DRC-2009-006884 - 0901a06880153367

Page 1 of 1
Sonja Robinson
From:
To:
Date:
CC:
David Frydenlund <[e-mail]>
Loren Morton <[e-mail]>, Tliomas Rushing ii <[e-mail]>
12/15/2009 9:59 AM
"Douglas Oliver Jr." <[e-mail]>, Harold Roberts
<[e-mail]>, Dane Finerfrock <DFTNERFROCK@utah,gov>
Attachments: Dec 1, 2009 Lr with Response , improved Fig 2. 3, 4 12,15,09.pdf
Loren and Tom,
Attached is our Dec 1, 2009 Response letter on the infiltration report, with improved Figures 1, and 3, as well as
the Improved Figure 4 that was previously sent to you.
Dave
David Frydenlund
'/.'fje P\e.R\(.'.er\[ Rn:iui,i:oi\ '^ffa'-i'^s dnd Crmn.'-^el
T: [phone] | f: [phone]
1050 I7(h Street, Suite 950, Denver. CO 30265
DENISON MINES (USA) CORP
www.denisonnnines.com
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fiIe;//C:.Documents and Settings\Sdrobinson\Loca1 Settings\Temp\XPgrpwise\4B27979A... 12/16/2009
OENISO
MINES
December 1,2009
VIA PDF and FEDERAL EXPRESS
Mr.Dane L.Finerfrock
Executive Secretary
Utah Radiation Control Board
State of Utah Department of Environmental Quality
168 North 1950 West
Salt Lake City,UT 84114-4850
Denison Mines (USA)Corp.
1050 17th Street,Suite950
Denver,CO 80265
USA
Tel:[phone]
Fax:[phone]
www.denisonmines.com
Re:White Mesa Uranium Mill;Groundwater Discharge Permit No.UGW370004 -
Infiltration and Contaminant Transport Modeling Report:Confirmatory Action
Letter Regarding Deliverables
Dear Mr.Finerfrock:
Reference is made to your November 16,2009 Confirmatory Action Letter Regarding
Deliverables,which sets out delivery dates for four items relating to the White Mesa Mill
Infiltration and Contaminant Transport Modeling Report.
Enclosed please find a Memorandum dated December 1,2008 prepared by MWH Americas,
Inc.which provides Denison's responses to the Utah Division of Radiation Control's February 2,
2009 Request for information letter.
If you have any questions or require any further information,please contact the undersigned.
Yours!"
Da~.\~ry enlund
Vice Presiden Regulatory Affairs and Counsel
cc:Ron F.Hochstein
Harold R.Roberts
Steven D.Landau
David E.Turk
Douglas Oliver,MWH
BUILDING A BETTER WORLD
TO:
FROM:
DATE:
SUBJECT:
David Frydenlund &Harold Roberts
Denison Mines (USA)Corp
Ryan Jakubowski &Douglas Oliver
December 1,2009
Revised responses to the Utah Division of Radiation Control's February 2,2009
comments on the Denison Mines (USA)Corp.November 2007 Infiltration and
Contaminant Transport Report
This memorandum provides responses to the comments submitted on February 2nd,2009 by
Thomas Rushing of the Utah Department of Environmental Quality (UDEQ)Division of
Radiation Control (DRC)to Denison Mines (USA)Corp.(Denison)on the November 2007
"Infiltration and Contaminant Transport Modeling Report,White Mesa Mill Site,Blanding,
Utah"(2007 ICTM Report).The 2007 ICTM Report was prepared by MWH Americas,Inc.
(MWH),for Denison and submitted to the DRC to fulfill White Mesa Mill's Ground Water
Discharge Permit No.UGW370004.The DRC's comments are provided in italics followed by
our responses in regular font.
To facilitate discussion and provide clarification regarding the DRC's comments,a meeting was
held between the DRC,Denison,and MWH on March 31 St,2009 at the DRC's office in Salt
Lake City,Utah.The meeting was attended by Loren Morton and Thomas Rushing from the
DRC,David Frydenlund and Harold Roberts from Denison,and Doug Oliver,Ryan Jakubowski,
and Phil Crouse from MWH.A follow-up meeting was held on September 2nd,2009 at the
DRC's office in Salt Lake City.This meeting was attended by Loren Morton,David Rupp,and
Thomas Rushing from the DRC,David Frydenlund and Harold Roberts from Denison,and Doug
Oliver,Ryan Jakubowski,and Ed Redente from MWH.These responses reflect the discussions
and agreements from both meetings.
White Mesa Cell 1
Per the ICTM,White Mesa states that CellI will not contain mill tailings upon decommissioning
ofthe site.It appears that Cell 1 has not been included in the ICTM based on the determination
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 interpretationfor the following reasons:
Comment 1.The demolition and decommissioning wastes in question will be contaminated with
by-product material (See Utah Administrative Code (UAC)R313-12-3).
Response:As discussed during the March 31st and September 2nd meetings,the demolition and
decommissioning (D &D)waste material will be contained in an approved facility.A portion of
10619 So Jordan Gateway
Suite 100
South Jordan.Utah 84095
TEL [phone]
FAX [phone]
www.mwholnnAI.r.nm
David Frydenlund &Harold Roberts
December I,2009
Page 2 of36
the current CellI evaporation pond (a lO-acre area on the southern side of CellI,adjacent to the
Cell 2 benn)may be used to contain construction debris from decommissioning of the mill site.
Disposal of the decommissioning material in Cell 1 is identified as a contingency in case other
cells (e.g.,Ce1l4B)do not have adequate storage for such material.The bottom of CellI would
be constructed with a liner comprised of compacted clay while the top of Cell 1 would be
constructed with an evapotranspiration (ET)cover as proposed for Cells 2,3,4A,and 4B.The
current CellI evaporation pond area would be opened to the west for surface water drainage.
Prior to reclamation or placement of any demolition debris (dry)in Cell 1,the solution in the
Cell 1 pond will be evaporated dry and the evaporate crystals,sediment,geomembrane liner, and
any contaminated underlay (foundation)material will be relocated to another cell (e.g.,Cell 4A
or 4B).A radiation survey (with field instrumentation and lab data for correlation)would be
completed and any contamination remaining beneath the former evaporation pond would be
remediated with the end goal being clean closure (soil cleanup standard based on radium-226 and
natural uranium).
Comment 2.Denison Mines U.S.A.has not made a demonstration that the wastes in celli meet
the definition of "de-minimus"effect on local ground water quality,pursuant to UAC R3i7-6-
6.2(A)(25),nor has the Executive Secretary approved such a demonstration.As a result,Celli
does not qualify for Permit-by Rule status.
Therefore,because contaminated materials from mill site decommissioning,including by-
product material will be disposed in Celli,Celli 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 application that demonstrates how Cell i will have a "de-minimus"effect on
local ground water quality pursuant to UAC R-3i7-6-6.2(A)(25.)
Response:If Cell 1 is necessary for D &D disposal,it will be covered with an ET cover.The
design will be the same as the cover proposed for the other cells.Consequently,the rate of
infiltration would be equivalent to the values presented for the other cells.As discussed during
the March 31 st and September 2nd meetings,modeling of potential water and solute transport
through Cell 1 into the underlying vadose zone and resulting impacts to groundwater will be
evaluated and included in the revised ICTM Report.The material will be modeled as a relatively~'dry,unsaturated high-permeability porous media (sandy loam)at a uranium concentration/source
term chemistry equivalent to the value used for the tailings.
David Frydenlund &Harold Roberts
December 1,2009
Page 3 of36
Model Platforms
DUSA needs to describe in detail each of the various models used in the ICTM,and
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 ofgreat magnitude.
Also,the ICTM does not include calculations used for Model 3 Giraud and Bonaparte
analytical equations,this is especially confusing in relation to Cells 2 and 3 where
predictionsfrom a Knight Piesold Report have been used.
Based on DRC review DUSA needs to provide more information regarding the
conceptual modeling approach:
Comment 1.Denison Mines U.S.A.has not provided adequate justification that the
conceptual approach ofusing several sequenced,compartmentalized models (HYDRUS,
PHREEQC,and MODFLOW)provides a representative or conservative simulation of
tailings cell performance.Provide an expanded discussion ofthe conceptual modeling
approach,and more specifically;
Response:An expanded discussion of the modeling approach and justification for using
several sequenced,compartmentalized analytical and numerical models is discussed
below,and will be included in the revised ICTM Report.During the March 31st meeting
it was agreed that no single model could be used to simulate all of the physical and
chemical processes at one time in a simultaneous manner.Whether the models provide a
representative or conservative simulation of tailings cell performance depends on the
assumptions incorporated during the conceptualization and parameterization process.We
feel that the simulations are based on realistic or conservative assumptions,which will
become evident as the various comments are addressed throughout this memorandum.
