Colloid-facilitated migration of plutonium in fractured rock has been implicated in both field and laboratory studies . Other reactive radionuclides may also experience enhanced mobility due to groundwater colloids. Model prediction of this process is necessary for assessment of contaminant boundaries in systems for which radionuclides are already in the groundwater and for performance assessment of potential repositories for radioactive waste. Therefore, a reactive transport model is developed and parameterized using results from controlled laboratory fracture column experiments. Silica, montmorillonite and clinoptilolite colloids are used in the experiments along with plutonium and Tritium . . The goal of the numerical model is to identify and parameterize the physical and chemical processes that affect the colloid-facilitated transport of plutonium in the fractures. The parameters used in this model are similar in form to those that might be used in a field-scale transport model

Lu, G. (Guoping)

Reimus, P. W. (Paul William)

Viswanathan, H. S. (Hari Selvi)

Ware, S. D. (Stuart D.)

Wolfsberg, A. V. (Andrew V.)

UNT Digital Library

LA-UR- 0-2, - e=-) DApproved fo r public release;distribution is unlimited.Title :Author(s) :Submitted to:Colloid Facilitated Transport in Fractured Rocks: ParameterEstimation and Comparison with Experimental DataHar i S . V i swanathanAndrew V . WolfsbergPaul W . ReimusDoug WareGuoping L u10th International High-Level Radioactive WasteManagement ConferenceLos AlamosNATIONAL LABORATOR YLos Alamos National Laboratory, an affirmative action/equal opportunity employer, is operated by the University of California for the U .S .Department of Energy under contract W-7405-E NG-36 . B y acceptance of this article, the publisher recognizes that the U .S. Governmentretains a nonexclusive, royalty-free license to publish or reproduce the published form of this contribution, or to allow others to do so, for U .S.Government purposes. Los Alamos National Laboratory requests that the publisher identify this article as work performed under theauspices of the U .S. D epartment of Energy. Los A lamos National Laboratory strongly supports academic freedom and a researcher's right topublish ; as an institution, however, the Laboratory does not endorse the viewpoint of a publication or guarantee its technical correctness .Form 836 (8/00)Colloid Facilitated Transport in Fractured Rocks : Parameter Estimationand Comparison with Experimental Dat aHari S . Viswanthan', Andrew V. Wolfsberg', Paul W . Reimus', Doug Ware' and Guoping Lug1 Los Alamos National Laboratory, Los Alamos, NM, USA .2 Lawrence Berkeley National Laboratory, Berkeley, CA, US Amigration followed the discovery of lowABSTRACT concentrations of plutonium (Pu) in groundwate rColloid-facilitated migration of plutonium in si nifcantl further downstream from anfractured rock has been implicated in both fiel dand laboratory studies . Other reactiveradionuclides may also experience enhancedmobility due to groundwater colloids . Modelprediction of this process is necessary forassessment of contaminant boundaries in systemsfor which radionuclides are already in thegroundwater and for performance assessment ofpotential repositories for radioactive waste .Therefore, a reactive transport model is developedand parameterized using results from controlledlaboratory fracture column experiments .Silica, montmorillonite and clinoptilolite colloidsare used in the experiments along with plutoniumand Tritium . . The goal of the numerical model isto identify and parameterize the physical andchemical processes that affect the colloid-facilitated transport of plutonium in the fractures.The parameters used in this model are similar inform to those that might be used in a field-scaletransport model .INTRODUCTIONSome contaminants in groundwater interactstrongly with immobile minerals which retardtheir movement relative to groundwater flow .However, they may also sorb to colloids,suspended pa rt icles in the submicometer sizerange often resulting from weathering ofimmobile minerals, thus enhancing their mobility .g junderground nuclear test source than would bepredicted for aqueous solute transport (1) . Theplutonium