Spatial distribution characteristics of mercury in the soils and native earthworms (Bimastos parvus) of the leachate-contaminated zone around a traditional landfill

Abstract The contents and spatial distribution of mercury (Hg), including soil-Hg fractionation and Hg-containing native earthworm Bimastos parvus ( B. parvus ) species, were investigated in the leachate-contaminated zone of a large traditional landfill, Japan. Soil-Hg was fractionated into 5 categories: F1/water soluble Hg (Hg-w), F2/human stomach acid soluble Hg (Hg-h), F3/organic-chelated (Hg-o), F4/elemental Hg (Hg-e), and F5/mercuric sulfide (Hg-s). The total mercury (T-Hg) and methylmercury (MeHg) of native B. parvus , and the geochemical properties of soils were examined in this study. Soil T-Hg concentration ranged between 0.227 and 2.919 mg kg −1 dry weight (dw). The T-Hg and MeHg concentrations of B. parvus species ranged from 1.242 to 6.775 mg kg −1 dw and from 0.031 to 0.218 mg kg −1 dw, respectively. Percentages of soil-Hg fractions were in the order of F3/Hg-o > F4/ Hg-e > F5/Hg-s > F1/Hg-w > F2/Hg-h, and the fractions of Hg-o and Hg-e were 55.50% and 35.31%, respectively. Similar distributions and close correlations between the levels of B. parvus Hg and soil Hg-o, Hg-e, and Hg-s were observed in this study. The distribution of Hg in B. parvus was associated with soil organic matter (SOM) content and particle size (sand, clay); however, it was not correlated with Hg-w or Hg-h. The results indicated that easily bioavailable and soluble Hg fractions (Hg-w, Hg-h) of the soil were not appropriate to illustrate the distribution of Hg in native B. parvus . Instead, the stable soil-Hg fractions (Hg-o, Hg-e, and Hg-s) demonstrated good relationships of spatial distribution with B. parvus Hg in leachate-contaminated soil. It is advisable to preclude the evaluation of Hg biological distribution using soluble Hg fractions only. Stable Hg fractions in leachate-contaminated soil should also be included for assessing the biological distribution of Hg in leachate-contaminated soils.