modelling the impact of physical and chemical heterogeneity on solute leaching in pyritic overburden...

Ecological Engineering 17 (2001) 91–101

Modelling the impact of physical and chemical heterogeneityon solute leaching in pyritic overburden mine spoils

Horst H. Gerke a,*, John W. Molson b,1, Emil O. Frind b

a Institut fur Bodenlandschaftsforschung, Zentrum fur Agrarlandschafts- und Landnutzungsforschung (ZALF) e.V.,Eberswalder Straße 84, D-15374 Muncheberg, Germany

b Department of Earth Sciences, Uni6ersity of Waterloo, Waterloo, Ont., Canada N2L 3G1

Accepted 19 August 2000

Abstract

Spatial variability of physical and chemical properties may affect acidification and acid mine drainage insulfide-bearing overburden mine spoils. This study compares heterogeneous with vertically layered and horizontallyaveraged spatial distributions of chemical components in a generic 2D-vertical spoil cross-section with heterogeneousdistributions of water and air contents and of water flux densities. The initial spatial distributions of the fraction ofsulfur and the pre-oxidized zones follow autocorrelated random functions. The initial background chemistrydistribution is correlated to the degree of pre-oxidation using five classes of differing initial chemical systems.Processes considered include variably saturated water flow, oxygen diffusion, shrinking-core kinetics of pyriteoxidation, multicomponent reactive solute transport, and geochemical equilibrium reactions between aqueous andmineral components. Spatial distributions were generated using the SGSIM-GSLIB geostatistical simulation al-gorithm and estimated parameters. Results show differences in vertical concentration profiles of chemical componentsand in integrated breakthrough curves at 30 m depth between the heterogeneous and the layered case for both—highand low—sulfur content scenarios. Differences are relatively large in the beginning and with respect to thedistribution of solid phase components. A heterogeneous sulfide mineral distribution results in vertical spreading ofthe oxidation front while precipitation and dissolution of the secondary minerals affects the acid mine drainage, bylocally retarding and releasing the solutes. © 2001 Elsevier Science B.V. All rights reserved.

Keywords: Mine spoils; Spatial variability; Reactive transport; Acid mine drainage; Sulfide oxidation; Shrinking core model;Numerical modelling

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1. Introduction

Spoils in the Lusatian mining district (e.g. Huttlet al., 1996) are formed as a result of open-pitmining of brown coal (lignite). The overburdenconsists of a mixture of Holocene, Pleistocene,and Tertiary sediments. The Tertiary sediments

* Corresponding author. Fax: +49-33423-82280.E-mail addresses: [email protected] (H.H. Gerke), mol-

[email protected] (J.W. Molson).1 Fax: 519-746-7484.

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H.H. Gerke et al. / Ecological Engineering 17 (2001) 91–10192

contain sulfide minerals, such as pyrite and marc-asite. After aeration, oxidative weathering ofsulfides leads to spoil acidification and the genera-tion of acid mine drainage (e.g. Van Berk andWisotzky, 1995; Evangelou, 1995; Wisotzky,1996a,b) and creates acidic zones with relativelyhigh iron and sulfate concentrations (e.g. Prein,1994; Wisotzky, 1996a,b). Suggestions for reduc-ing sulfide oxidation, acidification, and acid minedrainage focus on techniques to limit oxygentransport into the spoils, e.g. by covering withplastic sheets, organic matter, sand, clay or bysurface compaction (Kolling, 1990; Prein, 1994),to reduce the drainage rate by creating low perme-ability zones (Kolling, 1990), or to increase thebuffering capacity in the spoil heaps by applyingcarbonate rock, flue ash, or lime (e.g. Katzur,1977), among others. However, the large-scaleand long-term effects of such measures can noteasily be tested under real field situations. Thus,predictions are largely based on laboratory exper-iments and theoretical models. Mine spoil acidifi-cation studies are essential for the development ofadequate modelling tools. Long-term predictionsof contaminant evolution from spoil piles havebeen based on one-dimensional (1D) vertical un-saturated (e.g. Schwan et al., 1988; Prein, 1994) or2D and three-dimensional (3D) saturated flowand transport simulations (e.g. Walter et al.,1994a,b; Brand, 1996), on batch or column perco-lation experiments (e.g. Karathanasis et al., 1990;Kolling, 1990; Wisotzky, 1994), or on coupling of1D oxygen diffusion with pyrite oxidation and 2Dsolute transport (Wunderly et al., 1996). However,in most of these model approaches, the impact ofthe spatial variability on spoil properties has notbeen addressed. Field observations, however, sug-gest a spatial variability of physical and chemicalproperties of the spoil heaps resulting from over-burden mixing and the incorporation of pre-oxi-dized temporary surfaces (e.g. Latif, 1993; Prein,1994; Wisotzky, 1994). Sulfide oxidation may beimpacted by spatial heterogeneity especiallywithin the vadose zone where water flow andoxygen transport occurs under variably-saturatedconditions.

