CONTROLS ON ARSENIC SPECIATION IN A FRACTURED BEDROCK AQUIFER

Arsenic speciation and oxidation-reduction potential were measured in 28 samples from a fractured bedrock aquifer in south central New Hampshire. Previous work has established that arsenic is most likely derived from geologic materials, specifically arsenic sulfide and oxide minerals. Oxidation/reduction potential was measured on-site using a flow through cell, and arsenic species determined in the laboratory using continuous flow online hydride generation ultraviolet oxidation with magnetic sector ICP-MS detection. The distribution of arsenic species ranged from <1% to >99% As(III) within the entire dataset. Twelve samples are dominated by either As(III) or As(V), and plot within the stability field for their respective species. However, sixteen samples with significant concentrations (>20%) of both species plot in both fields. These samples are out of equilibrium, and may represent either mixing of waters with different oxidation/reduction status, or movement of water into a different redox environment.

Mixing of two (or more) waters might occur where fractures intersect naturally or where drilled wells connect otherwise unconnected fracture systems. Waters could mix in the well casing immediately before sampling, resulting in arsenic species that would appear to be out of equilibrium with their environment. Despite the appearance of disequilibrium, arsenic species may have been in equilibrium with their original fracture environment, however the arsenic couple does not respond as rapidly as other couples which define the measured Eh conditions. Movement of water from one redox zone to another could also include situations where pumping would draw deeper, less oxygenated waters towards the surface, and expose them to higher partial pressures of oxygen near the well casing. These waters would then begin to equilibrate with the new redox environment, and arsenic species would begin to adjust accordingly. Either of these scenarios could adequately explain why samples with mixed arsenic species appear out of equilibrium.