Arsenic in a fractured slate aquifer system, New England, USA: Influence of bedrock geochemistry, groundwater flow paths, redox and ion exchange

Ryan, Peter C.; Kim, Jonathan J.; Mango, Helen; Hattori, Kéiko; Thompson, Ali

doi:10.1016/j.apgeochem.2013.09.010

Cited by 30 publications

(14 citation statements)

References 31 publications

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“…Concentrations of arsenic measured in the Sw pyrites (up to 1,944 mg kg −1 ) are consistent with concentrations measured in pyrites present in low grade slates in nearby southwestern Vermont (Ryan et al, 2013) where calculated As concentrations in pyrite are between 236 and 2,074 mg kg −1 indicating an enrichment of arsenic in the shale protolith occurred as marine depositional conditions became more anoxic. Mango and Ryan (this issue) present a convincing case through the use of sulfur isotopes to show that the sulfur isotopic composition of sedimentary/diagenetic pyrite in their marine protolith reflects the change in redox conditions during protolith deposition and arsenic uptake in pyrites increased as sedimentary conditions became more anoxic.…”

Section: Discussionsupporting

confidence: 80%

“…However, Yang et al, (2009) show a west to east spatial correlation for decreasing groundwater As but unfortunately the groundwaters in the low grade Sw and SOv rocks were not directly sampled as part of their study. Arsenian pyrite oxidation is implied as an arsenic mobilization mechanism in As-rich black shales from Vermont by Ryan et al (2013). Strong correlations among Fe, SO 4 and As in groundwater were observed.…”

Section: Discussionmentioning

confidence: 89%

“…Therefore, it is plausible that regional scale fluid migration common in prograde metamorphism can also concentrate or re-distribute arsenic in the upper crust. Few studies, however, have addressed arsenic abundance in regionally metamorphosed rocks where elevated arsenic concentrations in groundwater are naturally occurring (Ayotte et al, 2003; Ryan et al, 2013) and are not derived from anthropogenic contamination. While metamorphic aquifers may not exhibit groundwater yields comparable to sedimentary aquifers they often provide important sources of drinking water to rural communities relying on private wells.…”

Section: Introductionmentioning

confidence: 99%

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Heterogeneous arsenic enrichment in meta-sedimentary rocks in central Maine, United States

O’Shea

Stransky

Leitheiser

et al. 2015

Science of The Total Environment

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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=36) than the neighboring Vassalboro Group (mean As 19.1 mg kg−1, median 6.0 mg kg−1, n=36). 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 1,944 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.

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Section: Discussionsupporting

confidence: 80%

Section: Discussionmentioning

confidence: 89%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Heterogeneous arsenic enrichment in meta-sedimentary rocks in central Maine, United States

O’Shea

Stransky

Leitheiser

et al. 2015

Science of The Total Environment

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“…(Color online. ) is mostly fixed in sulfide minerals (e.g., Ryan et al 2013). For As to be incorporated in the APS minerals, it must be oxidized to As 5+ , which is stable in oxidizing, near-surface conditions (Smedley and Kinniburgh 2002).…”

Section: Nature Of Fluids Responsible For Aps Mineral Formationmentioning

confidence: 99%

Compositional variation and timing of aluminum phosphate-sulfate minerals in the basement rocks along the P2 fault and in association with the McArthur River uranium deposit, Athabasca Basin, Saskatchewan, Canada