Differences in pyrite oxidation rates dependingon the spatial distribution, e.g. layering of alka-

line and pyritic material, have been reported byseveral authors (Evangelou, 1995). Gerke et al.(1998) analyzed effects of spatial heterogeneity inmine spoils by comparing the results of a generic2D heterogeneous system with those obtainedwith a 1D homogeneous system with averageproperties. The assumed relatively large porosi-ties, oxygen diffusion, and water flow rates in thescenarios of Gerke et al. (1998), however, resultedin similar oxidation fronts for both the scenarios.In addition, the 1D and 2D scenarios were notdirectly comparable since the layered structurewas not adequately considered in the homoge-neous case. Thus, the impact of heterogeneitycould not clearly be distinguished.

In this study, we use the modelling system ofGerke et al. (1998) to analyze the effect of thespatial distribution of chemical properties forcomparable initial chemistry and for reduced oxy-gen transport rates. For a generic 2D cross–sec-tion located in the unsaturated zone, we comparerandom with layered distributions of initial solidphase sulfur content and preacidified zones foridentical 2D heterogeneous water flow, solutetransport, and oxygen diffusion fields. The objec-tive here is to study effects of the spatial distribu-tion of preoxidized zones and pyrite and buffermineral contents on the movement of acidity andoxidation products in overburden mine spoils.

2. Material and methods

We use models for describing water flow, multi-component solute transport, geochemical equi-librium reactions, oxygen transport, pyrite (FeS2)oxidation, and geostatistical simulations for gen-erating random fields of physical and chemicalparameters. Variably-saturated water flow is de-scribed with the 2D Darcian-based Richard’sequation and solved using the SWMS-2D numeri-cal finite element code (Simunek et al., 1994). 2Dconvective-dispersive solute transport for multiplecomponents and geochemical equilibrium reac-tions are described using the MINTRAN code(Walter et al., 1994a), which sequentially couplesa finite element transport module PLUME2D(Frind et al., 1990) with the geochemical equi-

H.H. Gerke et al. / Ecological Engineering 17 (2001) 91–101 93

librium code MINTEQA2 (Allison et al., 1990).2D diffusive transport of oxygen is coupled with asink term for describing oxygen consumption bypyrite oxidation which is described using a shrink-ing core model (Davis and Ritchie, 1986) as fol-lows (Gerke et al., 1998);

ua

( [O2]a(t

=uaDa

�(2[O2]a(x2 +

( 2 [O2]a(z2

�−Dw

3(1−u)R2

� rc

R−rc

� [O2]aH

(1)

drc

dt= −

Dw(1−u)ors

Rrc(R−rc)