As discussed during the March 31st meeting,the sensitivity analysis will be expanded to
evaluate the effects ofuncertainty in more parameters.This work will be documented in
the appendices of the revised ICTM Report to keep the body of the report streamlined.
To predict the potential movement of water and solutes from the tailings cells,and
demonstrate that the cover design is adequate to protect human health and the
environment,the physical and chemical processes expected to govern the potential
transport of water and solutes in unsaturated,variably saturated,and saturated porous
media were simulated with different models.
The use of several sequenced,compartmentalized models (HYDRUS,PHREEQC,
MODFLOW,and MT3DMS)is justified for a number of technical and practical reasons,
which are discussed below:
• A single numerical model that is capable of simulating,in a simultaneous manner,
the different physical and chemical processes that could potentially occur within
the cover,the tailings,the liner,the unsaturated bedrock vadose zone,and the
saturated perched aquifer does not exist.
•Each program's (HYDRUS,PHREEQC,MODFLOW,MT3DMS)capabilities,in
David Frydenlund &Harold Roberts
December I,2009
Page 4 of36
regard to simulating the potential physical and chemical processes that could
occur at the site,were selected to maximize functionality.
•Considering the complex hydrogeologic and geochemical conditions that occur at
the site,a sequenced modeling approach that utilized independent numerical
models was necessary to prescribe the initial and boundary conditions used to
solve the differential equations (e.g.,Richards'Equation).
•Different parts of the system operate under different timeframes (e.g.,water flow
through the cover requires much smaller time steps than flow through the bedrock
vadose zone beneath the cells).Computationally,it is far more efficient to have
the infiltration model of the cover separate from the flow and transport model of
the underlying vadose zone.
Numerical models are programmed to simulate specific processes (e.g.,unsaturated flow,
surface complexation,dewatering,etc.),which requires the user to understand the
inherent limitations of the different programs.For example,while MODFLOW is well
structured to simulate tailings dewatering,the program is not designed to simulate
infiltration and the transport of solutes under unsaturated conditions;and despite the fact
that HYDRUS can simulate saturated flow conditions,the model is not specifically
designed to simulate drawdown from a pumping well or surface complexation reactions.
Therefore,by necessity,different numerical models (HYDRUS,PHREEQC,
MODFLOW,MT3DMS)were required to simulate the different processes in a sequenced
manner.Even if one computer program was capable ofsimulating the different processes
in a parsimonious manner using a single numerical model,numerous submodels would
have to be developed and used in a sequenced fashion in order to assign initial conditions
and for efficiency given the differences in timescales of various processes.
Consequently,the model platform was structured to ensure that the physical and chemical
processes were adequately represented in a realistic and judicious manner.
While we agree that inaccurate assignment of parameters values from one model could
propagate to the other models,the effects of trickle-down "errors"would not be
eliminated if a single model were constructed for the entire effort.Error associated with
uncertainty in conceptualization and input parameter values would remain regardless of
whether multiple coupled models or a single comprehensive model were used.
Sensitivity analyses were completed to quantify model prediction uncertainty due to
estimating values for input parameters (see Section 4.5 and Table 4-3 in the 2007 ICTM
Report).Additional sensitivity analyses will be performed as discussed in the March 31 st
meeting.The results of these sensitivity analyses will be summarized in the revised
ICTM Report.
As discussed during the March 31st and September 2nd meetings,a simplified flowchart
will be included in the revised ICTM Report to provide a broad overview of the models
used (see Figure 1).
The method of Giroud and Bonaparte was used to calculate potential leakage for various
head values (water levels in the tailings)through the liner.These calculations will be
included in an appendix of the revised ICTM Report.As discussed during the September
David Frydenlund &Harold Roberts
December I,2009
Page 5 of36
2nd meeting,the pseudo layer will be removed from the model and potential water flux
rates through the liners will be used as an upper boundary condition (time-dependent
flux)for predicting flow and transport through the vadose zone to the perched aquifer.
Removal of the pseudo layer,formerly used to represent the geomembrane liner,will also
result in reduced complexity in the modeling approach.
Comment 2.Provide verification that the use ofa 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.
Response:As described on Pages 3-1 through 3-4 of the 2007 ICTM Report,HYDRUS,
a variably saturated coupled flow and transport model,was used to simulate the flow and
transport of water and solutes for a variety of hydrogeologic conditions.As part of the
revised ICTM Report,HYDRUS will be used to predict the:
•Potential flow of water through the cover under unsaturated conditions;and
•Potential flow and transport of water and solutes from the liner through the
bedrock vadose zone toward the perched aquifer under variably saturated
conditions.
The transport equation is coupled to the flow equation through the velocity term.
David Frydenlund &Harold Roberts
December 1,2009
Page 6 of36
Cover Design (Modell)
The ICTM states that the current Nuclear Regulatory Commission (NRC)approved cover
design will undergo a modification to provide a vegetated cover system maximizing
evapotranspiration (ET).DRC agrees that,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 the ICTM does not adequately address:1.Long term degradation
ofcover materials including the radon barrier due to weathering and intrusion impacts,
2.Long term vegetation establishment of indigenous plant species based on local
lysimetry (ET)data and 3.Potential suiface ponding or runoff impacts fonn intense or
chronic storm events.Please resolve the following issues:
Comment 1.The modified cover design eliminates the cobble suiface layer.Please
justify that long tenn suiface erosion will not occur with the new suiface layer.Specify
topsoil gradations (e.g.admixture components)which will be used to replace the rip-rap
layer.
Response:The preliminary cover design proposed in the 2007 ICTM Report included
replacing the cobble/rip-rap layer with 6 inches of a soil-gravel admixture.The proposed
cover design replaces only the top slope (surface with a 0.2%slope).The sideslope
design (surface with a 20%slope)has not been altered,and the cobble layer will likely be
the same as in the original design (TITAN Environmental,1996).To support the
preliminary cover design used to model infiltration through the cover,the gravel-soil
mixture was assumed to be composed of 25%gravel by weight.The preliminary
conceptual cover design is used in the ICTM report to evaluate potential impacts from the
tailings cells.There has been and will continue to be iterations between the infiltration
modeling and the preliminary cover design.Once the ICTM report is approved by the
State,Denison will prepare a detailed cover design,which will include the requested
design specifications.Following completion of the detailed cover design,Denison will
prepare a revised reclamation plan.
Vegetated covers that contain an admixture of soil and gravel within the surface layer
have been constructed in order to promote revegetation,reduce runoff,and minimize
erosion of the surface cover materials (e.g.,Waugh et al.,2008).Successful revegetation
reduces the percentage of bare ground and disperses raindrop energy,which tends to
reduce overland flow velocities and the potential for generation of runoff and erosion of
the surface layer.The presence of gravel-sized particles within the soil layer serves to
minimize surface water and wind erosion ofnonvegetated surfaces.Engineering controls
will be included in the Final Cover Design to ensure that long term surface erosion of the
cover does not occur.
Comment 2.The modified cover includes a top vegetated layer.The ICTM only
describes the vegetation type as grasses.Please specify the grass species that will be
used and justify long tenn growth on the cover including reasoning for not including an
evaluation of the long tenn sustainability and potential for in growth ofother natural
species or plant succession (e.g.indigenous brush.)Also,provide additional infonnation
David Frydenlund &Harold Roberts
December I,2009
Page 7 of36
regarding root structures and plant type inputs entered into the HYDRUS l-D surface
layer model regarding long term establishment ofvegetation.Provide the basis for any
assumptions made,including documented studies of local vegetation/soil models (e.g.
lysimetry)to support claims.
Response:As discussed during the September 2nd meeting,the revised ICTM Report
will include more details to support short-term establishment of vegetation and long-term
sustainability of the ET cover's plant community (e.g.,potential for plant succession and
ability to withstand various perturbations).Empirical data regarding the ecological
characteristics of the species mix (rooting depth and root distribution)and established
plant community (percent cover)will be summarized from the literature and nearby
lysimeter studies (Monticello site).The empirical data will be used to parameterize the
numerical model and predict the ET cover's performance over the long term.A range of
parameter values will be included to determine which parameters may be more sensitive
in predicting flow through the cover.Documentation used to support the parameter
values ofthe established plant community will be included in the revised ICTM Report.
