1 Introduction

Sulfidic mine tailings need to be covered in order to prevent oxygen and water from reacting with pyrite (FeS2), forming acidic and metal-rich drainage water (Holmström 2000). Because large areas need to be treated, wood fly ashes from power heating stations and sewage sludge from waste water treatment plants are suggested as replacements for soil as cover material. The fly ash can form durable sealing layers due to its self-hardening properties (Steenari et al. 1999) and can possibly prevent the formation of acidic drainage water due to its high alkalinity. Sewage sludge contains nutrients and organic matter and can be used either as an organic amendment supporting the establishment of vegetation (Sopper 1993) or as a barrier against oxygen diffusion to the mine tailings (Peppas et al. 2000). However, both wood fly ash and sewage sludge contain metals (Steenari et al. 1999; Wang et al. 2005), and sewage sludge contains high levels of nitrogen and phosphorus (Petersen et al. 2003), which may leach from the system and harm the environment.

Plants can be utilized for phytostabilization of contaminated soils (Pilon-Smits 2005) and can be used as part of the treatment of mine tailings (Clemensson-Lindell et al. 1992; Elander et al. 1998) or be used for the prevention of nutrient leakage from sewage sludge (Hasselgren 1998; Santibáñez et al. 2007). However, the combination of sewage sludge, fly ash, and mine tailings leads to complex environments that can be affected by plant roots differently from each material on its own. In studies where plants have been used for the treatment of mine tailings, including fly ash and/or sewage sludge, the focus has often been on metal uptake in the plants (Stjernman Forsberg and Ledin 2006) or the beneficial effects of the cover material on plant growth (Theodoratos et al. 2000; Brown et al. 2003). Fewer studies have addressed the effects of plants on the release of metals and nutrients from the materials. Plants can strongly affect the material in which they are growing (e.g., reviewed by Gregory 2006), e.g., by changing the pH (Stoltz and Greger 2002; Neuschütz et al. 2006), resulting in an often considerably higher availability of elements in the rhizosphere than in the bulk soil (Séguin et al. 2004). The uptake of metals by roots can not only decrease the metal levels in the soil (Greger and Landberg 1999) but can also cause a transfer of metals to grazing animals if the transport of metals from roots to shoots is high. Plants can further affect the mobility of elements by taking up water and thereby changing the oxygen conditions in the growth material.

The leakage of elements may also be affected by the order in which the different cover materials are applied at the tailings or whether the fly ash is constructed as a compacted layer or spread in a loose form. Fly ash (from coal combustion) has been found to exert a preventive effect on metal leaching from mine waste by causing metal precipitation (Pérez-López et al. 2007) or alkalinization or adsorption of metals to Al and Fe (hydro)oxides (Kumpiene et al. 2007) after mixing with the mine tailings. The results of experiments involving the addition of sewage sludge to mine tailings are more varied. Some researchers have found that the sludge can prevent metal leaching, by forming physical and chemical barriers against oxygen diffusion (Peppas et al. 2000), causing metals to precipitate (Gibert et al. 2004) or bind to organic matter (Holtzclaw et al. 1978). Others have reported that sewage sludge addition results in increased leakage of metals due to the reduction of Fe-(oxy)-hydroxides by organic acids (Ribet et al. 1995) or stimulation of iron-oxidizing bacteria (Cravotta 1998). The nutrient leaching from the sewage sludge, in turn, can be substantial (Stehouwer et al. 2006), but can be decreased if the sludge is mixed with the mine tailings (Santibáñez et al. 2007) or fly ash (Topaç et al. 2008). Mixtures of fly ash and sewage sludge have also been used to form a dense sealing layer that can prevent oxygen diffusion to the tailings (Hallberg et al. 2005). However, the addition of sewage sludge to sealing layers of fly ash can result in increased root penetration down to the mine tailings (Neuschütz et al. 2006); thus, it may be more suitable to apply the materials as separate layers.

The aim of this study is to explore how vegetation affects the metal and nutrient content of the leachate from mine tailings covered with layers of fly ash and sewage sludge. The energy crop Phalaris arundinacea L. (reed canary grass) was used since it has been shown to be able to establish and grow in sewage sludge on top of mine tailings (Evanylo et al. 2005). Different models were tested with either ash or sludge as the first covering material in direct contact with the mine tailings and with fly ash applied as a compact or porous layer. The hypothesis was that plants would decrease the release of metals and nutrients and that the addition of ash, compact or not, close to the tailings would result in lower metal release than conditions where sludge is placed in direct contact with the tailings. This was tested in two greenhouse experiments, lasting for 10 or 12 weeks. Additionally, the uptake of metals in energy grass that had been grown in sludge on top of ash and/or mine tailings was analyzed to see if metal accumulation could pose a risk to grazing animals.

