Seasonal impacts on arsenic mobility and geochemistry in streams surrounding a gold mineralization area, Paracatu, Brazil

Abstract

Arsenic (As) is a trace element in the Earth's crust that often occurs with gold mineralization, where it is found in association with sulfide-bearing minerals such as pyrite (FeS2). Our study evaluated the mobility of As in creeks that traverse a gold mineralization area in Paracatu, Brazil. The results relate As mobility in water draining actively mined areas to season, wet and dry, and to the impact of in-stream channels lined with limestone that are used to neutralize acid mine drainage. Passage of the creek water through channels lined with limestone decreased soluble As to concentrations below the drinking water threshold in 13 out of 31 samples (10 μg.L−1). The soluble As concentrations were generally higher during the wet season. Arsenic speciation in water was mainly influenced by the season and by the proximity to mineralization zones in the areas that are actively mined. The organic arsenic species DMA (dimethylarsinate) was detected near the mineralized area during the wet season. Sediment-associated As varied with season and with sampling site, but was consistently high at one headwater spring; the highest observed value was 8750 mg As kg−1. We applied selective sequential extraction techniques to assess the portioning of As in the creek sediments. Two of the most mobile fractions of sediment-associated As, the adsorbed and very poorly crystalline fractions, showed a strong seasonal dependence which supported mobilization of As during the wet season. The amorphous Fe oxide fraction followed the same trend but contributed less As, and As associated with the crystalline Fe oxide fraction showed little fluctuation with season.