leaching induced changes in substrate and solution chemistry of mine soil microcosms

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LEACHING INDUCED CHANGES IN SUBSTRATE AND SOLUTION CHEMISTRY OF MINE SOIL MICROCOSMS WOLFGANG SCHAAF Dept. of Soil Protection and Recultivation, Brandenburg University of Technology, P.O. Box 101344, D-03013 Cottbus, Germany (e-mail: [email protected], fax: (33) 355 692323) (Received 21 August 2001; accepted 3 June 2002) Abstract. Leaching of soluble salts formed as the result of pyrite oxidation and primary mineral weathering is a major process in mine soil development. A microcosm experiment was designed to study leaching rates from mine soil columns under controlled laboratory conditions. Objectives of this experiment were to investigate the effect of leaching and the effect of fly ash amelioration on mid- to long-term chemical soil properties, and to test whether the results are qualitatively comparable to long-term field studies along a site chronosequence. The leaching experiment was conducted over a period of 850 days representing a kind of time- lapse picture due to high water fluxes. Leaching resulted in more favourable mid- to long-term properties of mine site topsoils, especially a reduced risk for acidity and salt stress. Ash amelioration decreases leaching rates by increasing pH and Al and Fe precipitation. It could be shown that the results of the column leaching studies are qualitatively in good agreement with field observations at least for long-term considerations. By enhancing the leaching process mid- to long-term chemical soil properties can be estimated. Keywords: acid mine drainage, element budget, fly ash, gypsum, lignite, pyrite, soil columns, soil development, weathering 1. Introduction Spoil substrates in the post-mining landscape of Lusatia, Germany, are character- ized to a large extent by their lignite and pyrite contents stemming from Tertiary sediments (Hüttl, 1998). The majority of these substrates are sands and loamy sands with low nutrient contents and poor water retention capacities (Katzur and Haubold-Rosar, 1996a). The dumped overburden substrates differ in their geo- chemistry from the undisturbed geological parent material of the region. It is there- fore expected that under these disturbed conditions new and different types of soils and ecosystems will develop. The pyrite content of these substrates exposed to the atmosphere leads to a high production of acidity and consequently to very phytotoxic site conditions, high primary mineral weathering and secondary salt precipitation. To ameliorate these sites, large amounts of ash from lignite power plants (up to 1000 t ha 1 ) were used prior to 1990 for neutralization of actual and potential acidity (Katzur and Haubold-Rosar, 1996b). Water, Air, and Soil Pollution 3: 139–152, 2003. © 2003 Kluwer Academic Publishers. Printed in the Netherlands.

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Page 1: Leaching Induced Changes in Substrate and Solution Chemistry of Mine Soil Microcosms

LEACHING INDUCED CHANGES IN SUBSTRATE AND SOLUTIONCHEMISTRY OF MINE SOIL MICROCOSMS

WOLFGANG SCHAAFDept. of Soil Protection and Recultivation, Brandenburg University of Technology, P.O. Box101344, D-03013 Cottbus, Germany (e-mail: [email protected], fax: (33) 355 692323)

(Received 21 August 2001; accepted 3 June 2002)

Abstract. Leaching of soluble salts formed as the result of pyrite oxidation and primary mineralweathering is a major process in mine soil development. A microcosm experiment was designed tostudy leaching rates from mine soil columns under controlled laboratory conditions. Objectives ofthis experiment were to investigate the effect of leaching and the effect of fly ash amelioration on mid-to long-term chemical soil properties, and to test whether the results are qualitatively comparable tolong-term field studies along a site chronosequence.

The leaching experiment was conducted over a period of 850 days representing a kind of time-lapse picture due to high water fluxes. Leaching resulted in more favourable mid- to long-termproperties of mine site topsoils, especially a reduced risk for acidity and salt stress. Ash ameliorationdecreases leaching rates by increasing pH and Al and Fe precipitation. It could be shown that theresults of the column leaching studies are qualitatively in good agreement with field observations atleast for long-term considerations. By enhancing the leaching process mid- to long-term chemicalsoil properties can be estimated.

Keywords: acid mine drainage, element budget, fly ash, gypsum, lignite, pyrite, soil columns, soildevelopment, weathering

1. Introduction

Spoil substrates in the post-mining landscape of Lusatia, Germany, are character-ized to a large extent by their lignite and pyrite contents stemming from Tertiarysediments (Hüttl, 1998). The majority of these substrates are sands and loamysands with low nutrient contents and poor water retention capacities (Katzur andHaubold-Rosar, 1996a). The dumped overburden substrates differ in their geo-chemistry from the undisturbed geological parent material of the region. It is there-fore expected that under these disturbed conditions new and different types of soilsand ecosystems will develop.

