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TECHNICAL REPORTS

Waste Management

The Disposal of a Lime Water Treatment Residue on Soil and Spoil Material from a Coalmine

A Glasshouse Investigation

^a Soil Science, School of Environmental Sciences, Univ. of KwaZulu-Natal, Private Bag X01, Scottsville, 3209, South Africa
^b Agricultural Research Council, c/o Univ. of KwaZulu-Natal, Private Bag X01, Scottsville, 3209, South Africa
^c College of Agriculture, Engineering and Science, Univ. of KwaZulu-Natal, Private Bag X01, Scottsville, 3209, South Africa

^* Corresponding author (hughesj{at}ukzn.ac.za)

Received for publication July 10, 2006.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION DISCUSSION CONCLUSIONS REFERENCES

 
Eragrostis tef (Zucc.), Cenchrus ciliaris L., and Digitaria eriantha Steud. were grown in a soil (Psammentic Haplustalf) and spoil material from a coalmine both treated with a lime water treatment residue (WTR) at rates of 0, 50, 100, 200, and 400 g kg^–1. The yield of the grasses, from the sum of the three harvests, and concentrations of B, Ca, Cu, K, Fe, Mg, Mn, N, Na, P, and Zn in foliage from the second harvest were determined. The yield of grasses grown in the soil decreased exponentially as WTR application increased. The yields of C. ciliaris, D. eriantha, and E. tef (in the 400 g kg^–1 WTR treated soil) decreased by 74.4, 78.7, and 59.8%, respectively, when compared with the control treatments. In the spoil, the yield of E. tef and D. eriantha decreased by 13.6% and and D. eriantha by 23.9%, while an increase was observed for C. ciliaris (45.4%), at the highest WTR application rate. No relationship existed between yield of E. tef and WTR application rate when grown in the spoil, while a weak negative linear relationship (p < 0.05) was found for D. eriantha and a positive linear relationship existed for C. ciliaris. Magnesium concentrations of the grasses were positively correlated to WTR application rate. Grasses grown in the soil had higher Na concentrations, while those grown in the spoil typically had higher B, N, and Zn concentrations. The decreases in yield were attributed to nutrient deficiencies (notably Zn), induced by high WTR application rates that led to high substrate pH. Disposal of high rates of WTR on the mine materials was not recommended.

Abbreviations: ANOVA, analysis of variance • CCE, calcium carbonate equivalence • CEC, cation exchange capacity • DAE, days after establishment • EC, electrical conductivity • OC, organic carbon • RDA, redundancy analysis • WTR, water treatment residue

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION DISCUSSION CONCLUSIONS REFERENCES

 
THE production of potable water from turbid primary sources requires the removal of suspended and dissolved solids, organic matter, and other contaminants. The resulting material has been called "water treatment sludge" but this term is too similar to "wastewater treatment sludge" (otherwise known as sewage sludge or biosolids) from which it differs radically. The term "water treatment residue" (WTR) is similar to the term "water treatment residual" used by some authors, but the simpler form is preferred here. Landfill has been the traditional method of disposal for WTR but increasingly, due to constraints of space and economics, land disposal (sometimes called land treatment) is becoming the preferred disposal method (Basta, 2000).

The use of industrial byproducts to ameliorate mine wastes for plant growth and simultaneously to act as a disposal option for the byproducts has been reported extensively in the literature. Sopper (1992) reviewed a number of studies that considered the use of sewage sludge to improve soil properties and plant growth on mine dumps. Other materials used on mine tailings have included fly ash (Taylor and Schuman, 1988; Welden et al., 1999; Bhumbla et al., 2000; Seoane and Leiros, 2001); sawdust (Roberts et al., 1988); and manures and papermill sludges (Haering et al., 2000).

Very few studies have examined the use of WTR on mined land. Dayton and Basta (2001) considered the use of WTR as a soil substitute and the potential of WTR to aid in mined land reclamation; however, they did not specifically test the effects of applying WTR to mined land. Van Rensburg and Morgenthal (2003) examined the alkalizing effect of a lime WTR on a variety of acid mine waste materials. They found that the lime WTR had a strong acid-neutralizing capacity. They also reported on volunteer plants that germinated in the pots used in their study and preliminary germination studies. However, they did not indicate the effect of WTR on plant growth or nutrient uptake.

