DRC-2009-006469 - 0901a0688014f6a9

lONM HI'NTSMAN..IR
(iii\ i-)ilii)
GARY HERBERT
l.ienlL-'iiitii (ri/vt-rr}'ir
IXc^Po^^i'^oo^^-l^i
State of Utah
Department of
Environmenlal Quality
Richard W Sprutl
E.wi-iili'.f Dircrtur
Oi\ isioii of U,i(iintit.in Conlrol
Dnnc L. Fincrtrock
Din-rior
Febriiiiry 2, 2009
CERTIFIED MAIL
(Return Receipt Requested)
David C. Frydenlund
Vice President. Regulatory Affair.'^ and Counsel
Denison Mines (U.SA) Coip.
1050 17^ Street
Suite 950
Denver, CO 80265
Subject: White Mesa Lraiiium Mill, Ground Water Discharge Perniit No. UGW370004.
Infiltration and Contaminant Transport Modeling Report: DRC Review
Comments, Request for Additional Informalion
The Utah Division of Radiaiion Control (DRC) has completed a preliminary review of the "Infiltration and
Contaminant Tran.sport Modeling Report. White Mesa Mill Site, Blanding, l.Itah (ICTM)" prepared for
Denison Mines (USA) Corp. by MWH Americas Inc. and dated November. 2007.
Based on the review DRC is requesting additiona! information regarding the ICTM as listed below:
White Mesa Cell 1
Per the ICTM, White Me.sa state.^; tliat Cell I will not contain mill tailings upon decommissioning of the
sue. h appears that Cell I has not been included in the ICTM based on the determination by Denison
Mines U.S.A. that il does not meet the permit language requirement of a closed tailings ceil.
DRC does not agree wilh this interpretation for the following reasons:
1. The demolition and decommissioning wastes in question will be contaminated with by-product
material (See Utah Admini.stralive Code (UAC) R313-12-3.)
2. Denison Mines U.S.A. has no[ made a demonstration thai lhe wastes in cell 1 meet lhe
definition of "de-minimus"' effect on local ground water quality, pursuant to UAC R3I7-6-
6.2(A)(25), nor has the E\eculive Secrelar>' approved such a demonstration. As a result. Cell i
does not qualify for Permit-by Rule status.
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Dave Freydenlund
Denison Mines
Infiltration and Contaminant Transpoit Plan Review
Page 2
Therefore, because contaminated materials from mill site decommissioning, including by-product
material will be disposed in Cell I. Cell I is required to be modeled, and included in the ICTM.
Alternatively, Denison Mines U.S.A. may submit and secure Execulive Secretary approval of an
application that demonstrates how Cell 1 will have a "de-minimus'" effect on local ground water
quality pursuant to UAC R-3!7-6-6.2(A)(25.)
Model Platforms RFI
DUSA needs to describe in detail each ofthe various models used in the ICTM. andjustify the use of
independent sequenced modeling platforms. DRC noted that input enors from one model to the next could
result in trickle down errors of great magnitude. Also, the TCTM does not include calculations used for
Model 3 Giroud and Bonaparte analytical equations, Ihis is especially confusing in relation to Cells 2 and 3
w here predictions from a Knight Piesold Report have been used.
The flowchart below displays DRC understanding of b different models within the overall TCTM:
Model 1 ~ Conceptual Model forthe
Vegetated Cover Utilizing HYDRUS 1-D.
Conceptual Model Calculates Flux
through the Cover.
Model 2 -Time to reach pseudo-steady stale
conditions in the tailings which equates to 3 ft
of saturated conditions in cells 2 and 3 and 1
ft of saturated conditions in cells 4A and 4B
using MODFLOW, Once Pseudo Steady
State is achieved time 0 is set for further
modeling.
Model 3 - Giroud &. Bonaparte 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 neutralizalion potential and total HFO quantities) and
existing tailings data. Model is limited to single ion reactions. K^ and Retardation
coefficients are calculated using chemical speciation data in lhe PHREEQC Program for
input into HYDRUS Model 5.