2 Materials and Methods

2.1 Sewage Sludge, Fly Ashes, and Mine Tailings

The properties of the sewage sludge, fly ash, and mine tailings used in the experiments are given in Table 1. The sewage sludge (anaerobic digested and dewatered sludge) was collected on two occasions from a wastewater treatment plant in Stockholm using iron sulfate as a precipitation agent. To facilitate the establishment of plants, the sludge was spread on a plastic sheet, kept in darkness for 3 weeks, mixed manually, and sieved (<10 mm) before planting. The sludge used in experiment 1 had been stored in sealed plastic buckets for 5 months. Fly ash was obtained from two paperboard factories in Sweden that incinerate biofuel in circulating fluidized bed boilers. The mine waste was collected at Gillervattnet mine tailings impoundment in Boliden, Sweden. For the first experiment, unweathered mine tailings collected below the weathered surface at the tailings impoundments were used. For the second experiment, partly weathered tailings collected 5 to 10 cm below the surface were used. The ashes and mine tailings were homogenized manually and sieved (<2 mm) before the experiments.

Table 1 Characteristics of the sewage sludge (n = 3–6), wood fly ashes, and mine tailings (n = 1) used in experiments 1 and 2
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2.2 Plant Material and Growth Conditions

The perennial grass P. arundinacea (reed canary grass; seeds from Svalöf Weibull, Sweden) was used since this plant has shown to grow vigorously in sewage sludge (Evanylo et al. 2005) from year to year even at latitudes with long periods of frost, e.g., at Boliden (64°5′ N, 20°2′ E), Sweden (unpublished data). Therefore, the grass may be suitable as stabilizing plant on covered mine tailings impoundments that often offer harsh growth conditions. Furthermore, due to its strong growth, the grass can be used as biofuel crop (Venendaal et al. 1997) and become of economical value. In the experiments, two varieties of P. arundinacea were used (cv Bamse for experiment 1 and cv Palaton for experiment 2) since the same variety was not available at start of both experiments. The experiments were performed in a greenhouse with a day/night temperature of 19°C/17°C and 18 h of illumination with light intensity of minimum 120 µmol m−2 s−1. These conditions were chosen to stimulate a high growth of the grass and do only occur during summer in the areas where mine tailing impoundments are situated in Sweden. No experiments were performed under conditions with lower temperatures.

2.3 Experiment 1

In this experiment, the effect of plants on the leachate and the uptake of metals from sewage sludge on top of thin porous layers of fly ash and/or mine tailings were studied. Containers with a volume of 1 L were constructed using semitransparent polyethylene (PEHD) plastic bottles (Witre, Mölndal, Sweden) without bottoms and inverted (Fig. 1). Transparent polypropylene funnels covered with a polyamide filter, pore size 25 µm (Sintab, Oxie, Sweden) fixed with glass silicon (Casco, Sweden), were mounted to collect drainage water. A 0.20-m long silicon tube (8 mm inside diameter) was connected to the funnel and used for regulating the water flow from the containers. All materials were acid-washed (5% HNO3) before use. To prevent formation of algae in the sludge, the containers were covered with black plastic.

Fig. 1
figure 1

Schematic picture of containers used in experiment 1, with sewage sludge, with or without fly ash and/or mine tailings

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Six replicate containers were prepared with and without plants and with the following treatments: (1) sludge, (2) sludge on top of ash, (3) sludge on top of tailings, or (4) sludge on top of ash on tailings. The dry weight (and depth) of the material in each container was as follows: sewage sludge, 100 g (12 to 14 cm); fly ash, 40 g (1.5 to 2.0 cm); and mine tailings, 70 g (1.0 to 1.5 cm). Before the start of the experiment, the fly ash was mixed with water (1:1 w/w), cured for 4 weeks, crushed in a mortar, and sieved (<2 mm). The mine tailings were dried at 75°C for 10 h before adding to the containers. After the addition of the materials, 4-week-old seedlings of P. arundinacea were transferred to half of the containers. The seedlings used for each container originated from 1.0 g of seeds and had at the time of transfer an approximate shoot biomass of 1.2 g (dry weight).

