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www.vadosezonejournal.org433Reproduced from Vadose Zone Journal. Published by Soil Science Society of America. All copyrights reserved.Aqueous Complexation ReactionsTable 3. Arsenic aqueous complex thermodynamic values(Appelo and Postma, 1993; Welch and Stollenwerk, 2003).The set of As aqueous complexes that has been addedComplexation Reaction Log K Dh r (kcal molto the geochemical database built into UNSATCHEM)Arsenian Pyrite–FeAs x S 2-x (0 , X , 1)Sorbed speciesAs(III), As(V)for which the rate of iron hydroxide precipitation is expressedin moles per second at 208C (Langmuir, 1997).O 2 species P O2is given in Table 2; the values of the thermodynamicH 3 As (III) O 3 W H 2 As (III) O 32 1 H 1 29.228 6.56H 3 As (III) O 3 W HAs (III) Oparameters associated with these reactions are given in3 1 2H 1 221.33 14.19932H 3 As (III) O 3 W As (III) O 3 1 3H 1 234.744 20.25Table 3. Since these complexes affect the overall mass H 3 As (V) O 4 W H 2 As (V) O 2 4 1 H 1 22.243 21.69Hbalance, two additional equations for the two As oxidationstates considered are3 As (V) O 4 W HAs (V) O 22 4 1 2H 1 29.1 20.92H 3 As (V) O 4 W As (V) O 32 4 1 3H 1 220.597 3.43As(III) T5 [H 2 AsO 2 ] 1 3 [HAsO22]3 [AsO32]31 [H 3 AsO o ] [13] 8.3 (Krauskopf and Bird, 1995). Because of this high3 pH environment, the dissolution of pyrite by Fe 31 isAs(V) T5 [H 2 AsO 2 ] 1 4 [HAsO22]4 [AsO32]minimized as a result of the rapid precipitation of Fe 314 to Fe(OH) 3 by the following reaction:1 [H 3 AsO o ] [14]4where variables with the subscript T represent the totalFe 31 1 1 O 2 1 5 H 4 22O!Fe(OH) 31 2H 1 [19]analytical concentrations in solution of the particularspecies, and where the brackets refer to molalities (molkg 21 ). In addition to these mass-balance equations, theoverall charge-balance equation, including As complexes,is expressed aswhich, in the flow-through environment of the heapleachfacility, is assumed to be irreversible. Therefore,the reactions given in Eq. [17] and [18] are ignored, andthe primary dissolution reaction for pyrite in CarlinTrend type deposits, as given in Eq. [16], is controlled by2[Ca 21 ] 1 2[Mg 21 ] 1 [Na 1 ] 1 [K 1 ] 1 [CaHCO 1 ] the availability of O 2 .3The acidity generated from the oxidation of pyrite1 [MgHCO 1 ] 1 3 [H1 ] 2 2[CO 22 3 [HCO2] 2 3 2[SO22]4 (Eq. [16]) and the hydrolysis of ferric iron (Eq. [18]) is2 [Cl 2 ] 2 [OH 2 ] 2 [NaCO 2 ] 2 3 [NaSO2] 2 4 [KSO2]buffered by the dissolution of calcite and the precipitationof gypsum. In Carlin Trend heap-leach facilities, the42 [H 2 AsO 2 ] 2 3 2[HAsO22]3 3[AsO32]3 2AsO 2 ] 4 amount of carbonate mass far exceeds the total mass of2 2[HAsO 22 4 3[AsO32 4] 5 0 [15] sulfides in the heap, and a high pH environment shouldprevail at all times.Simulating the oxidation of pyrite and the precipitationof ferric hydroxide requires the inclusion of kineticPrecipitation–Dissolution ReactionsInorganic pyrite dissolution is described by a series ofreactions that encompass a wide pH range. These reactionsare commonly written as (Langmuir, 1997)versions of these reactions. An experimental rate lawfor Eq. [16] was determined in a stirred reactor in acontrolled laboratory environment, and is given byFeS 2 1 7 O (Williamson and Rimstidt, 1994) as2 1 H2 2 O W Fe 21 1 2SO 2241 2H 1 [16]d(FeS 2 )FeS 2 1 14Fe 31 1 8H 2 O W 15Fe 21 1 2SO 2241 16H 15 10 (mO 2) 0:528:10 [20]dt(mH 1 ) 0:11[17]where the rate of pyrite dissolution is expressed in molesFe 21 1 1 O 2 1 H 1 W Fe 31 1 1 H 4 22O [18] per square meter per second. This rate law is particularlyuseful to the present modeling effort, as it depends onlyThe Carlin Trend heaps represent a somewhat unique on pO 2 and pH. An alternative rate law for pyriteenvironment to describe pyrite oxidation due to the oxidation in moist air proposed by Jerz and Rimstidtpresence of significant amounts of carbonate minerals (2004) is of interest because it is conceptually applicablerelative to the low mass percentages of sulfide minerals. to a variably saturated environment. The Jerz andThe equilibrium pH for the solution in equilibrium withboth gypsum and calcite minerals varies between 7.8 andRimstidt (2004) experiment was performed under stagnantflow conditions and with initially unoxidized pyrite.These experimental conditions are not analogousto a field-scale heap-leach facility, and therefore theTable 2. Components added to UNSATCHEM to simulate Asgeochemistry.Williamson and Rimstidt (1994) rate expression is incorporatedinto UNSATCHEM.Component typeComponentsAt high pH, Fe(OH) 3 is relatively insoluble, andAqueous ions As 31 ,As 51 ,Fe 21precipitates by Eq. [19]. An empirical rate law for this22 32AqueousH 2 AsO 32 ,H 3 AsO°, 3 HAsO 3 , AsO 3 ,H 2 AsO 42 ,complexesHAsO 22 4 , AsO 32 4 ,H 3 AsO°4reaction is given byMineral species Fe(OH) 3Pyrite minerals:d[Fe(OH) 3]5 210 25:98 (Fe21 )Pyrite–FeSdt(H 1 ) P 22 O2Arsenopyrite–FeAsS[21]

