ARSENIC SPECIES IN THE HUMBER-PLUME, UK

Dissolved inorganic arsenic (DIA), monomethylarsenic (MMA) and dimethy larsenic (DMA) have been determined in the water column and sediment p orewaters of the Humber Plume, during December 1988, May 1990 and Augu st 1990. In winter 1988 only DIA was present, whereas 2 and 18% of tot al arsenic was in the methylated forms in spring and summer 1990, resp ectively. Partition coefficients (Kd) for suspended particles and DIA were in the range 2-25 X 10(3) 1 kg(-1) and were compatible with value s obtained in the English Channel. Concentrations of DIA, MMA and DMA in porewaters from bottom sediments of the plume were 5-20 times great er than in the overlying water column. Estimates of porewater diffusio nal fluxes for DIA were in the range 2-30 mu mol m(-2) a(-1), whereas for MMA and DMA they were 30-110 nmol m(-2) a(-1). A two-dimensional h ydrodynamic model was used to determine the fluxes of arsenic species in the Humber Plume. The DIA fluxes across the plume boundaries were a pproximately two orders of magnitude greater than the estuarine flux o f DIA and decreased from 10(4) kg day(-1) in winter to 5 X 10(3) kg da y(-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 M MA during the periods of investigation and for DMA in spring. DMA was exported to the North Sea in summer. Diffusional fluxes of DIA from se diment porewaters increased the concentrations of DIA by about 10% dur ing the spring and summer. Arsenic assimilation by phytoplankton accou nted 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. Cop yright (C) 1996 Elsevier Science Ltd