The pyrite content of these substrates exposed to the atmosphere leads to a highproduction of acidity and consequently to very phytotoxic site conditions, highprimary mineral weathering and secondary salt precipitation. To ameliorate thesesites, large amounts of ash from lignite power plants (up to 1000 t ha−1) wereused prior to 1990 for neutralization of actual and potential acidity (Katzur andHaubold-Rosar, 1996b).

Water, Air, and Soil Pollution 3: 139–152, 2003.© 2003 Kluwer Academic Publishers. Printed in the Netherlands.

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The element budgets of the afforested new ecosystems were studied in a false-time series approach under field conditions (Gast et al., 2001; Schaaf, 2001). Itcould be shown that for the experimental sites, covering a period of 5 to 55 yearsafter dumping, the element flux rates differ considerably compared to non-minedsites of the region (Schaaf, 2001). Especially at younger sites element output rates(e.g. for Ca, Mg, Al, Fe and S) can reach extraordinary high values in the orderof tons per hectare and year. It was shown that these high fluxes decline with time(Knoche et al., 1999).

Leaching of soluble salts formed as the result of pyrite oxidation and primarymineral weathering is a major process in mine soil development. Whereas in theliterature these processes are mostly studied with respect to the potential for acidmine drainage formation (e.g. Banks et al., 1997; Evangelou, 1995; Geldenhuisand Bell, 1998; Karathanasis et al., 1988; Karathansis et al., 1990; Miller et al.,1996; Wieder 1993), the objective of this experiment was to investigate the effectof leaching on mid- to long-term chemical soil properties and element cycling.

Due to the prevailing regional climate with low mean annual precipitation (550mm yr−1; Preußner, 1998), leaching is a very slow process under field conditions.Besides seasonal and inter-annual variations of precipitation, leaching rates are in-fluenced by differences in evapotranspiration of the growing stands at the afforestedsites, in the geochemical composition of the original substrate at different sites (e.g.pyrite content), and by the high intra-site spatial heterogeneity.

A microcosm experiment with soil columns was therefore designed to studyleaching rates under controlled laboratory conditions. The following questionsshould be addressed:

• How does leaching affect chemical soil properties? It is hypothesized thatsoil compartments where pyrite oxidation is completed and soluble salts areleached represent a quasi-stable soil status after a highly dynamic initial phase.

• What is the effect of fly ash amelioration on leaching rates, gypsum dissolu-tion, and solid phase soil properties?

• Are the results of a controlled soil column study at least qualitatively compar-able to long-term trends in development under field conditions? If so, columnstudies could be used to derive parameters for modelling and to estimate thelong-term soil properties for a larger number of substrate mixtures reducingthe high efforts of field measurements.

2. Materials and methods

Soil microcosms were used for studying element fluxes of soil columns undercontrolled conditions. The automated soil microcosm system (Blechschmidt et al.,2000; Hantschel et al., 1994) contains soil columns, an irrigation system, a suctionunit, a leachate sampling system, a flow unit, a gas chromatograph, a gas multi-plexer valve, a control unit, and data logging. The soil microcosm system consists

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TABLE I

Chemical characteristics of the mine soil substrate and the fly ash used in the microcosmexperiment (Blechschmidt et al., 2000)

pH EC CTa NT

a STa Cab Mgb CECact. BS

(H2O) (mS cm−1) ←−−−−−− (g kg−1) −−−−−−→ (cmol(c) kg−1) (%)

Substrate 3.0 1.5 27.3 0.57 5.7 5.7 0.7 2.35 31.5

Fly ash 12.0 9.5 4.8 0.71 7.1 79.2 17.1 n.a. n.a.

a Total element analyzer (Leco).b HNO3 extract.CECact. = actual cation exchange capacity (BaCl2 extract); BS = base saturation; n.a. = notanalyzed.

of closed Plexiglas columns (height 30 cm, ∅ 14.4 cm) in a climate chamber. Thetemperature was kept constant at 5 ◦C for the first 3 weeks and was then increasedto 10 ◦C (mean air temperature of the region: 8.4 ◦ C, Preußner, 1998).