434 VADOSE ZONE J., VOL. 5, FEBRUARY 2006Reproduced from Vadose Zone Journal. Published by Soil Science Society of America. All copyrights reserved.The occurrence of As as a minor constituent in pyriteis a possibility and has been documented in preciousmetals heaps around the world (Jambor, 1994). The oxidationof arsenian pyrite can be written asFeAs x S 22x 1 ð 7 2 22x Þ O 2 1 H 2 O!Fe 12 1 (22x)SO 2241 2H 1 1xAs aq [22]where x is the fraction of As present in pyrite. Note thatsetting x 5 0 results in the pyrite oxidation reactiongiven in Eq. [16], and setting x 5 1 results in a representationfor the oxidation of the mineral arsenopyrite,FeAsS. The generalized reaction in Eq. [22] is includedin UNSATCHEM as a master equation for pyrite, arsenopyrite,and arsenian pyrite oxidation and is invokedby specifying the As fraction as an input parameter. Thedissolution rate for this reaction is assumed to proceedunder the same rate law as pure-phase pyrite as given inEq. [20].Arsenic SorptionThe partitioning of As mass between the aqueous andsorbed phases is accomplished using the two pHdependentisotherm expressions described in Deckeret al. (2006). The first expression is a Sips-type, orLangmuir–Freundlich, isotherm that is pH dependent.1 (a S C e ) nSq e 5 q m [23]1 1 de 2epH 1 1 (a S C e ) nSThis expression encompasses two analytically measuredparameters, C e and pH, and one calculated parameter,q e . There are three isotherm-fitting parametersfrom the Sips formulation, q m , n s , and a s , and incorporatingpH dependence adds two additional fitting parameters,d and e. The simulations included in this studysimplified this model to a Langmuir form by assigningn s 5 1. In addition, a Keren-type pH-dependent isothermis also incorporated into UNSATCHEMK L C eq e 5 q m1 1 K L C eA 1 B 3 10 (pH214)K L 5[24]1 1 C 3 10 (pH214) 1 D 3 [10 (pH214) ] 2where A, B, C, and D are empirically determinedconstants.Oxygen TransportA critical component to modeling As sorption isincluding redox chemistry to enable speciating As inboth the aqueous and sorbed phases. The experimentalsorption results given in Decker et al. (2006) demonstratethat the sorption behavior of As(III) and As(V)exhibits significant pH dependence. The speciation ofAs(III) and As(V) is controlled by redox chemistry.Therefore, a reactive transport model for As should beinclusive of redox processes. The inclusion of redoxchemistry is particularly important to a realistic inclusionof both As(III) and As(V) sorption behavior.Water chemistry data for the two facilities were neitheravailable nor obtainable due to the anonymous nature ofthe donations of rock material from the mining industry.Therefore, a surrogate redox chemistry approach wastaken. An approximation of the redox potential can bemade through dissolved oxygen concentration in thatreducing conditions are characterized by low oxygen concentrations,and oxidizing conditions are characterizedby relatively higher oxygen concentrations. This generalizedredox chemistry approach is incorporated intoUNSATCHEM for the singular purpose of controllingthe oxidation state of As. In practice, this is accomplishedby specifying the critical oxygen concentration that definesthe threshold between oxidizing and reducing conditionsas an input variable. This variable in turn is used todetermine the speciation of As on a node-by-node basisat each time step. Arsenic speciation is allowed to proceedthrough this oxygen surrogate in a reversible manner.The use of oxygen and iron as a redox surrogate tocontrol As redox chemistry presumes that the geochemistryassociated with Fe, O 2 , and As is coupled. In thepresence of sulfide mineralogy, this assumption isprobably reasonable since the coupled interaction of O 2and Fe drives the redox chemistry. However, in theabsence of sulfide mineralogy, the use of the partialpressure for O 2 may not be a satisfactory redox surrogatefor As, and explicit As redox chemistry may need to beincorporated into UNSATCHEM to adequately representthis type of domain. The Carlin Trend ore depositsare associated with sulfide mineralogy, and therefore theassumption is made that the use of O 2 is a reasonablesurrogate representation of the complex redox chemistryfor As.The oxygen transport modeling approach is similar tothat used for carbon dioxide transport (Šimůnek andSuarez, 1993). The gas transport mass conservationequation is]C T]t5 2 ]]z (J da 1 J dw 1 J ca 1 J cw ) 1 P [25]where J da describes the O 2 flux caused by diffusion in thegas phase, J dw is the O 2 flux caused by dispersion in thedissolved phase, J ca is the O 2 flux caused by convectionin the gas phase, and J cw is the O 2 flux caused byconvection in the dissolved phase. The term C T is thetotal volumetric concentration of O 2 , and P is a source–sink term. These terms are defined asJ da 5 2u a D a]C a]zJ dw 5 2u w D w]C w]zJ ca 5 q a C aJ cw 5 q w C wC T 5 C a u a 1 C w u a [26]where C w and C a are the volumetric concentrations ofO 2 in the dissolved and gas phases, respectively, D a isthe effective soil matrix diffusion coefficient of O 2 in the