During the experiment, the containers were watered three times a week with 100 mL deionized water, for a total of 3.0 L to each container. The tubes at the bottom were fixed with their openings 3 cm above the bottom of the containers to slow down the water percolation rate. After 3 and 10 weeks, samples of leachates were collected after the containers had been given 200 mL of deionized water, which had been kept in the containers for 60 min by closing the tubes. The collected samples were filtered through 0.45-µm syringe filters (Filtropur S, Sarstedt, Germany) poured in plastic bottles and analyzed for pH and electrical conductivity. Samples for nutrient analysis were stored at −20°C, and those for metal analyses were stored at 4°C after addition of HNO3 (5 µL mL−1).

After 10 weeks, the experiment was terminated, and the plants were harvested. At this stage, the dry biomass of plant shoots in each container had increased by about a factor of 10 since the start of the experiment. Samples of sewage sludge were taken from the middle of the containers, where sludge had been without contact with ash or tailings. The sludge was weighed and either dried at 105°C for 72 h (to constant weight) for determination of water, metal, and P content, at 60°C for analysis of N and C content, or at 21°C for analysis of pH and conductivity. The plant roots were thoroughly washed with deionized water over a sieve (<2 mm). Shoots and roots were washed with 20 mM ethylene-diamine-tetraacetic acid for 5 s, rinsed two times with deionized water, and dried at 105°C for 24 h for determination of dry weight and later analysis of metal content. For analysis of metals, roots that had been in contact with sewage sludge, but not ash or tailings were collected.

2.4 Experiment 2

In this experiment, the effect of P. arundinacea on the composition of the leachate from two different covering techniques were tested, in which either sludge or ash was placed in direct contact with the mine tailings. For this purpose, boxes made of transparent polypropylene, with a volume of 3 L (0.14 m width, 0.20 m length, 0.17 m height; Hammarplast AB, Sweden) were used (Fig. 2). On one of the short walls, three triangular holes (height 0.87 cm) were made 1.8 cm above the bottom, with 1.0 cm distance between them. Inside the holes, a filter of polyamide with 25 µm pore size (Sintab Produkt AB, Oxie, Sweden) and a layer of glass wool (Merck, Darmstadt, Germany) were mounted. The boxes were placed in non-transparent boxes of the same size, with a 3-cm space left at the bottom in which drainage water could be collected. The space between the boxes at the rim was sealed with polystyrene. The following treatments were prepared, each in four replicates: (1) mine tailings without cover; (2) tailings covered with a two-layer cover of ash and sludge, with or without plants; and (3) tailings covered with a three-layer cover of sludge, ash, and sludge, with or without plants. At the bottom of all boxes, a 2-cm-deep layer (290 g dry weight) of partly weathered mine tailings was placed. The ash layer was made of 230 g (dry weight) fly ash mixed with 250 mL deionized water. The amount of sludge was 345 g (dry weight), applied in one or two layers. In treatments with plants, 2.0 g of P. arundinacea seeds was sown in the sewage sludge at the start of the experiment.

Fig. 2
figure 2

Schematic picture of the containers used in experiment 2 and the two different cover techniques with sewage sludge and fly ash on top of mine tailings. The depth of each layer is given in centimeter

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Three times a week, the boxes were watered with deionized water; the amount was gradually increased from 83 to 167 mL during the experimental time to avoid desiccation. In total, 4.9 L was added to each container during the experiment, corresponding to a yearly precipitation of 758 mm, a value that is within the range of Swedish conditions (SMHI 2008). Once every second week, the leachate was collected, weighed, and analyzed for pH, poured in plastic bottles, acidified with concentrated HNO3 (5 µL mL−1), and stored at 4°C. Metal analysis was performed on samples collected after 6 and 12 weeks. The experiment was terminated after 12 weeks. Plant roots were carefully separated and washed, and the roots and shoots were dried at 105°C for determination of dry weight. Sewage sludge from the pots was weighed, homogenized, and dried at 105°C for 72 h (to constant weight) for determination of water content. The penetration resistance of the ash layers was analyzed with a cone penetrometer (semiangle, 15° and diameter, 4.1 mm) driven 10 mm down into the material.

