Chemical Geology, 68 (1988) 89-98 89 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands [31 ARSENIC REGENERATION FROM ESTUARINE SEDIMENTS OF THE BOTHNIAN BAY, SWEDEN NILS G. HOLM Department of Geology, University of Stockholm, S-106 91 Stockholm (Sweden) (Accepted for publication November 12, 1987) Abstract Holm, N.G., 1988. Arsenic regeneration from estuarine sediments of the Bothnian Bay, Sweden. Chem. Geol., 68: 89 -98. Sediments of the Bothnian Bay between northern Sweden and northern Finland are contaminated by As, giving rise to concentrations in the percent range. The enclosure of a sediment and bottom water by benthic chambers resulted in chemical reduction of the water and the oxidized sediment surface layer, and a flux of reduced As from the FeOOH phase of the sediment. The release of As could be described by zero- or pseudo-zero-order reaction kinetics. The measured flux of As into the water at 20°C was 2.1 mg m 2 day ~As. 1. Introduction caused by high river load of Fe and Mn (Ingri and Pont6r, 1986). For the same reason the The aim of the project reported in this paper natural background level of As at, for instance, was to determine mechanisms for the remobi- the indicated "unpolluted" station 205 of the lization of As from estuarine sediments. The Bothnian Bay sediments is high, i.e. of the or- distribution of As and transition metals among der 200-400 mg kg- 1 (Fig. 1 ) (Holm, 1983). sediment components has also been studied. La Peintre (1954) showed that arsenate spe- The rate of release, especially during a negative cies coprecipitate with Fe-oxide-hydroxides. redox turnover, i.e. a change from oxidizing to BostrSm and Valdes (1969) remarked that As reducing conditions, has been measured. The covaries with Fe and Mn in ocean-floor sedi- study is part of a regional survey of the sedi- ments. It may therefore be expected a priori that ments of the Gulf of Bothnia. The survey in- most of the As of the Bothnian Bay is present cluded sediment sampling from ship and as pentavalent arsenate in the Fe (III) phase of chemical analysis of As, Pb, Fe, Mn, and some the sediments. of the other transition metals (Holm, 1983). The Bothnian Bay is a large estuary in the The Gulf of Bothnia between Sweden and sense that the salinity is 2-3 g kg-1. The sedi- Finland (Fig. 1 ), and especially the Bothnian ment geochemistry is complicated by the pres- Bay in its extreme north, is rich in ferroman- ence of a base-metal ore smelter at RSnnsk~ir ganese sediments (Winterhalter, 1966; Bos- close to the town of Skellefte& in Sweden (Fig. trSm et al., 1978, 1982; Hallberg, 1979). This is 2). The smelter processes concentrates of com- 0009-2541/88/$03.50 © 1988 Elsevier Science Publishers B.V.