,
Chilled Mirror Hygrometer and/or Centrifuge" Includes methods A. hanging columns
suitable for making determinations for suctions in the range of 0 to 80 kPa, B. Pressure
Chamber with Volumetric Measurement and C. Pressure Chamber with Gravimetric
Measurement suitable for suctions in the range of 0 to 1500 kPa, D. Chilled Mirror
Hygrometer suitable for determination for suctions in the range of 500 kPa to 100 MPa
and E. Centrifuge Method suitable for determinations in the range 0 to 120 kPa.
Method A. - coarse soils with little fines that drain readily
Methods B. and C. - finer soils which retain water more tightiy
Method D. ~ used when suctions near samration are not required and commonly is
employed to define the dry end of the soil water characteristic curve (>1000 kPa)
Method E. - typically used for coarser soils where an appreciable amount of water can be
extracted with suctions up to 120 kPa
Methods A-C - Yield soil water characteristic curves in terms of matric suction. Various
suctions are applied to the soil and the conesponding water contents are measured. Two
different procedures are used to apply the suction, Method A - tbe matric suction is
applied by reducing the pore water pressure while maintaining the pore gas pressure at
the atmospheric condition, Methods B and C - the pore water pressure is maintained at
atmospheric pressure and the pore gas pressure is raised lo apply the suction via the axis
translation principal. For all three methods, saturated soil specimens are placed in
contact with water saturated porous plate or membrane. The matric suction is applied by
one of the two aforementioned procedures. Application ofthe matric suction causes
water to flow from the specimen until the equilibrium water content corresponding to the
Denison Mines
White Mesa Uranium Mill
Infiltration and Contaminant Transport Modeling Report
Review Memo Appendix A. ASTM Core Analysis Methods Summary
Page 2
apphed suction ids reached. Equilibrium is established by monitoring when water ceases
to flow from the specimen. Several equilibria are established at successive matric
suctions to construct a soil water characteristic curve. The water content corresponding
to the applied suction is determined in one of two ways. For A and B, the volume of
water expelled is measured using a capillary tube. The water content is then determined
based on the known initial water content of the specimen and the volume of water
expelled. Method C. the waler content is measured gravimetrically by weighing the
specimen, after removal from the apparatus.
Method D - Yields soil water characteristic curve in terms of total suction. Water
content of the soil is controlled and the corresponding suctions are measured. Based on
this, two approaches are used, In one approach a set of specimens are prepared that are
essentially identical, but have different water contents. Water contents are selected that
span the range of water contents that will be used to define the soil water characteristic
curve. In the other approach a single specimen is used. The specimen is tested, dried to a
lower water content and then tested again, this process is repeated until suctions have
been measured at all ofthe desired water contents.
Method E. - Yields a soil water characteristic curve in terms of matric suction or
capillary pressure. Different matric suctions are appUed by varying the angular velocity -
of the sample and measuring volmne of waler displaced from the soil
MOISTURE COISTENT
ASTM D2216 - "Standard Test Methods for Laboratory Delermination of Water Content
of Soil and Rock by Mass" Two test method which cover the laboratory determination of
the water content by mass of soil, rock and similar materials where the reduction in mass
by drying is due to loss of water. Due to drying difficulties with materials such as
gypsum or dissolved solids the test may require different drying temperatures or humidity
conditions. Assumptions are made that mass lost in oven drying is water content.
CALCULATED POROSITY
ASTM D 2435 - "Standard Test Methods for One-Dimensional Consolidation Properties
of Soils Using Incremental Loading" In this test method a soil specimen is restrained
laterally and loaded axially with total stress increments. Each stress increment is
maintained until excess pore waler pressures are completely dissipated. During the
consolidation process, measurements are made of change in the specimen height and the
data are used to delermine the relationship between the effective stress and void ratio or
strain, and the rate at which consolidation can occur by evaluating the coefficient of
consoUdation.
Denison Mines
White Mesa Uranium Mil!