was found with inorganic colloidalparticles, suggesting that it was carried by thecolloids .Radionuclides are a class of contaminants forwhich colloid-facilitated transpo rt may be ofparticular significance. Increased interest inunderstanding colloid-facilitated radionuclideColloid-facilitated radionuclide migration ingroundwater has recently been included in theassessment of the performance of the potentialgeologic repository at Yucca Mountain, Nevada .One reason why geologic repositories areconsidered for the disposal of spent nuclear fuel isthe strong affinity of many radionuclides to sorbonto immobile minerals. However, the presenceof colloids may enhance radionuclide mobility ofsome component of source releases in thegroundwater. This may have an impact on theefficiency of geologic media to act as a naturalbarrier .In this study, we develop a reactive transportmodel for colloid-facilitated plutonium transportin fractures based upon laborato ry columnexperiments that utilize natural colloids, Pu andtritium (3H) . Using the column experiments andthe numerical model, we seek to identify thephysical and chemical processes and parametersthat affect colloid-facilitated transport ofplutonium transport in fractured rocks .EXPERIMENTAL SUM MARYA series of colloid-facilitated plutonium transportexperiments were conducted in naturally fracturedrock cores . Prior to these experiments, solublePu(V) was sorbed onto inorganic montmorilloniteand silica colloids, and Pu(IV) was sorbed ontoinorganic clinoptilolite colloids . Some of the Pu-colloid solution was then injected into saturated,fractured rock cores, through which steady waterflow (natural water from well U-20WW at NITS,or a synthetic water with the same major ionchemistry) had been established . The colloidmass concentrations in the injection solutionsranged from -140 to 200 mg/L in all experiments,and Pu concentrations were on the order of IO "M. The percent of total Pu initially sorbed ontocolloids in the feed solutions ranged from 70%for the Cheto-montmorillonite colloids to 90-100% (actually measured at -90% in batchexperiments, but the column results suggest closerto 100%) for the clinoptilolite colloids. Inaddition, tritiated water was injected with the Pu-colloid solution, thus providing a non-reactivetracer to determine diffusion, dispersion, andfracture aperture size . Figure I shows a schematicof the experiments .Twelve transport experiments involving Pu(V)and Cheto-montmorillonite, Otay-montmorillonite, and silica colloids wereconducted in two separate fractured cores fromborehole UE-20c at the NTS . The two cores wereobtained from within 7 feet of each other in aheavily fractured interval of the Topopah SpringTuff unit . One core was from 2851 ft below thesurface and the other from 2858 ft below thesurface . Two transport experiments involvingPu(IV) and clinoptilolite colloids were conductedin two separate fractured cores from boreholesPM I and PM2 . The PM 1 core was from 4823 ftbelow the surface while the PM2 core was from4177 R below the surface . The fractured coreswere cut perpendicularly to their axes such thatthe fracture bisected the length of the resultingcore section . The cores ranged in length from 12 .6to 2 1 .9 cm, and they all had a diameter of 8 .7 cm .For each of the cores from UE-20C, experimentswere conducted with high and low flow rates foreach of the different colloids: Cheto-montmorillonite, Otay-montmorillonite, and silicawith Pu(V) . Similarly, high and low flow rateexperiments were conducted with clinoptilolitecolloids with Pu(IV) for the cores from PMI andPM'? . Thus, for each colloid type, four differentexperiments with Pu were conducted .Normal ize d Concentrations. >.~• • '.r, ~o •!• `. n e. :. . ~, .:. '..Figure 1 . Schematic of laboratory experiments .Example results s h own for experimentcondu c ted with m ontmor i lloniteco lloid s .CONCEPTUAL MODE LThe chemical and physical processes controllingplutonium mobility in the experimental columnsinclude (I) plutonium aqueous speciation ; (2)plutonium sorption and desorption on colloids ;(3) filtration of colloids on the fracture walls ; (4)solute diffusion into the matrix ; and (5) surfacecomplexation and ion exchange of plutoniumwith fracture minerals and matrix minerals (seeFigure _') . The five processes functioncompetitively with each other ; the first twoincreasing the mobility of plutonium, the latterthree reducing mobility . Additionally, the fractureaperture affects the fluid velocity for a given flowrate and the proximity of solutes in the fracture tothe fracture wall where diffusion into the matrix,sorption of solutes to fracture minerals, andfiltration of colloids occurs .MATRIX PU0.ea (ox), .