MOBILIZATION OF ARSENIC IN GROUNDWATER AQUIFERS IN NEWFOUNDLAND, CANADA

Approximately 28% of the population in Newfoundland, Canada relies on groundwater as a source of potable water. Arsenic (As) concentrations in some domestic water wells in Newfoundland exceed the maximum acceptable concentration (>10 µg/L) recommended by Health Canada. It has been speculated that the As in groundwater in Newfoundland originates from the dissolution of natural aquifer solid materials, however, this has never been confirmed. This research is intended to identify the potential sources of groundwater As in Newfoundland, and to assess water chemistry conditions that influence As mobilization (release from solid phases to groundwater). Subsurface glacial till samples, collected from different locations in Newfoundland, were used as representative aquifer solid materials and were examined with acid digestion, sequential extraction, and batch dissolution methods to understand the mechanisms that control As mobilization. Acid digestion experiments showed As concentration in till samples ranges from 19 to 21 mg/kg. Sequential extraction experiments revealed that a considerable amount of As (5mg/kg) in till samples is in labile form, suggesting these As could release to groundwater under proper conditions. Our batch dissolution experiments showed release of As from till samples is influenced by water pH, redox potential (Eh), and siderophore trihydroxamate desferroxamine B (Dfob) (an iron chelator) concentration. In the pH range of 3-8, very low concentration (3-4 µg/L) of As was released. However, significant amount (25-30 µg/L) of As was mobilized at a higher pH of 10. At high Eh (+200 to +300 mV), slight dissolution of As and Fe was observed. At lower Eh (+50 to -100 mV), elevated concentrations of As and Fe were released, indicating As release is due to reductive dissolution of minerals. A siderophore concentration of 500 µM considerably enhanced the mobilization of As and Fe, signifying the released As in those till samples was associated with Fe minerals. SEM/EDX and XRD analysis of solid till samples confirmed the presence of As and Fe minerals. Our results suggest reductive dissolution of As and Fe minerals could contribute significantly to groundwater As contamination in Newfoundland, and that water chemistry conditions such as pH, Eh and Dfob control the rate of As release to groundwater.