www.vadosezonejournal.org435Reproduced from Vadose Zone Journal. Published by Soil Science Society of America. All copyrights reserved.gas phase, D w is the effective soil matrix dispersioncoefficient of O 2 in the dissolved phase, q a is the soil airflux, q w is the soil water flux, and u a is the volumetric aircontent. Since UNSATCHEM does not consider coupledwater and air movement, the flux of air, q a , isunknown, and thus must be approximated somehowusing additional assumptions. One possibility is to assumethat the advective transport of O 2 in response tothe total pressure gradient is negligible compared withO 2 diffusion, and therefore to assume a stagnant gasphase in which only diffusive transport occurs (q a 5 0).Another possibility is to assume that because of themuch lower viscosity of air in comparison with water, arelatively small pressure gradient will lead to significantgas flow. Changes in the total volume of water in the soilprofile caused by water flow are thus assumed to beimmediately matched by corresponding changes in thegas volume, leading to air fluxes. This simple model thustakes into account “rainfall effects” on soil aeration,which was estimated to contribute about 7 to 9% of normalaeration (Jury and Horton, 2004). Jury and Horton(2004) also stated that other factors, such as wind, barometricpressure changes, and temperature effects, donot contribute to more than about 1% of normal aeration,and thus may be neglected in the majority of applications.Although the modified UNSATCHEM modelcan consider simplified gas convection, the examplesgiven in this paper are run under steady-state, constantfluid flux conditions. Therefore, the contribution to thetotal oxygen flux from convection in the air phaseis neglected.Sulfide waste rock dumps in other mining districtsaround the world can contain sulfide mass fractions of 2to 30% (Ritchie, 1994). In some of these cases, convectiveoxygen flux is known to substantially contributeto the overall oxygen mass budget as a result of theexothermic heat of reaction produced from the dissolutionof sulfides (Ritchie, 1994). However, it is unlikelythat convective O 2 flux is a major feature of the CarlinTrend heap-leach facilities for three reasons. First, thesulfide concentration of the ores, at 2.5% by weight, is solow that significant heat generation is not likely to occur.In fact, following the analysis given in Ritchie (1994), it ishighly likely that a temperature rise of ,28C could beexpected. Second, the heaps are constructed in a waythat limits the likelihood of vertically continuous highconductivityfeatures such as commonly are found inend-dumped waste-rock piles that provide preferentialflowpaths for gas transport from the toe of a facility tothe upper reaches of the dump. Third, the continuousflux of downward-directed fluid applied at ambienttemperature at the surface would moderate the thermalregime in such heap-leach facilities, thus damping theupward convective flow of air and heat. Therefore,neither this thermally driven convective flow nor theheat generated during pyrite dissolution is included inthis version of UNSATCHEM.The term P in Eq. [25] represents a sink term foroxygen resulting from the consumption of oxygen bychemical reactions such as those given in Eq. [16], [19],and [22]. No sources of oxygen other than the atmosphereare included in this formulation. Henry’s Lawgoverns the distribution of gas between aqueous andgaseous phasesC w 5 K O2RTC a [27]where K O2is the Henry’s Law constant (1.26 3 10 23 molbar 21 (Langmuir, 1997)), R is the universal gas constant(8.314 kg m 2 s 22 K 21 mol 21 ), and T is the absolutetemperature (K). The dissolution–precipitation reactionsgiven in Eq. [16], [19], and [22] include water, and itis assumed that these reactions occur in the presence ofa water film. Therefore, Henry’s Law is used to movegaseous oxygen to the aqueous phase such that the reactionsin Eq. [16], [19], and [22] can occur.The effective dispersion coefficients for O 2 in soils arecalculated from the molecular diffusion coefficients foroxygen in free air and the dispersion coefficient in water.UNSATCHEM calculates an effective diffusion coefficientfor gas transport by multiplying the moleculardiffusion coefficient by a tortuosity factor. The resultingeffective dispersion coefficients, D w , and diffusion coefficients,D a , are then written asD w 5 D ws H w 1 l w| q wu w| 5 D wsu 7 / 3wu 2 s1 l w| q wu w| [28]u 7 / 3aD a 5 D as H a 5 D as [29]u 2 swhere D ws 5 2.10 3 10 29 m 2 s 21 and D as 5 2.01 3 10 25m 2 s 21 are the molecular diffusion coefficients for O 2 inthe dissolved and gas phases from Glinski and Stepniewski(1985), H a and H w are the tortuosity factors inboth phases, and l w is the dispersivity in the dissolvedphase (Šimůnek et al., 1996).Numerical Solution StrategyThe numerical solution of Richards’ equation isaccomplished with the use of a mass-lumped, finiteelementmethod that is reduced to a finite-differencescheme (Šimůnek et al., 1996). This finite-differencenumerical approach is combined with a Picard iterativesolution technique to solve the time-derivative inRichards’ equation. The numerical solutions to thesolute, heat, and gas transport equations are accomplishedwith the Galerkin finite element method. Theflow and transport models are inclusive of a full suiteof boundary conditions including specified head andconcentration (Dirichlet), or specified flux (Neumann).An iterative solution is used to solve the chemicalsystem and is discussed in the UNSATCHEM manual(Šimůnek et al., 1996). The iteration process must beaccomplished at each node, for each time step, and betweenthe solute transport and equilibrium componentsof the model. The solution process at each time stepproceeds by iteratively solving the flow equation andsolving the transport equations for temperature, CO 2 ,and then multicomponent solute transport. The geochemicalmodule is invoked at each time step, for eachnode, to redistribute mass between solid, aqueous, andsorbed phases.

