Distribution of mercury and methylmercury in the sediments of a lowland river system

Concentrations of total mercury (T-Hg) and methylmercury (MeHg) were determined in surficial sediments from the River Yare, Norfolk, UK over the period 1986-1998, to assess the temporal and spatial trends of contamination arising from a historical point source discharge. The results demonstrate that the spatial distribution pattern follows that of a distinct pollution plume in the sediments with an initial increase 2-3 km downstream from the point source discharge at Whitlingham sewage treatment works (STW) outfall. Temporally, this pollution plume remains evident, although there has been an overall decline in the mean T-Hg concentrations from 5.4 in 1986 to 1.8 mg kg(-3) in 1998. The major factor for this improvement has been the reduction in discharge of metals from the STW, together with burial by a fresh overlay of uncontaminated sediments. MeHg concentrations also exhibited a dispersed pollution plume. Mean concentrations of MeHg fluctuated during the study period between 3.3 and 8.5 mu g kg(-1). There has been no concomitant decline in MeHg concentrations over this period. MeHg was found to constitute less than 1% of T-Hg concentrations in the sediments. The weak correlations observed between T-Hg and MeHg concentrations indicate external environmental factors have a significant influence on determining MeHg concentrations in the sedimentary compartment.