A number of studies have considered the effect of various types of WTR on plant growth and nutrient uptake, but these have focused on agricultural applications. Rengasamy et al. (1980) found that maize (Zea mays L.) grown in WTR-amended soil gave the highest yield at WTR application rates of 2 Mg ha^–1, while the 20 Mg ha^–1 treatment had a yield intermediate to the control and the 2 Mg ha^–1 treatment. Bugbee and Frink (1985) tested the growth of lettuce (Lactuca sativa) and marigolds (Tagetes cv. Lemondrop) in WTR-amended potting materials. They found that lettuce showed P deficiencies when grown in WTR-amended material. The marigolds had reduced yields at higher rates of WTR. A concurrent study examined the impact of liquid WTR applied to forested land (Bugbee and Frink, 1985). While the pH of the soil increased by 0.5 to 1.0 pH unit, no effect was measured on tree growth or nutrient uptake. They concluded that the WTR they tested was not toxic to plants, but may lead to P deficiencies at high application rates under glasshouse conditions.

Elliott and Singer (1988) tested the growth of tomato (Lycospersicum esculentum) in an acid silt loam soil treated with an Fe-WTR at rates of 0 to 100 g kg^–1. They found that the WTR increased soil pH from 5.3 in the control to 8.0 in the highest WTR-amended treatment. This was attributed to the high pH and moderate to high calcium carbonate equivalence (CCE) of the WTR (9.3 and 53.0%, respectively). A reduction in uptake of Cd, Zn, Cu, and Ni by the tomato shoots was observed and was attributed to the high pH. Heil and Barbarick (1989) tested the growth of sorghum-sudangrass [Sorghum bicolor L. Moench ‘NB280S’–S. sudanese (Piper) Stapf] in alum and Fe-WTR amended soils. They found that low application rates (5 and 10 g kg^–1) of either gave the highest yields, without causing P deficiencies. They also found that Fe deficiencies were corrected when low rates of Fe-WTR and higher rates of alum WTR were used on a calcareous soil. Trace metal concentrations were not considered to be high.

Lucas et al. (1994) found that alum WTR reduced tall fescue (Festuca arundinacea) growth in a greenhouse study. Although they found WTR additions led to P deficiencies, these were overcome by doubling the recommended P fertilization rate. They reported that Mn and Cu concentrations were elevated in plant tissue at the higher WTR rates, but neither this nor the resulting reduced yield was cause for concern. Notably, while pot experiments may indicate how the WTR may affect plant growth, these results are not always directly transferable to field conditions.

Skene et al. (1995) compared the growth and elemental composition of the foliage of broad beans (Phaseolus vulgaris) grown in a soil treated with either an alum or a polymer WTR applied at rates of 20, 40, and 100 g kg^–1. They concluded that these WTRs may have potential as growth media, but would require fertilization for optimal plant growth. They found that the polymer WTR supplied some macronutrients and did not immobilize P as much as the alum WTR. Basta et al. (2000) examined P uptake by bermudagrass (Cynodon dactylon) grown in three different WTR-amended soils. They found that an addition of 200 mg P kg^–1 did not increase P availability in the WTRs, but it did increase plant concentrations. They concluded that the WTRs exhibited behavior similar to allophanic soils that require large P additions to improve the growth of crops.

Ahmed et al. (1997) conducted laboratory, glasshouse, and field studies and found that the alum WTR used resulted in no detrimental impacts on soil properties. They also reported on a number of favorable characteristics, including increased N availability, neutral to alkaline pH, and reasonable CCE. They found that the yield of "lawngrass" generally increased with WTR application up to rates of 1600 Mg ha^–1 in both pot and field experiments. They did note, however, that in the pot experiment a rate of 800 Mg ha^–1 and in the field experiment a rate of 1600 Mg ha^–1 inhibited seed germination. This was attributed to the coarse nature of the WTR, but they suggested that the self-mulching nature of the material would cause rapid breakdown into finer particles. This would possibly improve seed contact with the soil and improve germination. This is in line with the comments of Skene et al. (1995) and Hughes et al. (2005). Ahmed et al. (1997) also found that, in the pot experiment, P concentrations in plant tissue decreased with increasing application rates of WTR, but this was not evident in the field. They concluded that, while in the short-term WTR additions could lead to P deficiencies, the additions did not appear to present any long-term P deficiency problems in the field. They also report on concerns about Al toxicity, but found that background soil concentrations were above those of the WTR examined. Elliott and Dempsey (1991) suggested that, as soil Al concentrations are typically high, the factors controlling Al solubility (such as pH) will determine potential toxicity. As many WTRs are reported to increase pH in the soil (due to some liming potential), the potential for Al toxicity is low.

Geertsema et al. (1994) examined the effect of an alum WTR applied to pine plantation research plots. They found that it had no effect on soil, ground water, or growth of pine trees 30 mo after application, although they noted that in the short-term (8 mo after WTR application) N levels in the ground water were elevated over the control treatment. They also indicated that, while short-term concerns regarding P deficiencies are reported, it is unlikely to be problematic in the longer term, especially at the field scale.