Disturbed soil samples of lignite and pyrite containing spoil substrate weretaken, sieved (<5 mm), homogenized and analyzed (Table I). Soil gypsum contentat the site varied between 1.5% and 1.8% (Neumann, 1999). Data from Meyer(2000) suggest that only <1% of the total sulfur still occurred in form of pyrite.Four microcosms were filled to a height of 23 cm with the soil material compactedto a bulk density of 1.5 Mg m−3. Disturbed samples were used to fill the columnssince the material itself was dumped only 5 years ago and the sandy mine soilstypically have no or only poor structure. Another four columns were filled withameliorated substrate mixed with power plant fly ash (2.5 g ash 100 g−1 soil)according to the lime requirement calculated by ABA using the NAG-test (Millerand Jeffery, 1996).

The columns were irrigated with spray nozzles (3.1 mm day−1) connected topressure tanks using artificial rain water with the average composition of bulkprecipitation (Wilden et al., 2001) of the area (pH 4.6, 0.018 mmol NO3 L−1,0.002 mmol NH4 L−1, 0.195 mmol Ca L−1, 0.083 mmol Na L−1, 0.049 mmol MgL−1, 0.013 mmol K L−1, 0.106 mmol SO4 L−1, 0.051 mmol Cl L−1). Irrigationheight was chosen to represent twice the typical bulk precipitation to enhanceleaching. The amount of irrigation was triggered by computer controlled solenoidvalves. At the bottom of the columns leachate was sampled using a low pressurehead (6 kPa) draining the coarse pores to maintain unsaturated conditions. Theleachate was collected in glass bottles. At the beginning of the experiment, columnleachate was sampled weekly and after two months in biweekly intervals. The solu-tion samples were analyzed for pH (Beckmann pH34 glass electrode), electricalconductivity (EC, Hanna HI 8733), SO4, NO3, Cl (Dionex 120 IC), Ca, Mg, Fe,Al (ICP-AES, Unicam 939), K (AAS, Unicam 701), NH4 (Rapid Flow Analyzer,

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Perstorp Analytical 7050), and dissolved organic carbon (DOC, Shimadzu TOC5000 Analyzer).

All microcosms were leached for 300 days and analyzed for amount and chem-ical composition of the leachate. After that, two columns per treatment were re-moved, cut into several layers and analyzed for pH and EC distribution within thesoil. Leaching of two remaining columns per treatment was continued for a totalof 850 days. Afterwards the columns were cut in layers of 2 cm thickness and thesoil was again analyzed for pH, EC, actual cation exchange capacity (Schaaf et al.,2001), base saturation, acid neutralization capacity (ANC; pH-stat. titration), andgypsum content (Schlichting et al., 1995).

3. Results

After 2 weeks, a constant and stable water flux was established through thecolumns. Over the total period of 850 days with an irrigation amount of 2635mm, mean total leachate was 2352 mm 2347 mm from the control and the fly ashtreatments, respectively. During the experimental period daily output fluxes of theeight columns varied insignificantly between 2.2 and 3.6 mm d−1. Mean leachateconcentrations per sampling date were therefore not volume weighted.

Figure 1 shows the leachate composition during the first 300 days. Ash ap-plication increased the leachate pH by two units to 4.5–5.0. Most of the elementsshowed elevated concentrations in the beginning that declined over the first 60–70days to relatively constant levels for the rest of the period. EC decreased in thesame period from > 8 mS cm−1 to 2.5 and 3.0 mS cm−1 for the control and theameliorated columns, respectively. pH increase after ash application reduced thesolubility of Al and Fe to values close to or below the detection limit. The controlmicrocosms also showed high initial leaching of Zn. Mg concentrations were highafter ash application with initial values of 80 mmol L−1 although the leachate fromcontrol columns also showed increased concentrations in the beginning. After 120days concentrations were close to zero for both treatments. In contrast, Ca con-centrations increased during the first several weeks and leachates from the controlcolumns showed even higher values compared to the ash treatment during the first120 days. After that time, leachate concentrations remained at similar values forall columns. Initial SO4 concentrations were also higher for the control for the firstseven weeks and decreased to constant values around 20 mmol SO4 L−1 for the restof the experiment for both treatments. DOC concentrations remained at slightlyhigher levels for the control compared to the ash ameliorated columns over theinvestigated period. In the initial phase, NH4 was also leached from the substrate insubstantial concentrations with higher values for the control. Nitrate concentrationsshowed a peak after 50 days and decreased afterwards. During the same period adecrease in chloride concentrations was recorded.

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Figure 1. pH, EC and element concentrations in the leachate from soil columns over a period of 300days (symbols are mean values and error bars indicate standard deviation of four replicates for thecontrol and ash amelioration treatment, respectively).

