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

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  • 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: schaaf@tu-cottbus.de, 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 (Httl, 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 ha1) wereused prior to 1990 for neutralization of actual and potential acidity (Katzur andHaubold-Rosar, 1996b).

    Water, Air, and Soil Pollution 3: 139152, 2003. 2003 Kluwer Academic Publishers. Printed in the Netherlands.

  • 140 WOFGANG SCHAAF

    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 yr1; Preuner, 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

  • LEACHING OF MINE SOIL MICROCOSMS 141

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

    pH EC CT a NT a ST a Cab Mgb CECact. BS(H2O) (mS cm1) (g kg1) (cmol(c) kg1) (%)

    Substrate 3.0 1.5 27.3 0.57 5.7 5.7 0.7 2.35 31.5Fly 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, Preuner, 1998).

    Disturbed soil samples of lignite and pyrite containing spoil substrate weretaken, sieved (

  • 142 WOFGANG SCHAAF

    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 d1. 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 4.55.0. Most of the elementsshowed elevated concentrations in the beginning that declined over the first 6070days to relatively constant levels for the rest of the period. EC decreased in thesame period from > 8 mS cm1 to 2.5 and 3.0 mS cm1 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 L1 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 L1 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.

  • LEACHING OF MINE SOIL MICROCOSMS 143

    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).

  • 144 WOFGANG SCHAAF

    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 67 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 (

  • LEACHING OF MINE SOIL MICROCOSMS 145

    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)

  • 146 WOFGANG SCHAAF

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  • LEACHING OF MINE SOIL MICROCOSMS 147

    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 24 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 yr1was 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 de...

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