[O2]aH

(2)

where u(x,z) is porosity and ua(x,z) is the volu-metric air content of the spoil (dimensions,L3L−3), x is the horizontal and z the verticaldimension (L), t is time (T), [O2]a= f(t) is theoxygen concentration in the air phase (ML−3),Da(x,z) is the local diffusion coefficient of oxygenin the air phase (L2T−1), Dw=0.001 m2 per yearis an effective diffusion coefficient (L2T−1), whichincorporates characteristic diffusional propertiesof both the water film and the oxidized shell ofthe idealized spherical pyrite-bearing particle,H=26.3 is Henry’s constant, R is the averageradius of particles (L), rc is the average radius ofthe unreacted core (L), rs= fsrb is the mass ofsulphur per unit volume of soil (ML−3), with fs

being the proportion of sulphur in the solid phaseof the soil material (MSulfur/Msolids) and rb the soilbulk density (ML−3), and o is a weighted averageof the mass ratio of oxygen, O2, to sulfur, S,consumed on the basis of the reaction stoichiome-try (Wunderly et al., 1996)

FeS2+H2O+72

O2 �Fe2+ +2SO42− +2H+

(3)

Fe2+ +14

O2+H+�Fe3+ +12H2O (4)

FeS2+12

H2O+154

O2�Fe3+ +2SO42− +H+

(5)

The concentration of [Fe2+] in the pore wateris calculated using an equilibrium relationshipbetween the total concentration of iron, [Fet], and

that of [Fe2+]. The pyrite oxidation model issolved using a Newton-Raphson iterative numeri-cal scheme. The simplifying kinetic pyrite oxida-tion model assumes that the whole spoil materialconsists of uniformly spherical particles of radius,R, which are covered by a thin film of water.Individual spheres are homogeneous mixtures ofall solid minerals having a certain pyritic sulfurcontent, fs(x, z), which can be spatially variable inthe 2D-domain. At each location, oxygen fromthe spoil air may diffuse across the water film tothe reaction site located at a radius rc(t) inside ofthe assumed spherical particles. The amount ofoxidation products (SO4, H+, Fe2+, Fe3+) enter-ing the liquid phase is calculated from the changein radius, rc, of the unreacted part of the spheresduring each time interval according to the stoi-chiometry (3)–(5).

The validity of this model for the Lusatianoverburden sediments has not yet been tested.Micromorphological studies (Heinkele et al.,1999) show that in Lusatian mine soils, pyriteexists in the form of individual cubic minerals andclusters, the so-called framboids, consisting of alarge number of individual minerals. We also didnot consider here that secondary minerals, e.g.gypsum, may precipitate near the primary mineralsurfaces forming coatings or larger pore regionswith reduced porosity, and thereby changingpyrite oxidation kinetics and oxygen diffusionwith time, among other factors. Note that al-though the oxidation model contains microscalelengths information, it describes the macroscopicor average behavior of a certain spoil volume,represented by the kinetics of fictitious sphereshaving average properties. Thus, this model can-not consider any spatial variability occurring be-low a length scale of around 0.2–0.4 m within the2D-domain. All the complex microscopic-scale ef-fects can only be included in this model througheffective parameter values, which requires experi-mental calibration.

Realizations of 2D spatial distributions of thehydraulic properties (i.e. the scaling coefficient,aK, of the hydraulic conductivity, K (Vogel et al.,1991)), the fraction of sulfur, fs, the initial degreeof pre-oxidation (i.e. the relation, rc/R) are gener-ated with a sequential Gaussian non-conditioned


Kriging approach using the SGSIM module of theGSLIB program (Deutsch and Journel, 1992).

For a single realization of the generic 2D verti-cal spoil cross-section of 40 m width and 50 mdepth, the SWMS-2D flow model is used to simu-late heterogeneous 2D fields of water flow veloc-ities, q(x,y) (LT−1) (Fig. 1) and volumetric water,uw(x,y), and air, ua(x,y), contents (L3L−3) assum-ing a steady annual average uniform infiltrationrate of 0.189 m per year (Prein, 1994). The 2Dspatial distributions of q, uw, and ua are used forreactive transport modelling. The oxidation prod-ucts from the shrinking core model (1)–(2) areadded to the solutes already present in the solu-tion. After chemical speciation, equilibration, andbuffering, using the MINTEQA2 code, the majorchemical components are transported downwardsby advection and dispersion using theMINTRAN code. A more detailed description of

the modelling system can be found in Gerke et al.(1998).

