The fate of lead in soils contaminated with lead shot

Abstract

A study of the interactions between lead (Pb) shot and the soil, and the fate of Pb at shooting ranges was conducted in order to address the lack of research on these issues at the current time. Surveys of the spatial and vertical distributions of Pb in the soil were carried out at selected Canterbury clay target shooting ranges. These confirmed that maximum soil Pb concentrations at New Zealand ranges are comparable to those reported overseas, which commonly exceed 10,000 mg kg-1, while in New Zealand the guideline limit for Pb in soil is 300 mg kg-1. There was evidence of Pb movement from the contaminated surface-soil zone into underlying soil. While sufficient Pb had been solubilised from the Pb shot to cause substantially elevated soil Pb concentrations, the majority (68-99%) of the total Pb at each site was present as intact Pb shot >2 mm. An incubation experiment was designed to assess the rate of oxidation of Pb shot and subsequent transfer of Pb to the soil under various environmental conditions. The onset of Pb shot oxidation and subsequent development of the corrosion crust surrounding individual Pb pellets was relatively rapid. Corrosion products in the corrosion crust and precipitated directly onto the soil solid phase readily dissolved over a soil pH range of approximately 4.5-7. Soil solution and fine earth (<1 mm) Pb concentrations became substantially elevated within 6 months, and there was evidence of the development of a quasi-equilibrium within 24 months. Soil pH, moisture content and temperature impacted on these processes. The data generated by the incubation experiment was used to predict the speciation of Pb in soil solutions using GEOCHEM-PC modelling software. Mechanisms of control of Pb solubility were deduced from equilibrium solubility diagrams constructed with the aid of the speciation calculations. The solubility diagrams were strongly suggestive of Pb solubility regulation by the pH-dependent solubility of corrosion products associated with the corrosion crust or soil solid phase. The dominant mineral controlling Pb solubility appears to be PbCO3. Thus, the presence of Pb shot in the soil alters the control of solubility from adsorption mechanisms in uncontaminated soil to precipitation-dissolution mechanisms in contaminated soil. High soil pH reduces, but does not prevent, the solubility of Pb shot corrosion products. A lysimeter leaching experiment, using intact soil cores collected from three shooting ranges, was carried out to assess the potential for Pb mobility. Leachates from contaminated soil contained elevated Pb concentrations which were sustained over multiple leaching events. Solubility diagrams confirmed that Pb mobilisation was caused by the dissolution of readily soluble Pb shot corrosion products. A large pool of potentially soluble Pb is generated by the readily soluble fine earth Pb and Pb minerals in the corrosion crust. There was a close relationship between leachate Pb concentrations and soluble organic carbon dynamics, indicating the importance of organic complexation on Pb mobility. The results indicate there is a high risk of chronic Pb leaching from contaminated shooting ranges. Natural attenuation by subsoil is expected to delay movement of Pb down the profile, but Pb shot deposition onto young soils with little attenuation capacity could generate substantial movement of Pb. Management options were considered for soil contaminated with Pb shot. Phosphate addition to Pb-contaminated soil aims to precipitate Pb as relatively insoluble pyromorphite minerals. This study confirmed the effectiveness of phosphate immobilisation of Pb in a preliminary incubation study using fine earth with elevated Pb concentrations. Lysimeters were then collected from three shooting ranges and subjected to leaching following the application of phosphate. Where molar P:Pb ratios were sufficient, substantial reduction in the concentrations of Pb in leachates was observed, confirming that phosphate immobilisation could be used at shooting ranges to reduce Pb mobility. In addition to reducing the mobility of soluble Pb, phosphate addition has the potential to encourage bulk transformations of soluble Pb minerals in the corrosion crust and soil into much less soluble pyromorphite compounds. The use of phosphate immobilisation will be limited by soil vulnerability to phosphorus loss, and environmental sensitivity to the eutrophic effect of phosphorus. The results of this thesis contribute valuable information on the fate of Pb at CTS ranges. With this new knowledge, the effectiveness of range and contamination management techniques currently suggested in the literature is discussed.