In general these studies found that small amounts of WTR can be added to soils without any serious effect on either soil chemical properties or the crop. Problems reported have included the possibility of inducing P deficiency and elevated N concentrations in drainage water. A positive effect has been a reduction in uptake of heavy metals by plants that has generally been explained by the increase in pH as a result of the addition of WTR.

In some instances, land disposal offers a potentially viable option for discarding a waste to land without adversely impacting the land. Furthermore, the land-applied waste may potentially improve the properties of the substrate, especially if applied to disturbed or degraded land. In this instance, the application of the WTR onto the mine materials may be a suitable disposal option, reducing the need to lagoon or store large quantities of waste WTR. Disposal of WTR at a coalmine will only be possible if the application of WTR has either no impact or is beneficial, assisting in reclamation work. The mine has a net acid-producing potential, thus a long-term benefit of applying a lime WTR may be to reduce acid mine drainage. The close proximity of the water treatment facility to the coalmine also enhanced the proposition of cost-effective disposal of the WTR. This investigation examined the effects of applying high rates of a lime WTR on selected overburden materials from a coalmine and on the growth and nutrient uptake of three grass species under glasshouse conditions. Biomass production of the grasses was measured and compared using regression analysis, while differences in nutrient concentrations of the grasses were assessed using a multivariate statistical technique.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION DISCUSSION CONCLUSIONS REFERENCES

 
Material Collection and Preparation
The materials collected from the coalmine [Vereeniging, South Africa (26°70' S; 28°00' E)] consisted of a sandy material (soil) removed before mining (3 to 6 m in depth) and the overburden material from above the coal seams that consisted of shale interspersed with calcareous nodules and coal fragments (spoil). While the mine termed the sandy soil "topsoil" (which was stockpiled for use in post-mining reclamation), it consisted of a mixture of A and B horizon material. Pre-stripping, the soil was classified as a Psammentic Haplustalf (Soil Survey Staff, 2006). Dry WTR was collected from Rand Water's disposal site (Panfontein, Vereeniging). The materials were air-dried, and the WTR and soil mechanically milled and sieved (<2 mm). The bulk of the spoil material consisted of large fragments (>20 mm) and was crushed to pass through an 8.5-mm sieve for the pot experiment; a subsample was crushed to <2 mm for laboratory analysis. This size fraction (8.5 mm for the spoil) was selected as a compromise between practicality of use in the pot experiment and ease at which the material could be reduced in coarseness.

Chemical and Physical Analyses
All laboratory analyses were conducted on the <2-mm material. Particle size distribution was determined by the pipette method (Gee and Bauder, 1986). The pH was measured in distilled water and 1 M KCl (1:2.5 soil/solution ratio) using a Radiometer PHM210 pH meter with a standard glass electrode. Electrical conductivity (EC) was measured at 25°C using a Radiometer CDM83 electrical conductivity meter in a 1:5 WTR/water suspension (U.S. Salinity Laboratory Staff, 1954). Extractable bases were measured by saturating with Sr²⁺ and cation exchange capacity (CEC) by subsequent replacement with NH₄⁺ (Hughes and Girdlestone, 1994). Extracts were analyzed by atomic absorption spectrophotometry (AAS, Varian SpectraAA-200), using the manufacturer's recommended working parameters. Total N was determined on <0.5-mm samples using a LECO combustion analyzer (LECO, St. Joseph, MI). Nitrate and ammonium were extracted with 2 M KCl (Maynard and Kalra, 1993), and solution concentrations were determined colorimetrically using a TRAACS 2000 continuous flow auto-analyser (IRAMA, Milwaukie, OR). Plant-available P was extracted with ammonium bicarbonate (0.25 M buffered at pH 8.0) solution and determined colorimetrically (The Non-Affiliated Soil Analysis Work Committee, 1990) on a Varian Cary 1E UV-Visible spectrophotometer (Varian, Cary, NC). Exchangeable acidity and exchangeable Al were extracted according to Sims (1996), with Al being measured by AAS. Organic carbon (OC) was digested by potassium dichromate oxidation and determined titrimetrically (Walkley, 1947). Calcium carbonate equivalence of the WTR was determined according to Jackson (1958). Total elemental composition was measured by X-ray fluorescence spectrometry. Plant available Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, and Zn were extracted with diethylene-triamine-penta-acetic acid (DTPA; Liang and Karamanos, 1993) and determined by AAS.

In addition, P adsorption isotherms were determined for the WTR, soil, and spoil. An adaptation of the method presented by Basta et al. (2000) was used. Twenty-five mL of a range of P solutions (0, 2, 4, 8, 16, and 32 mg P L^–1) were added in a 0.01 M CaCl₂ matrix to 1 g of material. The mixtures were shaken on a reciprocating shaker for 18 h, allowed to settle for a few minutes, and the supernatant filtered and analyzed for P colorimetrically (The Non-Affiliated Soil Analysis Work Committee, 1990).

