Methylmercury levels in cover soils from two landfills in Xi’an and Shanghai, China: Implications for mercury methylation potentials

Abstract

Landfills are considered important sources of mercury for surrounding ecosystems. Methylmercury (MeHg) levels in waste layers have been studied extensively; however, the levels of MeHg in cover soils remain undefined. Here, total mercury (THg) and MeHg concentrations in surface cover soils and soil cores from two landfills in China and possible factors affecting Hg methylation were studied. The mean MeHg concentration in surface cover soils from both landfills was 0.048 ng g−1, suggesting that cover soil layers are not active sites of MeHg production. Soil MeHg concentrations in both landfills were affected little by closure time. In the Jiangcungou landfill, no correlations between MeHg concentration and the measured environmental factors (e.g., THg, soil pH, organic matter (OM), and S) were observed that indicated that these parameters might have indirect effects on MeHg concentration. However, in the Laogang landfill, significant correlations were found between MeHg concentration and the measured environmental factors. The results showed that MeHg concentration in the surface cover soil from area D of the Laogang landfill is regulated mainly by soil pH, OM, and S, and that its vertical distribution in areas C and D is regulated mainly by soil pH and soil OM, respectively. These findings fill a knowledge gap regarding MeHg levels in cover soils and they advance our understanding of Hg cycling in landfills, presenting positive implications for landfill management and risk assessment of MeHg.