In HYDRUS,the uptake of water by plants is simulated through a sink term in the
Richards'Equation.The mathematical description of how HYDRUS simulates root
water uptake is described on Pages 2-16 and 2-17 of the 2007 ICTM Report.Root water
uptake will vary as a function ofthe soil water pressure head within the rooting zone,the
normalized plant root distribution function (i.e.,density ofroots),and the rate of potential
transpiration (PT).The rate of PT is assigned as part of the atmospheric upper boundary
condition for the cover model,which HYDRUS then uses to compute the actual
transpiration (AT)as a function of time and space within the rooting zone.For example,
when conditions are extremely dry (i.e.,less than the wilting point)or extremely wet (i.e.,
near saturation)plants cease to uptake water,and the AT would be zero.At intermediate
soil water conditions,the AT would be a fraction of the PT.As discussed on Page 3-8 of
the 2007 ICTM Report,the water stress response function for grass was selected from the
default database in HYDRUS.The database does not distinguish between different
species of grass.Water stress response parameters at high suctions are considered to be
conservative because transpiration is assumed to cease at soil water pressures below the
assumed wilting point of -8,000 cm,and plants in many arid and semiarid environments
commonly maintain transpiration under significantly drier conditions (soil water tensions
as low as -40,000 cm).For example,crested wheatgrass,a proposed species for the
cover,can survive in soil water conditions where the soil water pressure ranges between-
20,000 and -40,000 cm (Chabot and Mooney,1985;Brown,1995).
Comment 3.The HYDRUS inputs for ETfrom 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.Example of needed information include the specific
vegetation type or species,cover density and rooting depths and density,leafarea index,
and a description ofcalculations performed in the HYDRUS model and self-sustainability
ofthe vegetation at the White Mesa Facility.Provide specific reference to studies used to
support the long term vegetation establishment and assumed ET component.
David Frydenlund &Harold Roberts
December 1,2009
Page 8 of36
Response:Infonnation regarding the vegetative component of the ET cover (vegetation
type,percent cover,rooting depths and root density)and self-sustainability of the
vegetated cover were discussed in the response to Cover Design Comment 2 above.
These details will be added to the revised ICTM Report.
As discussed during the March 31st meeting,potential evapotranspiration (PET)was
calculated for each day of the year using measured air temperature (maximum and
minimum)and estimated extraterrestrial radiation between 1932 and 1988.This 57-year
period was selected because it contained a nearly continuous dataset.As described in the
2007 ICTM Report,the entire period of record for the Blanding weather station ranges
from December 1904 through December 2005,but contained large intervals of missing
data before 1932 and after 1988.
As discussed during the March 31st meeting,PET for each day during the 57-year climate
record was calculated using the Hargreave's Equation.This approach was selected
because long-tenn meteorological data (e.g.,wind speed)were not available,and the
Hargreave's Equation can be used as a substitute for the Penman-Monteith Equation.
The calculations assume a hypothetical grass reference crop with sufficient access to
water such that the amount ofPET is controlled by site-specific climatic conditions (e.g.,
air temperature,day of year,solar declination).Potential evaporation (PE)from the soil
surface and potential transpiration (PT)from roots are partitioned from the PET based on
the percent vegetative cover.As described above,HYDRUS uses the PT to compute the
AT based on the soil water pressure within the rooting zone;actual evaporation (AE)is
controlled by the minimum pressure head allowed at the surface.The months of
December through February have been assigned a transpiration rate of zero,and only
evaporation will be simulated in the HYDRUS model during these timeframes.
Comment 4.The modified cover does not appear to include design to avoid biointrusion
into the underlying compacted clay,radon barrier.Please justify why this is not a
concern for long term degradation,including justification that the 3itfrost barrier layer
provides adequate protection 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 barrier.
Response:As discussed during the September 2nd meeting,the potential for biointrusion
to occur at the site will be evaluated as part of the revised ICTM Report.Implications
that may result from biointrusion due to burrowing plants and animals,such as increased
hydraulic conductivity,will be evaluated.
Comment 5.Provide documentation ofa cover systemfrost depth analysis and maximum
projected frost penetration depth in the ICTM to justify that the 3 it frost barrier
thickness is adequate.Provide reference to local and/or regional studies to support all
claims.
Response:TITAN Environmental (1996)completed a freeze/thaw evaluation based on
site-specific conditions which indicated that the anticipated maximum depth of frost
David FrydenIund &Harold Roberts
December 1,2009
Page 9 of36
penetration was 6.8 inches.Therefore,the preliminary assumption of a O.5-ft (6-inch)
thick soil-gravel surface layer,combined with a minimum 3-ft thick water storage/frost
barrier layer,would be protective of impacts that may result from frost heaving.
Consequently,the shallow frost penetration depth is not expected to affect the results of
the infiltration and radon barrier models..
Comment 6.Provide justification why the daily average precipitation rates used for the
modeling assumption are representative (or conservative)of true field conditions and
storm intensity effects.Specifically,the HYDRUS model distribution ofa daily average
precipitation over a 24 hour period negates storm intensity effects which are pertinent to
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 ofthese processes may change hydrologic properties ofthe cover and justify how
these 2 and 3-dimensional phenomena can be modeled with a i-dimensional model
(HYDRUS i-D).
Response:As presented in the September 2nd meeting,a series of model simulations that
used daily and hourly input data were evaluated to determine whether simulating storm
intensity and surface ponding results in increased water flux entering the tailings.The
results will be summarized in an appendix of the revised ICTM Report.The use of a one-
dimensional model will also be justified.
Comment 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 internal
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.
Response:The references on ET cover design studies cited in the 2007 ICTM Report
contained a mixture of covers constructed with a variety of design configurations
including both covers with capillary breaks and monolithic covers.The publications
referenced in the 2007 ICTM Report represent a fraction of the studies that have been
completed to assess the performance of alternative cover designs.For example,as of
August 2008,a U.S.EPA online database ofET covers revealed 40 demonstration and 36
full-scale projects throughout the United States (U.S.EPA,2009).Of these sites,62
contained monolithic ET covers and 23 contained capillary barrier ET covers (U.S.EPA,
2009).The inclusion of a capillary barrier is designed to increase the storage capacity of
the overlying fine-grained water storage layer (Stormont and Morris,1998;Khire et aI.,
2000);the capillary barrier is not designed to act as a drainage layer,unless it is designed
with anisotropy to promote drainage (Dwyer,2003).
A capillary barrier was not included in the proposed design because the monolithic
storage layer was deemed sufficient to limit infiltration through the cover.A sensitivity
analysis was performed for the various cover designs (original conventional design,ET
cover with capillary break,and monolithic ET cover).Results from this sensitivity
analysis will be included in an appendix to the revised ICTM Report.The ET cover with
David Frydenlund &Harold Roberts
December 1,2009
Page 10 of36
capillary break and monolithic ET cover were predicted to perform similarly,and both
significantly out-performed the conventional cover.An internal drainage layer was not
included in the design because the majority of precipitation would be consumed within
the cover,and the relatively flat slope of the surface would not dictate installation of a
drainage layer.A surface runoff layer was included in the design (i.e.,soil-gravel
admixture).The cover presented in the 2007 ICTM Report is conceptual and preliminary
in nature and was assumed for modeling purposes.Final design details will be provided
in the revised Cover Design.
Comment 8.Include discussion regarding how the HYDRUS-ID modeling inputs
account for any long term changes in 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.)
Response:Potential impacts due to freeze-thaw damage were discussed in the response
to Cover Design Comment 5 above.Different sensitivity analyses will be completed to
evaluate reduced performance of the ET cover due to biointrusion.Additionally,reduced
performance,and potential omission of the compacted clay layer,will also be evaluated
with the infiltration model.The results of this sensitivity analysis will be presented in the
appendices of the revised ICTM Report.The compacted clay layer would only be
removed from the design if supported by the radon attenuation modeling.
Comment 9.Provide additional information regarding 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.)
Response:As described in the response to Cover Design Comment 3 above,PET was
calculated for each day of the year for the 57-year climate record;as a result,seasonal
variations in ET were accounted for in the model (i.e.,PET during summer is greater than
PET during winter).The amount of PE,PT,and precipitation for each day ofthe climate
record was prescribed as part of the atmospheric upper boundary condition for the ET
cover model.The seasonal variations in ET affect the amount of water stored within the
cover.
Snow accumulation was not simulated in HYDRUS because the site is not expected to
develop a large snowpack.The average annual depth of snow reported for the Blanding
weather station (which is located at a slightly higher elevation as compared to the White
Mesa Mill)was approximately 2.5 cm.In terms ofwater content,all water from snow is
assumed to enter the top of the model domain as water;thus, this assumption would tend
to predict slightly more conservative results because no evaporation or sublimation of
snow was accounted for (i.e.,all water from snow is assumed to enter the soil at the top
of the cover).
Comment 10.Explain andjustify why the HYDRUS model assumes the FML to be 1foot
David Frydenlund &Harold Roberts
December 1,2009
Page 11 of36
thick.
Response:Originally,a thicker pseudo layer,as compared to the actual liner thickness,
was necessary to achieve numerical convergence;to counteract the increased thickness a
higher hydraulic conductivity was assigned to this layer.However,as discussed during
the September 2nd meeting,the pseudo layer will be removed from the model and
potential water flux rates through the liners will be used as an upper boundary condition
for predicting flow and transport through the bedrock vadose zone beneath the tailings
cells.Removal of the pseudo layer,formerly used to represent the geomembrane liner,
will also result in reduced complexity in the modeling approach.
