ARSENIC IN GROUNDWATER WELLS IN GLACIAL DRIFT, NORTH-CENTRAL VERMONT

Groundwater wells in glacial drift in north-central Vermont contain arsenic concentrations above the EPA MCL of 10 ppb. These wells are situated within or adjacent to the Rowe-Hawley Belt (RHB), a sequence of early Paleozoic metasedimentary and metaigneous rocks in the suture zone of the Ordovician Taconian Orogeny. Included in the RHB are metasomatized ultramafic rocks (serpentinites and talc-magnesites) that occur along thrust slices. Whole rock arsenic concentrations of the dominant rock units in the RHB, in order of decreasing as concentration, include (1) meta-ultramafic rocks with As values as high as 1104 ppm in talc-magnesite and 449 ppm in serpentinite (mean As value = 93 ppm, N = 41), (2) phyllites that contain an average As concentration of 22 ppm (N = 34), including two isolated occurrences of 101 and 190 ppm As from below a thrust slice of As-rich talc-magnesite, and (3) greenstones (N = 33) that contain a mean As concentration of 4.1 ppm. The presence of elevated arsenic in wells that are glacially down-gradient from ultramafic bodies of the RHB raises the possibility that the arsenic is derived from ultramafic components in the till and outwash, a hypothesis that has also been suggested for the original source of arsenic in the Bengal fan (Guillot and Charlet, 2007). Accordingly, this study aims to evaluate the origins of arsenic in glacial drift groundwater by combined analysis of drift mineralogy and chemistry, well water chemistry, analysis of historic well chemistry records and geospatial analyses of aquifer composition and hydrogeology. Preliminary analyses of two case-study wells in Orleans County reveal possible variation in arsenic sources. First, regarding a well within the RHB, XRD data indicate the presence of serpentine and talc in nearby drift specimens, confirming the potential for ultramafic-derived As in wells producing water from glacial drift. Analysis of hydrochemistry and aquifer geochemistry will be used to further test this hypothesis. Second, in a well down-gradient of the RHB, the presence of chlorinated solvents and pesticides permits the hypothesis that the elevated As levels in that well are either anthropogenically-derived or modified.

Guillot, S., and Charlet, L., 2007, Bengal arsenic, an archive of Himalaya orogeny and paleohydrology, J Env Sci Health Part A, v. 42, p. 1-10.