Arsenic in a fractured slate aquifer system, New England, USA: Influence of bedrock geochemistry, groundwater flow paths, redox and ion exchange

Elevated As levels have been reported by the Vermont Geological Survey in groundwater from public and domestic bedrock wells in northwestern New England (USA). The study area in southwestern Vermont is underlain by pyrite-rich, organic-rich slates that were thrusted over carbonate and clastic sedimentary rocks of the continental shelf during the Ordovician Taconian Orogeny, and the distribution of wells with elevated As shows that they were completed in slates. Hydrochemical and bedrock geochemical analysis indicates that elevated As in the aquifer system is controlled by the following: (1) the presence of black slates that are rich in arsenian pyrite (200–2000ppm As); (2) release of As via the dissolution of As-rich pyrite; (3) geochemically-reducing and slightly alkaline conditions, where high As values occur at Eh<200mV and pH>7; and (4) physical hydrogeological parameters that foster low Eh and high pH, particularly long groundwater flow paths and low well yields (i.e. high residence time) which provides high rock to water ratios. Where all four factors affect As contents in groundwater, 72% of wells in a zone of distal groundwater flow/low-relief topography exceed 10μg/L (ppb) and 60% of wells in this zone exceed 25ppb As. Where flow paths are shorter in slates and groundwater has higher Eh and lower pH (i.e. in regions of higher-relief topography closer to recharge zones), only 3% of wells contain >10ppb As and none contain >25ppb.Overall, 28% (50/176) of low-elevation wells (<245meters above sea level [masl]) exceed 10ppb As; only 3% (2/60) of higher-elevation wells (245–600masl) exceed 10ppb As. Over the entire aquifer system, 22% of bedrock wells (52/236) exceed 10ppb and the mean As concentration is 12.4ppb. Strong positive correlations among Fe, SO4 and As in groundwater confirm that dissolution of pyrite is the dominant As source. Positive correlations among SO4, Na and As indicate that, in reducing (Eh<200mV) groundwater, Fe(II) is exchanged for Na on mineral surfaces following pyrite dissolution and As remains in solution; conversely, in oxidizing groundwater (recharge zones), Fe(II) is oxidized to Fe(III) and the subsequent formation of ferrihydrite removes As (V) from solution.