Naturally-Derived Arsenic in Fractured Slate: Influence of Bedrock Geochemistry, Groundwater Flow, Reduction-Oxidation and Ion Exchange

Tuesday, September 24, 2013: 11:10 a.m.
Peter Ryan , Geology, Middlebury College, Middlebury, VT
Jonathan Kim , Vermont Geological Survey, Montpelier, VT
Helen Mango , Department of Natural Sciences, Castleton State College, Castleton

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.

Peter Ryan, Geology, Middlebury College, Middlebury, VT
Peter Ryan teaches in the Geology Department and Environmental Studies Program at Middlebury College. Research interests include the geochemical and mineralogical analysis of bedrock-derived arsenic and uranium in Vermont ground water (and associated research on trace element speciation in minerals), and the other is the mineralogy and geochemistry of soils developed on terraces along the tectonically active Pacific coasts of Costa Rica and Ecuador.


Jonathan Kim, Vermont Geological Survey, Montpelier, VT
Jonathan Kim has been at the Vermont Geological Survey since 1996. He obtained his Ph.D. at SUNY at Buffalo in 1996. Research interests include the tectonic evolution of the northern Appalachians and geological constraints on bedrock aquifers in Vermont.


Helen Mango, Department of Natural Sciences, Castleton State College, Castleton
Helen Mango received her B.A. from Williams College and her M.S. and Ph.D. from Dartmouth College. She has been teaching geology and chemistry at Castleton since 1991. Her research interests include the geochemistry of ore deposits, the contamination of groundwater by metals such as uranium and arsenic, and geoscience education. She always includes undergraduates in her research, and often weaves the material directly into her courses.