Arsenic species in the Humber Plume, U.K.

Dissolved inorganic arsenic (DIA), monomethylarsenic (MMA) and dimethylarsenic (DMA) have been determined in the water column and sediment porewaters of the Humber Plume, during December 1988, May 1990 and August 1990. In winter 1988 only DIA was present, whereas 2 and 18% of total arsenic was in the methylated forms in spring and summer 1990, respectively. Partition coefficients ( Kd) for suspended particles and DIA were in the range 2–25 × 103 1 kg−1 and were compatible with values obtained in the English Channel. Concentrations of DIA, MMA and DMA in porewaters from bottom sediments of the plume were 5–20 times greater than in the overlying water column. Estimates of porewater diffusional fluxes for DIA were in the range 2–30 μmol m−2 a−1, whereas for MMA and DMA they were 30–110 nmol m−2 a−1. A two-dimensional hydrodynamic model was used to determine the fluxes of arsenic species in the Humber Plume. The DIA fluxes across the plume boundaries were approximately two orders of magnitude greater than the estuarine flux of DIA and decreased from 104 kg day−1 in winter to 5 × 103 kg day−1 in spring and summer. Mass balance calculations for the arsenic species in the plume showed that there were no net fluxes of DIA and MMA during the periods of investigation and for DMA in spring. DMA was exported to the North Sea in summer. Diffusional fluxes of DIA from sediment porewaters increased the concentrations of DIA by about 10% during the spring and summer. Arsenic assimilation by phytoplankton accounted for the decrease in DIA concentrations between winter and spring and phytoplankton degradation for the appearance of methylated species in summer. These processes are discussed in terms of their relevance to the improvements of arsenic ecosystem models for the North Sea.