Model 5 - Hydrus 1-D model is used for the modeling domain from fiux through the cover
10 flux inlo the saturated 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.
V
•^
Model 6 - MODFLOW and MT3DMS are used lo model horizonlal flow and transport
through the saturated zone (advection and dispersion.) Chloride and Sulfate are modeled
bused on the findings of Mtxiel 3. no other ions are considered in the saturated zone model.
Dave Freydenlund
Denison Mines
Infiltration and Conlaminani Transport Plan Review
Page 3
Based on DRC review DUS.A needs to provide more information regarding the conceptual modeling
approach:
1- Denison Mines Ll.S.A. has not provided adequate justification that the conceptual approach of
using several sequenced, compartmentalized models (HYDRUS. PHI^EQC, and
MODFT.OW) provides a represenlaiive or conservative simulation of tailings cell performance.
Provide an e.\panded discussion ofthe conceptual modeling approach, and more specifically;
2. Provide verification that the use of a variable saturated coupled How and transport model
(VSCF model) was considered in lhe conceptualized modeling phase and justification why the
VSCF modeling platform was not appropriate or necessary.
Cover Design (Model 1)
The ICTM states that the current Nuclear Regulatory C'ommission (NRC) approved cover design will
undergo a modification to provide a vegetated cover system maximizing evapotranspiration (ET). DRC
agrees Ihat. in concept, a vegetated cover of some kind may be appropriate for the White Mesa tailings
impoundments. DRC is concerned that the proposed cover in lhe TCTM does not adequately address: 1.
Long tenn degradation of cover materials including the radon barrier due to weathering and intrusion
impacts, 2. Long term vegetalion establishment of indigenous planl species based on local lysimetry (ET)
data, and 3. Potential surtace ponding or runoff jjiipacis from intense or chronic storm events. Please
resolve the following issues:
Request for Informalion:
1. The modified cover design eliminates the cobble surface layer. Please justify that long term
suriace erosion will not occur wilh 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 (he vegetation
type as grasses. Please specify lhe grass species [hat will be used and justify long tenn growth
on lhe cover including reasoning for not including an evaluation ofthe long term sustainability
and potential for in growth of other natural species or plant succession (e.g. indigenous brush.)
Also, provide addilional information regarding root structures and plant type inputs entered
into the HYDRUS I-D surface layer model regarding long term establishment of vegetation.
Provide the basis for any assumptions made, including documenled studies of local
vegetal ion/soil models (e.g. lysimetry) to support claims.
3. The HYDRUS inputs for ET from plant surfaces are unclear. Per appendi.\ C. a grass layer
was entered into the HYDRUS model. Please provide information regarding the details ofthis
entry. Examples of needed informiilion include the specilic vegetation type or species, cover
density and rooting depths and density, leaf area index, and a description of calculations
performed in the HYDRUS model and self sustainability ofthe vegetalion at the White Mesa
Fiicilily. Provide specific reference to studies used lo support the long term vegetalion
establishment and assumed ET component.
4. The modified cover does not appear lo include design to avoid biointrusion inlo the underlying
compacted clay, radon barrier. Please justify why this is not a concem for long term
Dave Freydenlund
Denison Mines
Infiltration and Contaminant Transport Plan Review
Page 4
degradation, including justification that the 3 fl frost barrier layer provides adequate proiection
from intrusion by roots, animals, etc. into the underlying layers. Alternatively, modify the
design to include a biointrusion layer at an appropriate location above the radon biurier.
5. Provide documentation of a cover system frost depth analysis and maximum projected frost
penetration depth in the TCTM to justify that the 3 ft frosl 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 forthe modeling
assumption are representative (or conservative) of true field conditions and storm intensity
effects. Specifically, the TTYDRUS model distribution of a daily average precipitation over a
24 hour period negates storm intensity effects which are peitinent lo the semi-arid environment
at White Mesa. This assumption 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 justify how ihe.se 2 and 3-dimensional phenomena can
be modeled with a l-dimensional model (HYDRUS 1-D.)
