title: Heterogeneous arsenic enrichment in meta-sedimentary rocks in central Maine, United States.
abstract: Arsenic is enriched up to 28 times the average crustal abundance of 4.8 mg kg(-1) for meta-sedimentary rocks of two adjacent formations in central Maine, USA where groundwater in the bedrock aquifer frequently contains elevated As levels. The Waterville Formation contains higher arsenic concentrations (mean As 32.9 mg kg(-1), median 12.1 mg kg(-1), n=38) than the neighboring Vassalboro Group (mean As 19.1 mg kg(-1), median 6.0 mg kg(-1), n=38). The Waterville Formation is a pelitic meta-sedimentary unit with abundant pyrite either visible or observed by scanning electron microprobe. Concentrations of As and S are strongly correlated (r=0.88, p<0.05) in the low grade phyllite rocks, and arsenic is detected up to 1944 mg kg(-1) in pyrite measured by electron microprobe. In contrast, statistically significant (p<0.05) correlations between concentrations of As and S are absent in the calcareous meta-sediments of the Vassalboro Group, consistent with the absence of arsenic-rich pyrite in the protolith. Metamorphism converts the arsenic-rich pyrite to arsenic-poor pyrrhotite (mean As 1 mg kg(-1), n=15) during de-sulfidation reactions: the resulting metamorphic rocks contain arsenic but little or no sulfur indicating that the arsenic is now in new mineral hosts. Secondary weathering products such as iron oxides may host As, yet the geochemical methods employed (oxidative and reductive leaching) do not conclusively indicate that arsenic is associated only with these. Instead, silicate minerals such as biotite and garnet are present in metamorphic zones where arsenic is enriched (up to 130.8 mg kg(-1) As) where S is 0%. Redistribution of already variable As in the protolith during metamorphism and contemporary water-rock interaction in the aquifers, all combine to contribute to a spatially heterogeneous groundwater arsenic distribution in bedrock aquifers.
authors.affiliation: Department of Marine Science and Environmental Studies, University of San Diego, 5998 Alcala Park, San Diego, CA 92110, United States; Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964, United States. Electronic address: bethoshea@sandiego.edu.
authors.forename: Beth
authors.initials: B
authors.lastname: O'Shea
authors.affiliation: Department of Marine Science and Environmental Studies, University of San Diego, 5998 Alcala Park, San Diego, CA 92110, United States.
authors.forename: Megan
authors.initials: M
authors.lastname: Stransky
authors.affiliation: Department of Marine Science and Environmental Studies, University of San Diego, 5998 Alcala Park, San Diego, CA 92110, United States.
authors.forename: Sara
authors.initials: S
authors.lastname: Leitheiser
authors.affiliation: School of Earth and Environmental Sciences, Queens College, City University of New York, 65-30 Kissena Blvd., Flushing, NY 11367, United States.
authors.forename: Patrick
authors.initials: P
authors.lastname: Brock
authors.affiliation: Maine Geological Survey, 93 State House Station, Augusta, ME 04333, United States.
authors.forename: Robert G
authors.initials: RG
authors.lastname: Marvinney
authors.affiliation: School of Earth and Environmental Sciences, Queens College, City University of New York, 65-30 Kissena Blvd., Flushing, NY 11367, United States; Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964, United States.
authors.forename: Yan
authors.initials: Y
authors.lastname: Zheng
mesh_terms: D001151:Arsenic / Q000032:analysis*; D004784:Environmental Monitoring; D019015:Geologic Sediments / Q000737:chemistry*; D060587:Groundwater / Q000737:chemistry*; D008281:Maine; D014874:Water Pollutants, Chemical / Q000032:analysis*
chemical_list: D014874:Water Pollutants, Chemical; D001151:Arsenic
keywords: Arsenic; Bedrock aquifer; Prograde metamorphism; Pyrite; Pyrrhotite; Sulfur