David Frydenlund &Harold Roberts
December I,2009
Page 12 of36
Water Balance (Models 1,2,5,and 6)
Per the ICTM summary of results,section 4,the tailings cells are expected to reach
steady state 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 needfor simplification ofthe model,especially as a starting point,
additional information is needed to insure that the model results are conservative.
Please resolve thefollowing issues:
Comment 1.Provide a cross section(s)depicting:1)The model layers and node
geometry used in each model,and,2)A plot of steady state water saturation with depth
from the top ofthe 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 to simulate the
hydraulic performance ofeach specific layer.
Response:The node geometries of the HYDRUS models were explained on Pages 3-5
and 3-6,and the model layers were depicted on the cross section schematic presented as
Figure 3-1 and described in Table 3-1 of the 2007 ICTM Report.Note that the cover
model will be modified slightly in the revised ICTM Report to reflect modifications to
the design from what was modeled for the 2007 ICTM Report..
As discussed during the September 2nd meeting,the tailings and tailing's liner systems
will be removed from the vadose zone model.A cross section showing the relationship
between the tailing cells and the underlying vadose zone,similar to the figure shown
during the March 31 st meeting,will be included in the revised ICTM Report.The
different models used to simulate hydraulic performance are summarized in the
simplified flowchart presented as Figure 1 in this memorandum.This figure (or a similar
figure)will be incorporated in the revised ICTM Report.A table summarizing the
hydrogeologic properties of the ET cover model and vadose zone model will be included
in the revised ICTM Report.
Time series and profile plots of water content and/or pressure head at salient intervals
from the vadose zone model will be included in the revised ICTM Report.While a
pseudo-steady-state is reached (no long-term trends,simply a repeating response),an
actual steady state is not reached in the cover model because the input function varies
through time (repeating 57-year record).
Comment 2.Provide graphs to demonstrate steady state water content at representative
depth intervals for each layer in the model to represent flux through the liner system
(recharge component)flux through bottom liner 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 anticipatedfluxes through the vadose zone,however,these
flux rates are not clearly associated with steady state predictions ofwater content.
David Frydenlund &Harold Roberts
December 1,2009
Page 13 of36
Response:Average flux through the cover system was not simulated.Instead,transient
flow through the cover was simulated (Figure 4-1 of the 2007 ICTM Report).Water flow
through the cover will reach a pseudo-steady state (periodic response with no long-term
trend)but not steady state because the water content and soil water pressure are expected
to vary with depth through time.However,a periodic signal would develop because the
57-year climate record was repeated to establish a 200-year simulation.Time series and
profile plots of pressure head and/or water content at salient intervals within the cover
will be included in the revised ICTM Report.
Potential water flux rates through the liners and through the vadose zone to the perched
aquifer will be included in the revised ICTM Report.The relation of these potential
water flux rates to the establishment of steady state or pseudo-steady state within the
vadose zone,and comparison to infiltration rates through the ET cover,will also be
discussed.
Comment 3.For the unsaturated tailings and vadose zone please explain andjustify why
the initial saturation in the HYDRUS model (ModelS)was entered as 0%.
Response:For the 2007 ICTM Report,initial water contents of the tailings and vadose
zone were not entered as 0%.The tailings were considered to be variably saturated
(initially,the bottom 4 feet were assumed to be fully saturated in Cells 2 and 3,and the
bottom 1 foot was assumed to be fully saturated for Cells 4A and 4B)while the vadose
zone was considered to be unsaturated with initial soil water contents based on modeling
of the vadose zone with an atmospheric boundary condition at the top rather than the
tailings cells.A profile plot of water content and/or pressure head within the vadose zone
through time will be included in the revised ICTM Report to provide additional
elucidation.
Comment 4.Please provide and justify the Van Genuchten/Mualem fitting parameters
used in the HYDRUS model for both the unsaturated tailings (Modell)and the vadose
zone materials (ModelS).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)
Response:The unsaturated and saturated hydraulic properties for the vadose zone
samples were estimated from laboratory measurements and through optimization.
Parameter values measured in the lab on vadose zone samples included the residual water
content (8 r,),saturated water content (81115),water contents at intermediate soil water
pressures 8(h),and the saturated hydraulic conductivity in the vertical direction (Ks).
Parameter values determined through optimization included the empirical fitting
parameters (n and a).The fitting parameters are considered to be empirical coefficients
that affect the shape of the hydraulic functions used to describe variations in water
content and hydraulic conductivity for different soil water pressures.The unsaturated
hydraulic properties [8(h)]and [K(h)]are highly nonlinear functions of the soil water
pressure (h).
David Frydenlund &Harold Roberts
December I,2009
Page 14 of36
Soil water retention characteristics for the vadose zone samples were determined by a
variety of methods including hanging column,pressure plate,water activity meter,and
relative humidity box methods to cover a range of pressure heads from 0 cm (saturated
water content)to -851,000 cm (residual water content).Unsaturated hydraulic properties
(parameters 8s,ex,and n)were then determined by fitting van Genuchten's (1980)
analytical model to the water retention data reported in DBS&A (2007)using the RETC
computer code developed by the U.S.Salinity Laboratory for the U.S.EPA (van
Genuchten et aI.,1991).RETC utilizes a nonlinear,least-squares parameter optimization
method to estimate the unknown variables.During parameter optimization,the program
is run many times in succession,each time incrementally varying the unknown variables
so as to minimize the sum of squared residuals until convergence is reached and the
measured data are matched.A comparison between the measured and model predicted
soil water retention curves for the vadose zone samples are plotted in Figure 2.Overall,
there is good agreement between the measured and optimized parameter values used to
describe 8(h).Data collected from MW-30 (44.0-44.5 ft)were not included because the
core experienced swelling and cracking after the saturated hydraulic conductivity test.
The hydraulic properties of the vadose zone samples are included in Table 1 of this
memorandum (as well as Table 2-1 ofthe 2007 ICTM Report).Justification for selection
of hydrogeologic parameters for the vadose zone is discussed in the response to
Comment 1 in the Flow Modeling section below.
As discussed during the September 2nd meeting,the tailings will be removed from the
vadose zone model.Therefore,only the saturated hydraulic conductivity of the tailings is
required to predict tailings dewatering for Cells 2 &3.The saturated hydraulic
conductivity was taken from a multiple well aquifer test completed at another uranium
mill tailings impoundment (Canon City),because site-specific measurements were
lacking for White Mesa.The saturated hydraulic conductivity ofthe tailings assumed for
White Mesa were based on measured values reported for the Cotter Corporation's Canon
City Mill tailings impoundment (MFG Inc.,2005).The mill tailings at Canon City (MFG
Inc.,2005)are considered to be representative of the mill tailings at White Mesa because
the average grain size distribution and DS/DlO values between the two sites is similar:
•Canon City:44%sand,52%silt,and 4%clay with aDs -0.002 mm.
•White Mesa:56%sand,34%silt,and 10%clay with a DlO -0.002 mm.
Gradation curves are not available for the Canon City Mill tailings;however,the
similarity in average grain size distribution (%sand,silt,clay)and DS/DlO values between
the two sites is expected to produce tailings materials that behave similarly
hydrogeologically.
To address the uncertainty associated with variability of hydraulic conductivity in the
tailings,a sensitivity analysis will be performed to evaluate the impacts that tailings
hydraulic conductivity has on the model predictions of dewatering within Cells 2 &3.
The results of this sensitivity analysis will be included in the revised ICTM Report.
David Frydenlund &Harold Roberts
December 1,2009
Page 15 of36
Comment 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-uniform sand layer.Emphasis is needed in explaining how and why a gravity driven,
head dependent sink term in the MODFLOW model (Model 2)is representative ofactual
slimes drain construction and operation,that includes but is not limited to episodic
operation ofthe slimes drain pump.
Response:The slimes drains in Cells 2 &3 are "burrito drains",meaning the sand is
placed in an envelope over the drains,rather than a continuous layer across the bottom of
the tailing cells.The I-foot thick sand layer in the MODFLOW model was only
simulated within the slimes drain array.This area is approximately 600 ft by 400 ft (5.4
acres)within the total Cell 2 area of 65 acres.To better represent the drains in the
MODFLOW model used to predict dewatering,the hydraulic conductivity field will be
revised such that the I-foot sand layer (with higher hydraulic conductivity)will be
restricted to the drain cells.The zone between the drain lines will be modeled with lower
hydraulic conductivity tailings.The results of this modeling will be included in the
revised ICTM Report.
