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Aqueous and mineralogical analysis of arsenic in the reduced, circumneutral groundwaters and sediments of the Fraser River Delta, British Columbia Bolton, Mark


An aqueous and mineralogical investigation of the groundwaters and sediments of the Fraser River delta, B C is presented. Groundwater and sediment samples were collected from two sites. The DND site is located in an upland area, approximately 3 km upgradient from the north arm of the Fraser River. The Kidd2 site is located adjacent to the north arm of the Fraser River. A saline water wedge extends inland from the bed of the Fraser River to the subsurface beneath the Kidd2 site. Concentrations of dissolved arsenic (As) are relatively low at the DND site, while concentrations of approximately 30 µg/L dissolved As are present within the saline water wedge, at the Kidd2 site. The aqueous geochemical data suggest that dissolved As is generated by the reduction of hydrous ferric oxide (HFO) minerals. The data also suggest that the elevated concentrations of dissolved As are correlated to the mass loading that occurs as dense saline water infiltrates from the base of the Fraser River into the adjacent aquifer. Sediment samples from both sites were analysed using a sequential extraction procedure (SEP) that was designed to target specific pools of solid-phase As. Results of the SEP analyses indicate that solid-phase As is present in the sediments at trace concentrations. However, processes such as sorption and the precipitation of arsenical sulphide minerals do not appear to be sufficient to mitigate the relatively elevated concentrations of dissolved As at the Kidd2 site. If concentrations of dissolved As at the Kidd2 site were to remain constant at 30 µg/L, the time required to flush the solid-phase As that is associated with HFO minerals from the sediments is estimated to be over 24,750 years. It would take significantly longer for the HFO-associated solid-phase As to be flushed from the upland areas of the delta, where mass loading of reactants to the groundwater is significantly lower. The results of this investigation indicate that a disturbance to the geochemistry of the Fraser River delta has the potential to generate significant concentrations of dissolved As, possibly as high as those encountered in other deltaic environments such as Bangladesh.

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