Distribution of mercury in soil and its concentration in runoff from a biosolids-amended agricultural watershed.

Biosolids applications can significantly increase Hg concentrations in cultivated soils. The objective of this study was to quantify levels of mercury in soils and runoff from a biosolids-amended watershed. The study site was a terraced, cultivated watershed that received cumulative biosolids loadings of 0, 87, or 224 Mg ha -1 between 1974 and 1993. Snowmelt runoff samples were collected from the three treatment areas in the spring of 1995. Soils were collected along transects from the 0 and 224 Mg ha -1 biosolids treatment areas at depths of 0 to 15 and 15 to 30 cm. Mercury analysis of stored, freeze-dried biosolids samples showed that Hg concentrations during the 20-year study ranged from 12.4 mg kg -1 initially to 2.4 mg kg -1 near the end. Soil Hg concentrations were elevated in the surface (0-15 cm) and subsurface (15-30 cm) of the 224 Mg ha -1 biosolids-treated terrace relative to the control. Mercury concentrations in the 0- to 15-cm soil depth ranged from 30 to 50 μg kg -1 for the control terrace and 180 to 390 μg kg -1 for the 224 Mg ha -1 biosolids-treated terrace. Concentrations were lower in the 15- to 30-cm depth. Total Hg concentrations in snowmelt from the control terrace ranged from 9.2 to 27.9 ng L -1 and 19.8 to 44.8 ng L -1 for the biosolids-treated terraces. Most Hg was associated with particulates > 0.45 μm. Mercury concentrations were elevated in grass tissue growing near the watershed's runoff lagoon.