Infiltration and Contaminant Transport Modeling Report
Review Memo Appendix A. ASTM Core Analysis Methods Summary
Page 3
PRESSURE PLATE
ASTM D2325 - "Standard Test Metiiod for Capillary-Moisttire Relationships for Coarse
and Medium Textured Soils by Porous Plate Apparatus" This test method covers the
detennination of capillary moisture relationships for coarse and medium textured soils as
indicated by the soil-moisture tension relations for tensions between 10 and 101 kPa (.1
and } atm. Under equilibrium conditions, moisture tension is defined as tbe equivalent
negative gage pressure or suction corresponding lo soil moisture content. This test
method determines the equilibrium moisture content retained in a soil subjected to a
given soil water tension. This test method is not suitable for fine textured materials.
This test measures water content at different pressures across a range.
SATURATED HYDRAULIC CONDUCTIVITY FALLING HEAD RISING TAIL
ASTM D 5084 "Standard Test Methods for Measurement of HydrauUc Conductivity of
Saturated Porous Materials Using a Flexible Wall Permeameter**
Metiiod A - Constant Head
Method B - Falling Head, constant tailwater elevation
Method C - Falhng Head, rising tailwater elevation
Metiiod D - Constant Rate of Flow
Method E - Constant Volume - Constant Head
Method F - Constant Volume Falling Head
Test methods per Darcys law at saturated pressure head greater than or equal to 0
(atmospheric.)
As noted in the ASTM - The correlation between small sample size K measurements are
seldom applicable lo field scale measurements. The systems are designed to determine
the hydraulic conductivity of a material as rapidly as possible.
.APPENDIX B - ICTM Plots of Flux Rates and Breakthrough Curves
DRC NOTE: Figure 4-1 Plotted from HYDRUS Model 1 Output
DRC NOTE: Figure 4-2 Plotted from MODFLOW Model 2 Output
o
in
050
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(1113) pv3H uncsajj
DRC NOTE: Figure 4-3 Plotted from MODFLOW Model 2 Output
o: o
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(^p/taa) ajry xnij jajiA\
DRC NOTE: Figure 4-4 Plotted from HYDRUS Model 4 Output
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(q/Sm) goi)Kjju»RO:> spuoiq;;)
DRC NOTE: Figure 4-4 Plotted from HYDRUS Model 4 Output
DRC NOTE: Figure 4-4 Plotted from HYDRUS Model 4 Output
d.<
m m
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DRC NOTE: Figure 4-4 Plotted from HYDRUS Model 4 Output
APPENDDC C - CORE NEUTRALIZATION DATA
Laboratories, Inc.
2773 Downtiill Drive Sleamboat Springs. CO 80487(800) 33'i-5493
Inorganic Analytical
Results
MWH America's Inc.
Projecl ID:
Sampte ID: L61917-01
ACZ Sample ID: L64240-01
Date Sampled: 08/03/07 09:55
Date Received; 08/03/07
Sample Matrix: Soil
Soil AnalysiE
Parainel9r
Acid NeutralizatJon
Potential (calc)
Neutralization
PotenLal as CaCOS
EPA Method
MB00/2-7B-054 1.3
MeOO/2-7a-05A 3.2.3
Soil Preparaiton
Crush and Pulverize USDA No. 1,1972
Result .Qual XQ Units MDL PQL .Oate Analyst
1
0.1
t CaC03/Kt 1 5 08/08/07 16:22 calc
% 0.1 0.5 08/04/07 9:36 Iwt
EPAMctlipd Result- r. Qual'XQ Unitsr —MDL "PQL " " rDale --:. Analyst
08/03/07 14:00 Iwt
REPIN.02.06.05.01 Please refer to Qualifier Reports for defaiV.
L62140: Pa?e2of 17
Laboratories, Inc.
2773 Downtiill Dnvf SteBmboel Sorings, CO 30487(800) 334-5493
MWH America's Inc.
