Sewage sludges are significant sources of N, P, organic matter, and other elements for plant nutrition. Soil and spoil application of such residues is one of the most efficient ways of nutrient recovery. However, the benefits from sewage sludge soil application have to be weighed against sanitary and environmental risks such as excessive nitrate leaching, which may contaminate water. Sanitary and environmental regulations have banned human consumption of waters with nitrate concentrations > 10 mg L-1. Therefore, this work aimed to measure the risk of groundwater contamination by nitrate produced in a mined spoil treated with 120 t ha-1 of domestic sewage sludge on wet basis. The amended spoil leached solutions with concentrations up to 78 mg L-1 of nitrate, which poses risks to underground water. Under the experimental conditions set in this work, the groundwater table has to rest at 4.5 m depth or deeper to allow the leachate diluting through the soil profile to concentrations < 10 mg L-1, if the soil remains bare. A grass cover on the amended spoil decreased by 40% the amount of nitrate in the leachate and it would allow a groundwater table at 2.5 m depth without an excessive nitrate input from the top sewage sludge amended layer.
biosolids; nitrate; mineralization; nitritication; underground water