---rYo ,F RACTU RE0 wo, ( co,7C„'~ R laa i x pio n site 0 Coll o id 'V► Frx u ra Co ate `Fi gure 2 . Schematic of conceptual model forplutonium and colloid transport infracture sNUMERICAL MODE LThe tritium breakthrough curves obtained for UE-20C columns 2851 and 2 858 (referred to fromhere on as 2851 and 2858) were interpreted usingthe computer m odel FZ ELAP (2) and the PM I andPM 2 breakth ro ugh curve s were interpreted u s in gthe comput er m odel FEHM (3,4) . The s esimulati o ns y ield e s timates of effective halfape rtures , m a tri x diffu s i o n coefficients, and so lutedi s pers i v ity p a rameters . These pa rameters arethen u sed directly in estimating the reactivetranspo rt parameters .We simulate the colloid-facilitated plutoniumtransport column experiments with a discretefracture model using FF,IIM . The model utilizesa computational grid normal to the fracture tocapture both the fracture and matrix and thechemical and physical processes described in theconceptual model (Figure 2) . In the fractures,advective transport, first-order kinetic Pu sorptiononto colloids, equilibrium Pu sorption ontofracture minerals, and first-order kinetic colloidfiltration all affect Pu migration . In the matrix, thedominant processes affecting Pu mobility arediffusion and equilibrium sorption onto matrixminerals .The experiments are modeled using a_'-D x-, y-grid . The first column in the grid represents thefracture and its width is set to the fractureaperture, which is obtained from the model fits tothe tritium data . The remaining columns representthe matrix and are finely spaced to capture matrixdiffusion . Along the length of the model, the gridis evenly discretized at a fine enough resolution tominimize numerical errors . Each column requiresa unique grid with the column's dimensions,aperture, flow rate, input concentrations ofplutonium and colloid, diffusion coefficient, anddispersivity . Parameters for sorption of Pu ontosilica colloids should be the same regardless ofwhich column is used. Likewise, sorptionparameters for Pu onto fracture minerals andmatrix minerals for a specific column should bethe same regardless of flow rate and whichcolloids are involved in the particular experiment .Automatic parameter estimation is used to findmodel parameters that lead to the best matchesbetween simulated and observed concentrationbreakthrough curves . Thus, the goal of parameterestimation is to reduce the difference betweenobserved and simulated results by adjusting themodel parameters . In this study, we couple thePEST (Parameter ESTimation) software package(5) with the reactive transport model, describedabove .In a parameter estimation simulation, PEST runsFEIIM as many times as necessary to determinethe optimal set of parameters . PEST uses theGauss-Marquardt-Levenberg algorithm (GML) toadjust the FEHM parameters while reducing anobjective function . In this case the objectivefunction is the sum of the squared residuals (SSR)where the residual is the difference in theobserved and simulated concentrations . Bycomparing parameter changes with objectivefunction improvement achieved with eachiteration, PEST determines how and if to proceedwith additional optimization iterations .R ESULTSThe simulations seek to identify the optimal set oftransport parameters for multiple experiments inwhich the same type of colloid was used .Therefore, four different parameter estimationsimulations were conducted for each of the tourdifferent colloids . In each parameter simulationset, results for two flow rates on two differentcolumns, hence a total of four results, are used .For each of these sets of simulations, six modelparameters are estimated . They include I )forward rate of Pu sorption onto the colloid, 2)reverse rate of Pu desorption off of the colloid, 3)matrix sorption Kd in the first column, 4) fracturesorption Kd in the first column, 5) matrix sorptionKd in the second column, and 6) fracture sorptionKd in the second column . Figure 3 shows thereactions and which parameters are being fit .