436 VADOSE ZONE J., VOL. 5, FEBRUARY 2006Reproduced from Vadose Zone Journal. Published by Soil Science Society of America. All copyrights reserved.The governing solute transport equation (Eq. [8]) includesthree time-derivative components: total aqueousconcentration, sorbed concentration, and mineral-phaseconcentration. The solute transport equation is highlynonlinear because of these terms and must be solvediteratively. Iteration continues until the differences betweenthe calculated aqueous concentrations for the newiteration and the previous iteration are less than a prescribedconcentration tolerance.MODEL DEMONSTRATIONThree examples of variably saturated transport inheap-leach systems are presented to illustrate theimportance of incorporating reactive geochemistry intotransport simulations that attempt to estimate As fluxesat long time-scales. Because the model examples dealwith variably saturated media, gas-phase transport ofoxygen and carbon dioxide is an important processcontrolling redox chemistry, pH, and mineral dissolution.Mineral dissolution and precipitation reactions controlredox chemistry, which in turn directly control As speciation.Since As sorption behavior is species and pHdependent, the overall reactive transport of As in porousmedia is a highly complex interactive process that isdependent on mineralogy, geochemistry, and the physicsof flow in porous media. The examples are intended todemonstrate that a more representative model of Astransport is obtained through the integration of flowphysics, geochemistry, and reactive transport than is possiblewith simple conservative or linear isotherm approaches.While these examples are not representationsof a specific heap-leach facility, they provide a range ofbehavior that might be expected for three possibleclosure scenarios that are believed to be applicable toCarlin Trend ores. These examples include the followingheap facility closure scenarios: no surface treatment, anengineered surface treatment to reduce the flux of infiltratingwater, and a more sophisticated cover that reducesinfiltrating water and gas flux.Problem DefinitionThe problem definition for these three examples isshown in Fig. 1. The 30-m-tall, one-dimensional profilesare the same in all three cases, with the exception of a57-cm-thick clay-loam cover that is built into the 30-mprofile in Example 3 (Fig. 1c). The hydraulic propertiesfor the heap material are reported in Table 4, and aretaken from a study conducted on heap facility materialfrom the Carlin Trend (Decker and Tyler, 1999a). Thehydraulic properties for the clay-loam cover in the thirdexample are taken from the soils database included withUNSATCHEM and HYDRUS-1D. The steady-state,upper flux boundary conditions are used to simplify theinterpretation of the reactive transport results. Thelower flux boundary condition is free drainage, or zeropressure head gradient. An alternative lower boundarycondition that can be used in UNSATCHEM is a seepageface, which allows for the development of a saturatedzone. Incorporating this boundary condition intoFig. 1. Schematics of the three model examples.a heap facility model is appropriate when the effects ofthe heap liner are of interest. For the examples shownhere, it is assumed that the location of the heap liner isdeeper than 30 m.The geochemical conditions within the heap and thecovers are identical for all three examples and are summarizedin Tables 5, 6, and 7. Table 5 includes the initialaqueous geochemical conditions within the interstitialwater of the heap as well as the geochemistry of therecharging water from precipitation. The initial total Asmass (Table 6) is identical for all examples and bothisotherm formulation types. However, the initial aqueousAs concentrations (Table 5) are different for theSips- and Keren-type isotherms as a result of the differencein fit between each of the isotherm types and theexperimental sorption data. The Sips- and Keren-typeisotherm parameters used in the examples are given inTable 7 and are derived from As sorption experimentsconducted on heap facility rock (Decker et al., 2006).The remaining minerals listed in Table 6 are intended toTable 4. Soil hydraulic parameters for the heap material and gasfluxcover.ParameterUnitHeap material,all examples Gas-flux cover, Example 3u r 0.07 0.095u s 0.4 0.41a cm 21 0.05 0.019n 3.2 1.31K s cm d 21 114.57 6.24r b gcm 23 1.5 1.5D L cm 100 100

www.vadosezonejournal.org437Reproduced from Vadose Zone Journal. Published by Soil Science Society of America. All copyrights reserved.Table 5. Initial and boundary liquid-phase concentrations andother geochemistry parameters.ParameterUnitsInitial heapinterstitial waterRecharge waterCa 12 mmol L 21 20 0.5Mg 12 mmol L 21 0.225 0.025Na 11 mmol L 21 5 0.05K 11 mmol L 21 0.05 0.05Alkalinity mmol L 21 37.5 0.05SO4 22 mmol L 21 3 0.025Cl 21 mmol L 21 2 0.05As(III) Sips mmol L 21 (mg L 21 ) 4.731 (354) 0IsothermAs(III) Keren mmol L 21 (mg L 21 ) 3.330 (250) 0IsothermAs(V) mmol L 21 (mg L 21 ) 0 (0) 0 (0)pH 8.76pO 2 atm 0 0.2Tracer 0 1.0be representative of an average, carbonate-hosted,sulfide-bearing ore. In particular, the 20% by weightcarbonate and 2.5% by weight pyrite are characteristicsof a carbonate-hosted ore body. The inclusion of As inthe pyrite mass is meant to represent the mineral originof As that has been reported in several heap facilities.The gas transport boundary conditions for all threeexamples are modeled as constant partial pressures inthe atmosphere, with the top of the heap profilerepresented with values of 0.2 atm for O 2 and 10 -3.5 atmfor CO 2 and the bottom of the profile set to 0 atm forboth gases. Thus, gas transport into the heap profile islimited to the top surface.The geographical region encompassing the CarlinTrend is characterized by an arid climate with lowannual precipitation with correspondingly high evapotranspirationrates that can be several times larger thanthe average annual precipitation. However, the largerange in heap facility site elevation complicates thisbroad categorization, with the possibility of significantsnowmelt recharge occurring at high-elevation sites inthe winter months (Kampf et al., 2002). Controlling rechargeflux into heap facilities is a key component oflong-term management of both the remaining mineralresource in the facility, and in ensuring effective environmentalstewardship.Mine operations are required to close heap-leachfacilities such that a number of environmental regulationsare met. This is typically accomplished through aregimen of rinsing leachate fluids from the heap withfresh water and establishing a long-term fluids managementplan to control water discharge for the facility. TheTable 6. Initial conditions for mineral- and sorbed-phase concentrations.costs associated with the fluids management plan are ofsuch magnitude that designing an effective closure planthat includes understanding the impacts of engineeringsolutions to limit the precipitation flux into the top of theheap facility on the chemistry of the discharge water iskey to minimizing