Anomalous arsenic concentrations in groundwaters of an island community, Bowen Island, British Columbia

Recently, occurrences of elevated arsenic concentrations in bedrock groundwaters used for individual and municipal water supplies have been recognised along the mainland coast of southern British Columbia, Canada. An area on Bowen Island (Queen Charlotte Heights community) was chosen to investigate the source(s) of arsenic, geochemical processes controlling its aqueous uptake, the role of geology and structure in the formation of these waters, and the use of hydrogeochemical survey methods for detecting arsenious groundwater regions. Pyrite-arsenopyrite-Cu mineralisation in the Queen Charlotte Heights area is present as veins associated with NE-trending fault zones and as disseminations in marginal breccia and stockwork zones of a ryhodacite porphyry stock. Near surface mineralisation was later altered by supergene events to form minor pyrite-arsenopyrite in a matrix of limonite and clay. Geochemical factors that control the concentration and mobility of arsenic in these groundwaters include: (a) anion exchange of arsenic from clay minerals during cation divalent/univalent exchange processes (water softening) which leads to waters of high pH and high anion exchange capacities; (b) desorption of arsenic from Fe and Al oxyhydroxide minerals during pronounced shifts in pH from acid to alkaline conditions brought on by the water softening process; and (c) stepwise oxidation of arsenopyrite mineralisation to form arsenious sulphate-bearing groundwaters. The inconsistent behaviour of the As3+ and As5+ ions during the complex water-rock interaction processes described above, precludes the use of detailed chemical analyses to predict relative abundances of the more toxic As3+ species. For a full appreciation of the toxic impact of groundwaters containing total arsenic concentrations that are above regulatory guidelines an arsenic speciation analysis is required.