The design of the slimes drain system consists of perforated PVC pipe that drains to an
extraction sump.Groundwater flow to this array is gravity driven and dependent on the
head difference between the surrounding material and the perforated pipe.Operation of
the slimes drain extraction pump is only necessary to extract the groundwater driven into
this array to maintain a head difference.Essentially,this system acts as a field drain
array.The MODFLOW Drain package was developed specifically to simulate this sort
of gravity driven,head dependent drain system,and MWH feels that this is the most
appropriate sink term for simulation of the slimes drains.A thorough quantitative
explanation of the MODFLOW Drain package is presented in A Modular Three-
Dimensional Finite-Difference Ground-Water Flow Model:U.S.Geological Survey
Techniques of Water-Resources Investigations,book 6,chap.Al (McDonald and
Harbaugh,1988).
Episodic operation of this pump may be necessary,and will be dependent on several
variables including the pump size and the hydraulic conductance of the drain array.The
model simulations did not attempt to predict this periodicity,the simulations only
attempted to give an average water level in the sump through time while the tailings cell
was dewatered.
Comment 6.Provide calculations and plots ofdata reflecting expected/estimated leakage
rates from the single FML (viafactory defects)under tailings Cells 2 and 3 {e.g.specific
calculations of Giroud and Bonaparte (Model 3)as representative of White Mesa
materials and installation and/or leakage rates estimated by field study.]Also,provide
details of Giroud and Bonaparte Model for Cells 4A and 4B (Model 3)as previously
approved by DRC and appropriately applied to the ICTM modeling.
David Frydenlund &Harold Roberts
December 1,2009
Page 160f36
Response:As discussed during the September 2nd meeting,Giroud-Bonaparte equations
and assumptions used to calculate potential water flux rates through the liners beneath
Cells 2 &3 and Cells 4A &4B will be included in the revised ICTM Report.
David Frydenlund &Harold Roberts
December 1,2009
Page 17 of36
Flow Modeling
Conceptual modeling ofthe saturated and unsaturated flow is not clear,in particular,the
ICTM does not clarify characteristic flow curves associated with expected saturation in
the vadose zone or clear associations between transient and steady state flow.Please
resolve the following issues:
Comment 1.Provide additional clarification regarding the 3 vadose zone intervals used
in the HYDRUS model (Model 5),including specific descriptions of each to discuss
different hydraulic characteristics of each zone.Provide additional justification
regarding the thickness of the 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.
Response:As discussed during the March 31st and September 2nd meetings,additional
clarification regarding the selection of vadose zone intervals,both for hydrogeology and
geochemistry,will be included in the revised ICTM Report.A cross section illustrating
the monitoring wells with available core and the depth intervals of samples used for
hydrogeologic and geochemical characterization is plotted in Figure 4.
Geologic logs indicate that the predominate lithology in the vadose zone is sandstone
(Dakota Sandstone and Burro Canyon Formation).While predominantly sandstone,
between the bottom ofthe tailings cells (-30 feet bgs)and the top ofthe Brushy Basin
Member shale (-103-127 feet bgs),there is 3 feet of siltstone and 4 feet of conglomerate,
on average.Five sandstone samples were analyzed for hydraulic properties.One sample,
considered to represent the transition from siltstone to sandstone (MW-23 74.3-74.6 ft),
was also analyzed for hydraulic properties.No conglomerate layers were analyzed for
hydraulic properties because the core samples from this rock type generally consist of
irregular shaped,angular pieces with variable sorting and clast sizes.Furthermore,it is
likely that the conglomerate behaves hydraulically very similarly to the sandstone
because the matrix is sandstone and the clasts are generally small gravel,in low
percentages (less than 30%).
We anticipate using a single set of hydraulic properties for the vadose zone assuming the
properties of sandstone.This assumption is considered appropriate because the saturated
and unsaturated hydraulic properties of the samples are quite similar to one another
(Figures 2 and 3)with the exception of MW-23 (74.3-74.6 ft).MW-23 (74.3-74.6 ft)had
a smaller storage capacity and a slightly lower saturated hydraulic conductivity compared
to the other samples.Assignment of a single set of hydrogeologic properties should not
significantly affect the model results given the similarity in unsaturated hydraulic
properties [8(h)]and [K(h)]for all samples (i.e.,there were no large differences in soil-
water retention curves or unsaturated hydraulic conductivity curves for the materials
tested).We anticipate using the hydraulic properties from MW-23 (55.5-56.0 ft)during
hydraulic modeling because the hydraulic functions are intermediate as compared to the
other samples.
DavidFrydenlund &Harold Roberts
December 1,2009
Page 18 of36
The number of geochemical layers assigned to the vadose zone model will be supported
by a statistical evaluation of the mineralogical data,and will be included as an appendix
in the revised ICTM Report.Based on a preliminary evaluation of the geochemical
properties (ANP and HFO results for 34 normal and 4 duplicate samples as shown on
Figure 4),it is anticipated that uniform geochemical properties will be assigned for the
vadose zone.
As described in the 2007 ICTM Report,the thickness of the vadose zone beneath Cells 2
&3 (42 ft)and Cells 4A &4B (40 ft)was based on the minimum separation distance
between the bottom elevation of the cell and the distance to the water table.As a
comparison,the average vadose zone thickness beneath Cell 2,Cell 3,and Ce1l4A were
63 ft,66 ft,and 56 ft.Data used to support the vadose zone thicknesses are included in
Table 2 of this memorandum.
Comment 2.Provide information regarding error flags on the flow modeling 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 ofthese errors on the
model output.DRC noted that several different types oferror messages were logged on
the HYDRUS output decks.It would be usefulfor Denison to include a table in the ICTM
summarizing the location ofthese errorflags and explaining why the messages occurred,
their impact on model output results,and potential errors which may be present in the
output data.
Response:MWH was not aware of any error flags that were generated while running the
HYDRUS computer program for any of the model simulations presented in the 2007
ICTM Report.MWH will coordinate with the DRC to resolve this issue.Water and
solute mass balances were reported in the model output files electronically as transmitted
in Appendix C of the 2007 ICTM Report.Data regarding water and solute mass balance
performance are contained in each folder for each simulation in the file titled
"BALANCE.OUT".Mass balance information for the HYDRUS models will be
included in the revised ICTM Report.
No error flags were generated while running the MODFLOW or MT3DMS models for
any of the simulations presented in the 2007 ICTM Report.Mass balance errors for the
MODFLOW and MT3DMS simulations will be included in the revised ICTM Report.
Comment 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.Please explain how the flux through
the bottom liner during operations and during slimes drain dewatering (transient flow
period)is accountedfor.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 justify this
position and explain why contaminant mass released to the foundation during the
operational phase ofthe tailings cells can be ignored in the transport predictions.This is
David Frydenlund &Harold Roberts
December I,2009
Page 19 of36
particularly important in light ofthe fact that the highest driving heads are likely found
during the operation ofthe tailings cells.
Response:As discussed during the March 318t and September 2nd meetings,the
operational phase of the White Mesa Mill will be included with the dewatering and post-
reclamation phases to predict potential impacts that may arise due to the transport of
solutes through the vadose zone.Results of this modeling will be included in the revised
ICTM Report.However,as discussed in the 2007 ICTM Report,it is important to note
that to date there is no evidence to support the assumption that the migration of tailings
solutions through the liners is occurring.Model simulations will include an
approximately 40-year operational period followed by the 200-year post closure period.
Details will be provided in the revised ICTM Report.However,if the results of the
operational phase mask the results that are needed to evaluate the performance of the
cover system (i.e.,the impacts to groundwater quality as a result of different cover
designs are not discemable),the operational phase may be excluded for better
comparison ofcover design performance.
Comment 4.Provide a graph depicting model layers associated with the HYDRUS
Vadose Zone flow model (ModelS)to depict steady state water contents in each layer.
Response:The response to this comment was provided previously in this memorandum
under the Water Balance Model Comment 2 above.The water content within the vadose
zone layers did not change significantly from year to year,as presented in the 2007 ICTM
Report.Time series and profile plots of pressure head and/or water content at salient
intervals within the bedrock vadose zone will be included in the revised ICTM Report.
Comment 5.Provide a specific discussion regarding differential flow characteristics
which define the layers in the model (e.g.differences in permeability,flow direction,head
distributions,etc.)This could be achieved through graphical representation and
additional explanation ofTable 3-1 in the ICTM.
Response:We anticipate assigning a single set of hydraulic properties to represent the
vadose zone,as described in the response to Comment 1 in the Flow Modeling Section
above.Soil water characteristic curves (SWCCs)for the different vadose zone samples
are plotted in Figure 2 and 3 ofthis memorandum.
Comment 6.Justify the use ofa head dependent sink term in the MODFLOW model for
tailings dewatering (Model 2).Explain and quantitatively justify how the simulation is
representative ofpump dependent slimes dewatering systems at each ofthe tailings cells.
Response:The response to this comment was provided previously in this memorandum
under the Water Balance Model Comment 5 above.
David Frydenlund &Harold Roberts
December I,2009
Page 20 of36
Comment 7.Explain andjustify why HYDRUS Model 5 breakthrough curves were based
on a lowerflux rate than predictedfor other upstream models (i.e.Model 3.)