2.5 Chemical Analysis of Substrates

Analysis of pH (Metrohm 744 pH Meter) and electrical conductivity (Schott Handylab Multi 12) was performed after air drying (48 h) and sieving (<2 mm) of the materials, which were then shaken with water at a solid/liquid ratio of 1:5 (v/v) for 5 min and left standing for 5 h. Total N was analyzed in sludge (<2 mm) via high-temperature combustion (950 ºC) and thermal conductivity detection (LECO CHNS 932, St Joseph, MI, USA). Total P was determined after the sludge samples had been ignited at 550°C for 2 h, shaken for 16 h with 0.5 M H2SO4, filtered, and analyzed spectrophotometrically at 680 nm for PO4–P after reaction with AmMo, SnCl2, and ascorbic acid according to a modified Fiske–Subbarow method (Lindeman 1958). The organic matter of the sludge was determined as loss on ignition after 2-h combustion at 550°C and for fly ash and mine tailings at 1,000°C (the latter analysis was performed by Analytica AB). For metal analysis, dried plant tissues from experiment 1 were wet-digested in HNO3 and HClO4 (7:3, v/v) in a 20-h heating program reaching 225°C. The reference material Energy grass (NJV 94-4; from the Swedish University of Agricultural Sciences) was used to validate the digestion procedure and the metal analysis. Dried sludge was wet-digested in 7 M HNO3 for 30 min at 120°C before dilution and analysis of metals. The metal analysis of sludge and plant tissues from experiment 1 was performed with an atomic absorption spectrophotometer (Varian SpectraAA-100), with a flame technique for Zn and a furnace for Cd and Cu. Standards were added for each sample to eliminate the interaction of the sample matrix. The metal content of sludge used in experiment 2 was analyzed by the accredited laboratory at Stockholm Vatten, Sweden, by means of ICP-AES (inductively coupled plasma-atomic emission spectrometry). The metal and S content in the fly ash and mine tailings were analyzed by the accredited laboratory Analytica AB, by means of ICP-AES, or inductively coupled plasma sector field mass.

2.6 Chemical Analysis of Leachate

Analysis of pH and electrical conductivity was performed in leachate samples the same day that they were taken. Metals were analyzed with an atomic absorption spectrophotometer (Varian SpectraAA-100), with a flame technique for Zn and a furnace for Cd and Cu. Nutrient analysis was performed spectrophotometrically at 640 nm for NH4–N at 220 nm for NO3–N (APHA, AWWA, WEF 1995) and at 880 nm for PO4–P (Murphy and Riley 1962).

2.7 Statistical Analysis

Statistical calculations were performed using the software Statistica version 8.0 (StatSoft Inc. 2008). Differences between treatments were analyzed by ANOVA and multiple comparisons of means using the Tukey honestly significant differences test at the 5% significance level. If the data were not normally distributed, it was either log-transformed or analyzed using the non-parametric Kruskal–Wallis ANOVA. Correlations were analyzed using Spearman’s rank correlation.

3 Results and Discussion

3.1 Effect of Plants on Metal and Nutrient Leaching

The effect of plants on the leachate from mine tailings depended on how the cover was constructed. The data from the two experiments cannot be directly compared with each other due to differences in experimental setup and used materials. However, conclusions from each experiment can be drawn. In our first experiment, where crushed fly ash was applied as a porous layer between the mine tailings and the sewage sludge, the presence of plants caused high metal levels in the leachate at the end of the experiment (Table 2). In the second experiment, where the ash was cured and compacted in the boxes, no effects of plants could be observed on the concentrations of metals in the leachate (Fig. 3). Instead, in this case, the plants had a preventive effect on metal leaching by decreasing the total amount of drainage water (Fig. 3). A probable explanation of this difference in performance between the two experiments is that in experiment 1, the porous ash layer allowed roots and air to move down into the mine tailings, causing increased weathering of the tailings, while in experiment 2, the compacted fly ash effectively sealed the mine tailings. The penetration resistance of the sealing layers in experiment 2 ranged between 2.6 and 5.0 MPa, values that are considered high enough to restrict root growth in most soils (Bengough and Mullins 1990; SEPA 1999). The fly ash layer used in experiment 1 was too porous to be measured.

