Arsenic Biogeochemistry in the Humber Estuary, U.K.

Sampling campaigns have been carried out in the Humber Estuary to assess the spatial and temporal distributions of arsenic species and their fluxes to the North Sea. Axial concentrations of dissolved inorganic arsenic displayed conservative behaviour down-estuary of an industrial source. There was no evidence of methylated species in the low-salinity region in mid-summer, due to high turbidities, which impacted on phytoplankton activity. At the estuary entrance dissolved inorganic arsenic concentrations were directly related to salinity over each of four tidal cycles, covering seasonal variability in estuarine conditions. Arsenic partition coefficients, for suspended particulate matter, showed no systematic trend with salinity but were linearly related to iron inputs, possibly from industrial sources. During spring, in the lower estuary, dissolved inorganic arsenic was accompanied by trace quantities ( 0•03 μg l-1) of dimethylarsenic. In mid-summer, when the water temperatures were 18 °C and at high salinity, both dimethylarsenic and monomethylarsenic were present, with the former species having the greater concentrations. The fraction of total arsenic as methylated species increased from 0% in winter to 2% in spring to 12% in mid-summer. Seasonal fluxes of dissolved inorganic arsenic increased from 55 kg day-1 in winter to 20 kg day-1 in spring and 13 kg day-1 in mid-summer but fluxes of dissolved methylated species were <1 kg day-1. Particulate fluxes were in the range 70-100 kg day-1 in winter, 13 kg day-1 in spring and <1 kg day-1 in mid-summer. The significance of arsenic fluxes to the North Sea is discussed, together with the predictive modelling of arsenic behaviour in the Humber Estuary and Plume.