We assume here that the spoil pile consists oftwo distinct layers of sandy overburden sedi-ments. The top parts of the layers have beenleveled, compacted, and exposed for a consider-able time, resulting in an advanced stage of oxida-tion. The initial oxygen concentration in theair-filled porosity throughout the spoil is assumedto be 0.0375 g/lair. The 50 m deep and 40 m wide2D cross-section is discretized using 101×130nodes. Grid spacing is uniform horizontally(Dx=0.4 m) and varies vertically from Dz=0.2m near the top to Dz=0.4 m near the bottom.Numerical time steps were 0.1 year for transportand B0.05 year for the oxidation model. Prelimi-nary tests of the effect of the numerical discretiza-tion on the results indicated that decreasing thespatial resolution below Dz=0.2 m will changethe simulated 2D flow field only marginally.

In the top layer (0–10 m depth), the averagevalues are u=0.3 m3m−3 for porosity, uw=0.133m3m−3 for volumetric water content, and rb=1855.0 kg/m3 for bulk density. In the second layer(10–50 m depth), we use u=0.33 m3m−3, uw=0.1 m3m−3, and rb=1775.5 kg/m3. We assume arelatively strong degree of pre-oxidation at thesurface of the cross-section and in the upper partof the second layer, i.e., the intact core radius, rc,is linearly increasing from rc/R=0.2 at the sur-face to rc/R=0.7 at 2 m depth and from rc/R=0.3 at 10.4 m to the average value of rc/R=0.75at 12 m depth. In the rest of the spoil, the averagevalue of rc/R=0.75 is used in the layered casewhile in the heterogeneous case, rc/R is randomlydistributed (Fig. 2) as described in Gerke et al.(1998). The particle radius of the oxidation modelis assumed to be R=0.001 m.

In the layered scenarios, we use values of fs=0.01 and fs=0.001 (i.e. sulfur 1 and 0.1% byweight) uniformly throughout the cross-section,while in the heterogeneous scenarios, fs follows arandom distribution according to a sphericalsemivariogram model with correlation lengths of4 m in the upper and 1.2 m in the lower part, anugget of 0.1 (see Gerke et al., 1998), and alognormal frequency distribution with averagevalues identical to those of the layered case aboveand a variance of 1.0.

Fig. 1. The 2D-spatial distribution of water flow velocities, q,for a steady and homogeneous infiltration rate of 0.189 m peryear within the 50 m deep and 40 m wide domain resultingfrom unsaturated flow simulations based on a geostatisticallygenerated hydraulic conductivity field. The length of the ar-rows is proportional to the value of q.


Fig. 2. The 2D spatial distribution of the relative radius,Rr=rc/R, of the unoxidized core used in the coupled pyriteoxidation and reactive transport simulations. The relativelylighter gray areas indicate the more pre-oxidized zones whichreflect temporary surfaces, e.g. at 10 m depth, and incorpo-rated sediments that have already been exposed to the atmo-sphere prior to dumping. In the upper 10 m of the2D-cross-section, we assumed relatively smaller correlationlengths than in the bottom part.

scenarios, most of the cross-section is initially inchemistry class 4 at pH-values around 4.5, exceptfor the top parts of the first and second layer. Inthe heterogeneous scenario, all the five initialchemistry classes occur within the cross-section.Initial concentrations for those components of the5 chemistry classes not given in Table 1 are (fromlow to high pH-values) 0.2, 2, 2, 2, and 1.1 forNa, 0.5, 0.5, 0.5, 0.5, and 0.5 for Cl, 0.8, 0.3, 0.3,0.3, and 0.3 for Mn, and 10−5, 0.1, 0.1, 0.1, and0.1 for H4SiO4, respectively, here in 10−3 molesper liter (mmol/l) liquid phase.

