Ecological Risk Reduction of Lead-Contaminated Mining Sites by the In-Situ Phosphate Treatments

摘要

Elevated lead (Pb) in soil resulting from mining or smelting activities is well known to threaten terrestrial ecosystem by phyto-toxicity and surface-runoff or leaching processes. Restoration of site vegetation and improvement of water quality in contaminated areas are publicly concerned and environmentally challenged. In an effort to safeguard human and ecosystem from the contamination in the Jasper County Superfund Site located in the southwest Missouri, field plot experiments were conducted at three mining sites (urban, mill, and mine areas) to investigate the impacts of the in-situ phosphate treatments on the Pb uptake by plants and water quality in the areas. Urban soils and mine wastes were treated on site with soluble phosphate fertilizer and various types of phosphate-enriched biosolids, and plant tissues and surface or ground waters were collected from the fields post treatments and analyzed for Pb and P concentrations by ICP-OES and microbial toxicity by a bioassay. Results indicated that the phosphate treatments that attempt to induce the immobilization reactions of soil Pb would also significantly reduce Pb uptake by plants and improve water quality in treated areas. Lead concentration in plant tissues was found positively correlated with bioaccessible Pb in the soils. As a result of the treatments, the microbial toxicities of the waters were significantly lowered, and aqueous Pb and P concentrations in both surface and shallow ground water were below the USEPA criteria of TCLP water quality, though aqueous P in the water was moderately elevated. The Pb risk reductions to plants and waters were likely achieved by reduced Pb solubility in the soils through the treatment-induced transformation of labile Pb species to relatively insoluble compounds, most probably formation of pyromorphites. This field study demonstrated that the soil treatment using phosphate-based amendments could be potentially used as a cost-effective and environmental-sound remedial alternative for reducing the human health and ecological risks associated with soil Pb.