Behaviour of mercury species in isolated estuarine sediment samples—growth and decay of methyl mercury during storage

Abstract

WE have investigated the levels of methyl mercury found in mercury-containing sediments from the River Mersey Estuary, England, as part of our work on the determination and transport of mercury in the environment¹. We now wish to draw attention to the unexpected behaviour of sediment-bound methyl mercury on storage. In the initial stages of this study we experienced difficulty in obtaining consistent methyl mercury values; supposedly identical samples analysed at intervals of a few days gave markedly different results. We, therefore, followed the levels of methyl mercury in selected sediments over a period, to determine if any change was occurring on storage. We found that the amounts of methyl mercury observed in the stored sediments did not remain constant; initially there is a rise in the amount of methyl mercury observed, and then, after about ten days, the amount present begins to decline to levels which in general only approximate to those originally present. We have observed this phenomenon in nearly all of the Mersey sediments samples we have examined; in two cases where the total mercury levels were < 1.5 µg g⁻¹, a rise in methyl mercury levels was not detected. In general the sediments were in fact found to contain quite high total mercury levels (usually between 5 and 8 µg g⁻¹) and this feature may be a requirement for the effect to be observed.