costs and maximizing the effectivenessof the management plan. The three examples presentedillustrate the long-term effects on As concentration andmass flux from precipitation and gas fluxes at the heapfacility surface.Example 1: Uncovered Heap-Leach FacilityThe first example represents an uncovered heap facilitywith a relatively large steady-state recharge fluxrate of 300 mm yr 21 that is representative of a highelevationenvironment with significant snowmelt recharge.The calculated steady-state water content is0.097, which corresponds to a pore volume of water of291 cm 3 and a time to infiltrate a single pore volume of9.7 yr (Fig. 2h). The initial pH of the profile is set at 8.7 torepresent a heap that is recovering from a period of goldproduction wherein the pH is maintained at very highlevels (in excess of pH 10) to prevent the loss of cyanide(Bartlett, 1992) (Fig. 2e). This pH rapidly decreases tolower pH values due to the influence of carbonate mineraldissolution, sulfide mineral dissolution, and gypsumprecipitation reactions. As arsenian pyrite is dissolved(Fig. 2b), the acid generated is buffered through thedissolution of calcite (Fig. 2a). The dissolution of arsenianpyrite is limited by the rate of oxygen transportthrough the relationship given in Eq. [20]. Oxygen transport(Fig. 2g) in turn is limited by the effective dispersioncoefficient for oxygen. The linear profiles shown inFig. 2g are indicative of an arsenian pyrite oxidation ratethat is in excess of the oxygen transport rate. Sulfate isproduced through the dissolution of arsenian pyrite(Fig. 2f) and is precipitated as gypsum (Fig. 2c). Theaqueous iron generated from arsenian pyrite dissolutionis precipitated as iron hydroxide through the kineticrelationship in Eq. [21] (Fig. 2d).Arsenic is initially distributed as As(III) between theaqueous and sorbed phases in a proportion determinedby the Sips-type isotherm, (Eq. [23]), and for the parametervalues given in Table 7 for an initial pH of 8.76(Fig. 2i). The speciation of As from As(III) to As(V)occurs along a moving redox front that is coincident withthe oxygen transport and arsenian pyrite oxidationfronts. In Fig. 2i and 2j, the presence of the oxidationParameter Unit Keren isotherm Sips isothermCalcite mol kg 21 (g kg 21 ) 2 (200) 2 (200)Gypsum mol kg 21 (g kg 21 ) 0.005 (0.68) 0.005 (0.68)Pyrite mol kg 21 (g kg 21 ) 0.208 (25) 0.208 (25)Fe(OH) 3 mol kg 21 (g kg 21 ) 0 0Pyrite surface area m 2 g 21 7.5 3 10 28 7.5 3 10 28Arsenian pyrite fraction 0.00075 0.00075Sorbed As(III) mmol kg 21 (mg kg 21 ) 1666.663 (124.8691) 1666.662 (124.8689)Sorbed As(V) mmol kg 21 (mg kg 21 ) 0 (0) 0 (0)Total As(III) mass(aqueous and sorbed)mmol kg 21 (mg kg 21 ) 1666.667 (124.8693) 1666.667 (124.8693)

438 VADOSE ZONE J., VOL. 5, FEBRUARY 2006Reproduced from Vadose Zone Journal. Published by Soil Science Society of America. All copyrights reserved.Table 7. pH-dependent isotherm surface model parameters. Unitsfor the calculated isotherm surface plot are: C e , mg L 21 ; q e , mgg 21 ;pH5 2log[H 1 ].Model Parameter As(V) As(III)Sips type d 0.000004891 4.925e 21.404 0.4211q m (mg kg rock ) 960 000 337 000a s (L mg 21 ) 0.001151 0.002011n s 1.0 1.0Keren type A (L mg 21 ) 0.0007962 0.0002831B (L mg 21 ) 46.99 6297C 548 600 3 526 000D 0.1250 1.003q m (mg kg rock ) 1 200 000 377 000front is graphically illustrated by the horizontal linesrunning from a nonzero As concentration value to the yaxis. The As(III) results illustrate the position of theoxidation front at 25, 50, and 100 yr (Fig. 2i). At 50 yr, theoxidation front has reached a depth of 15 m. From thesurface to 15 m the available As exists as As(V) as shownin Fig. 2j, while below 15 m reducing conditions exist andthe available As exists as As(III) (Fig. 2i). The graphicalrepresentation of the oxidation front shown in Fig. 2i and2j is expressed again as a continuous time series at thebottom of the heap profile (Fig. 2k). From the onset ofthe simulation until 115 yr, the discharging As from theheap profile is the reduced form, As(III). At 115 yr, theoxidation front reaches the bottom of the profile as aresult of the complete oxidation of arsenian pyrite. As aresult, the redox conditions at the bottom of the profiletransition from reducing to oxidizing, and the dischargingAs is then present as As(V). The dramatic decrease indischarge As concentration following the arrival of theoxidation front is a direct result of the pH-dependentsorption characteristics of As(III) and As(V). The rocksorption capacity is much larger for As(V) than As(III),leading to a larger mass of As(V) bound to the rockmineral surfaces and a proportionally smaller aqueousconcentration. This characteristic is apparent by the veryFig. 2. Selected results for an uncovered heap-leach facility when the Sips-type isotherm for As sorption was used.

www.vadosezonejournal.org439Reproduced from Vadose Zone Journal. Published by Soil Science Society of America. All copyrights reserved.slow decline in aqueous As(V) concentrations throughoutthe heap profile with time (Fig. 2j).The shapes of the As(III) and As(V) concentrationprofiles are determined by the dissolution of arsenianpyrite and the geochemistry that controls pH. The dissolutionof arsenian pyrite releases As, which is subjectedto the aqueous complexation reactions for As(III)or As(V) and is then distributed between the sorbedand aqueous phases. This distribution is dependent onoxidation state and pH. The large As(V) sorption capacityof the rock results in an apparent relatively stableconcentration profile following the early time redoxfront propagation and related pH behavior. The substantiallysmaller sorption capacity for As(III) results ina more dynamic aqueous concentration response bothwithin the profile and at the lower boundary. Thismobility characteristic has been observed for As(V)and As(III) in saturated environments (Welch andStollenwerk, 2003).Example 2: Heap Facility with Fluid Flux CoverAn approach to limit fluid flux into a heap-leachfacility is to augment the surface with low conductivitymaterials or install a cover system that matches fluid fluxwith a plant community that is capable of respiring all ormost of this fluid (Albright et al., 2004). The secondexample infers the performance of a surface augmentationor evapotranspiration cover system by limiting thefluid flux into the top of the simulated heap facility profileto 50 mm yr 21 . The reduction in infiltrating flux bya factor of six is a reasonable performance for a non-RCRA cover using on-site borrowed materials. The resultsof this simulation are shown in Fig. 3. The calculatedsteady-state water content is 0.076, which corresponds toa pore volume of 228 cm 3 and a time to infiltrate a singlepore volume of 45.6 yr (Fig. 3h). The larger volumetricair content associated with this reduced water contentresults in larger air-phase diffusion-driven transportFig. 3. Selected results for a heap-leach facility with a surface treatment to limit fluid flux into the top of the heap when the Keren-type isotherm forAs sorption was used.

