Chemical transformation and fate of lead in shooting range soils found in eastern and northern Botswana

Abstract

Ammunition used for military training and exercises has been found to contain high content of lead (Pb). The Pb present in the ammunition projectiles undergoes an array of transformation and weathering reactions when it reaches the shooting range soils. Seven military shooting ranges in the eastern and northern Botswana were used for this study. Soil samples were collected at each of the seven shooting ranges at the berm (stop butt), target line, 50 and 100 m from the berm. Pb concentrations were determined using ICP-OES and in all of the shooting ranges investigated, the highest concentrations were found in the berm soils. The weathering products of the Pb bullets and shots at all the seven shooting ranges studied were predominantly carbonates: specifically, cerussite (PbCO3) and hydrocerussite (Pb3(CO3)2(OH)2). The XRD results agreed well with the fractionation studies which showed that the carbonate bound Pb was the most predominant in all the seven shooting ranges investigated. The presence of high concentrations of carbonate bound Pb culminated in high concentrations of Pb in the shooting range soils due to dissolution of carbonate Pb at lower pH ranges. MAT R2 berm soils accumulated the highest concentrations of total Pb of up to 20882 ± 5419 mg/kg. In all the shooting ranges studied, total Pb concentrations far exceeded the United States Environmental Protection Agency (USEPA) critical level of 400 mg/kg. All the seven shooting ranges studied also failed the Synthetic Precipitation Leaching Procedure (SPLP) with SPLP-Pb concentrations exceeding the USEPA 0.015 mg/kg critical level of hazardous waste, posing a pollution threat to surface and groundwater. Mobility of Pb in the berm soils in all the shooting ranges was found to be over 90% implying high Pb lability. The bioavailability index of Pb was found to be in the range 60-90%, an indication that most of the Pb was available for plant uptake in all the shooting ranges studied.