The pH (in water) of fresh mixtures was also determined as described previously.

Establishment of the Pot Experiment
Plastic pots (2.8 L; 0.2 m i.d. and 0.17 m height) were filled with soil or spoil mixed with WTR at rates of 50, 100, 200, and 400 g kg^–1. In the case of the soil mixtures, the pots were filled with 3.8 kg of dry material, while 3.9 kg of the spoil mixtures were used. This gave approximate bulk densities of 1360 and 1390 kg m^–3 for the soil and spoil mixtures, respectively. These bulk densities were the natural settling densities of the material when placed in the pots. The WTR application range is equivalent to approximately 130 to 1040 Mg ha^–1. Pure soil or spoil treatments were also included (control pots), packed to the same bulk density as the mixtures. To approximate the fertilizer currently applied at the mine (i.e., 1 Mg ha^–1 of 2:3:2 N/P/K and 1 Mg ha^–1 of single superphosphate), all treatments received about 0.05 g N, 0.05 g P, and 0.16 g K per Kg spoil or soil as NH₄H₂PO₄ and KH₂PO₄.

The three grass species chosen were Digitaria eriantha Steud., Cenchrus ciliaris L., and Eragrostis tef (Zuccagni) Trotter, as they are all currently used at the mine for rehabilitation practices. Eragrostis tef is a fast growing annual. Cenchrus ciliaris and D. eriantha are slower establishing, but more persistent perennial species. Approximately 10 seeds of each grass species were germinated in each pot. Seedlings were thinned to three plants pot^–1 3 wk after germination.

The experiment was arranged in a randomized complete block design with three replicates. Each pot was placed on a collecting tray to prevent loss of sediment and water. Initially the pots were watered every 2 to 3 d with the frequency of watering increasing as plants grew larger and evapotranspiration increased. The mean glasshouse temperature range was from 14 to 28°C (min = 10°C, max = 31°C).

Grass Harvests
The plants were harvested three times, and the total mean yield determined by summing the yields of individual harvests. The first harvest was 45 d after establishment (DAE), the second at 115 DAE, and the final harvest at 150 DAE (for C. ciliaris and D. eriantha). The E. tef treatments were allowed to grow for an additional 21 d (harvested 171 DAE) because of slower growth than the other two species. In the case of C. ciliaris and D. eriantha, plants were allowed to regrow after the first two harvests; but E. tef was reseeded after each harvest, as this is an annual species. The plants were cut approximately 10 mm from the substrate surface at each harvest, and the plant material was placed in paper bags and dried at 65°C for 2 d in a forced-draft oven. Dry mass yield was determined for each pot, and these data were used to calculate total yields for each pot. The pots were fertilized after each harvest with the N/P/K fertilizer.

Plant material from the second harvest was analyzed for B, Ca, Cu, K, Mg, Mn, Na, P, Pb, and Zn by the Soil Fertility and Analytical Services Laboratory (KwaZulu-Natal Department of Agriculture and Environmental Affairs, Cedara, South Africa). In brief, the method is as follows: a subsample of plant material was ashed at 450°C, the ash was dissolved in HCl and filtered, the supernatant was analyzed for Ca, Cu, K, Mg, Mn, Na, Pb, and Zn by AAS; and B and P were determined colorimetrically.

The replicates were bulked for chemical analysis because of limited sample in some instances. The remaining plant material was analyzed for total N using a LECO combustion analyzer. Harvests 1 and 3 were not considered as some treatments had insufficient sample for complete analysis.

After the final harvest, the material in each pot (with roots removed) was air-dried, mixed, and milled to pass through a 2-mm sieve. The pH (in water) of the substrates was determined as described previously.

Statistical Analyses
The effect of substrate type across WTR application rates within a grass species were compared by two-way analysis of variance (ANOVA; Genstat 8.1). The best relationships between the yield of each grass and WTR application rate, for either soil or spoil, were determined by the regression and curve fitting functions in Genstat 8.1, using the coefficient of determination (r²) as the criteria for best fit. Data were not transformed because the assumptions of normality and equality of variance, for ANOVA and regression analysis, were met.

Redundancy analysis [RDA, CANOCO 4.5 (ter Braak and milauer, 1997)] was used to examine the effects of substrate (soil or spoil), WTR application rates, and their interaction on the nutrient concentration profile of the harvested foliage of each of the grass species. Redundancy analysis is the canonical form of principal component analysis that extracts dimensions of (joint) variability in measured variables (nutrients, in this study) that are a function of specified independent variables (i.e., WTR rate and substrate) (Lep and milauer, 2003). This analysis was only performed on data from the second harvest as this was the only complete data set. Leaf nutrient concentrations were "centered" and standardized to a mean of zero and a standard deviation of one for the analysis. A Monte-Carlo permutation test (n = 500) was conducted to test the significance of the effects of WTR application rate, substrate type, and their interaction on variation in leaf nutrient concentrations among samples (ter Braak and milauer, 1997).

