Effects of dry and heavy rainfall periods on arsenic species and behaviour in the aquatic environment adjacent a mining area in South Korea

This study investigated the spatiotemporal variation of arsenic (As) distribution, species and its behaviour in the aquatic environment changed by extended dry and heavy rainfall periods in the area adjacent an abandoned gold mine, South Korea. As appears to be transported from the mine wastes to Guryong stream through groundwater baseflow and leachate. The oxidation-reduction potential (ORP) conditions changed spatially and temporally following dry and heavy rainfall periods, and rainfalls caused decrease in the stream pH. As mainly existed as arsenates (AsO₄^3-) bound with H, Ca, and Fe in water. In groundwater, the lower the pH and the ORP, the higher the proportions of acid species of arsenate (HAsO₄^2-, H₂AsO₄^-) and arsenite (H₃AsO₃), resulting in increased As mobility and toxicity, respectively. In stream, the primary influencing factor of As variation is the ORP. Under oxidizing conditions, As in stream could precipitate as amorphous FeAsO₄·2HH₂O and FeOOH in the streambed. Then, the ORP decrease could remobilize As by redissolution and desorption of As bound to bed sediments. Thus, streambed sediments acted as a temporary sink-and-source for As, and extension of source areas accompanied with physical transport after heavy rainfalls.