The mobility of arsenic in a Canadian freshwater system receiving gold mine effluents

Abstract

The mobility of arsenic in freshwater systems is dictated by its partitioning between the solid and dissolved fractions in the sediments and their interstitial waters. Arsenic is largely associated with ferric iron in the form of oxyhydroxides in oxic waters and sediments as arsenate As(V). In deeper anaerobic sediments arsenic is released from iron oxyhydroxides because of the reduction of iron from the ferric to the more soluble ferrous state. Reducing environments can also be encountered in sediments relatively close to the sediment-water interface when there are high rates of biological activity that consume oxygen and create a reducing environment. For the past several years we have examined the relationships between organic carbon content of surface sediments, bottom water anoxia, redox zonation of sediments and the release of arsenic from freshwater sediments to the overlying waters. These studies have been performed using limnocorrals to isolate columns of water and their underlying sediments in Balmer Lake, a shallow freshwater system in Central Canada that has served as the final repository for tailings from two gold mines for more than 40 years. The results indicate that surface sediments with higher organic carbon content are more susceptible to developing late season bottom water anoxia that can facilitate the subsequent release of arsenic from sediments to the overlying water. These results have implications for metal mining operations where reduced metal loadings from effluents or mine closure are expected to result in higher biological productivity and greater organic matter deposition to sediments.