440 VADOSE ZONE J., VOL. 5, FEBRUARY 2006Reproduced from Vadose Zone Journal. Published by Soil Science Society of America. All copyrights reserved.rates of oxygen (Fig. 3g). As a consequence, the oxygenlimited arsenian pyrite oxidation front also moves faster(Fig. 3b). The corresponding mineral phases also exhibitsubstantially faster dissolution or precipitation front velocitiesfor calcite (Fig. 3a), gypsum (Fig. 3c), and ironhydroxide (Fig. 3d). The dissolution of calcite preventsthe acidification of the profile during pyrite dissolution,maintaining a near-neutral pH profile (Fig. 3e).Arsenic is initially distributed as As(III) between theaqueous and sorbed phases in a proportion determinedby the Keren-type isotherm (24) and for the parametervalues given in Table 7 for an initial pH of 8.76 (Fig. 2i).The As redox front velocity is identical to the pyriteoxidation and oxygen front velocities (Fig. 3i and 3j). Aconsequence of the higher arsenian pyrite oxidation rateis the conversion from As(III) to As(V) at the bottomof the profile after 50 yr (Fig. 3k), 65 yr earlier than inthe previous, uncovered example (Fig. 2k). The rate ofdecline in As(V) aqueous concentrations near the topof the profile is substantially reduced compared to thefirst example as a consequence of reducing the meteoricwater flux by a factor of six (Fig. 3j).Example 3: Heap Facility with Fluid andGas Flux CoverThe third example is representative of a cover systemthat is designed to limit both fluid and gas flux into theheap facility through the top surface. While a typicalcarbonate-hosted heap facility has more than enoughcarbonate mineral mass to neutralize the smaller massfraction of sulfide minerals, reducing the oxidation ratecould slow the release of As from As-bearing mineralphases. In addition, the development of large-scalepreferential flowpaths in a heap structure could reducethe effectiveness of the carbonate mineral mass toFig. 4. Selected results for a heap-leach facility with a cover to limit fluid and gas fluxes into the top of the heap. Arsenic sorption is described usingthe Sips-type isotherm.

www.vadosezonejournal.org441Reproduced from Vadose Zone Journal. Published by Soil Science Society of America. All copyrights reserved.neutralize the acid generated from sulfide mineraloxidation. In either case, a cover system may be warrantedto control the fluid and chemical flux at thefacility discharge.The incorporation of sophisticated earthen cover designsinto heap facilities for the purposes of reducing theflux of oxidizing gases into a sulfide heap or waste-rockdump represents an interesting alternative to controllingsulfide mineral oxidation rates (Kim and Benson, 2004).This approach incorporates an earthen cover systemthat reduces the gas-phase flux by maintaining a highwater content in the cover, thereby reducing the airphasecontent and producing a corresponding reductionin the diffusion coefficient for oxygen as given in Eq.[29]. While the concept of reducing the rate of sulfidemineral oxidation to reduce the dissolution rate of metalsbearing minerals is well founded, the impact of thisdesign on the geochemistry of a sulfide bearing heapfacility is not well understood. The coupled relationshipsbetween oxygen concentration, sulfide mineral oxidationrate, redox- and pH-dependent As aqueous complexation,and water–rock sorption reactions producehighly nonlinear geochemical behavior. To demonstratethe efficacy of a gas-flux cover and the nonlinear geochemicalresponse within a heap facility to the installationof a gas-flux cover, this final example is presented.The gas-flux cover is simulated with an 80-cm-thicklayer of clay loam incorporated into the top of the 30-mprofile, while the fluid flux cover is inferred as in theprevious two examples by applying a steady-state rechargeflux of 50 mm yr 21 . The hydraulic and geochemicalproperties of the heap facility rock are identical tothe previous examples (Tables 5 and 6), and the Sips-typeisotherm is used to describe As sorption (Table 7). Theresults are shown in Fig. 4. The oxygen flux rate isdramatically reduced as compared with the previousexamples (Fig. 4g). The partial pressure of oxygen onlyapproaches atmospheric values in the cover, while withinthe profile, the oxidation of arsenian pyrite proceedsmuch faster than the oxygen flux rate, maintaining lowpartial pressures of oxygen within the profile. The reducedoxygen flux rate slows the arsenian pyrite oxidationrate such that the oxidation front has only reached20 m after 200 yr (Fig. 4b). The remaining mineralogythat is dependent on arsenian pyrite oxidation chemistryis similarly affected (Fig. 4a, 4c, and 4d).The reduced gas flux cover significantly reduces theflux of oxygen through the top of the simulated heapprofile such that reducing conditions are maintainedwithin the profile for the entire 200-yr simulation. As aresult, the initial reducing conditions and As(III) speciationare maintained throughout the profile (Fig. 4i),with the exception of the cover, wherein conditions aresuch that As occurs as As(V) (Fig. 4j). The cover preventsthe complete oxidation of the sulfide mineral masswithin the heap profile, and maintains redox conditionsFig. 5. Time series of aqueous As concentrations at the lower boundary discharge point for all examples and both isotherm types.

