Distribution of heavy metals in the River Yare and its associated broads I. Mercury and methylmercury

Abstract

A detailed sediment sampling program of the River Yare, Norfolk, and adjoining Broad systems was undertaken to evaluate the magnitude, spatial distribution and vertical extent of mercury contamination arising from a point source discharge. Particle size fractionation, total organic loadings and textural properties of the sediments were determined to provide an insight into the potential factors dictating mercury accumulation. Total mercury concentrations ranged between 0.05 and 32.9 mg kg −1 with mean transect loadings of 0.81–11.9 mg kg −1, which corresponded to 2–30-fold increases in sediment mercury concentrations over published background levels. Mercury contamination displayed the characteristic profile of a pollution plume derived from a point source, despite spatial variability which reflected the controls imposed by channel morphology, underlying geology, sediment type and catchment hydrology. Mercury occurred predominantly in surface deposits in upstream areas, principally in association with the 2000–1000 μm size fraction, but shifted towards subsurface enrichment in the downstream reach. Mercury association with smaller grain size fractions occurred with increased distance from the point source, at depth within core profiles and within the shallower Broad systems. Methyl mercury concentrations ranged between <0.7 and 13.2 μg kg −1 in surficial sediments, which represented between 0.01 and 0.2% of the total mercury burden. A crude mercury mass balance indicated that, since industrial mercury discharge began in 1964, 50% of the total released could be accounted for in the sedimentary compartment.