Quantitative assessment of static versus dynamic environmental pollution risk from lead pollution at shooting ranges

Abstract

Lead pollution of soil arising from use of leaded ammunition in shooting ranges has received considerable attention from researchers, scientists, policy makers and legislatures. This is due to the adverse health effects associated with exposure to Pb. Eight military shooting ranges found in Botswana were used in this research project. All the eight shooting range soils were polluted with Pb with total Pb concentrations in the range 13-38 386 mg/kg. TAB shooting range soils accumulated the highest concentration of Pb. Most of the shooting ranges studied had Pb loading of over 1000 mg/kg, far surpasing the United States Environmental Protection Agency (USEPA) set maximum contaminant limt (MCL) value of 400 mg/kg. Fractionation studies established that Pb existed mostly in the carbonate form and the X-ray diffraction (XRD) patterns showed higher proportions of the cerussite (PbCO3) and hydrocerussite (PbCO3H2O). The SPLP-Pb concentrations in all the eight shooting ranges were higher than the USEPA set critical limit of 0.015 mg/kg, which indicates possible contamination of water sources. Determining total Pb concentration in shooting range soils only gives information on the amount of Pb stored in the soil but does not quantify the pollution risk to the environment. The amount of Pb deposited in shooting range soil may be in inert form, posing low risk to biota. To quantify the environmental pollution risk, pollution risk indices have been found to be suitable for this task. Contamination factor (CF), enrichment factor (EF) and potential ecological risk index (PERI) provided a good quantitative measure of environmental pollution risk posed by Pb accumulated in shooting range soils compared to total Pb concentration. TAB shooting range posed the highest risk to environmental pollution with highest CF of over 40000 indicating very high contamination and this correlated well with the amount of Pb deposited into the shooting range. In contrast, TSH shooting range experienced lower total concetration of Pb compared to MAK shooting range. However, TSH shooting range displayed a higher contamination factor (CF~9360) compared to MAK with CF ~ 3585 and corresponding total Pb concentrations of 13309.06 mg/kg and 25,193.19 mg/kg respectively. Similarly, TSH experienced higher enrichment factor (EF) than MAK with EF ~ 4916 and 2432 respectively. This means that TSH shooting range posed higher environmental pollution risk than MAK even though it accumulated lower total Pb concentration compared to MAK. It should be appreciated that total Pb concentration alone does not give a complete assessment of the environmental pollution risk posed by Pb in shooting range soils. Total concentration gives a static assessment of the pollution risk and does not provide insight into the effects of physicochemical properties of the environment or its holding capacity on total Pbxix concentration. Therefore, it is important to treat environmental pollution risk as a dynamic evolution process such that concentration of Pb in the environment would change with changing soil physicochemical properties and the holding capacity of the environment. The delayed geochemical hazard (DGH) model was used to show that environmental pollution risk from Pb is a dynamic process as opposed to static. The DGH model describes the potential transformation and release of Pb partitioned in the different fractions of the soil that may be inert but becoming reactive when the conditions of the soil such as pH, moisture and organic matter become favourable or when the Pb deposited into the soil exceeds the soil holding capacity. This model was applied to the TAB shooting range which is highly polluted with Pb. The findings indicated that TAB shooting range may be classified as high-risk area of Pb DGH with over 65% of Pb capable of being transformed into the mobile and bioavailable chemical forms. The high Pb deposition into shooting range soils calls for immediate remedial action. The use of chemical amendments, Pb accumulating plants and physical techniques such as soil sieving have proven to minimize Pb pollution at shooting ranges.