Evaluation of nitrate leaching from mine tailings amended with biosolids under Mediterranean type climate conditions

Mine tailings are difficult to revegetate due to the lack of organic matter, severe nutrient limitations, and potential metal toxicity. Biosolids has been shown to be favorable for improving properties of mine tailings. The rates of biosolids required to reclaim mine tailings (up to 250 dry t ha - 1 ) may produce conditions where significant amounts of nitrates can leach into groundwater. Leaching column experiments were conducted to determine the influences of biosolids placement and plant cover on nitrate leaching from biosolids-amended mine tailings. PVC columns packed with 7.7 kg of tailings were treated with 168 g of biosolids (approximately 270 kg mineral N ha - 1 ). Biosolids were either placed on the surface or mixed with the tailings and half of the columns were seeded with perennial ryegrass (Lolium perenne, L.). Columns were drip irrigated at a rate of 758 mm of water y - 1 . This rate was twice the average precipitation for Central Chile. All leachates were collected weekly for up to 21 weeks and analyzed for nitrate, pH, electrical conductivity, and chemical oxygen demand. The electrical conductivity and nitrate concentration of percolates decreased with time, while the pH remained constant. In some cases the percolate had nitrate concentrations greater than the maximum amount allowed for human consumption (10 mg NO 3 – N 1 - 1 ). Vegetation cover and mixing the biosolids with tailings reduced NO 3 – N concentrations in the percolate.