ULTRAMAFIC SOURCE OF ARSENIC IN A FRACTURED BEDROCK AQUIFER

Whole rock geochemistry, hydrochemistry, X-ray diffraction (XRD), sequential chemical extraction (SCE) and Fourier transform infrared spectroscopy (FTIR) indicate that elevated arsenic (as high as 300 ppb) in groundwater from public and private domestic bedrock wells in northern Vermont is derived from metasomatized ultramafic rock. Serpentinites and talc-magnesite ultramafics occur along thrust slices in the Rowe-Hawley Belt (RHB), a sequence of early Paleozoic metasedimentary and metaigneous rocks that were originally juxtaposed in the suture zone for the Ordovician Taconian Orogeny. Whole rock compositions of meta-ultramafic samples (N = 41) contain As values as high as 1104 ppm in talc-magnesite and 449 ppm in serpentinite, with a mean As value of 93 ppm. Phyllites contain an average As concentration of 22 ppm (N = 34), a value that includes two isolated high concentrations of 101 and 190 ppm As from well cuttings below a thrust slice of As-rich talc-magnesite, and greenstones (N = 33) contain a mean As concentration of 4.1 ppm. Three monitoring wells installed in a recharge area dominated by serpentinite and talc-magnesite contain elevated As (3-10 ppb) and a Mg-HCO₃ signature that reflects water compositions dominated by dissolution of serpentinite and talc-magnesite. SCE paired with XRD indicates that As is released when antigorite dissolves, implying that the As is contained in antigorite structural sites. While XRD indicates that the serpentines are antigorite, preliminary FTIR data reveal basal Si-O stretching vibrations that are intermediate to antigorite (985 cm^-1) and lizardite (950 cm^-1), indicating unusual tetrahedral configuration. These specimens also contain a broad 880 - 890 cm^-1 peak whose intensity is proportional to As concentration in the antigorites and is coincident with As-O stretching vibrations produced by arsenate. Thus, shifting and broadening of antigorite Si-O stretching vibrations may be caused by altered Si-O bond distance due to structural rearrangements that accompany As substituted into tetrahedral sites. Similar shifts in talc spectra were also observed. The 0.34 Ǻ radius of As(V) in tetrahedral coordination is intermediate to Si (0.26 Ǻ) and Al (0.39 Ǻ), and charge balance can be achieved by paired As-Al for Si substitution, or by octahedral vacancies.