Response:This is incorrect.The long-term average water flux rate that transported
solutes through the vadose zone exceeded the predicted water flux rate for other upstream
models,such as the cover,as described in the 2007 ICTM Report.A comparison of flux
rates for different parts of the system (cover,liner,vadose zone)will be included in the
revised ICTM Report.
Comment 8.Provide and justify the specific calculations used to determine the
geomembrane flux rates for Cells 2 and 3.
Response:As discussed during the September 2nd meeting,the methodology,
assumptions,and data pertaining to the Giroud-Bonaparte calculations used to determine
potential flux rates through the geomembrane liners will be included as an appendix to
support the revised ICTM Report.
David Frydenlund &Harold Roberts
December 1,2009
Page 21 of36
Contaminant Transport Modeling
DRC has significant concerns with the current contaminant transport modeling,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 5fails to provide a reasonably conservative evaluation ofthe
physical system.This is primarily due to lack of empirical data to represent chemical
partitioning and retardation through the vadose zone.The failure ofHYDRUS Model 5
is further exasperated downstream in MODFLOW Model 6 where non-conservative
HYDRUS inputs are used for the development of Cl and S04 breakthrough curves.
Please resolve thefollowing concerns:
Comment 1.Regarding the evaluation ofthe tailings cell wastewater chemistry,please
explain and justify the use of average concentrations (max.concentrations should be
used)especially in light ofthe use ofa non-weighted data set asfound 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 recent University of Utah samples taken
from the tailings cells.DRC also noted that no tailings cell wastewater data newer than
the 2004 Statement of Bases (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 concerned that auto-correlation is
present in the data set used for the ICTM.Please revise the evaluation of the tailings
wastewater and include conservative estimates of the wastewater contaminants along
with a full statistical review andjustification ofthe data set used.
Response:An updated source term chemistry for the tailings pore water will be
submitted with the revised ICTM Report.MWH feels that tailings pore water in slimes
drains are more representative of solutions that would remain in the tailings cells during
operations and at closure because these solutions would have had sufficient time to
equilibrate with the tailings.Furthermore,water extracted from the slimes drains,as
opposed to samples grabbed from surface ponds,is not affected as much by evaporation
and addition/recirculation of mill process water;evaporation and recirculation of mill
process water would tend to create a variable source term chemistry that is dissimilar to
and not representative of the slimes drain water.
A statistical analysis of all existing tailings slimes drain data collected by the University
of Utah (Hurst and Solomon,2008)and by Denison will be completed.However,the
University of Utah data may not be used in the revised ICTM because complete ion
charge balance data are not available.The results of this evaluation will be used to set
David Frydenlund &Harold Roberts
December I,2009
Page 22 of36
initial conditions for the HYDRUS contaminant transport model and to guide a sensitivity
analysis to evaluate the effects of tailings pore water chemistry on potential impacts to
groundwater.
The University of Utah data were not incorporated into the 2007 ICTM Report because
their report was submitted in May 2008 (Hurst and Solomon,2008),approximately six
months after the ICTM Report (MWH,2007)was submitted by Denison to the DRC.
Comment 2.Clarify whether flux rates from the tailings cell liners,used to evaluate the
200 year contaminant transport breakthrough curves include differentiating flux during
transient phases ofdewatering heads on the bottom liner.Include references to specific
model jiles/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 results ofthe 200 year breakthrough curve modeling.
Response:The 2007 ICTM Report did not consider the operational phase during
dewatering of Cells 2 &3.However,as discussed during the March 31 st and September
2nd meetings,the operational and dewatering phases will be considered during the
analysis of potential impacts to groundwater for the revised ICTM Report.However,if
the results of the operational phase mask the results that are needed to evaluate the
performance of the cover system (i.e.,the impacts to groundwater quality as a result of
different cover designs are not discernable),the operational phase may be excluded for
better comparison ofcover design performance.
Comment 3.Per the ITCM,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 ofthe laboratory data
sheets including QAlQC verification of results and statistical analysis of the data set
used.Provide concentration data which was used for adsorption calculations and an
explanation ofthe calculations made.Please demonstrate how these laboratory tests and
calculations were peiformed using standardized methods recognized by the regulatory
and relevant technical communities.
Response:Summary statistics of the geochemical data measured from vadose zone
samples as well as the original laboratory data sheets will be included as an appendix to
the revised ICTM Report.Spreadsheets summarizing the adsorption calculations in
addition to the source term chemistry will also be provided.
An explanation of the adsorption calculations was explained on Pages B-6 and B-7 in
Appendix B ofthe 2007 ICTM Report,but additional clarification of the calculations will
be included in the revised report.
The adsorption calculations and leaching procedure used to obtain the mass of HFO
incorporated into the ICTM Report were similar to the approach adopted for the Naturita
UMTRA Site as prepared by the U.S.Geological Survey for the U.S.Nuclear Regulatory
David Frydenlund &Harold Roberts
December 1,2009
Page 23 of36
Commission as part of NUREG/CR-6820 (Davis and Curtis,2003;Davis et aI.,2004).
The approach used in NUREG/CR-6820 to estimate the mass of BFO was also similar to
an approach adopted by the U.S.EPA Environmental Research Laboratory (Loux et aI.,
1989).The procedure consisted of subjecting samples of crushed bedrock to short-term
leaching in which the chemical extractions with hydroxylamine-hydrochloride acid were
expected to completely dissolve amorphous-mineral phases (e.g.,ferrihydrite/hydrous
ferric oxide;BFO)and partially dissolve some crystalline minerals (e.g.,goethite).The
solution acts as a reducing agent converting iron from the solid phase (ferric iron;Fe+3)to
an aqueous phase (ferrous iron;Fe+2).The concentrations of dissolved iron reported in
the leachate were then back-converted to the mass of BFO originally present in the rock
sample (i.e.,mg of HFO per kg of rock).Note that no data or values from the Naturita
site were assumed for the White Mesa site -only the approach was used.
The acid neutralization measurements were based on U.S.EPA method M600/2-78-054
3.2.3.
Comment 4.Provide additional justification that the laboratory results of digested core
data are representative or conservative of the mineralogy of the vadose zone.Also,
please prepare spreadsheets summarizing laboratory data results for the core samples
encompassing all results characterizing the mineralogy and attending descriptive
statistics.
Response:As discussed during the September 2nd meeting,MWH collected additional
samples for geochemical characterization.In total,34 samples and 4 duplicates collected
from cores from four different wells were analyzed for BFO and ANP (Figure 4 from this
memorandum).The samples were collected on -5-foot centers beginning approximately
30 feet below ground surface.Samples were collected approximately every 7 -10 feet
below about 65 feet.Tables and figures (e.g.,Figure 4 from this memorandum)showing
the sample locations and corresponding results will be included in the revised ICTM
Report.A thorough statistical review of the geochemical data will be included as an
appendix to the revised ICTM Report,and will be used to justify the assignment of
geochemical layers for the vadose zone model.
The ANP data are considered to be conservative because the test only measures fast-
reacting calcium-bearing carbonate minerals.The ANP test does not account for slow-
reacting (slow-neutralizing)aluminosilicate minerals,which are present in the vadose
zone and expected to partially contribute to the neutralization of acidity during the
potential migration of tailings pore water (e.g.,Eary and Williamson,2006).
HFO is the only solid phase that is credited as a potential sorption site of uranium and
other trace elements,which is a conservative assumption because other phases (hematite,
quartz,clays)also participate in surface complexation reactions.The calculated Kd'S will
be varied as part of the expanded sensitivity analysis to be included in the revised ICTM
Report.
David Frydenlund &Harold Roberts
December 1,2009
Page 240f36
Comment 5.Additional clarification is needed regarding the modeling of tailings
leachates and bulk reactions with the receiving aquifer matrix.Please peiform
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.DRC anticipates
that the development ofempirical Kd coefficients and Retardation Factors will supersede
the use ofthe PHREEQC ion sorption model.
Response:MWH does not feel that laboratory-based ~tests are appropriate for a
number of technical reasons discussed in the September 2nd meeting and outlined below.
•Model simulations are preferred considering the complexity of the tailings
solutions,aquifer matrix chemistry,and potential water-rock reactions that may
occur along a flow path in the subsurface beneath the site.
•Empirical determinations of Kd's were originally developed to quantify the
sorption of organic compounds and alkali/alkaline earth cations whose speciation
and sorption is nearly insensitive to changes in solution chemistry.
•Empirical determinations of ~'s are not an adequate metric for determining the
sorption of uranium and other trace elements because these species are strongly
controlled by the chemical reactions and expected solution chemistry.