Table 2 Experiment 1: electrical conductivity (EC) and concentrations of dissolved Cd, Cu, and Zn in leachate from sewage sludge with (+) or without (−) plants on top of fly ash and/or mine tailings
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Fig. 3
figure 3

Experiment 2: metal concentrations and pH in leachate and amount of leachate collected during weeks 5–6 and 11–12, respectively, from boxes with mine tailings with or without two types of covers and with or without plants (Phalaris arundinacea). The covers consisted either of fly ash covered with sewage sludge (sludge/ash) or of a three-layer cover of sludge/ash/sludge (n = 4, ±SE). Bars with different letters (a–c) differ significantly (p < 0.05) from each other

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In experiment 1, the high metal concentrations in the leachate collected at week 10 from containers with sludge over top of ash and tailings was accompanied by decreased pH (Table 3) and increased electrical conductivity (Table 2). This supports the theory that the mine tailings in these containers had started weathering, causing a release of sulfates, hydrogen, and metal ions (Rimstidt and Vaughan 2003). In leachate collected at week 3, there were no such trends concerning effects of plants (not shown). In those samples, the levels of Cd and Zn from both planted and unplanted containers were similar to those from unplanted containers collected at week 10 (Table 2), while Cu levels and pH were slightly, but not significantly, higher. Unfortunately, leachate from containers with only mine tailings and sewage sludge could only be collected at the first sampling occasion, after which the water flow through the containers was stopped by the clogging of the filter. In planted containers with sludge on top of tailings, however, there was a general trend of lower metal concentrations in the leachate compared with those that also included an ash layer (Table 2). This could be a result of a lower growth of plants when no ash was included (Table 4), resulting in a decreased water uptake and thereby increased water content in the sludge, preventing oxygen from reaching the mine tailings. The water content of the sludge correlated negatively with the biomass of the shoots (r = −0.84) and roots (r = −0.91) of the plants, as well as with the concentration of Cu (but not Cd and Zn) in the leachate (r = −0.49). In general, the water content of the sludge at the end of the experiment was high in all unplanted treatments and ranged between 783 and 807 mg kg−1 (based on the wet weight of the sludge), compared with a water content of 366 to 656 mg kg−1 in sludge where plants had been growing. Another factor that may have had an impact on metal release from the mine tailings is root growth into the mine tailings, which was observed in containers both with and without a fly ash layer. The presence of roots is known to affect the solubility of elements in soil; for instance, organic acids found in root exudates have been demonstrated to have a mobilizing effect on metals in mine tailings (Burckhard et al. 1995).

Table 3 Experiment 1: pH and concentrations of ammonium–N, nitrate–N, and phosphate–P in leachate from sewage sludge with (+) or without (−) plants on top of fly ash and/or mine tailings
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Table 4 Experiment 1: biomass and concentrations of metals in shoots and roots of P. arundinacea after 10 weeks of growth in sewage sludge on top of fly ash and/or mine tailings (n = 6–7 ± SE)
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The analysis of nutrient concentrations in the leachate at the end of the first experiment revealed that plants, at least at this stage, had a preventive effect on nutrient release, in particular when fly ash was present (Table 3). This could be a combined result of nutrient uptake by the plants and adsorption processes in the fly ash. In a study by Topaç et al. (2008), fly ash was found to act as a strong adsorbent for nutrients in wastewater. In contrast to the Cu levels in the leachate, the concentrations of ammonium, nitrate, and phosphate in the leachate correlated positively with the water content of the sludge at the end of the experiment (r = 0.87, r = 0.35, and r = 0.60, respectively), possibly indicating a relationship between water uptake and uptake of nutrients. Although no effects of plants were observed on the P, N, or C content of the sludge (not shown), the biomass of the plants positively correlated with the C/N ratio (r = 0.75), suggesting that increased growth of plants causes a lower proportion of N in the sludge by taking up N or by affecting the decomposition of the sludge. The content of total N in the sludge decreased during the course of the experiment in all treatments from 38.5 to 19–30 mg kg−1 (dry weight), and the C/N ratio increased from 6.5 to 8.6–9.0, while the P content did not change.