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION DISCUSSION CONCLUSIONS REFERENCES

 
Chemical and Physical Properties
The physicochemical properties of the WTR have been previously described by Titshall and Hughes (2005), but are included here for convenience. The soil material was acidic, whereas the spoil and WTR were alkaline (Table 1). Electrical conductivity was lowest for soil, though the values for the spoil and WTR were similar. These values are below the range reported for low-salinity irrigation water (0–40 mS m^–1; Keren, 2000).

Both the soil and spoil had high amounts of sand (>70%), with the soil also containing a moderately high amount of clay (Table 1). The WTR consisted almost entirely of clay and fine silt (62.9 and 35.9%, respectively). The particle size distribution of the spoil was probably related to the intensity of the crushing process used to break the material down to a usable size fraction. This size fraction is unlikely to reflect the particle size distribution of the spoil under field conditions. The spoil consists of excavated overburden parent material, being predominantly shale. The coarse spoil weathered rapidly to finer fragments once exposed to environmental weathering processes (personal observation). Dry sieving of the <8.5 mm spoil material used in the pot experiment showed that 31.1% of this material was >4.8 mm, 31.9% was between 2.0 and 4.8 mm, and 36.9% was <2.0 mm.

X-ray fluorescence spectrometry data (Tables 2, 3, and 4) indicate that the spoil and soil consisted mainly of Si and Al, and the WTR was dominated by Ca with considerably lower Si. Concentrations of other elements were low, except for S in the spoil. This suggests that the spoil may have acid-producing potential, especially if this element is in pyritic form (Carrucio et al., 1988). Although the WTR had much higher total Ca than Mg, extractable Mg was higher than Ca (Table 1), indicating that the Mg was in a more labile form. Plant-available metal (DTPA-extractable) concentrations are unlikely to lead to toxicity problems (Table 5).

View larger version (12K): [in this window] [in a new window]   Fig. 1. Mean total yield (± SE, three replicates) for three harvests of Eragrostis tef foliage grown in soil or spoil material treated with water treatment residue (WTR) at application rates of 0, 50, 100, 200, and 400 g kg–1. Best fit line for the soil treatment is shown (Yield = 7.54 + (17.11 x 0.983WTR rate); r2 = 0.88***). No significant (p > 0.05) relationship between yield and WTR application rate for spoil treatments was found.

View larger version (15K): [in this window] [in a new window]   Fig. 2. Mean total yield (± SE, three replicates) for three harvests of Digitaria eriantha foliage grown in soil or spoil material treated with water treatment residue (WTR) at application rates of 0, 50, 100, 200, and 400 g kg–1. Best fit lines for soil (Yield = 7.14 + (14.87 x 0.945WTR rate); r2 = 0.77***) and spoil (Yield = (–0.018 x WTR rate) + 26.78; r2 = 0.29*) treatments are shown.

View larger version (18K): [in this window] [in a new window]   Fig. 3. Mean total yield (± SE, three replicates) for three harvests of Cenchrus ciliaris foliage grown in the soil material treated with water treatment residue (WTR) at application rates of 0, 50, 100, 200, and 400 g kg–1. Best fit lines for soil (Yield = 4.28 + (14.78 x 0.997WTR rate); r2 = 0.63**) and spoil (Yield = (0.03 x WTR rate) + 21.01; r2 = 0.23*) treatments are shown.

Digitaria eriantha
The yield of D. eriantha decreased exponentially with increasing WTR application rates when grown in soil, but decreased gradually and linearly when grown in spoil substrate (Fig. 2). The effect of WTR on the growth of D. eriantha in the soil is marked, as even at the lowest WTR application rate (50 g kg^–1) there is more than a 14 g pot^–1 loss in yield (63.9% decrease) from the yield of the control treatment. At the highest WTR application rate (400 g kg^–1), the yield decreased by 78.7% (Fig. 2). In the case of the spoil substrate there was only a 23.0 and 23.9% decrease in yield as was indicated at the two highest WTR application rates (200 and 400 g kg^–1, respectively; Fig. 2). The yield of the control was similar to that of the 50 g kg^–1 WTR treatment (26.9 and 27.1 g pot^–1, respectively). The relatively small decline in yield of the 100 g kg^–1 WTR treatment (1.9 g pot^–1) was not considered to be a problem, and application rates up to 100 g kg^–1 appear feasible in the spoil substrate.