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SO4 concentrations remained at constant levels for both treatments also fromday 300 to 850 (Figure 2). At the end of that period, leachate EC of the controlcolumns was decreased to the level of ash ameliorated microcosms. Despite acontinued difference in pH, leachate of the control showed a trend towards higherpH values, whereas the ash treatment showed the opposite trend. After the initialincrease described above Ca concentrations of both treatments remained similarfrom day 200 to 780. Afterwards concentrations decreased, slightly more so forthe control than for the ameliorated columns.

The data from this microcosm experiment were used to calculate simple elementbudgets for the columns (Table II). Ash application increased Ca and Mg stores by31% and 58%, respectively. S stores were only slightly increased. Element inputloads with irrigation water were negligible compared to leachate output rates. After850 days (2635 mm irrigation) large amounts of these stores were leached from thecolumns. Due to higher initial concentrations, the S output from the control washigher compared to the ameliorated columns. At the end of the experiment 71%of the Mg applied with the fly ash had been leached from the columns. Since totalCa output was similar for both treatments there was obviously no additional lossof applied Ca. The supply of Ca by ash application could result in further gypsumprecipitation and thereby could also reduce or delay SO4 leaching.

After 300 days of continuous leaching the columns showed clear depth gra-dients in pH and EC (Figure 3). In both treatments, soil pH in all depths washigher compared to the initial values (Table I). Throughout the microcosms pHhad increased by two units due to ash amelioration. In the uppermost 6–7 cm of thecolumns pH values were 0.5 units higher in the control and up to one unit higherin the ash treatment. However, there was a sharp gradient to the lower part of themicrocosms. EC was drastically decreased in the upper part of the columns (<0.1mS cm−1) and increased rapidly below this depth indicating leaching of solublesalts.

TABLE II

Element storage and fluxes of the microcosm experiment

All values in Mg ha−1 23 cm−1 Control + fly ash

soil depth Ca Mg S Ca Mg S

Storage 19.7 2.4 19.7 25.9 3.8 19.8

Input by irrigation 0.2 <0.1 <0.1 0.2 <0.1 <0.1

Output after 300 days 5.1 0.4 7.9 4.9 1.4 6.2

Output after 850 days 12.9 0.4a 14.8 13.0 1.4a 13.0

Output (– input) in % store 64 17 75 49 37 66

a Magnesium output after 300 days was no longer analyzed since Mg concentrations werebelow the detection limit after 150 days (see Figure 1).

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Figure 2. pH, EC, Ca and SO4 concentrations in the leachate from soil columns over a period of 850days (symbols are mean values and error bars indicate standard deviation of two replicates for thecontrol and ash amelioration treatment, respectively)

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After another 550 days the leaching zone had developed through the major partof the columns reaching about 2 cm deeper in the control compared to the ashtreatment (Figure 3). Below this, a steep gradient was again found, and only thebottom 2–4 cm still showed high EC values. pH values in the columns increasedfrom bottom to top and were generally higher than after 300 days for the control.

Only below 16 cm depth, gypsum could be detected. The lower gypsum con-tents in the control microcosms reflect the higher amount of leaching in this treat-ment and the higher initial contents after Ca application of fly ash. Both cationexchange capacity and base saturation were clearly increased by ash amelioration.In the control the upper part of the column substrate marked by higher pH valuesshowed a 30% higher ANC compared to the bottom of the columns correspondingto an increase in base saturation by about 100%.

4. Discussion

The results from the microcosm experiment confirm the overall high element con-centrations in the leachate from these types of spoil substrates. Similar data werereported from field samples of soil solution at different post-mining sites of Lusatia(Embacher, 2000; Knoche et al., 1999; Schaaf et al., 1999; Wilden et al., 2001).The elevated concentrations of almost all elements analyzed during the first fewweeks, especially in the unameliorated control, reflect the high potential of thesubstrate for acid mine drainage (AMD) formation.

Drastically decreasing solution concentrations with time are also found in chro-nosequence studies of mine soils developed from comparable substrate (Knoche etal., 1999; Schaaf, 2001). Since the irrigation and leaching rates in the microcosmexperiment were kept constant and were at a much higher level than under fieldconditions, the results are only comparable on a relative basis. Due to the lowamounts of precipitation in the Lusatian region, soil water fluxes in the field areextremely low, especially at afforested sites with high water losses due to inter-ception and evapotranspiration (Scherzer, 2001). By leaching 2635 mm over thetotal experimental period of 850 days, an annual water flux of 1130 mm yr−1

was established in the columns. In contrast, under field conditions we calculatedannual soil water fluxes between 50 and 200 mm (Scherzer, 2001). Hence undermicrocosm conditions the leaching rates were 5- to 20-fold higher than under fieldconditions.