At the top, the inflow boundary condition is ofthe Cauchy type. The concentrations in the inflow(mmol/l) are assumed to be Ca=1.1, Mg=6.7,Na=1.3, K=0.067, Cl=1.0, CO3=1.4, SO4=7.5, Mn=0.001, Fe2+ =0.017, Fe3+ =0.000012,H4SiO4=10−5, and Al=5×10−6. The inflowboundary condition for the oxygen diffusionmodel is of Dirichlet type, with the oxygen con-centration of the inflowing air corresponding toan atmospheric concentration of about 0.265 g/lair. At the bottom boundary, we assume gravity-induced free drainage while no flow occurs acrossthe left and right boundaries.

Note that we do not consider any temporalchange in porosity or hydraulic or transport prop-erties due to secondary mineral precipitation/dis-solution or mine spoil consolidation.

3. Results and discussion

The concentrations in Figs. 3–6 are obtainedby horizontally averaging the local solute massesweighted by the local water contents over the 40m width of the cross-section. The vertical profilesof the horizontally averaged unoxidized core radiiand oxygen, aqueous, and solid phase concentra-tions for the relatively low average sulfur content,fs=0.001, and the 2D heterogeneous (Fig. 3) andlayered (Fig. 4) systems shows effects of the initialchemical distribution already during the first 30years. The oxidation front reaches a depth of10–12 m after 30 years of simulation time. The O2

and rc-profiles of the layered case show a piston-type front deeper movement of the pyrite oxida-tion front with time. Note that the oxygen

Twelve aqueous components are selected fortransported reactive Ca, Mg, Na, K, Cl, CO3,SO4, Mn, Fe(II), Fe(III), H4SiO4, and Al. Inaddition, seven solid mineral phases are consid-ered: calcite (CaCO3), gypsum (CaSO4·H2O), fer-rihydrite (Fe(OH)3), gibbsite (Al(OH)3),ALOHSO4, K-Jarosite (KFe(SO4)2(OH)6), andamorphous silica (SiO2). All of these minerals arepermitted to precipitate. The initial distribution ofthe aqueous and solid phase chemical componentsin the 2D cross-section is assumed to be corre-lated with the degree of pre-oxidation, rc/R, ac-cording to five classes for which equilibratedchemical systems have been calculated for a rangeof pH-values between 1.8 and 7.2 (Table 1) usingthe MINTEQA2 model. The five backgroundchemistry classes are used to generate a simplifiedspatial pattern of the initial chemical systems inmore and less pre-oxidized spoil regions reflectedby the rc/R-distribution (Fig. 2). In the layered


diffusion in the layered case is also spatially dis-tributed since the diffusion coefficient depend onheterogeneously distributed water and air con-tents. In the heterogeneous case (Fig. 3), theoxidation front after 30 years seems to be about 2m higher and the rc-front within the oxidationzone to be steeper, i.e. extended over a largervertical zone, compared to the layered case (Fig.

4). The effect is caused only by differences in thespatial distribution of the solid sulfur content andof the initial chemistry distribution. Since flowand transport rates are identical for both cases,the vertical concentration profiles of the aqueouscomponents (SO4, Fe3+, Al) show relatively smalldifferences between the heterogeneous and layeredcases. After 10 years, the concentrations of SO4,

Table 1Initial aqueous and solid phase concentrations (both in mmol/lpore water) in the 2D cross-section based on five classes of the initialunoxidized grain radius ratio of rc/R

rc/R

0.75–0.820.0–0.35 0.82–1.00.35–0.6 0.6–0.75

4Ca 20 4 4 4Mg 3.288810

5.145.14 5.145.1410−3K10−13 0.2 0.20.2CO3 0.2

100 50SO4 42 38 10Fe2+ 0.0250.0250.0250.0250.01

2 8Fe3+ 10 0.16.169.250Al 5.5 1

20 10 101Fe(OH)3 301 5Al(OH)3 10 20 20

0 0CaSO4·H2O 0 0 01000CaCO3 00

0 00ALOHSO4 0 000 00K-Jarosite 0

10010 100100 100SiO2

pH 7.21.8 3.0 3.2 4.5

Fig. 3. Vertical profiles of horizontally averaged unoxidized core radii rc, oxygen concentrations O2, and aqueous and solid phasecomponent concentrations for the heterogeneous system and the relatively low average sulfur content fs=0.001.