Cenchrus ciliaris
The yield of C. ciliaris grown in soil decreased exponentially with increasing application rate of WTR (p < 0.01), but the decline was not as marked as for the other two grass species (Fig. 3). When grown on spoil, the yield of C. ciliaris increased linearly (p < 0.05) with increasing amounts of WTR (Fig. 3). In the soil substrate, only the 400 g kg^–1 WTR treatment had a marked decrease in yield, being less than half that of the control treatment (Fig. 3), while the 50 g kg^–1 treatment decreased 20.0% from the control yield. The yields of the 100 and 200 g kg^–1 WTR treatments were similar (Fig. 3). In the spoil substrate, the yield of the 200 g kg^–1 WTR treatment was about 1.2-fold greater and in the 400 g kg^–1 WTR treatment the yield was about 1.5-fold greater than the control treatment (Fig. 3). However, small decreases in yield from the control treatments were noted for the 50 and 100 g kg^–1 WTR treatments. The variability between replicates of the control and the 50, 100, and 200 g kg^–1 WTR treatments was high and is reflected in the poor fit of the regression equation (Fig. 3).

Plant Analyses
Concentrations of Ca, K, Mg, N, Na, and P in the foliage of the grasses for the second harvest and typical macronutrient concentrations for turf grasses (Bennett, 1993) and Eragrostis curvula (Schrad.) and Festuca arundinacea (Shreb.) (Miles, 1994) are presented in Table 8. Trace nutrient concentrations (Zn, Cu, Mn, Fe, and B) for the grasses are given in Table 9. Figures 4, 5, and 6 show the RDA triplots for E. tef, C. ciliaris, and D. eriantha, respectively. Details on interpreting RDA triplots are given by Lep and milauer (2003), but a brief description is given here. The direction of an arrow indicates the maximum direction of change of a variable in ordination space. The angle between arrows gives an approximation of correlation, where small angles suggest a positive correlation, right angles indicate zero or low correlation, and large angles or opposing arrows indicate a negative correlation. The positioning of centroids in ordination space indicate the similarity between means of samples (in the soil or spoil), and the arrow for WTR indicates it is the direction of increasing WTR application rate on foliar nutrient concentration.

View larger version (19K): [in this window] [in a new window]   Fig. 4. Redundancy analysis (RDA) triplot of second harvest foliage samples (circles) for Eragrostis tef, leaf nutrients (arrows), and amount of water treatment residue (WTR) applied, at rates of 0, 50, 100, 200, and 400 g kg–1, to soil (centroid is indicated by open triangle) and spoil (centroid indicated by closed triangle). WTR-treated spoil represented by shaded circles and WTR-treated soil by open circles. Eigenvalues for axes 1 (horizontal) and 2 (vertical) were 0.415 and 0.167 respectively, cumulatively representing 58.2% of the total variability in leaf nutrient composition.

View larger version (19K): [in this window] [in a new window]   Fig. 5. Redundancy analysis (RDA) triplot of second harvest foliage samples (circles) for Digitaria eriantha, leaf nutrients (arrows) and amount of water treatment residue (WTR) applied, at rates of 0, 50, 100, 200, and 400 g kg–1, to soil (centroid in indicated by open triangle) and spoil (centroid indicated by closed triangle). WTR-treated spoil represented by shaded circles and WTR-treated soil by open circles. Eigenvalues for axes 1 (horizontal) and 2 (vertical) were 0.436 and 0.178 respectively, cumulatively representing 61.4% of the total variability in leaf nutrient composition.

View larger version (19K): [in this window] [in a new window]   Fig. 6. Redundancy analysis (RDA) triplot of second harvest foliage samples (circles) for Cenchrus ciliaris, leaf nutrients (arrows) and amount of water treatment residue (WTR) applied, at rates of 0, 50, 100, 200, and 400 g kg–1, to soil (centroid is indicated by open triangle) and spoil (centroid indicated by closed triangle). WTR-treated spoil represented by shaded circles and WTR-treated soil by open circles. Eigenvalues for axes 1 (horizontal) and 2 (vertical) were 0.326 and 0.138 respectively, cumulatively representing 46.4% of the total variability in leaf nutrient composition.

Digitaria eriantha
For D. eriantha, the individual effects of substrate and WTR application rate were found to be significant (p = 0.018 and 0.016, respectively), though the interaction was not significant (p = 0.230). A positive correlation was found between WTR application rate and Mg content of the foliage (Fig. 5), though this relationship was not as marked as for E. tef (Fig. 4). Potassium and Mn concentrations were weakly correlated to WTR application rate, while B, Fe, N, P, Zn, and Na concentrations were generally not correlated to WTR application rate (Fig. 5). Again, grasses grown in soil had higher Fe and Na concentrations, while those grown in the spoil tended to have higher N, Zn, and B concentrations. In this instance P concentrations were generally higher in plants grown in soil than in those grown in spoil (Fig. 5).

