NATURAL ARSENIC IN LAKES, STREAMS AND FISH IN A METASEDIMENT AREA IN NORTHERN SWEDEN

Precambrian fine-grained sediments contain elevated contents of arsenic in the form of arsenopyrite. Mean content in 30 samples is 50 mg/kg while the median content is 15 mg/kg. This arsenic is mobilized under reducing conditions in wetlands and emerges in streams along with ferrous iron. Ferrihydrite formed in the streams, is a very good scavenger of arsenic. Only locally in smaller drains and streams up to 100 microg/l is found in the water. However, sandy stream sediments coated with ferric precipitates contain up to 500 mg/kg As and precipitates of ferrihydrate up to 5000 mg/kg (0.5 %).

The arsenic concentrations in lakes in the area is elevated over a regional background and some lakes do contain up to 20 microg/l As. Benthic macroinvertebrates show elevated contents of As, however to a considerable extent in the form of organic As as arsenobetaine, MMA and DMA. Other aquatic macroinvertebrates like water striders do not show elevated As. Even in lakes with elevated As the fish does not show excess As. Fractionation of the water by filtration (0.2 microm) and dialysis with 10 kDa and 1 kDa membranes show a clear correlation between Fe and As indicating that the As in lake water is tied up to suspended and colloidal Fe-particles. Thus even when elevated As occurs in lake water it does not seem to be bioavailable. In spite of high concentrations in stream sediments the fish is found to be suitable for human consumption.

Naturally occurring arsenic, in the metasediments is subject to mobilisation and immobilisation by the land use. Forest drainage which periodically has been very intensive tend to oxidise the sulphides in the local till, while obstruction of the ditches by vegetation debris from trees etc tend to mobilise the As by reduction of ferrihydrates. In deeper lakes there is a migration of both As and Fe towards the sediment surface where it is generally repricipitated or mobilised into the water column, but then in the form of particles and colloids with a low bioavailability.