Sedimentary Phosphorus and Arsenic Inactivation in an Alum-Treated Lake

High fluxes of sedimentary phosphorus (P) in Spy Pond in Arlington, MA have increased the rate of eutrophication and diminished the recreational value of the pond. The pond sediments are also contaminated with arsenic, which was applied in the 1950s and 1960s to control macrophyte growth. Arsenic (As) is chemically similar to P and remobilizes from the sediments under the same conditions that give rise to P remobilization. The remobilization of P and As are controlled by redox conditions in the bottom waters of the pond: reducing conditions cause P and As adsorbed to sediments to desorb and enter the water column from late spring until fall. To control P remobilization, the pond was treated with alum in May 2004. The goal of our study was to determine whether alum was also effective for controlling As remobilization. Data was collected to monitor the change in average peak concentrations of P and As before and after treatment. The pond is separated into two basins, north and south, by an island and shallow sill. The two basins have unique water chemistry characteristics and the responses to the alum treatment differ. In the south basin, the P concentration ratio before and after treatment was 13:1, while the ratio for As was only 2:1. The ratios for P and As in the north basin were 5:1 and 3:1, respectively. Data collected from Spy Pond indicate As(III) was not significantly inactivated by alum. The effectiveness of the alum to control As remobilization is dependent on the As redox state and competition with P for adsorption sites.