Cenchrus ciliaris
The effect of substrate type was found to have a significant (p = 0.026) effect on nutrient uptake in C. ciliaris, though the effect of WTR application rate was only marginally significant (p = 0.056). As was the case for the other two grass species, the interaction effect of substrate by WTR application rate was not significant (p = 0.454). A weak positive correlation was found between WTR application rate and the concentration of Mg and Cu in the foliage (Fig. 6). Generally, the concentrations of other elements were not correlated to WTR application rate, though a slight negative relationship was evident with P. As before, the grasses grown in soil typically had higher Na and Fe concentrations, while those in spoil had higher Zn, B, and N concentrations.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION DISCUSSION CONCLUSIONS REFERENCES

 
Calcium uptake was not markedly influenced by WTR application rate, even though large additions of total Ca were made to the substrates. While the total Ca content of the WTR was high (Table 2), extractable amounts were considerably lower (Table 1). Furthermore, the extractable (available) Ca contents of the spoil and soil were not markedly different from the WTR (Table 1), masking the impact that the WTR may have had even at the high application rates. Generally, the Ca concentrations were lower than those reported by Bennett (1993) for turf grasses, but similar to those reported by Miles (1994) for E. curvula and F. arundinacea (Table 8). In contrast, Mg concentrations increased with increasing WTR addition, regardless of substrate type (Fig. 4, 5, and 6; Table 8). This was attributed to the considerably higher extractable Mg concentrations of the WTR (Table 1), possibly leading to luxury uptake by the grasses. This is reflected in the Mg concentrations of the grasses tested here that generally exceed typical Mg concentrations (Bennett, 1993; Miles, 1994; Table 8). Excess Mg can lead to suppression of Ca (Rengel, 2000) and Mn uptake (Bergmann, 1992), which may negatively impact plant growth (Bergmann, 1992). At the highest WTR application rate (400 g kg^–1) for all grasses, Mg concentrations were about three- to sixfold greater than Ca concentrations. This may, in part, have led to the general decline in yields noted for some of the treatments, especially at high WTR application rates. This was, however, not apparent for E. tef or C. ciliaris grown in the spoil treatment. In the control treatments, Ca and Mg concentrations were similar.

Potassium concentrations tended to be high (typically >25 g kg^–1), although the pattern of uptake was variable between grass species (Table 8). This was attributed to K fertilizer additions and variable uptake by the plants, rather than a consequence of WTR application. Generally, tissue P concentration decreased for E. tef and C. ciliaris, but not for D. eriantha, as WTR addition increased. This was probably due to the high P sorbing capacity of the WTR (Table 6). Digitaria eriantha had higher P concentrations than the other grasses grown in the soil at all WTR application rates. This suggests that under those conditions D. eriantha was able to take up greater amounts of P, though the mechanism for this is not clear. No consistent pattern for N uptake was evident, except that the grasses grown in spoil typically had higher N concentrations than the same grass species grown in soil. While N was added as fertilizer, additional N was probably supplied by decomposition of organic material present in the spoil (Table 1).

Generally trace metal concentrations decrease with an increase in soil pH (Gray et al., 1998). This is attributed to increased sorption onto negative exchange sites and formation of insoluble hydrous metal oxides. The effect of WTR addition on Mn uptake was marked compared with the untreated control soil. Though the relationship between WTR application rate and Mn uptake was not clear, apparently the addition of WTR led to a reduction in Mn uptake by all grasses, regardless of substrate, when compared with the control soil. Two mechanisms may account for this. The first is the increase in pH of the substrates when treated with WTR (Table 7), and the second is reduced uptake induced by competition with Mg ions (Bergmann, 1992). The Mn concentrations exceeded typical ranges for grasses (Table 9) only in the control treatments. The higher concentration of Mn in plants grown in the control soil (compared with the control spoil) was attributed to the slightly higher availability of Mn in the soil, which is reflected in the DTPA extraction (Table 5).

The increase in Fe concentrations with increasing WTR application rate was attributed to moderate amounts of available Fe in the WTR due to the use of FeCl₃ at the treatment plant (Hughes et al., 2005). While Fe deficiency is typical of calcareous or over-limed media (Mortvedt, 2000), this was not apparent. This may be due to the high availability of Fe in the WTR (Table 5), though Fe uptake was not strongly correlated to WTR application rate (Fig. 4, 5, and 6). The Fe concentrations reported here are generally within the sufficiency range (60–300 mg kg^–1) reported by Brown (1982), but were often greater than the typical range for grasses (Bennett, 1993; Table 9).

