Aqueous and mineralogical analysis of arsenic in the reduced, circumneutral groundwater and sediments of the lower Fraser River delta, British Columbia, Canada

Naturally occurring As in the groundwater and Holocene sediments of the lower Fraser River delta was studied. At each of two sites in the delta, vertical profiles of groundwater geochemistry were characterized and sediment cores were collected for petrographic analysis, quantification of elemental abundances, and sequential extraction analysis to characterize the pools of solid-phase As. Total solid-phase As concentrations were similar to those found in other deltaic environments, ranging from 2.2 to 2.6 μg/g at a site of groundwater recharge with approximately 4 m of peat at the surface, to 2.7–4.1 μg/g at a site adjacent to the Fraser River where saline water is encountered within the aquifer at depth. Arsenic that was weakly bound or could be mobilized by reduction of Fe oxides constituted between 20% and 50% of the total As in the sediments. Groundwater As concentrations showed distinct vertical trends, and peaked at 9 μg/L at the peat-influenced site and between 29 and 32 μg/L at the site with saline water at depth. At both sites, reduction of dissolved organic matter appears to be responsible for reducing conditions, high dissolved Fe (up to 230 mg/L), high HCO 3 - , and the presence of NH 4 + and PO 4 3 - . The peat-impacted site implies that a direct source of dissolved organic C from peat is not a sufficient condition for significant As release into groundwater in the Fraser River delta.