Effect of soil type on distribution and bioaccessibility of metal contaminants in shooting range soils

Abstract

Shooting ranges from Department of Defence sites around Australia were investigated for extent of metal contamination. Shooting range soils contained concentrations ranging from 399 to 10,403mg/kg Pb, 6.57 to 252mg/kg Sb, 28.7 to 1250mg/kg Cu, 5.63 to 153mg/kg Zn, 1.35 to 8.8mg/kg Ni and 3.08 to 15.8mg/kg As. Metal(loid)s were primarily concentrated in the stop butt and the surface soil (0–10cm). The distribution of contamination reflected firing activity, soil properties, climate and management practices. Climatic variations among sites in Australia are significant, with a temperate climate in the south and tropical climate with high rainfall in the north. Up to 8% of total Pb resided in soil fines (<0.075mm), due to the fragmentation of bullets on impact. Distribution and bioaccessibility varied between each site. Acidic Townsville soil had the highest proportion of water extractable Pb at 10%, compared to the alkaline Murray Bridge with only 2% Pb water extractable. Soil properties such as CEC, pH and dissolved organic carbon influence mobility. This is reflected in the subsoil concentrations of Pb in Townsville and Darwin which are up to 30 and 46% of surface concentration in the subsoil respectively. Similarly bioaccessibility is influenced by soil properties and ranges from 46% in Townsville to 70% in Perth. Acidic pH promotes dissolution of secondary minerals and the downward movement of Pb in the profile. The secondary Pb minerals formed as a result of weathering in these soils were cerussite, hydrocerussite, pyromorphite, galena and anglesite. Copper oxide was also reported on fragments from bullet jackets. These results have implications for range management.