Zinc deficiencies are frequently reported in soils with pH > 6.0, especially if OC content is low (Mortvedt, 2000). The higher OC content of the spoil (Table 1), along with the higher availability of Zn in the spoil (Table 5), is likely to be the cause of the higher Zn concentrations of plants grown in the spoil (Table 9). This was also reflected in the RDA triplots for all the grass species (Fig. 4, 5, and 6). Lower availability and immobilization of Zn due to high pH led to lower uptake from the soil. In the case of plants grown in the soil, deficiencies occurred as foliage concentrations were often well below the sufficiency range reported by Bennett (1993; Table 9). While Zn concentrations were greater than the upper range suggested by Bennett (1993) when grown in the spoil, that author reported that some grasses had not shown toxicity symptoms with Zn concentrations of up to 3000 mg kg^–1. Copper uptake was generally weakly correlated to WTR application rate for all the grass species (Fig. 4, 5, and 6), and no clear relationship was evident (Table 9). Concentrations were low, however, and in some instances deficiencies may have occurred (Table 9). Boron uptake was variable, but clearly plants grown in the spoil had considerably higher B uptake than those grown in the soil (Table 9). We also noted that, in the spoil treatments, B uptake decreased with increasing WTR application rate (Table 9), though this trend was not clearly reflected in the RDA triplots (Fig. 4, 5, and 6). We speculate that the high B concentrations of plants grown in the spoil was due to high amounts of B in the spoil material, though this was not measured. The decrease in B uptake with increasing WTR application was probably due to the increase in pH (Mortvedt, 2000).

Generally, these results indicate that nutrients were present in sufficient amounts to support the growth of the grasses, although poor growth was observed in a number of treatments. The unfavorable Mg to Ca ratio and low Zn availability may have reduced yields in some, but not all, treatments. The low availability of Zn in the soil was the likely cause of the lower yields of grasses grown in the soil compared with the grasses grown in the spoil. Also, unmeasured trace elements were possibly either at toxic or deficient concentrations. The high pH of the WTR may have led to trace nutrients becoming unavailable to plants, or weathering of the material may have released some trace elements at toxic concentrations. However, no symptoms of toxicity or deficiency were visually apparent.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION DISCUSSION CONCLUSIONS REFERENCES

 
Plant performance data suggest that application of this WTR to the soil should be avoided or applied at a minimum rate (50 g kg^–1), possibly lower than tested here. However, the WTR can be applied at the maximum rate (400 g kg^–1) on the spoil material, without apparently causing a significant loss in production of the grasses tested. Cenchrus ciliaris was the only species to benefit markedly from WTR addition when grown in the spoil material. The other spoil treatments showed marginal decreases in total yield with increasing WTR addition, but these decreases were small and so are not considered to be problematic in terms of rehabilitation procedures. High pH and unfavorable Ca to Mg ratios, especially in the soil, were the cause for the overall reductions in yield. Cenchrus ciliaris seems to tolerate the effect of WTR better than either E. tef or D. eriantha, and in the spoil substrate its growth may be improved.

However, as a precautionary approach, the high rates of WTR application used here are generally not recommended. In terms of disposing of the WTR, this is an unfavorable situation as lower application rates imply reduced disposal capacity. However, this needs to be considered in the context of the mine rehabilitation schedule, where vegetation growth and cover needs to be promoted. The soil and spoil substrates collected for use in this investigation were not highly weathered. Given sufficient time and environmental exposure, greater acidity may be generated by the materials. The mine has a net acid-producing potential, typical of collieries. In this instance, the neutralizing capacity of the WTR may be more beneficial and indeed improve plant growth. Use of the WTR as a liming material (at low application rates relative to those used here) may be more beneficial under such circumstances. For instance, to achieve liming application rates of 2 to 10 Mg ha^–1, WTR would be applied at rates of about 5 to 26 g kg^–1. Repeated applications may then be possible, depending on the extent of soil acidification. However, this will then equate to longer time periods for the disposal of the WTR. Considering that the coalmine has a finite existence and that there is likely to be an increase in WTR production (due to increasing demands for potable water), this may not be a feasible long-term disposal option.

	ACKNOWLEDGMENTS

 
The authors gratefully acknowledge funding from the Water Research Commission, Pretoria (Project number K5/1148). We are especially grateful to P. Smit of the coalmine and J. Parsons of Rand Water, as well as their colleagues, for their assistance. Also a note of thanks to the Animal Science Discipline (total plant N analyses) and the Geology Discipline (XRF analyses), University of KwaZulu-Natal.

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TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION DISCUSSION CONCLUSIONS REFERENCES

 

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