Discriminating between Sources of Arsenic in the Sediments of a Tidal Waterway, Tacoma, Washington

Abstract

Various industries located along the Hylebos Waterway in Tacoma, WA, and runoff from slag used for road ballast are potential sources of elevated arsenic concentrations found in the waterway sediments. To discriminate between specific sources of As to the waterway, the history of arsenic deposition was reconstructed using 137Cs sediment age-dating combined with characterization of arsenic solid phases. In a sediment core collected near a former powdered metals facility, As occurred primarily in metal sulfides, reaching peak concentrations in the mid-1950s, consistent with the operational history of that facility. In a second core collected near a pesticide plant that historically discharged soluble arsenic, total arsenic was present predominantly as surface-bound species in sediment pre-dating the mid-1980s, decreasing in recent sediments coincident with initiation of remedial measures in 1981. Ferro-alloy and smelter slags were recent and optically distinguishable. The combination of optical microscopy and electron and laser-ion microprobe techniques in conjunction with historical information and radioisotopic analysis provides a powerful tool to differentiate between metal sources to sediments.