("11101d . Column I~~~~<~ • , . h L ~• ~~Il ' x'11K ,,PuUp;PU Q~pu~Wt 1 . •Figure 3 . Schematic comparing plutoniumtransport in the presence of the samecolloid type in two different columns .Plutonium-colloid reaction parametersexpected to he the same whereassorption parameters to fracture andmatrix minerals expected to differbetween the two columns .The first-order kinetic model parameters forattachment and detachment of colloids to fracturesurfaces generally provides ve ry good matchesbetween simulated and obse rved colloidb reakthrough data . Holding the colloida ttachment and det achment parameter s fixed ,PEST simulation s are conducted to e s timate theparameters fo r plut onium so rpti o n t o co ll o ids andimmobile minerals .The Cheto-montmorillonite experiments arematched very well for all four experiments (SeeFigures 4 and 5) . In contrast, there are substantialdifferences in the plutonium breakthrough databetween the two columns when silica colloidswere used (see Figures 6 and 7) . The peaks forboth high- and low-flow experiments aresubstantially larger for column 2858 than 2851 .Thus the parameter estimation process ischallenged to come up with one set of kineticparameters for plutonium sorption on silicacolloids for all experiments yet different matrixand fracture Kds for the two different columns .The resulting fit is a compromise that overpredicts the peaks in one set of experiments andunder predicts them in the others, minimizing theobjective function for the set of tour experiments .Fits to the Otay-montmorillonire colloidexperiments capture the trends reasonably well,but again show the compromises associated withfitting four experiments with one set ofparameters (see Figures 8 and 9) . Theclinoptilolite colloid experiments yield thegreatest mass of plutonium in the effluent (seeFigures 10 and I I ) . The desorption rate ofplutonium from clinoptilolite colloids is muchsmaller than for the other colloids . In fact, theparameter estimation process evolved such that itidentified the maximum desorption rate to be sosmall that no substantial desorption could occur .In other words, the best match between model andexperimental results involves no desorption ofplutonium over the length and time scalesassociated with the experimental columns .U Q ~D .I. Ne o n Fro .r oe . i x qn r b .D .I. - low Fb«r o a . i _ io . v io .Fi gure 4 . P u breakthrough curves : Cheto-montmorillonite experiment, column"_'851, high and low flow rates .01 50pUDeb - Hq h f bwModa l - Hgh FlowDan -Low FlowMadN -Low F b wFi gure 5. Pu breakthrough curves : Cheto-montmorillonite experiment, column 2558, hig ha nd l ow flow rates .020 1 '0V O 1U0 0`Dad - M iph FbwModel - High FlowDate - Low FlowModel - L wv FlowT ime (hrs. )Figure 6. Pu breakthrough curves : silicaexperiment, column ?851, high andlow flow rates .0 2o 1 s k?0V O1U0 .05Da ta - High FlowModel - High FlowDa ta • Low FlowModel - Low F lo wa015005 Ooh NIgA FbwOV 01UM ode l - H lph FbwData - Lav FlowModel - Low F b wFigur e 8 . P u breakthrough curves : Otay-montmorillonite experiment, columnX851, high and l ow flow rates .0201 5V p ~ JU0 OSData -High FlowM ode l - High FlowData - L ow flowM ode l - Low F l owFigure 7. Pu breakthrough curves : silic aexperiment, column 2858, high and Fi gure9 . Pu breakthrough curves : Otay-flow rates. montmorillonite experiment, column1858, high and low flow rates .Data - High v la-MoNI- Hq~fbwDMa - Lav FlowMotlN - Law Flowo sivi, lMo~ os h1o zo f ~ iFigure 10 . Pu breakthrough curves : clinoptiloliteexperiment, column PM 1, high andlow flow rates .Da4 M lph Fbw0 7MoOS I . High FlowDAY - Lnw FbwOB Modal LO. Flowo !u 05 I ~'o . ~ _ . _.0 3o . ~ . ..Fi gure 11 . Pu breakthrough curves : clinoptiloliteexperiment, column PMT, high andl ow flow rates .SUMMARYThe reactive transport model uses empiricalparameters (e .g . Ind and k,) that describe sorptionand desorption of plutonium an colloids andimmobile minerals