Role of climate and geography in arsenic mobility and risk at an artisanal mining site in an urbanized semi-arid environment.

Ultra-enriched arsenic (As) concentrations >30% occur at an abandoned artisanal mine in an urban park in San Diego, California, presenting a complex risk to the public. This study uses geochemistry in consideration with climate and geography to evaluate As transport away from the mine. Sediment As concentrations reach 2320 mg/kg (483 times background crustal concentrations; n = 73) along drainage pathways and sequential extraction experiments indicate As overwhelmingly partitions into the least mobile phase (mean 83% As in residual fraction; n = 30); there is little redistribution of As from primary minerals into secondary or dissolved phases - a potentially positive outcome for managing the risk - despite the sediments being exposed to a century of weathering. Dissolved As transport does occur, with intermittent rain events producing As up to 272 μg/L in runoff. Both sediment and water As decrease to background concentrations within 1,000 m of the mines, influenced by the encroaching urbanization, and the semi-arid climate which limits weathering and transport. Similar patterns of As migration downgradient of abandoned mines occur at other mining sites in arid and semi-arid regions; however, off-site As transport at other sites extends far greater distances. This study indicates that a combination of geochemistry, geomorphology, climate, and urbanization can work together to retard the surface transport of As from artisanal and un-remediated mine sites; and helps inform environmental management at this site and others like it.