Fig. 4. Vertical profiles of horizontally averaged unoxidized core radii rc, oxygen concentrations O2, and aqueous and solid phasecomponent concentrations for the layered system and the relatively low average sulfur content fs=0.001.

Fig. 5. Vertical profiles of horizontally averaged unoxidized core radii rc, oxygen concentrations O2, and aqueous and solid phasecomponent concentrations for the heterogeneous system and the relatively high sulfur content fs=0.01.

Fe3+, and Al are highest shortly below the oxida-tion zone, while after 30 years, the concentrationsare more equally distributed and dispersed overthe profile, and the release of oxidation productsis decreasing. The pH-profile of the heterogeneouscase (Fig. 3) shows a more smeared shape withdepth with slightly higher values than that of thelayered case (Fig. 4).

The largest differences between layered (Figs. 4and 6) and heterogeneous (Figs. 3 and 5) chem-istry cases can be observed when looking at theamount and the spatial distribution of the solidcomponents. Note that gypsum did not precipi-

tate under conditions simulated here. In the het-erogeneous cases, relatively more gibbsite isdissolved in the upper part of the profile andpresent in the bottom part and the concentrationsof ferrihydrite are higher, while ALOHSO4 andK-Jarosite are mostly lower than in the layeredcase. The ALOHSO4 profiles indicate dissolutionwithin and precipitation above the pyrite oxida-tion zone, the latter is more pronounced in theheterogeneous cases (Figs. 3 and 5). The solidcomponents retain, according to their solubilityequlilibria, certain amounts of oxidation productsin the upper parts of the spoil profile. Dissolution


of minerals by the infiltrating rain water is justbeginning at the top of the profile, e.g. see profilesof K-Jarosite. Differences in the concentrationprofiles between the two cases can be expected tocontinue with time as long as pyrite oxidationcontinues and spatially distributed secondary min-erals have not been washed out. If the boundaryconditions remain unchanged for much longertimes, the final hypothetical total amount of acidgeneration and sulfate release will, of course, besimilar for both cases since the overall content ofsulfides is nearly identical.

The vertical profiles of the averaged values forthe high sulfur content scenario, fs=0.01, for theheterogeneous (Fig. 5) and layered (Fig. 6) caseshow relatively smaller vertical movement of theoxidation front and higher aqueous componentconcentrations than for the low sulfur scenariodescribed above. The oxidation front reaches about5 m depth after 30 years. The rc-profiles are,however, markedly different in shape indicating aspreading of the oxidation front for the heteroge-neous sulfur distribution. The peak concentrationsfor SO4, Fe3+, and Al are somehow controlled bythe solubility equilibria of the mineral phases.Concentrations are even more dispersed over thewhole profile than in the low sulfur simulations.

The integrated breakthrough curves in Fig. 7 are

the solute concentrations in the seepage water, herefor example at 30 m depth, obtained by horizon-tally averaging the local vertical mass flux weightedby the local water flux rates. The breakthroughcurves show the effect of the chemical heterogeneityon the leachate concentrations for the low (left partof Fig. 7) and the high (right side of Fig. 7) fractionof sulfur scenario. In the 30-year simulation period,the pH-value gradually drops from about 4.5 to 2.5in the low and to below pH 2 in the high sulfurscenario. In both scenarios, the pH-values in theheterogeneous cases are initially lower but laterbecome higher as in the layered cases. The SO4 andFe3+ breakthrough curves look relatively similaralthough in the heterogeneous cases of both scenar-ios, the concentrations are initially slightly higherthan in the layered cases. These initial differencesprobably reflect the leaching from pre-oxidizedzones which already contains a relatively highinitial sulfate concentrations and which have beenincorporated in the spoil closely above the 30 mdepth level. The steep increase in concentration ofSO4 and Fe3+ after about 10 years begins about 1year earlier in the heterogeneous than in layeredcase.