7. It was noted that the TCTM references ET cover design studies and articles in which the
majority of the top cover designs include a capillary banierand iiilemal 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 hydraulic conductivity in the vegetated surface 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 addilional information regarding seasonal varialions 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 andjustify why the HYDRUS model assumes the FML to be I foot thick.
Water Baliince (Models 1, 2, 5, and 6)
Per the ICTM summary of results, section 4, the tailings cells are expected to reach steady slale after 200
years. DRC noted that the model was based on several simplifying assumptions given the 1-D aspect of
the HYDRUS model and sparse site data. While DRC understands the need for simplification ofthe
model, especially as a starting point, additional information is needed to insure that the model results are
conservative. Please resolve the following issues:
Request for Infonnation
1. Provide a cross section(s) depicting: I) The model layers and node geometry used in each
model, and. 2) A plot of steady stale water saturation wilh depth from the top of the cell cover
surfaces (Cell's 2, 3, 4, 4A) vertically downward to the top of the water table. Identify on the
cross section(s) which models were used lo simulate the hydraulic performance ofeach
specific layer.
Dave Freydenlund
Denison Mines
Infiltration and Containinant Tianspoit Plan Review
Page 5
2. Provide graphs to demonstrate steady state water content al representative depth intervals for
each layer in the model lo represent flux through the liner system (recharge component), fiux
llirough boKoin liner for each cell, and flux through vadose zone lo the perched aquifer for
each cell Tbased on steady slate saturation) and the modeled lime (years) needed lo reach
steady state. The ICTM includes average flux through the cover system and anticipated fluxes
through the vadose zone, however, these fiux rates are noi clearly associated with steady stale
predictions of water content.
3. For the unsaturated tailings and vadose zone please explain andjustify why the initial
saturation in the HYDRUS model (Model 5) was entered as OC?..
4. Flense provide andjustify the Van Genuchten/Mualem fitting parameters used in (he
HYDRUS model for both the unsaturated laiUngs (Mt.">del 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 cell deposition.)
5. The slimes drain layer shown in cross section for Cells 2 and 3 depicts a uniform sand layer,
per cell design and construction specifications this layer is not uniform across the bottom of
each cell. Explain potential input differences based on a non-unitbrm sand layer. Emphasis is
needed in e.\j>laining how and why a gravily driven, head dependent sink term in the
MODFLOW model (Model 2) is representative of actual slimes drain construciion and
operation, thai includes but is not limited to episodic operation ofthe slimes drain pump.
6. Provide calculaiions and plots of data reflecting expected/estimated leakage rales from the
single FML (via factory defects) under tailings Cells 2 and 3 [e.g. specific calculations oi
Giroud and Bonaparte (Model 3) as representative of White Mesa materials and installation,
and/or leakage rales estimated by field study.1 Also, provide details of Giroud and Bonaparte
Model for Cells 4A and4B (Model 3) as previously approved by DRC and appropriately
iipplied (o (he ICTM modeling.
Flow Modeling
Conceptual modeling ofthe saturated and unsaturated fiow is not clear, in panicuiar. lhe ICTM does not
clarify characteristic fiow curves associated with expected saturation in the vadose zone or clear
associations between transient and steady state fiow. Please resolve the following issues:
Request for Informalion
1. Provide additional clarification regarding the 3 vadose zone intervals used in the HY'DRUS
HKxlel (Model 5). including specific descriptions ofeach to discuss the different hydraulic
characteristics ofeach zone. Provide additional justification regarding the thickness ofthe
Ames. Provide a stratigraphic cro.ss .section ofthe vadose zone intervals in the TCTM. ba.sed on
core log inineralogical evaluation and/or other data. For ease of review, provide the vadose
^one interval descriptions, summary of data, and stratigraphic representation in a single section
within the repoit.