Amelioration with fly ash resulted in the desired pH increase throughout thewhole experimental period and proved the suitability of the NAG-test that wasused to calculate lime requirement. The elevated pH drastically decreased metalleaching from the beginning of the experiment especially for Al and Fe, but alsofor Zn, probably due to their precipitation in the form of oxides and hydroxides(Miller et al., 1996; Li, 1997). On the other hand, ash amelioration increased Mgleaching indicating a large loss of applied Mg.

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Due to the high total solution concentrations, the formation of ion pairs andcomplexes plays a dominating role in ion species distribution. Geochemical equi-librium calculations using PHREEQC (Parkhurst, 1995) indicate that >50% of thetotal Ca concentration is in form of CaSO4

◦. Other SO4 complexes including Fe,Al, and Mg can make up >75% of the total SO4 concentration (Schaaf et al., 1998).This could explain the higher initial concentrations of both Ca and SO4 in theleachate from the control columns with higher ionic strength and thereby increasedgypsum solubility (Schaaf, 2001). The slightly decreasing Ca concentrations ofboth treatments at the end of the experiment (Figure 2) are probably caused by thebeginning exhaustion of the soluble gypsum pool in the columns (Figure 3).

Ammonium is obviously released from lignite in the columns. High NH4 con-centrations were also reported by Katzur and Liebner (1995) and Cravotta (1998)in leachates from spoils containing lignite that was shown to have a high potentialfor NH4 sorption (Hons and Hossner, 1980). The delayed NO3 peak indicates thatsome of this NH4 could be nitrified even in the control, despite low pH values. Elev-ated NH4 concentrations were also found in soil solutions from very acid subsoilsof afforested dumps (Gast et al., 2001; Wilden et al., 2001). The fact that even sitesthat were dumped some 25 years ago showed these elevated NH4 concentrations incontrast to the quickly declining values in the column study might be attributed tothe high leaching rates in the microcosms. The mechanism of this release as well asthe original source of the NH4 is still unclear, but it could have a positive impact onthe nutritional status of these mine soils. Chloride shows a different temporal be-havior compared to all other analyzed ions. The decrease around day 50 cannot beexplained by pure concentration effects since water fluxes were constant through-out this period. Fly ash as a source for Cl release can be excluded since control andash treatment showed no differences. Geochemical calculation of saturation indicesgave no hints for temporal formation and subsequent dissolution of a chloridic solidphase. It remains unclear, whether the drop in chloride concentrations might be, forexample, the result of an antagonistic effect with nitrate.

The EC gradients within the microcosms at the end of the experiment reveal thehigh leaching of soluble salts. This development was enhanced by the high waterflux rates in the columns compared to field situations. Taking the flush of alreadydissolved and easily soluble salts that caused elevated EC in the leachate duringthe first 100 days (Figure 2) not into account, it can be estimated that it takesabout 125–140 mm of water percolation to leach 1 cm of soil free of salts (i.e.EC < 0.1 mS cm−1). At field conditions where soil water fluxes are much morevariable and also much lower these estimates would correspond to 0.3–1.6 cm soilyr−1 that could be leached. This assumption is supported by Neumann (1999), whofound similar steep EC gradients at depths between 10 and 20 cm at a 50 yearsold mine soil in Lusatia. Another difference between columns and field studies isthat in the microcosms water flux was constant over time but under field conditionsdistinct dry periods can be observed as an effect of both, summer drought and rootwater uptake. Thus, whereas the results of the column leaching experiment are well

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in agreement with field observations over long periods, i.e. decades, short-term(seasonal) effects are most likely much more pronounced under field conditionsdue to wetting/drying cycles resulting in repeated precipitation/dissolution mainlywithin the root zone.

