Naturally-Derived Arsenic in Fractured Slate: Influence of Bedrock Geochemistry, Groundwater Flow, Reduction-Oxidation and Ion Exchange
Groundwater hydrochemical and bedrock geochemical analysis indicates that elevated As (up to 155 ppb) in the Taconic slate aquifer system of southwestern Vermont is controlled by four main factors: (1) the presence of black slates rich in arsenian pyrite (with 200 – 2000 ppm As); (2) release of As via the oxidation of As-rich pyrite; (3) reducing conditions — the highest As values occur at Eh < 200 (and pH > 7); and (4) physical hydrogeological factors that foster low Eh and high pH, particularly long groundwater flow paths and low well yields (i.e. high residence time). Where all four of these factors affect groundwater, 72 % of wells in a zone of distal groundwater flow/low-relief topography exceed the US EPA MCL of 10 ppb and 60% of wells in this zone exceed 25 ppb As. Where flow paths are shorter and groundwater has higher Eh (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. Overall, 28 % (50/176) of wells with wellhead elevations between 60 and 245 masl exceed 10 ppb As; only 3 % (2/60) of wells (wellheads) situated between 245 and 600 masl exceed 10 ppb As. Over the entire aquifer system, 22 % of bedrock wells (52/236) exceed 10 ppb and the mean As concentration is 12.4 ppb. 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 < 200) groundwater, Fe(II) is exchanged for Na on mineral surfaces following pyrite dissolution and As remains in solution; in oxidizing groundwater (recharge zones), Fe(II) is oxidized to Fe(III) and the subsequent formation of Fe-hydroxides removes As from solution.