3.2 Metal Uptake in Plants

The levels of Cu and Zn, but not Cd, in the plant shoots reached the maximum tolerable levels for some foraging animals (e.g., sheep), which are (in mg kg−1 dry weight) 15 for Cu, 300 for Zn, and 10 for Cd, especially in the treatment with sludge applied on mine tailings without fly ash (Table 4). Metal levels stayed well below recommended limits for other animals such as cattle and swine (NRC 2005). These levels are recommendations for domestic animals during chronic exposure, assuming they have no other feed, and it is not likely that wild animals will base their entire food consumption on vegetation from one site. Therefore, the risk for toxic effects in grazing animals seems to be small, which is supported by the results from a study where no adverse effects were found in young calves that had been fed for 96 days with plants containing similar or higher metal concentrations as in the present study after growth in sewage sludge on mine waste (Stuczynski et al. 2007). The addition of fly ash below the sewage sludge had a preventive effect on Cu and Zn uptake in plants and at the same time stimulated both root and shoot growth of the plants (Table 4). The addition of ash may have increased the pH of the sludge, which could result in decreased availability of Cd, Cu, and Zn (Villar and Garcia 2002 and Crommentuijn et al. 1997). However, in our experiment, the total uptake of metals was significantly (p < 0.05) higher for all three metals in the roots, and for Cu in the shoots, in plants grown in sludge on ash compared with the cases where no ash was included (not shown). This indicates that the ash did not prevent uptake of metals, but rather stimulated growth to such an extent that the metal content in the plants became diluted. One effect of ash addition may be a reduced toxicity of NH4–N and salts in the sludge, as has been suggested by Wong and Su (1997) who saw an increase in seedlings emergence and biomass of Agropyron elongatum when (coal-) fly ash was added to sewage sludge compared with cultivation in only sludge. No effects of plants were observed on the total concentrations of metals in the sewage sludge (not shown).

3.3 Fly Ash or Sewage Sludge in Direct Contact with the Mine Tailings

In this short-term perspective of 3 months, it was obvious that the leakage of metals could be decreased if mine tailings were covered with fly ash and sewage sludge compared with those left uncovered (Fig. 3). The low pH in leachate from uncovered tailings indicated that a weathering process was taking place. When comparing the two different cover techniques, using either ash (two-layer cover) or sludge (three-layer cover) in direct contact with the mine tailings, there were, however, no differences concerning metal concentrations in the leachate. The pH in the leachate from these treatments was close to neutral and did not follow a clear pattern over time (Fig. 3). Nevertheless, the amount of leachate was lower when a two-layer cover with plants was used, compared with a three-layer cover. This was likely due to the layer of sewage sludge being deeper in the two-layer cover (Fig. 2), providing more nutrients and space for the plant roots and improved conditions for water uptake. The biomass (dry weight) at harvest of plants from the two-layer cover (roots, n = 4, ±SE 3.2 ± 0.2 g and shoots 11.3 ± 0.8 g) was also significantly higher compared with that of plants from the three-layer cover (roots 1.5 ± 0.3 g and shoots 5.8 ± 1.0 g).

In experiment 1, the metal concentrations tended to be higher in the leachate when ash was placed between the tailings and the vegetated sludge (Table 2). This contradicts the results from other studies where fly ash was found to prevent leakage of metals from mine waste (Kumpiene et al. 2007; Pérez-López et al. 2007). This difference may be due to the use in those studies of another type of fly ash, from combustion of coal instead of wood, and from the mixing of the fly ash with the mine waste, instead of applying it as a separate layer. An additional explanation for the results in the present study is that the growth of plants significantly increased when fly ash was present (Table 4), causing a higher water uptake by plants, a subsequent dryer cover material, and increased access of oxygen to the mine tailings.

4 Conclusions

The use of P. arundinacea in phytostabilization of a dry cover of sewage sludge and fly ash on mine tailings can prevent leakage of metals and nutrients, mainly by decreasing the amount of leachate. However, the construction of the cover is important. When a porous layer of fly ash was used between the mine tailings and sewage sludge, entry of plant roots and air into the tailings was possible, causing high metal concentrations in the leachate. If the fly ash was cured and compacted to prevent root growth, both a cover with fly ash in direct contact with the mine tailings and one with sludge/ash/sludge in a three-layer cover efficiently decreased the metal leakage compared with leaving mine tailings uncovered. The presence of fly ash was beneficial in that it was able to decrease the nutrient leakage and increase the biomass of plants, which can lead to increased water uptake and decreased amounts of leachate. These results should be confirmed by larger-scale and longer-term studies in field in which the efficiency of the system all around the year is considered. However, the results do suggest that a two-layer cover consisting of fly ash and sewage sludge is suitable as a cover and plant substrate in stabilization of mine tailings. It is important, though, that the ash layer is constructed using fly ash with a high ability to cure and create a dense sealing layer, preventing penetration by roots, water and air, and that the cover of sewage sludge is deep enough to hold water and support vegetation.