2. Provide inlormation regarding error fiags on lhe flow modeling runs (HY'DRUS 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 outptii. f)RC noted that several
Dave Freydenlund
Denison Mines
Infiltration and Conlaminani TYanspon Plan Review
Pago 6
different types of error messages were logged on the HYDRUS output decks. It would be
useful for Denison to include a table in ihe TCTM summarizing the location of these error flags
and explaining why the messages occuned, their impact on model output results, and potential
errors which may be present in the output data.
3. Provide clarification regarding HYURUS (Vadose Zone Model 5} and MODFLOW
(Dewatering Phase Model 6) interface during transient dewatering and corresponding fiux rales
through the botlom liner. Piease explain how the flux through the bottom liner during
operations and during slimes drain dewatering (transient fiow period) is accounted for.
According to language in the ICTM, it appears that wastewater entering ihc vadose zone
during this fime period is not included based on the assumption that it will have a minimal
effect on long tenn model outcomes. Please justify this position and explain why contaminant
mass released to the foundation duiing the operational phase ofthe tailings cells can be
ignored in the transport predictions. This is particularly important in light of lhe fact that the
highest driving heads are likely found during the operation ofthe tailings 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 fiow characteristics which define the layers
in the model (e.g. differences in penneability, flow direction, head distributions, etc.) This
could be achieved through graphical repre.senialion and additional explanation of Table 3-1 in
the 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 pump
dependent slimes dewatering systems at each ofthe tailings cells.
7. Explain and juslify why HYDRUS Model 5 breakthrough curves were based on a lower fiux
rale than predicted for olher upstream models (i.e. Model 3.)
8. Provide and juslify the specific calculations used to detennine the geomembrane fiux rates for
Cells 2 and 3.
ConUiminant Transport Modeling RFI
DRC has significant concerns with the current contaminant transport modeling. Models 5 and 6 per the
flow chart abo\e.
Two primary issues were evidenl ihrough DRC review; 1. Tailings wastewater chemistry calculations are
noi conservative, and 2. The PHREEQC ion sorption model is based largely on assumption and sketchy
field data which does noi adequately represent the receiving mineralogical matrix or geochemical
environmenL
The ICTM HYDRUS Model .5 fails lo provide a reasonably conservative evaluation ofthe physical system.
This is primarily due to a tack of empirical data to represent chemical partitioning and retardation through
the vadose zone. The failure of HYDRUS Model 5 is further exasperated downstream in MODFLOW
Mtxiel 6 where non-conservative HYDRUS inputs are used for the development of Cl and S04
breakthrough curves. Please resolve the following concems:
Dave Freydenlund
Denison Mines
Infiltration and Coniaminant Transport Pian Review
Page?
Request for Infonnation
1. Regarding the evaluation ofthe tailings cell wastewater chemistry, please explain andjustify
the use of average concentrations (max. concentrations should be used) especially in light of
lhe use of a non-weighted dala set as found in the Statement of Basis. Also, use ofthis table
may not be valid since the historic slimes drain samples are consistently much lower in
concentration than recenl liniversity ofUtah samples taken from the tailings ceils. DRC also
noted Ihat no tailings cell wastewater data newer than the 20(M Statement of Basis (such as
data included in the University of Utah Study document, "Summary of work completed, data
results, inlerprelalions and recommendations For the July 2007 Sampling Evenl Al lhe
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-
correlation is present in the data set used for the TCTM. Please revise the evaluation ofthe
tailings wastewater and include conservative estimates of the wastewater contaminants along
with a full statistical review and justification ofthe data .set used.
2. Clarify whether flux rates from the tailings cell Uners, u.sed to evaluate the 200 year
contaminant transport breakthrough curves include differentiating flux during uansienl phases
of dewatering heads on the botlom liner. Include references to specific model files/input decks
showing the differential heads ifapplicable. Ifthe transient dewalering phase is not included
in transport modeling provide justification why this would not impact the result ofthe 200 year
breaktltrough curve modeling.
