Numerous bedrock drinking water wells in the Taconic Allochthons (St. Catherines, Hatch Hill-W. Castleton, Pawlet, and Brezee fms) of southwestern Vermont contain dissolved As above the EPA MCL of 10 ppb. The 20 wells that could be accurately located have an average As concentration of 53 ppb, and we believe that the arsenic is derived from bedrock because (1) the distribution of wells with elevated As appears to correlate with rock type, and (2) nitrate concentrations in arsenic-affected wells are low (< 0.5 ppm).
A limited geochemical data set from each of the domestic wells with elevated As documents many wells with high Na levels (10 - 3840 ppm, mean = 380 ppm) without corresponding high Cl (<80 ppm); this suggests that road salt is not involved and that the high Na values were likely produced by cation exchange replacement of monovalent Na by divalent Ca on surfaces of illites and chlorites in the slates. Dissolved Fe (<100 - 2750 ppb) and Mn (<5-5418 ppb) in groundwater from As-affected wells vary widely, suggesting that redox conditions vary appreciably. At the same time, neither Fe nor Mn exhibit positive correlations with As, suggesting that As may be mobile under a wide range of redox conditions.
Previous research in the Taconic region suggests that the source of As is pyrite, which occurs in a variety of forms including (1) individual crystals within foliation, (2) as aggregates of finer-grained crystals and quartz within foliation, (3) as crystals parallel to foliation, and (4) as crystals that cross-cut quartz veins. As concentrations in pyrite range from 182-993 ppm. The low abundance of arsenopyrite argues that it is not a significant source of As. In several cases, wells with elevated arsenic were located within 50 m of wells with low arsenic; some of this variability was attributed to fracture control.
The objective of this study is to conduct a detailed hydrogeoochemical study of 25 bedrock wells to examine relationships among As and other groundwater parameters. We will analyze for 26 metals, 4 anions, alkalinity, hardness, temperature, pH, conductivity, DO, and ORP in order to assemble a geochemical groundwater model for this region.