Comparison of elemental contents in earthworm cast and soil from a mercury-contaminated site (Idrija area, Slovenia)

The aim of this paper was to test the new sampling media—earthworm casts in a highly contaminated area. The investigation was carried out at the ancient Hg ore roasting site Pšenk in the surroundings of Idrija, where extremely high Hg contents in soils and SOM were determined in previous investigations. 32 earthworm cast samples were collected in the research grid 30 × 30 m in order to compare the Hg contents and spatial distribution in earthworm casts to the values and distributions in SOM and soil (0–15 cm). Extremely elevated Hg concentrations were determined in earthworm casts from the studied area ranging from 5.4 to 4330 mg/kg with the median of 31 mg/kg. The Hg values in casts are somewhat lower than in soil (6.3–8600 mg/kg) and slightly higher compared to soil organic matter (SOM) (1.5–4200 mg/kg). Strong correlation (r² = 0.75) between Hg contents in casts and soil was found, while correlation between casts and SOM was positive but weaker (r² = 0.35). Spatial distribution of Hg in earthworm casts show the highest concentrations in the central part of investigated area, similar to the distribution in soil. Hg contents rapidly decrease from the center toward the margins of the studied area, where they reach values of less than 50 mg/kg. It was shown that Hg contents and dispersion in casts are comparable to those in soil, which indicates that at investigated area soil contamination is strongly reflected in contamination of earthworm casts.

► We compare Hg contents in soil, SOM and earthworm casts. ► The determined Hg contents are in the order of soil > earthworm casts > SOM. ► There is strong correlation between Hg contents in casts and soil. ► Soil contamination is strongly reflected in contamination of earthworm casts.