3. Per the ICTM, p. B-6. "water quality dala 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 ihe laboratoiy data sheets including QA/QC verificaliun of
results and statistical analysis of the data set used. Provide concentration data which was used
for adsorption calculations and an explanation of lhe calculations made. Please demonstrate
how these laboratory tests and calculations were perfonned using standardized methods
recognized by the regulatory and relevant technical communities..
4. Provide addilional juslificalion ihal the laboratory results of digested core data are
representative or conservative ofthe mineralogy of the vadose zone. Also, please prepare
spreadsheets suinmarizing laboratory data results for the core samples encompassing all results
characterizing lhe mineralogy and altending descriptive statistics.
5. Additional clarification is needed regarding the modeling of tailings leachates and bulk
reactions with the receiving aquifer matrix. Please perform laboratory tests for Kd using
standardized methods and representative soils/rock in the presence of multiple tailings leachate
samples with a range of contaminant concentrations to better characterize the geochemical
relationships. DR(] anticipates that the development of empirical Kj coefficients and
Retardation Factors will supersede the use ofthe PHREEQC ion sorption model.
6. Figure 4-8 ofthe ICTM, "Model-Predicted Dissolved Uranium Concentrations in Vadose Zone
Pore Water al 200 Years'" does not allow review ofthe migrafion of dissolved U below the
tailings impoundments to the Slale Ground Water Quality Standard (GWQS) concentration,
0.030 mg/L. The figure needs to be magnified (re-scaled) to demonstrate the travel distance of
Dave Freydenlund
Denison Mines
Infiltration and Contaminant Transport Plan Review
Pages
Uranium at or below the GWQS of 0.030 mg/L. Also., please insure that all breakthrough
curves which depict parameter (pollutant) concentrations (based on revised modeling) which
are subject to a State Water Quality Standard are included and are drawn to scale to show the
State Standard.
7. The current geochemical model does not appear to anticipate the effects of changes in
neutralization and HFO capability as minerals are consumed. Provide explanadon of how this
factor was considered in the model.
8. Per DRC review it was noted that 11 potential contaminants are listed with very low Kd values
predicted by PHREEQC. Although the breakthrough curves for CL and S04 are considered
conservative (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 curve modeling (per #6 above) please justify why the Kd and Rf used
for Uranium is conservative in relation to the geochemical environmenL
CoDclusioa
Generally, DRC review coticludes that empirical data is lacking to develop a representative or conservative
model of infiltration, flow and chemical transport. This is primarily due to a lack of several types of site
specific data or local data reference including, but not limited to: long term vegetation establishment inputs
(e.g. plant density, leaf index, and rooting stmcture, etc.), long term degradation of the system (e.g. cover
flow impacts from frost heaving, erosion, intmsion, etc.), tailings wastewater pollutant concentrations
based on sound statistical evaluation and representative sampling, initial vadose zone water content (based
on empirical core evaluation), vadose zone mineralogy, and geochemical Ka and R characteristics (based
on empirical data.)
Recommended Actions
Per a telephone conversation between Tom Rushing (DRC) and Dave Frydenlund (Denison Mines) a
meeting (via teleconference) has been tentatively arranged for March 2, 2009. The discussion will focus
on Q & A regarding the current conceptual model and modeling platforms, needed improvements in the
ICTM and needs for additional site specific data.
Piease contact me ifyou have concems or questions regarding this letter or the planned acdons at (801)
536-0080. Thank you.
Sincerely,
Thomas Rushing, P.O., Hydrogeologist
Geotechnical Services Section
cc: Steve Landau, Denison Mines U.S.A. Corp.
F:\Denison - White Mesa\InfiItraiion and Conaminant Tninsport\WhiieMesalCTMRFI.cloc
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David C Frydenlund
Vice President & General Counsel
Denison Mines (USA) Corp
1050 17th ST STE 950
Denver CO 80265
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David C Frydenlund
Vice President & General Counsel
Denison Mines (USA) Corp
1050 17th ST STE 950
Denver CO 80265
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