ARSENIC SPECIATION AND DISTRIBUTION IN THE CARNON ESTUARY FOLLOWING THE ACUTE DISCHARGE OF CONTAMINATED WATER FROM A DISUSED MINE

In February 1991 Wheal Jane, a tin mine in S.W. England, was closed, t he mine drainage pumps were removed and underground water levels were left to rise. By January 1992 the build up of water in the mine was su fficient to cause the Nangiles adit to burst, and contaminated mine wa ter containing high levels of arsenic overflowed into the Carnon River . The river in turn drained into an estuary system which reaches the s ea at Falmouth. A contingency plan was introduced, with water being pu mped from the mine and treated before discharge through a tailings dam . This brought the discharge from Nangiles under control by late Febru ary 1992, although flow recommenced from mid-April until early July 19 92. The immediate impact of the discharge and treatment operation on t he distribution and speciation of arsenic in the river was the presenc e of high concentrations of dissolved arsenite, with a level of 240 mu g As l(-1) being measured below the mine tailings dam. Elevated conce ntrations of arsenite, typically 9 mu g As l(-1), persisted in the sal ine region of the Carnon estuary. By the following July the levels of arsenite had dropped significantly in the river but the levels in Rest ronguet Creek remained essentially unchanged. In comparison with the a rsenite concentrations, the arsenate levels were comparatively low in the whole river/estuary system in the February following the initial d ischarge. A significant increase was however found in the saline regio n in the following July. Methylated arsenic species were absent from t he river but were present in the biologically productive saline region during the warmer summer sampling period. The July concentrations of dimethylarsenic ranged from 1.0 to 2.1 mu g As l(-1) and monomethyl-ar senic from 0.2 to 0.5 mu g As l(-1).