as well as colloid attachmentand detachment from fracture walls . Thus, theparameters used in this model are similar in formto those that might be used in a field-scaletransport model . Specific findings associated withthis study are :I . Simulations using parameters fit by RELAPand FEHM lead to good matches betweenmodel and experimental re sults .' . W e have u sed automatic parameterestimation and results from multiple columnexperiments to simultaneously estimate thetransport parameters tier sorption anddesorption kinetics of plutonium on fourdifferent types of colloids .3 . The parameter estimation simulati on s alsoy ield sorpti on parameter s for plutonium toimm obile mineral s in the four diffe rentco lumns c o n s idered .4 . The desorption rate of plutonium off o fcolloids is the most sensitive parameter .Small changes in this parameter lead to largechanges in model results . This is notsurprising due to the experimental design i nwhich colloids are effectively doped wit hplutonium prior to injection in the columns .Thus desorption off of the colloids govern sthe mass of plutonium transported bycolloids .5 . No free aqueous plutonium exits thecolumns . Thus, over the time and spacescales of the experiments, the sorption toimmobile minerals must he great enough toeffectively remove all tree plutonium fromaqueous solution .6 . Zeolite colloids have the smallest desorptionrate, suggesting that they would suppo rt theg re atest travel distances and times of Pu .7 .Leolite colloid experiments provide littleinformation on sorption rate to colloid or tothe fracture and matrix minerals because allplutonium that originates on the colloidsbreaks through on the colloids .8 . Other less sorptive colloids (Silica and Clay)appear to facilitate plutonium transport onlyfor small residence times, but they do provideinformation on matrix and fracture sorptiveproperties .AC ti NOWLE I )G F.h 1 f:NTSThis work was supported by the National NuclearSecurity Administration (NNSA) of the U .S .Department of Energy (DOE) as part of theUnderground Test Area (UGTA) Projectadministered out of the Nevada OperationsOffice . Los Alamos National Laboratory isoperated by the University of California for theNNSA/DOE under Contract W-7405-ENG-36 .The authors gratefully acknowledge AnnieKersting and Pihong Zhao of' LawrenceLivermore National Laboratory for providing theclinoptilolite colloids doped with Pu(IV) . Wewould also like to thank Ningping Lu forproviding all the Pu(V)-colloid solutions as wellas measurements of the fraction of Pu initiallysorbed to colloids in the solutions, and SteveKung and loana Anghel for conducting thecolloid concentration measurementsREFERENCES1) Kersting, A.B ., D . W . Efurd , D.L . Finnegan ,D .J . Rokop, D .K . Smith, and J . J . Thompson,Migration of plutonium in ground water at theNevada Test Si te, Nature , 397 , 56, 1999.2) Reimus, P .W. and M.J . Haga, Analysis oftracer responses in the BULLION forced-gradientexperiment , Los Alamo s National Laborato ryReport LA-13616-MS, Los Alamos NationalLaboratory , NM, 1999 .3) Zyvolo ski , G . A ., B . A Robinson , Z . V Da sh , andL . Trea se . Summary of the Model s and Method sfor the FEHM Application - A Finite-ElementHeat-and Mass-Transfer Code. Report LA - 13307-MS, Lo s Alamos, New Mexico : Los Alamo sNational Laborato ry, 1997a.4) Zyvolos ki , G . A ., B . A . Robin son , Z . V . Das hand L . T. Treace . User's Manual for the FEHMApplication - A Finite-Element Heat- and Mass-Tran sfer Code, Report LA- 13306-MS , LosAlamos, New Mexico : Los Alamos NationalLaborato ry, 1997b .5) Dohe rty, J ., 2000 . PEST, Model-IndependentParameter Estimation Users Manual . WatermarkNumerical Computing . http ://m e mbers .ozemail .com .a u /--wco mp/, distributed as freewaresoftware with documentation from SSPopadopulos & Associates, Inc .h ttp ://www .sspa.com/pest (CTRL confirmed July,2002) .

Colloid facilitated transport in fractured rocks : parameter estimation and comparison with experimental data.

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Colloid facilitated transport in fractured rocks : parameter estimation and comparison with experimental data.

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