Forms and fate of Cu in a source drinking water reservoir following CuSO 4 treatment

Abstract

Copper sulfate (CuSO 4) application to surface waters for nuisance algae control has been practised for decades, and the practice remains the most effective algicidal treatment for lakes, reservoirs and other managed water bodies. Despite its long history of use, questions remain about the fate of Cu in water bodies receiving CuSO 4 treatment, and the forms and availability of Cu within surface waters subject to high Cu loadings. This study quantifies the fate of Cu following CuSO 4 treatment to Lake Mathews, the terminal reservoir for the Colorado River Aqueduct and one of the source drinking water supply reservoirs for southern California, and evaluated the forms and kinetic lability of Cu in sediment samples. Based upon measured Cu concentrations, flows at the reservoir outlet, and known mass applied during treatment, mass balance calculations indicate that approximately 20% of the Cu was exported from the reservoir over a 70-day period following an 8.85 metric ton CuSO 4·5H 2O treatment on 23 October 1995. Rapid conversion of the dissolved Cu to particulate forms was observed, with significant accumulation of Cu in the sediments. Total sediment Cu contents ranged from approximately 10–600 μg/g dry-weight basis and decreased with increasing depth within the sediments, qualitatively following known historical Cu inputs to the reservoir. Sequential extraction and desorption kinetic experiments suggest that a significant portion of the sediment-borne Cu is associated with oxidizable and carbonate-bound phases which may release Cu to the water column, although significant release would occur only under extreme changes in water chemistry.