Persistence and Remobilization of Arsenic in Massachusetts (USA) Lakes Treated With Arsenical Herbicides

Abstract

From 1953 to 1969 the Commonwealth of Massachusetts funded a program to evaluate the effectiveness of sodium arsenite for treating algae and macrophytes in lakes. It is well known that arsenic (As) is persistent in the environment; however, little work has been done to determine its long-term fate in the treated lakes. Once sodium arsenite is added to lakes, As typically precipitates and accumulates in bottom sediments, but under certain conditions As in surface sediments can be remobilized to the water column under certain conditions. The goals of this study were to determine whether residual As is present in elevated concentrations in the surface sediments (top 0–15 cm) of treated lakes, and whether As is being significantly remobilized. Eleven lakes were studied: five were known to have been treated with sodium arsenite; three were suspected of having been treated; and three untreated lakes were used for reference. Surface sediment grab samples were collected at multiple locations in each lake and analyzed for total As. In addition, water samples were collected along a vertical transect at the deepest point in each lake and analyzed for total As. In three of the five treated lakes and one of the lakes suspected of having been treated, sediment As concentrations were >100 mg/kg, or about four-fold higher than background. Analysis of the water samples showed that As was being remobilized from sediments of both the treated and untreated lakes where reducing conditions existed. The highest concentrations of dissolved As were measured in the bottom waters of treated lakes at levels >100 μg/L, which is about 50-fold higher than background. Elevated As levels in the sediments of treated lakes could have implications for lake management.