•The quantification of potential reactions that may occur beneath the facility,
during the course of a laboratory experiment,is very difficult to reproduce,
especially taking into account the inherent variability in the geochemical reactions
that may occur during transport through the vadose zone
o Range in uranium concentration and other trace elements
o Range in neutralization potential
o Range in the mass and number ofsorbing phases
o Range in alkalinity of (partially)neutralized tailings solutions
o Range in water to rock proportions.
To counteract these difficulties,MWH decided to use a surface complexation model
developed to predict the adsorption of most contaminants ofconcern using the large body
of published literature that has evaluated the sorption of uranium and other metals onto
the surfaces of HFO (Dzombak and Morel,1990).The diffuse layer (sorption)database
developed by Dzombak and Morel (1990)was incorporated by the U.S.EPA into their
geochemical model MINTEQA2 (Allison et aI.,1991)and by the U.S.Geological Survey
into their geochemical model PHREEQC (Parkhurst and Appelo,1999).The diffuse
layer (sorption)database has been modified slightly by MWH to adjust the sorption
coefficients for uranyl because those values tended to overpredict the amount of
adsorption under low-pH conditions and underpredict the amount of adsorption under
high-pH conditions (Mahoney et aI.,2009).The agreement (R2 of -0.9)between the
final model selected parameters and the overall data set of 233 points,as represented in
14 data sets developed by five different research groups,show a consistency that supports
the general application of this revised model in describing uranyl adsorption onto HFO.
David Frydenlund &Harold Roberts
December I,2009
Page 25 of36
The surface complexation modeling approach incorporated into the ICTM Report is
functionally similar to the methodology developed by the U.S.Geological Survey for the
U.S.Nuclear Regulatory Commission,as presented in NUREG/CR-6820 (Davis and
Curtis,2003).As discussed in NUREG/CR-6820,the use of a surface complexation
model that incorporates linkages between surface and aqueous species through the
coupling of mass action equations and thermodynamic constraints is preferable to models
that rely on a constant partition coefficient (i.e.,single Kn)or empirical approaches (i.e.,
adsorption isotherms from batch tests).As previously discussed,additional samples were
collected from existing core in May 2009 and analyzed for ANP and HFO to improve the
geochemical characterization of the vadose zone.These data will be used to calculate
sorption coefficients (Kn's)and retardation factors for the underlying bedrock vadose
zone.As discussed during the March 31st meeting,sorption coefficients will be varied in
a sensitivity analysis to evaluate the effects of this uncertainty/variability on the model
results.The modeling will account for spatial variability in equilibrated solution
compositions and associated Kn's,which is preferred instead of a single ~model (Davis
and Curtis,2003;Zhu,2003;Bethke and Brady,2000).
Comment 6.Figure 4-8 of the ICTM,"Model-Predicted Dissolved Uranium
Concentrations in Vadose Zone Pore Water at 200 Years"does not allow review ofthe
migration of dissolved U below the tailings impoundments to the State Ground Water
Quality Standard (GWQS)concentration,0.030 mg/L.Thefigure needs to be magnified
(re-scaled)to demonstrate the travel distance ofUranium at orbelow the GWQS of0.030
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.
Response:For the revised ICTM Report the depth of uranium migration at levels less
than the GWQS will be presented.
Comment 7.The current geochemical model does not appear to anticipate the effects of
changes in neutralization and BFO capability as minerals are consumed.Provide
explanation ofhow thisfactor was considered in the model.
Response:As discussed during the March 31st and September 2nd meetings,the vadose
zone immediately below the liners will be discretized at a finer resolution to allow for
variability in pH (and sorption of uranium)as acidity would be neutralized.The amount
of sorption is limited to the calculated sorbent site densities of HFO (i.e.,finite number of
sorption sites).The amount of ANP is based on the measured mass of calcite.The
amount of calcite allowed to participate in neutralization reactions is mass limited.
Additional details will be included as an appendix to the revised ICTM Report.
Comment 8.Per DRC review it was noted that 11 potential contaminants are listed with
very low Kd values predicted by PBREEQC.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 ofpredicted concentrations with time.
In association with the additional breakthrough curve modeling (per #6 above)please
David Frydenlund &Harold Roberts
December 1,2009
Page 26 of36
justify why the Kd and Rfusedfor Uranium is conservative in relation to the geochemical
environment.
Response:A discussion of the calculated Kn's for different contaminants was discussed
on Page 4-8 of the 2007 ICTM Report;a similar discussion will be included in the
revised ICTM Report.Although some metals were predicted to have an intermediate to
low Kd-value,sorption and retardation ofthese species are likely to be significantly larger
than model-predicted values based on Kn's reported in the literature and the conservative
assumptions incorporated into the surface complexation modeling.
The amount of uranium sorption,in addition to other analytes of concern,was considered
to be conservative for a number of technical reasons,which were discussed in Appendix
B of the 2007 ICTM Report,and include:
•Sorption is only credited to occur onto the surface of a single mineral phase:iron-
hydroxide (i.e.,HFO).Surface complexation modeling for the Naturita UMTRA
Site suggests that additional mineral phases (e.g.,hematite,
montmorillonite/smectite,and quartz)would adsorb uranium.Furthermore,
adsorption of uranium onto the surfaces of aluminum and manganese hydroxides
is also expected to occur (e.g.,Langmuir,1997).All of these mineral phases in
addition to HFO are expected to be present,to some degree,as part of the vadose
zone mineralogy and participate in surface complexation reactions.
•Uranium adsorption was allowed to compete with other metals,which would
decrease the total amount ofuranium that could adsorb and lower the Kn.
•The thermodynamic database was updated to include additional aqueous species
including the calcium-uranyl (uranium[VI])-carbonate species (CaUOz(C03kz
and CazUOz(C03h).Inclusion of these aqueous species in the thermodynamic
database would significantly decrease the total amount of uranyl available to
participate in surface-complexation reactions (Fox et aI.,2006).
•Coprecipitation reactions were not accounted for during the geochemical
modeling.Coprecipitation of uranium (Abdelouas et aI.,1998)and metals onto
the surfaces of precipitating phases (e.g.,hydrous ferric oxide,sulfates,and
carbonates)was ignored,which could also serve as a sink for metals and reduce
transport mobility.
•Oxidizing conditions within the vadose zone were assumed which would prohibit
precipitation ofadditional uranium-bearing minerals.
A similar discussion of the conservative assumptions related to the sorption (Kd)and
retardation (R)ofuranium will be included in the revised ICTM Report.
Ground Water Compliance Limits (GWCLs)have been proposed for chloride and sulfate
on a well-by-well basis (INTERA,2007).As in the 2007 ICTM Report,the revised
ICTM Report will compare model-predicted concentrations of chloride and sulfate in the
perched aquifer to the proposed GWCLs (or GWQSs)for wells located downgradient
from the tailings cells.
David Frydenlund &Harold Roberts
December 1,2009
Page 27 of36
Conclusion
Generally,DRC review concludes that empirical data is lacking to develop a
representative or conservative model ofinfiltration,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,leafindex,and rooting structure,etc.),long term degradation ofthe system (e.g.
cover flow impact from frost heaving,erosion,intrusion,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 Kd and R characteristics (based on empirical data.)
Response:The responses discussed throughout this memorandum,including additional
analyses committed to at the March 31st and September 2nd meetings,and described
above,are intended to address DRC's concerns.The ICTM Report will be revised
accordingly such that potential impacts are predicted using a combination of
representative and conservative estimates to simulate potential infiltration through the
cover in addition to potential flow and chemical transport from the vadose zone to the
perched aquifer.
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 platfonns,needed improvements in the ICTM and needs for
additional site specific data.
Response:Meetings were held on March 31st and September 2nd,2009 in the DRC's
office in Salt Lake City.The first meeting was attended by Loren Morton and Thomas
Rushing from the DRC,David Frydenlund and Harold Roberts from Denison,and Doug
Oliver,Ryan Jakubowski,and Phil Crouse from MWH.The second meeting was
attended by Loren Morton,David Rupp,and Thomas Rushing from the DRC,David
Frydenlund and Harold Roberts from Denison,and Doug Oliver,Ryan Jakubowski,and
Ed Redente from MWH.
As discussed during the September 2nd meeting,MWH/Denison has submitted final
versions ofthe meeting minutes from the March 31 st and September 2nd meetings and this
final version of the response to comments.If the DRC observes any outstanding issues
that require addressing within the response to comments such information should be
conveyed to Denison to facilitate submittal of the revised ICTM Report.
David Frydenlund &Harold Roberts
December I,2009
Page 28 of36
Table 1.Summary ofunsaturated and saturated hydraulic properties of bedrock vadose zone core samples.