The CEC and BS of the control substrate is in the range of values found atvarious sites with comparable substrates. The lignite content in the sandy substratesgenerally increases CEC which was found to be 2–10 fold higher than for bothlignite-free spoils and undisturbed soils of the region (Schaaf et al., 2001). Thesame authors also reported significantly elevated CEC values in ash amelioratedtopsoils. Since BS in the ameliorated columns was mainly composed of Ca, sorp-tion at exchange sites could explain about 25% of the stored Ca amounts derivedfrom ash application. The still higher gypsum contents at the column bottom ofthe ameliorated microcosms compared to the control contribute another 40% to theincreased Ca store. The rest is probably stored as still undissolved CaO or CaCO3

in the fly ash.The reasons for the increased pH and ANC in the topsoil of the control mi-

crocosms at the end of the experiment is complex and not easily explained. In thetop part of the columns that show the highest leaching losses, soil pH was morethan one unit higher compared to the original substrate. One possible explanationfor this pH increase could be the leaching process, i.e. the removal of acidifyingreaction products by vertical transport. Further, the precipitation of some of thesecondary minerals frequently found in these substrates, like jarosite and schwert-mannite or jurbanite, can store acidity (Neumann, 1999). Geochemical modellingusing the leachate data from the microcosms indicate equillibria of the solutions ofboth treatments with gypsum and jurbanite (data not shown here). This storage ofacidity may only be temporary and dissolution or transformation would eventuallyrelease corresponding amounts of acidity again. Nevertheless, these combined pro-cesses of leaching, transformation and precipitation could contribute considerablyto the observed long-lasting effect of spoil amelioration (Katzur, 1998).

75% of total sulfur storage of the control was lost by leaching after 850 days(Table II). At the same time 64% of total Ca was leached. Overall Ca output wasnot increased by ash application. Hence at the end of the experiment Ca stores inthe microcosms were almost doubled compared to the control (12.9 and 6.8 Mg Caha−1, respectively). On the other hand, most of the Mg applied with the ash was lostby leaching which is most probably due to the high solubility of MgSO4. The MgOcomponent in the fly ash therefore does not contribute to long-term buffering ofthe acidifying potential of the substrates. Field experiments on young amelioratedspoils reported similar sulfur output rates (6.4 Mg SO4-S ha−1yr−1; Wilden et al.,2001). Since Ca fluxes are mainly controlled by gypsum solubility, Ca output ratesin the field were much lower due to lower water fluxes (750 kg Ca ha−1yr−1, 215mm yr−1). In contrast, Mg output rates from the layer that recently had been ashameliorated were again in the same range as in the microcosm experiment (600 kgMg ha−1yr−1).

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5. Conclusions

The leaching experiment of the microcosms represents a kind of time-lapse pictureof spoil internal processes due to the increased leaching rates. Generally, after 850days of leaching, the results of the microcosm study were in good agreement withfield findings of a site chronosequence covering more than 55 years (Gast et al.,2001).

Leaching and transformation of soluble weathering products has a distinct effecton the mid- to long-term chemical properties of the developing soils. Decreasingacidity, increasing base saturation and cation exchange capacity as well as decreas-ing solution concentrations are processes that result in the gradual improvement ofthe initial phytotoxic soil conditions. Ash application can enhance these processesconsiderably. Thus, leaching results in more favourable mid- to long-term proper-ties of mine site topsoils, especially a reduced risk for acidity and salt stress. Ashamelioration decreases leaching rates of Al and Fe by pH induced precipitation.Ash-born Mg is very easily leached from the soils and contributes only little toneutralization processes. Ash application results in increased gypsum precipitationand reduced gypsum solubility at least in the initial period. This situation resultsin initially slightly lower leaching rates for Ca and SO4, and in a longer periodnecessary for gypsum removal by leaching. The potential for nutrient storage inthe ameliorated top soils is clearly much higher compared to the natural sandysoils of the region (Schaaf et al., 2001).

It could be shown that the results of the column leaching studies are qualitativelyin good agreement with field observations at least for long-term considerations. Byenhancing the leaching process mid- to long-term chemical soil properties can beestimated. Therefore this method could be used to study both, other substrate ormixture types and amelioration measures to evaluate a wider range of future sitequalities in post-mining areas. Further research is needed to evaluate the effects ofhigh spatial heterogeneity typical for disturbed soils on preferential solute transportand small-scale variation in leaching rates and soil properties.

Acknowledgements

The work presented here was carried out as part of the research program ‘Restor-ation of soil ecological functions in the post-lignite mining landscape of Lusatiausing suitable waste materials’ (grant No. 0339646) jointly funded by the FederalMinistry of Education, Research and Technology (BMBF), the Lusatian LigniteAdministration Corp. (LMBV), and the Environmental Agency of Brandenburg.The author would like to thank Ralf Blechschmidt, who cared for the microcosmsduring the first year, and all technical staff at the laboratory. Last but not least, Iwant to thank the reviewers for their detailed contributions and suggestions thathelped to improve the manuscript considerably.

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