442 VADOSE ZONE J., VOL. 5, FEBRUARY 2006Reproduced from Vadose Zone Journal. Published by Soil Science Society of America. All copyrights reserved.such that As(III) speciation prevails for the duration ofthe simulation (Fig. 4k).SUMMARYThe three examples are intended to provide a range ofpossible behaviors that could be expected for As oxyaniontransport for the most likely heap facility closureoptions for a carbonate-rich, low-sulfide ore such asthose typical of the Carlin Trend operations. The Asbreakthrough curves at the bottom of the simulationprofile for the three examples are presented in Fig. 5. Thecumulative As mass released from the bottom of the profileis plotted in Fig. 6 for all three examples using theSips-type isotherm. Each of the three examples was runwith both the Sips-type and Keren-type isotherm formulations(Fig. 5). The differences between the isothermformulations are a result of the differences in the mathematicsof each and the overall fit to the experimental data(Decker et al., 2006). The sharp drop in As concentrationin Fig. 5 at 50 and 115 yr is a manifestation of theoxidation front arriving at the bottom of the simulatedheap profile, and As transitioning from As(III) to As(V).The arrival of the oxidation front is similarly the causeof the break in slope at the same times in Fig. 6. Thedifferences in release characteristics for As are a result ofthe coupled effects of variably saturated transport andwater–rock geochemistry.The uncovered example, with a fluid flux rate sixtimes higher than the other examples, produces thelargest cumulative As mass at the simulated outfalllocated at the bottom of the modeled domain. As a resultof the high fluid flux rate, aqueous As concentrationdrops rapidly as the sorbed As is rinsed from rock sorptionsites. The second example, with a simple fluid fluxcover, produces a proportionally smaller cumulative Asmass, and an overall higher As concentration at thebottom of the simulated heap profile. The reduced fluxrate results in a low volumetric water content and aproportionally larger air-phase volumetric content. Thisproduces a larger effective diffusion coefficient for oxygen,and a correspondingly faster sulfide mineral oxidationrate. The arrival of the oxidation front in thesecond example 65 yr earlier than the uncovered simulationis a direct result of modifying the rate of fluid fluxinto the simulated profile.The third example, with a fluid and gas flux cover,produces a larger cumulative As mass at the outfall thanthe simple, fluid-control cover example, despite the factthat both examples are simulated with the same infiltratingfluid flux. Because the gas flux cover prevents theoxidation of As(III) to As(V), and because As(III) issubstantially more mobile than As(V), the gas flux coverallows the faster release of As for a given fluid flux rate.A consequence of maintaining reducing conditions withinthe simulated profile is the persistent high concentrationof As at the outfall (Fig. 5).CONCLUSIONSAn existing variably saturated reactive transport codewas modified so that As transport in heap-leach facilitiescould be simulated. The code modifications included theincorporation of two pH-dependent isotherm formula-Fig. 6. Time series of cumulative aqueous As mass at the lower boundary discharge point for the three examples using the Sips-type isotherm.

www.vadosezonejournal.org443Reproduced from Vadose Zone Journal. Published by Soil Science Society of America. All copyrights reserved.tions for As, arsenian pyrite oxidation and related rock–water chemistry, and oxygen transport. The modifiedcode is capable of simulating the primary processescontrolling redox- and pH-dependent As transport. Onepossible application of the code is to estimate the performanceof heap-leach facility closure options that involvemodifying the hydrologic or geochemical regimeto control As release.The As transport capable version of UNSATCHEMis useful as an initial estimator for As release behaviorwith a modest cost in obtaining whole-rock As sorptionbehavior in the laboratory. The rapid estimation ofAs release behavior with a numerical solver that incorporatesthe primary physical and chemical processescontrolling transport is expected to be useful to the industrial,engineering, and regulatory communities. Additionalimprovements to UNSATCHEM could includethe incorporation of the Davis–Ritchie shrinking-corepyrite oxidation model, and the inclusion of site-specificsecondary mineralogy.Three examples were provided that are intended todemonstrate the code capabilities and the importance ofincluding the geochemistry that controls oxyanion behaviorin transport simulations. The results of the simulationsprovide a basis for a first round of engineering cost–benefit analysis using the solute (As in this case) cumulativemass and concentration behavior. If maintaininglow As concentration at the heap outfall is the primarymetric for heap facility management performance, thenthese simulations show that the heap should be oxidizedas quickly as possible to convert the mass of As to As(V)within the heap structure. Because the sorption capacityfor As(V) is higher than that for As(III), the release of Aswill occur over a longer period of time, but at much lowerconcentrations in an oxidized heap, as suggested by theresults from the second example. If the long-term costs ofconstructing and maintaining a discharge fluid-treatmentsystem are of primary concern, then optimizing the Asrelease behavior to achieve a particular As release functionthat is coupled to the treatment plant design is ofprimary importance and can be done with a higher degreeof realism by including the coupled physical and geochemicalprocesses that control As release behavior.REFERENCESAlbright, W.H., C.H. Benson, G.W. Gee, A.C. Roesler, T. Abichou, P.Apiwantragoon, B.F. Lyles, and S.A. Rock. 2004. Field waterbalance of landfill final covers. J. Environ. Qual. 33:2317–2332.Appelo, C.A.J., and D. Postma. 