GEOCHEMISTRY OF ARSENIC IN SEDIMENT PORE WATERS OF SHALLOW LAKES TREATING HEAVY METALS NEAR BUTTE, MONTANA

The Warm Springs Ponds Operable Unit (WSPOU) is a series of shallow (< 5 m) constructed lakes and wetlands located within the upper Clark Fork superfund complex, Montana, USA. The purpose of the WSPOU is to remove dissolved and particulate metals from Silver Bow Creek. Although lime is added during periods of ice cover or high flow, aquatic photosynthesis by plants and algae maintains pH > 9 during summer, and lime is not needed. Removal of copper and other heavy metals is excellent year-round. However, during the warmer months the WSPOU releases water with arsenic concentrations that exceed site-specific standards (0.02 mg/L). Whereas most of the arsenic enters the WSPOU as suspended sediment, most arsenic leaves the WSPOU in dissolved form.

This study employed diffusion samplers (peepers) to examine the geochemical controls on arsenic mobility in the WSPOU sediment. The pH and Eh of the sediment pore water dropped quickly in the top 10 cm of sediment, to values near 6.0 and 0 mV, respectively, and showed little seasonal variation. Concentrations of Fe(II), Mn(II), ammonium, phosphate, and bicarbonate increased dramatically with depth. Dissolved sulfide iwas locally present, but at relatively low concentrations (< 1 mg/L) fixed by precipitation of mackinawite/pyrite. Concentrations of dissolved As were approximately 10x higher in the pore water than in the overlying surface water, and were a mixture of As(III) and As(V) based on field speciation. Geochemical modeling suggests that vivianite and symplesite may exert solubility controls on the mobility of phosphate and arsenate, respectively, in the deeper sediment. Precipitation of Fe-sulfides, siderite, calcite, and other phases near the top of the sediment results in a black nepheloid layer up to 20 cm or more in thickness. This nepheloid layer is widespread within the WSPOU, and is a distinctive feature of the benthic ecosystem. Because As is more soluble than Fe in this layer, As is free to diffuse upwards into the water column. Calculated fluxes based on Fick’s Law suggest that much of the seasonal release of As from the WSPOU can be attributed to diffusion from sediment pore water, although other processes, such as groundwater discharge or pH- and/or temperature-dependent desorption reactions, may also be important.