LEAD IN GROUNDWATER DERIVED FROM A FRACTURED CARBONATE AQUIFER (CLARENDON SPRINGS FORMATION), NORTHWESTERN VERMONT

Elevated concentrations of Pb occur in groundwater associated with brecciated dolostone of the Cambrian-Ordovician Clarendon Springs Formation in the Champlain Valley of northwestern VT. 35 wells that produce from the Clarendon Springs Fm were analyzed by inductively coupled plasma-atomic emission spectrometry (ICP-AES) and ICP-Mass spectrometry (ICP-MS). Pb concentrations in acidified samples (pH ~ 1) range from < 1 ppb to 659 ppb, with a mean value of 95 ppb. 49% of wells that draw water from the Clarendon Springs Fm contained Pb at concentrations above the EPA MCL of 15 ppb, whereas only 30% of wells producing from other local carbonate and clastic aquifers contained Pb at concentrations > 15ppb. These findings are consistent with previous work in this area that identified the Clarendon Springs Fm as a source of Pb (Ryan et al. 2002) and U, Ra and Rn (Kim and Becker, 2001). Sulfides and hydroxides appear to be the dominant source of lead in the Clarendon Springs Formation. Partial rock dissolution and ICP-AES analysis shows that Pb ranges from 0 to 16 mg/kg, indicating relatively low Pb concentrations in carbonate minerals. Pb concentrations in groundwater increase by one to two orders of magnitude when water tables lower during times of drought, suggesting that Pb-bearing sulfides dissolve when oxygen penetrates the aquifer. Preliminary X-ray diffraction (XRD) analyses confirm the presence of pyrite, and more detailed XRD and ICP analyses of non-carbonate fractions will provide further information on the speciation of lead and the controls on its solubility in this bedrock aquifer system. Given the low solubility of Pb in carbonate aquifers, most Pb is removed by filtration, ion exchange and deposition, and is rarely detected at concentrations >15 ppb in drinking water.

Kim J, Becker L. 2001. Geologic context of elevated radionuclide occurrences in northwest Vermont. GSA Abst. Prog. 33(1), A60.

Ryan PC, Trombulak S, Aldrich L, Ausprey I, Brouwer B, Calvi B, Groff S, Hicks C, Johnston K, McBride M, Perlow L, Yazwinski M. 2002. A survey of groundwater composition in northern Addison County, Vermont: Geologic sources of contamination. Env. Sci. Practicum research report: Environmental Studies Program, Middlebury College, Middlebury, Vermont.