1993. Geochemistry, groundwater andpollution. A.A. Balkema, Brookfield, VT.Bartlett, R.W. 1992. Solution mining: Leaching and fluid recovery ofmaterials. Gordon and Breach Science Publishers, Philadelphia.Decker, D.L., C. Papelis, S.W. Tyler, M. Logsdon, and J. Šimůnek.2006. Arsenate and arsenite sorption on carbonate-hosted preciousmetals ore. Available at www.vadosezonejournal.org. Vadose ZoneJ. 5:xx–xx (this issue).Decker, D.L., and S.W. Tyler. 1999a. Evaluation of flow and solutetransport parameters for heap-leach recovery materials. J. Environ.Qual. 28:543–555.Decker, D.L., and S.W. Tyler. 1999b. Hydrodynamics and solutetransport in heap-leach mining. p. 1–13. In G.C. Miller and D.Y.Kosich (ed.) Closure, remediation and management of preciousmetals heap-leach facilities. Univ. of Nevada, Reno.Diodato, D.M., and R.R. Parizek. 1994. Unsaturated hydrogeologicproperties of reclaimed coal strip mines. Ground Water 32:108–118.Driesner, D., and A. Coyner. 2000. Major mines in Nevada 1999.Nevada Bureau of Mines and Geology, Reno.Glinski, J., and W. Stepniewski. 1985. Soil aeration and its role forplants. CRC Press, Boca Raton, FL.Hillel, D. 1998. Environmental soil physics. Academic Press, New York.Jambor, J.L. 1994. Mineralogy of sulfide-rich tailings and their oxidationproducts. p. 59–102. In J.L. Jambor and D.W. Blowes (ed.) Theenvironmental geochemistry of sulfide mine-wastes. Vol. 22. MineralogicalAssociation of Canada, Ottawa, Canada.Jerz, J.K., and J.D. Rimstidt. 2004. Pyrite oxidation in moist air.Geochim. Cosmochim. Acta 68:701–714.Jury, W.A., and R. Horton. 2004. Soil physics. 6th ed. John Wiley andSons, New York.Kampf, S., M. Salazar, and S. Tyler. 2002. Preliminary investigations ofeffluent drainage from mining heap-leach facilities. Available atwww.vadosezonejournal.org. Vadose Zone J. 1:186–196.Kim, H., and C.H. Benson. 2004. Contributions of advective anddiffusive oxygen transport through multilayer composite caps overmine waste. J. Contaminant Hydrol. 71:193–218.Krauskopf, K.B., and D.K. Bird. 1995. Introduction to Geochemistry.3rd ed. McGraw-Hill, New York.Langmuir, D. 1997. Aqueous environmental geochemistry. PrenticeHall, Upper Saddle River, NJ.Lichtner, P.C., and M.S. Seth. 1996. User’s manual for MULTIFLO:Part II MULTIFLO 1.0 and GEM 1.0, Multicomponent-multiphasereactive transport model. CNWRA 96–010.Lichtner, P.C., S. Yabusaki, K. Pruess, and C.I. Steefel. 2004. Role ofcompetitive cation exchange on chromatographic displacementof cesium in the vadose zone beneath the Hanford S/SX TankFarm. Available at www.vadosezonejournal.org. Vadose Zone J. 3:203–219.Mallants, D., B. Mohanty, A. Vervoort, and J. Feyen. 1997. Spatialanalysis of saturated hydraulic conductivity in a soil with macropores.Soil Technol. 10:115–131.Mayer, U., E.O. Frind, and D.W. Blowes. 2002. Multicomponentreactive transport modeling in variably saturated porous mediausing a generalized formulation for kinetically controlled reactions.Water Resour. Res. 38(9):1174. doi:10.1029/2001WR000862.Nichol, C.F. 2002. Transient flow and transport in unsaturatedheterogeneous media; field experiments in mine waste rock.Ph.D. diss. University of British Columbia, Vancouver, Canada.Nichol, C., L. Smith, and R. Beckie. 2003. Long-term measurement ofmatric suction using thermal conductivity sensors. Can. Geotech. J.40:587–597.Pruess, K. 1991. TOUGH2—A general purpose numerical simulatorfor multiphase fluid and heat flow. LBL-29400. Lawrence BerkeleyNatl. Lab., Berkeley, CA.Ritchie, A.I.M. 1994. Sulfide oxidation mechanisms: Controls andrates of oxygen transport. p. 201–246. In D.W. Blowes and J.L.Jambor (ed.) The environmental geochemistry of sulfide minewastes.Vol. 22. Mineralogical Association of Canada, Ottawa.Šimůnek, J., and D.L. Suarez. 1993. Modeling of carbon dioxidetransport and production in soil: 1. Model development. WaterResour. Res. 29:487–497.Šimůnek, J., and D.L. Suarez. 1994. Two-dimensional transport modelfor variably saturated porous media with major ion chemistry.Water Resour. Res. 30:1115–1133.Šimůnek, J., D.L. Suarez, and M. Šejna. 1996. The UNSATCHEMsoftware package for simulating the one-dimensional variablysaturated water flow, heat transport, carbon dioxide productionand transport, and multicomponent solute transport with major ionequilibrium and kinetic chemistry. Res. Rep. 141. U.S. SalinityLaboratory, Riverside, CA.Šimůnek, J., M. Šejna, and M.Th. van Genuchten. 1998. TheHYDRUS-1D software package for simulating the one-dimensionalmovement of water, heat, and multiple solutes in variably-saturatedmedia. Version 2.0. IGWMC-TPS-70. International Ground WaterModeling Center, Colorado School of Mines, Golden, CO.van Genuchten, M.Th. 1980. A closed-form equation for predicting thehydraulic conductivity of unsaturated soils. Soil Sci. Soc. Am. J. 44:892–898.

444 VADOSE ZONE J., VOL. 5, FEBRUARY 2006Viswanathan, H.S., B.A. Robinson, A.J. Valocchi, and I.R. Triay. 1998.A reactive transport model of neptunium migration from thepotential repository at Yucca Mountain. J. Hydrol. (Amsterdam)209:251–280.Welch, A.H., and K.G. Stollenwerk (ed.). 2003. Arsenic in groundwater. Kluwer Academic Publishers, Dordrecht, The Netherlands.Welch, A.H., D.B. Westjohn, D.R. Helsel, and R.B. Wanty. 2000.Arsenic in ground water of the United States: Occurrence andgeochemistry. Ground Water 38:589–604.Williamson, M.A., and J.D. Rimstidt. 1994. The kinetics andelectrochemical rate-determining step of aqueous pyrite oxidation.Geochim. Cosmochim. Acta 58:5443–5454.Reproduced from Vadose Zone Journal. Published by Soil Science Society of America. All copyrights reserved.