2010 GSA Denver Annual Meeting (31 October 3 November 2010)
Paper No. 88-11
Presentation Time: 11:05 AM-11:20 AM

ARSENIC IN CENTRAL MASSACHUSETTS BEDROCK AND GROUNDWATER

MCTIGUE, David F.1, STEIN, Carol L.1, BRANDON, William C.2, KOPERA, Joseph P.3, KESKULA, Anna J.4, and KOTEAS, G. Christopher5, (1) Gannett Fleming Inc, 15 Willard Road, New Ipswich, NH 03071, dmctigue@gfnet.com, (2) U.S. EPA, Region 1, 5 Post Office Square, Suite 100, Boston, MA 02109, (3) Massachusetts Geological Survey, Department of Geosciences, University of Massachusetts, 611 North Pleasant St, Amherst, MA 01003, (4) Department of Geosciences, University of Massachusetts, 611 North Pleasant Street, 233 Morrill Science Center, Amherst, MA 01003, (5) Geology and Environmental Science, Norwich University, 158 Harmon Drive, Northfield, VT 05663

Across the New England ‘arsenic belt,’ groundwater arsenic (As) concentrations often exceed the EPA’s 0.01-mg/L drinking water standard. In overburden groundwater at a site within this belt in north-central Massachusetts, As has been reported at levels up to 7.6 mg/L. Bedrock at the site consists of Silurian Central Maine Terrane metasediments intruded by the Devonian Ayer granodiorite and Chelmsford granite. Exchange of hydrothermal fluids between these lithologies during intrusion and later deformation, faulting, and metamorphism resulted in crystallization of arsenic-bearing minerals, including arsenopyrite. Quaternary deglaciation and unloading dilated joint systems in the bedrock, allowing increased exposure of the mineralogy to meteoric water. Several arsenopyrite alteration products (e.g., scorodite), of varying solubilities, precipitated on fracture surfaces and along grain boundaries between major phases.

In the emerging conceptual model for this site, groundwater is recharged in bedrock uplands and moves downgradient through the fracture network, becoming increasingly reducing as it moves along a flow path. Arsenic dissolved from arsenopyrite and arsenic-bearing alteration phases in bedrock remains in solution until the groundwater discharges to lowland areas hydraulically downgradient. In these adjacent lowlands, glacial sand and gravel overburden lies above the bedrock. When the reducing water reaches more oxidizing conditions, As-sorbing hydrous ferric oxides (HFO) precipitate out on the aquifer solids, resulting in accumulation of As in the deep overburden aquifer. A large landfill at this site, now closed and capped, imposed reducing conditions, and As is mobilized into groundwater by reductive dissolution of the HFO. The presence of elevated As in groundwater is consistent with arsenic-bearing phases generated in granitoids at depth during regional metamorphism, which were subsequently altered, and are being solubilized at present by the circulation of shallow groundwater through varying redox environments. This scenario is supported by geochemical and petrographic studies of the granitoids and the occurrence of the highest groundwater and soil arsenic concentrations in the adjacent deep overburden.

2010 GSA Denver Annual Meeting (31 October 3 November 2010)
General Information for this Meeting
Session No. 88
Geomicrobiology of Arsenic Transformation and Mobilization in Arsenic-Enriched Alluvial Aquifers and Subsequent Impact on Ecosystem as Well as Human Health
Colorado Convention Center: Mile High Ballroom 1AB
8:00 AM-12:00 PM, Monday, 1 November 2010

Geological Society of America Abstracts with Programs, Vol. 42, No. 5, p. 216

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