High resolution profiles for arsenic in the Seine Estuary. Seasonal variations and net fluxes to the English Channel

Three sampling cruises were conducted in the Seine Estuary from 1993 to 199 5 in varying hydrological and seasonal conditions. The site included all of the lower part of the river under the influence of tidal dynamics and the dilution plume in the Bale de Seine. Chemical speciation of arsenic showed high seasonal variations, especially in September when AsIII represented ar ound 50% of dissolved arsenic. The inclusion of organoarsenic compounds not accessible to direct analysis by hydride generation required preliminary m ineralisation of the samples. The ratio of dissolved to particulate arsenic distribution was controlled mainly by the iron content of particles, Biolo gical activity had an influence on chemical speciation and thus on the part ition coefficient (K-D 10(-3) = 6 +/- 1 in September and 12 +/- 0.9 in Febr uary). The zone of conservative mixing used for Seine River flow calculatio ns was limited to a salinity range of approximately 10-30. Dissolved arseni c concentrations extrapolated to null salinities were lower during the high -water period because of dilution (17.6 +/- 1.1 nM), and maximal during the low-water period in summer (35.7 +/- 0.9 nM). Mean arsenic export to the E nglish Channel was estimated at 33.2 +/- 6 T yr(-1) for dissolved arsenic. Observation of an arsenic output greater than the upstream input, as well a s a simultaneous increase in dissolved and particulate arsenic concentratio ns during the mixing of freshwater with seawater, strongly suggested the ex istence of an important intra-estuarine source of arsenic, either of indust rial origin or related to the transport and diagenesis of marine sediments. (C) 1999 Elsevier Science Ltd. All rights reserved.