ROLES OF BEDROCK GEOCHEMISTRY, GROUNDWATER FLOW PATHS, REDOX AND ION EXCHANGE ON DISTRIBUTION OF ARSENIC IN A FRACTURED SLATE AQUIFER SYSTEM, NEW ENGLAND, USA

Groundwater with elevated As (> 10 ppb) was identified in 23 % of bedrock water wells (N = 236) studied in the Taconic slate belt of southwestern Vermont. The bedrock of the study area is pyrite-rich, organic-rich black slates interbedded with green and purple slates and quartzites that were thrusted over carbonate and clastic sedimentary rocks of the continental shelf during the Ordovician Taconian Orogeny. 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 – 2000 ppm As in pyrite); (2) release of As via the dissolution of As-rich pyrite; (3) geochemically-reducing and slightly alkaline conditions, where high As values occur generally occur at Eh < 200 mV 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 10 mg/L (ppb) and 60% of wells in this zone exceed 25 ppb As. Where flow paths are shorter in rocks 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 > 10 ppb As and none contain > 25 ppb.