Fluxes and major transport routes of Arsenic in the Scheldt estuary

The estuary of the Scheldt was monitored for As during seven seasonally spread sampling campaigns. The resulting data indicated that As did not behave conservatively in the estuary; an important mobilisation of As was observed from the river up to halfway the estuary. Both input flux (from river to estuary) and output flux (from estuary to coastal zone) of dissolved As (AS(d)) were calculated from statistical interpretations of the longitudinal profiles. The size of the input flux changed equally to the river discharge, being at its highest in winter, during flood period, and at its lowest in summer, during dry period. The same could not be said for the output fluxes; in fact, as a consequence of the mobilisation, there was a large discrepancy between input and output fluxes and these differences appeared to depend largely on the season. In summer and early autumn, the estuary exported three times more dissolved As to the sea than it received from the river. In winter and spring, this amount was barely double. The additions in dissolved As during each sampling period were quantified spatially by applying an estuarine analogue of 21 sequential tanks assumed to be homogeneous, as well as by statistical estimates and calculations of dilution in each tank. A second model was used to model particulate As (As-p), dividing the estuary into four characteristic zones, in each of which the movements of suspended particulate matter (spm) were well defined. Since particulate particles are mainly transported as associated to suspended matter, the major routes of particulate As transport in each box could be determined from the knowledge of this spin transport. They appeared to be longitudinal, in downstream direction, together with a large flux of sedimentation of continental material in the 2-10 salinity zone. Such sedimentation greatly reduced the flux of particulate As to the coastal zone; the flux to the sea was only 16% of the initial particulate As flux from the river. By combining the findings of both models, literature data on As biogeochemistry and seasonal observations, some transport routes additional to the spm transport were suggested. (c) 2004 Elsevier B.V. All rights reserved.