Project ID;
Sample ID: L61917-02
Inorganic Analytical
Results
ACZ Sample ID
Date Sampled
Date Received
Sampie Matrix
L64240-02
0&03/07 09:55
08)03107
Soil
Soil Analysis
Parameter
Acid Neutralczalion
Potential (calc)
Neutraltzation
Potential as CaC03
.EPA Mettiod
M600/2-7B-0&4 1.3
M600/2-7B-054 3.2.3
Soil Preparstign
Crush and Pulverize USDA No. 1,1972
Result /..QuDl. XQ Units WIDL.PQL. Date Analyst
1
0.1
1 CaC03/Kt 1 5 08/08/0716:22 calc
% 0.1 0.5 08/04/07 10:07 Iwt
EPA Method Result . Qual XQ Units- WIDL' PQL • • Dale" " AnnlyM
08/03A)7 14:03 Iwt
REPIN.02.06.05 01 ' Please refer to Qualifier Reports for detail-
Lei) 40: Page 3 of 17
Laboratories, Inc.
2773 Downtiill Dnve Steamboel Springs, CO 80487(800} 3S'i-5493
Inorganic Analytical
Results
MWH Amenca's Inc.
Project ID:
Sample ID: L61917-03
ACZ Sample ID: L64240-03
Dale Sampled: 08/03/07 09:55
Date Received: 08/03/07
Sample Matrix: Soil
Soil AnalyBiE
Parameter • . i. . EPA Method
Acid Neiitralizatlon
Potential (calc)
Neutralization
Polential as CaC03
M60D/2-78-05'} 1.3
M600/2-78-054 3.2.3
Soil Preparation
Crush and Pulverize USDA No. 1, 1972
Result .. QiiBl XQ" . Units . .lUlDL PQL Date . Analyst
1 CBC03/Kt 1 5 08/08/07 16:22 calc
V 0.1 0.5 08/04/07 10:39 M
.•EPATJlclhod ~r Result- iCQualXQ"- Units". .MDL PQL . "^ Dale"
08/03^7 14:07 Iwt
REPIN.02.06.05.01 * Please refer lo Qualifier Reports for detail.
L62)40: Paee 4 of 17
Laboratories, Inc.
2773 Downhill Drive Steamboat Springs. CO 80487(800) 33^-5493
Inorganic Analytical
Results
MWH America's Inc,
Project ID:
Sample ID: L61917-04
ACZ Sample ID: L64240-04
Date Sampled: 08/03/07 09:55
Date Received: 08/03/07
Sample Matrix: Soil
Soil Anaiyais
Parameter
Acid Neutralization
Polential (calc)
NBUtraiization
Potential as CaC03
EPA Method
M600/2-76-054 1.3
M600/2-7e-Q54 3.2.3
Soil Preparation
Crush and Pulverize USDA No. 1.1972
EPA Method
Result Qual Xq Units . MDL PQL Date Analyst
A tCaC03/Kt 1 5 08/08/07 16:23 calc
0.4 B * % 0.1 0.5 08/04/0711;11 Iwt
Result Qual XQ Units MDL -PQL . Date 1 Analyst
08/03/07 14:11 Iwl
REPlN.02.0e .05.01 * Piease refer to Qualifier Reports for detail
L6:]40; Page 5 of 17
Laboratories, Inc.
2773 Downhill Drive SlaamboBl Springs. CO 80487(800) 334-5493
Inorganic Analytical
Results
MWH America's Inc.
Project ID:
Sample ID: L61917-05
ACZ Sample ID
Date Sampled
Date Received
Sample Matrix
L64240-05
08/03/07 09:55
08/03/07
Soil
Soil Analysis
Parameter
Acid Neutralizabon
Potential (calc)
Neutralization
Potential as CaCOS
EPA Method
M600/2-78-054 1.3
M60O/2-7e-054 3.2.3
Soil Preparation
Crush and Pulverize USDA No. 1, 1972
Result Qual XO. Units . MDL PQL . Dale . Analyst
0 t CaC03/Kt 1 5 08/08/07 16:23 calc
% 0.1 0.5 08/04/07 11:43 Iwt
EPA Method Result Qual XQ- Units MDL PQL Date . Analyst
08/03/07 14:15 iwt
REPIN.02.06.05.01 * PIsase refer ro Qualifier Reports for detail
L62J40: Page 6 of 17