CONNECTION BETWEEN ORDOVICIAN MANTLE METASOMATISM AND ARSENIC IN VERMONT GROUNDWATER

Elevated arsenic levels have been sporadically reported in groundwater from public and domestic bedrock wells in Vermont. In central and northern Vermont, a number of these wells are located in the Rowe-Hawley Belt (RHB), a tectonic assemblage of thrust slices composed of metasedimentary and metaigneous rocks originally juxtaposed in the suture zone for the Ordovician Taconian Orogeny. Within the RHB, slivers of serpentinized ultramafic rocks are common along fault surfaces and represent vestiges of metasomatically altered suprasubduction zone mantle. Within the RHB, private wells have been documented with As concentrations as high as 275 ppb.

Geochemical analysis reveals that of bedrock types in the region, ultramafic rocks contain appreciably higher concentrations of As than metasedimentary rocks and greenstones of the RHB. Serpentinites and talc-carbonate rocks contain 10 to 450 ppm As (mean = 63.7 ppm; N = 23), whereas metasedimentary rocks contain < 53 ppm As (mean = 8.6 ppm; N = 20) and greenstones contain < 69 ppm As (mean = 4.1 ppm; N = 33). SEM-EDS analysis of ultramafic thin sections could not identify a particular As-bearing trace mineral, and the ultramafic rocks contain no evidence of sulfides. Instead, the As appears to be contained in the serpentine. This observation is supported by recent work of Hattori et al. (2005, GCA 69, 5585-5596) who indicate that As is incorporated into serpentine (antigorite) during hydration and metamorphism of ultramafic rocks during downward mantle flow. As is substituted as As(V) for Si(IV) in antigorite tetrahedral layers, where charge balance is satisfied by paired substitution of As(V) & Al(III) for Si.

Three monitoring wells were recently installed in an area with elevated ground water As. One was completed in serpentinite and talc-carbonate of the Barnes Hill ultramafic body and the others in metasedimentary rocks that are structurally below. Field observations indicate that the talc-carbonate-serpentine contains a 2 m deeply-weathered saprolite beneath till, and field analysis indicates that ultramafic ground water is geochemically different than metasedimentary ground water – e.g. conductivity is 0.610 to 0.720 (ultramafic well) vs. 0.270 to 0.360 mS /cm (metasedimentary wells). Future work includes bedrock and ground water geochemical analysis of each well.