Residual Saturated Empirical Empirical (Vertical)DryWellillandCoreSaturated
Interval~easured Water Water Fitting Fitting Hydraulic Bulk
in Feet Below Content Content Parameter Parameter Conductivity Density
Ground Surface 9r (-)95 (-)a (cm·I )n (-)Ks (em/d)Pb (g1cm3)
MW-30 35.5-36.0 0.004 0.199 0.0266 1.348 69.9 1.98
MW-2355.5-56.0 0.003 0.184 0.0103 1.386 9.37 2.03
MW-2374.3-74.6 0.016 0.122 0.0003 1.354 2.47 2.33
MW-2382.7-82.9 0.003 0.160 0.0069 1.336 14.9 2.10
MW-23 103.3-103.5 0.006 0.205 0.0287 1.349 263 1.84
MW-30 44.0-44.5*0.032*0.264*0.0081*1.201*0.707 2.23
*Water retention parameters based on volume adjusted values because core cracked and swelled after conductivity testing.
David Frydenlund &Harold Roberts
December I,2009
Page 29 of36
Table 2.Vadose zone thickness beneath the tailings cells.
Tailings Location Bottom Elevation of Cell Nearest Water Table Vadose Zone ThicknessCellWellElevation
(--)(--)(ft above MSL)(--)(ft above MSL)(ft)
Cell 2 Cell 2 NW corner 5602 MW-24 5506 96
Cell 2 Between Cells 2&3 5592 MW-29 5511 81
Cell 2 Cell 2 N side 5595 MW-28 5541 54
Cell 2 Cell 2 NE corner 5605 TW4-20 5553 52
Cell 2 Between Cells 2&3 5582 MW-30 5535 47
Cell 2 Between Cells 2&3 5588 MW-31 5542 46
Cell 2 Ce1l2N side 5600 TW4-22 5571 29
Cell 3 Cell 3 SW corner 5585 MW-23 5495 90
Cell 3 Cell 3 S side 5585 MW-12 5500 85
Cell 3 Cell 3 S side 5577 MW-05 5502 75
Cell 3 Between Cells 2&3 5585 MW-29 5511 74
Cell 3 Cell 3 S side 5582 MW-lI 5518 64
Cell 3 Cell 3 SE corner 5592 MW-25 5535 57
Cell 3 Between Cells 2&3 5585 MW-31 5542 43
Cell 3 Between Cells 2&3 5577 MW-30 5535 42
Ce1l4A Cell 4A S side 5562 MW-14 5494 68
Cell4A Cell 4A SW corner 5557 MW-15 5493 64
Cell4A Cell 4A N side 5570 MW-ll 5518 52
Cell4A Cell 4A NE corner 5575 MW-25 5535 40
Notes:
1.Units for elevation are referenced to feet above mean sea level (ft above MSL).
2.Bottom elevations for Cells 2 &3 from D'Appolonia (1982).
3.Bottom elevations for Cell 4A from Geosyntec (2006).
4.Average water table elevations from 2007 Water year (calibration targets in GW flow model).
5.The vadose zone thickness was calculated as the difference between the cell bottom and the water table elevation.
6.The average vadose zone thickness for Cell 2 (excluding TW4-22),Cell 3,and Cell 4A were 63 ft,66 ft,and 56 ft.
7.TW4-22 excluded because this well is located upgradient ofCell 1.
David Frydenlund &Harold Roberts
December I,2009
Page 30 of36
Figure 1.Flowchart illustrating the primary models used to support the revised ICTM Report.
INFILTRATION MODEL OF
TAILINGS CELLS COVERS
(HYDRUS)
TAILINGS DEWATERING
MODEL
(MODFLOW)
!
POTENTIAL FLUX
THROUGH LINER
(GIROUD-BONAPARTE
EQUATIONS)
FLOW AND
TRANSPORT MODEL •OF VADOSE ZONE
(HYDRUS)
FLOW AND TRANSPORT
MODEL OF PERCHED
AQUIFER
(MODFLOW &MT3DMS)
GEOCHEMICAL
MODEL
(PHREEQC)
David Frydenlund & Harold Roberts
December 1, 2009
Page 31 of 36

Figure 2. Comparison between the measured and model predicted soil water retention curves
for the vadose zone samples.

0
1
10
100
1,000
10,000
100,000
1,000,000
0 0.05 0.1 0.15 0.2 0.25
pr
e
s
s
u
r
e
 he
a
d
 (‐cm
)
water content (‐)
MW‐30 35.5‐36.0 (Model Fit)MW‐30 35.5‐36 (Measured)
MW‐23 74.3‐74.6 (Model Fit)MW‐23 74.3‐74.6 (Measured)
MW‐23 55.5‐56.0 (Model Fit)MW‐23 55.5‐56.0 (Measured)
MW‐23 82.7‐82.9 (Model Fit)MW‐23 82.7‐82.9 (Measured)
MW‐23 103.3‐103.5 (Model Fit)MW‐23 103.3‐103.5 (Measured)
David Frydenlund & Harold Roberts
December 1, 2009
Page 32 of 36

Figure 3. Log hydraulic conductivity as a function of water content for the vadose zone
samples.

‐25
‐20
‐15
‐10
‐5
0
5
0 0.05 0.1 0.15 0.2 0.25
lo
g
 hy
d
r
a
u
l
i
c
 co
n
d
u
c
t
i
v
i
t
y
 (c
m
/
d
)
water content (‐)
MW‐30 35.5‐36.0 (Model Fit)
MW‐23 74.3‐74.6 (Model Fit)
MW‐23 55.5‐56.0 (Model Fit)
MW‐23 82.7‐82.9 (Model Fit)
MW‐23 103.3‐103.5 (Model Fit)
David Frydenlund & Harold Roberts
December 1, 2009
Page 33 of 36

Figure 4. Generalized cross section of monitoring wells and availability of core in the vicinity of the tailings cells. Sample intervals selected for hydrogeologic and geochemical characterization are also identified.

MW-23 MW-30 MW-24 MW-28 TW4-22
o
20
<.9
~o
Q)
CO
120
Q)
Q)
100 u..
60
80
40
TD=115
TD=110
TD=120
29.7 -29.9
34.9 -35.1 34.0 -34.2
40.0-40.2 39.4 -39.6
44.7 -44.9 456 -46.0
49.8 -49.9 50.0 -50.3
56.0 -56.2 55.5 -55.7
(dup)-."..
60.2 -60.3
63.4·63.5
69.8 -70.0
(dup)
73.0 -73.2 Sl
80.0 -80.3
(dup)84.0 -84.3
TD=110
Sl
::.H 35.5·36.0
37.5-38.0 -"::
313-31.5 -'."
50.0 -50.2 -'
53.9 -54.0 -':0'
59.8 -60.0 -'
43.0 -43.2 ===*43.2 -43.5 .H 44.0 -44.5
82.7 -82.9
103.3 -103.5
59.3 -59.5
49.3-49.5 -
53.0 -53.5
89.9 -90.0
63.8 -640
82.5 -82.7
68.9 -69.3
74.0 -74.3
99.8 -1000
103.0 -103.3(dup)
TD=132
140
10 Geochem samples
plus 1 duplicate
4 Hydrogeologic samples
7 Geochem samples
2 Hydrogeologic samples
9 Geochem samples
plus2 duplicates
No samples collected 8 Geochem samples
plus 1duplicate
<ID>MWH
Core not recovered
[J Sandstone
Siltstone
GJ Conglomerate
I Brushy Basin Shale
=sl Water Table (2007)
TD Total Depth
MW-23 MW-30 MW-24 MW-28 TW4-22
o
20
Q)
Q)
100 u..
120
80
60
40
TD=115
TD=110
TD=120
29.7 -29.9 ..
34.9 -35.1 34,0 -34.2
40.0 -40.2 39.4 -39.6
44.7 -44.9 456 -46.0
49.8 -49.9 50,0 -50.3
56.0 -56.2 55,5 -55.7
(dup)-,'-
60,2 -60.3
63.4 -63.5
69,8 -70.0
(dup)
73.0 -73.2 Sl.
80.0 -80.3
(dup)84,0 -84.3
TD=110
Sl.
313 -31.5 ->;'..
,'":H 35.5 -36.037,5 -38.0 -"..
43,0 -43.2 ===*'.:43,2 -43.5 ::.H 44.0 -44.5
50,0 -50.2 -::';
53,9 -54.0 --,
55,5-560
59,8 -60.0 -'
1033-103.5
59,3 -59,5
49,3-49,5 -
53,0 -53,5
68,9 -69,3
74,0 -74,3
82,5 -82,7
63,8 -64,0
89,9 -90.0
99.8 -100,0
103.0 -103,3
(dup)
TD=132
140
10 Geochem samples
plus 1 duplicate
4 Hydrogeologic samples
7 Geochem samples
2 Hydrogeologic samples
9 Geochem samptes
plus2 duplicates
No samples collected 8 Geochem samples
plus 1duplicate
<II»MWH
Core not recovered
DSandstone
Siltstone
GlConglomerate
I Brushy Basin Shale
=>:s£Water Table (2007)
TD Total Depth
David Frydenlund &Harold Roberts
December I,2009
Page 34 of36
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