The breakthrough curves for Al in Fig. 7 alsoshow a relatively small initial concentration peakin the heterogeneous case, probably resulting

Fig. 6. Vertical profiles of horizontally averaged unoxidized core radii rc, oxygen concentrations O2, and aqueous and solid phasecomponent concentrations for the layered system and the relatively high average sulfur content fs=0.01.


Fig. 7. Horizontally averaged breakthrough curves of selected chemical components at 30 m depth showing comparison between thecases of relatively low, fs=0.001, and high, fs=0.01, average sulfur contents and between the heterogeneous and the layered system.


from dissolution of locally distributed gibbsite.At later times, the Al concentration increasesmarkedly earlier in the layered than in the het-erogeneous case. The breakthrough curves of Caand CO3 are distinctly different for both casesand both sulfur content scenarios. In the hetero-geneous case, the initial concentration peaks inthe first years are probably caused by the disso-lution and leaching of CaCO3 which was presentclose to 30 m depth level. Note, that the calciteoriginates from the initial chemistry of class 5(Table 1) which is assumed to be correlated withrelatively non pre-oxidized spoil material. Notealso, that in the layered cases, calcite is notpresent because from the horizontal averaging ofthe rc/R-values, no initial chemistry of class 5occurs.

Further considerations, e.g. of the effects ofmineral precipitation and dissolution on changesin porosity and sulfide oxidation with time andrelated feedback mechanisms on water flow, so-lute transport, and oxygen diffusion, wouldprobably show increasing effects of heterogene-ity, but will considerably complicate the modelsystem and identification of individual effects.Already the analysis of this still relatively simplegeneric system indicates that only the spatial ar-rangement of components within the 2D do-main, under otherwise identical conditions, willaffect acid generation and leaching.

The spatial distributions of hydraulic andchemical properties within the mine spoil heapsare still rather qualitative and subjectively gener-ated such that they may fit hypothetical assump-tions. In future, however, the model system maybe combined with more realistic 2D-distributionsobtained from a spoil structure model in whichthe knowledge of geology of the intact outcropside, the mining technology, and mixing andparticle segregation processes are considered(e.g. Gerke et al., 1999).

4. Conclusions

The effect of spatial heterogeneity of physicaland chemical properties is significant not onlywith respect to concentrations of oxidation

products and leaching at the local or samplingscale but also when considering horizontally av-eraged concentrations of chemical componentsin vertical profiles and in breakthrough curves.The results show that differences in the spatialdistributions, i.e. layered compared to heteroge-neous initial chemistry, in a physically heteroge-neous pyritic mine spoil heap can, underotherwise identical conditions, lead to differencesin the temporal evolution of sulfide oxidationand acid mine leaching. The results suggest thatespecially the spatial distributions of the solidphase minerals, i.e. their particular location inthe spoil heap, play an important role becauseoxidation of sulfides and dissolution/precipita-tion of secondary minerals lead to a local re-lease and retardation of solutes. The time spanrequired for oxidation and salt leaching appearsto be longer in the case of heterogeneous com-pared to a layered distribution of sulfides andbuffer substances. A higher sulfur content in thespoil sediments slows down the oxidation frontand increases the Fe and SO4 – solute concen-trations and the mineral precipitation at differ-ent pH-values and locations. The simulationapproach still requires extensive experimentalwork for model validation. The method, how-ever, may be useful for estimating long-term ef-fects of the measures for spoil rehabilitation onacidification and salt leaching.

Acknowledgements

The authors are indebted to Prof. Dr. R.F.Huttl, head of the Department of Soil Protec-tion and Recultivation of the Brandenberg Uni-versity of Technology at Cottbus (BTUC),Germany, for initializing and critically support-ing this work. We thank Dr. U. Buczko fromthe BTUC for his help during the preparationof the manuscript and useful comments. Thiscollaborative research was made possible by atravel-grant from the Deutsche Forschungs-gemeinschaft, Bonn, and the Center of Excel-lence project, Ecology of post lignite miningLandscapes in Lusatia/Germany of the BTUC.


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modelling the impact of physical and chemical heterogeneity on solute leaching in pyritic overburden...

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