Geochemical identification of particulate lead pollution in shallow groundwater in inhabited areas in Kabwe, Zambia

Abstract

The town of Kabwe has been exposed to the most severe lead pollution due to mining activities in the 20th century. A large tailing (slag) hill containing over 1% of Pb is still standing within the mine's licensed area. We analysed the chemical composition and lead isotope ratios in seventeen shallow wells, five deep wells in the inhabitants' area, three ponds and three borehole wells in the area of the mine, and in the surrounding areas, we examined two samples from the soil's surface and nineteen 1-m deep core samples at each of the 15 sites. We found low dissolved Pb (<0.04 μg/L) but high particulate Pb concentrations (2–100 μg/L) in seventeen shallow groundwater samples (aquifer depth of ca. 10 meters). Five shallow wells exceeded the World Health Organization (WHO) guideline (10 μg/L) for total-Pb concentration. However, three of the five shallow wells had low dissolved Zn and sulphate ion concentrations (<1 μg/L; <50 mg/L), suggesting a low amount of Pb transfers through the aquifer. The isotopic water and soil samples data clarified that the Pb pollution in the shallow groundwaters is derived from the tailings deposit.Most of the shallow groundwater samples had negative cerium (Ce) anomaly in the rare earth element (REE) pattern, indicating contributions of colloidal transport from the dolomite aquifer in this area. However, PHREEQC modelling shows that migration by hydroxide colloids from the mining area does not explain the high particulate lead concentrations in the well water. The positive correlation between Pb/Ce and Pb/Al ratios in the particulates exhibited that the lead pollutants were derived from the aluminosilicates on the land. The estimated Pb contents in particulates in most shallow-well water were 700–2800 mg/L, and they distributed geographically similar to the Pb bulk concentration contour map of the surface soil. The analysis of the grain-size fractions in a 2-km transect soil demonstrated that Pb was eight times enriched in the silt fraction (<53 μm) than a very coarse sand fraction (1–2 mm). Therefore, we hypothesized that the particulates in the well water were mainly derived from the aeolian dust (>0.45 μm) at the well site. Further studies are required to examine the hypothesis.