Adlakha

Hattori

2015

American Mineralogist

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The Athabasca Basin hosts world class uranium deposits, such as the McArthur River deposit. This paper presents the occurrence of aluminum phosphate-sulfate (APS) minerals in the metasedimentary rocks along the P2 fault, the main ore-hosting fault of the McArthur River deposit. It compares the APS minerals along the P2 fault with those outside the fault, examined in this study, and those from other deposits of the Athabasca Basin and from other Paleo-to Mesoproterozoic basins worldwide.APS minerals are common along the P2 fault but rare outside of the P2 fault zone in the basement and along the unconformity between the Athabasca sandstones and the basement. The APS minerals along the P2 fault occur with sudoite (± illite, magnesiofoitite) and are zoned with Sr-, Ca-, and S-rich cores (solid solution between svanbergite, crandallite, and goyazite) and LREE-and P-rich rims (close to florencite composition). APS minerals in the Bleached Zone (altered rocks along the unconformity consisting predominantly of kaolin and illite) are Sr-, Ca-, and S-rich (high svanbergite component) and occur with kaolin. APS minerals in the Red-Green Zone (mingled red hematitic and green chloritic basement rocks below the Bleached Zone) occur with sudoite and clinochlore. They contain relict cores of LREE-and As-rich arsenoflorencite-(Ce) and rims of svanbergite-goyazite-crandallite solid solution.The occurrence of svanbergite-crandallite-goyazite along the unconformity suggests their formation by relatively oxidizing fluids during diagenesis of the overlying sandstones. The relict cores of arsenoflorencite-(Ce) in the Red-Green Zone are interpreted to be the product of paleo-weathering before the deposition of the Athabasca sandstones. Florencitic APS minerals are found along the entire studied strike length (7 km) of the P2 fault, including the ore zone and non-mineralized areas, but are absent outside the fault zone. The florencitic APS minerals contain low SO 4 2-in the ore zone, suggesting relatively reducing conditions during their crystallization. Zoned APS minerals (with svanbergitic cores and florencitic rims) proximal to ore contain elevated U (up to 16 ppm). These features suggest that diagenetic, oxidizing, and uranium-bearing fluids traveled along the P2 fault and became relatively reduced, especially within the ore zone. It also suggests florencitic APS minerals are contemporaneous with uranium mineralization. The restricted occurrence of florencitic APS mineral along the P2 fault in the basement suggests their use in identifying fertile basement structures associated with uranium mineralization.

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“…1), 28% (50/176) of low elevation wells (< 245 meters above sea level [masl]) exceed 10 µg/L As, whereas only 3% (2/60) of higher-elevation wells (245–600 masl) exceed 10 µg/L As in a slate aquifer with presence of arsenian pyrite (200–2,000 mg/kg As) (Ryan et al, 2013). Additionally, geochemically reducing and slightly alkaline conditions (pH > 7) are associated with high As values (Table 1).…”

Section: Risks From Arsenic In Private Well Water Of Northeast Unitmentioning

confidence: 99%

At the crossroads: Hazard assessment and reduction of health risks from arsenic in private well waters of the northeastern United States and Atlantic Canada

Zheng

Ayotte

2015

Science of The Total Environment

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This special issue contains 12 papers that report on new understanding of arsenic hydrogeochemistry, performance of household well water treatment systems, and testing and treatment behaviors of well users in several states of the northeastern region of the United States and Nova Scotia, Canada. The responsibility to ensure water safety of private wells falls on well owners. In the U.S., 43 million Americans, mostly from rural areas, use private wells. In order to reduce As exposure in rural populations that rely on private wells for drinking water, risk assessment, which includes estimation of population at risk of exposure to As above the EPA Maximum Contaminant Level, is helpful but insufficient because it does not identify individual households at risk. Persistent optimism bias among well owners against testing and barriers such as cost of treatment mean that a large percentage of the population will not act to reduce their exposure to harmful substances such as As. If households are in areas with known As occurrence, a potentially large percentage of well owners will remain unaware of their exposure. To ensure that everyone, including vulnerable populations such as low income families with children and pregnant women, is not exposed to arsenic in their drinking water, alternative action will be required and warrants further research.

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Arsenic in a fractured slate aquifer system, New England, USA: Influence of bedrock geochemistry, groundwater flow paths, redox and ion exchange

Cited by 30 publications

References 31 publications

Heterogeneous arsenic enrichment in meta-sedimentary rocks in central Maine, United States

Heterogeneous arsenic enrichment in meta-sedimentary rocks in central Maine, United States

Compositional variation and timing of aluminum phosphate-sulfate minerals in the basement rocks along the P2 fault and in association with the McArthur River uranium deposit, Athabasca Basin, Saskatchewan, Canada

At the crossroads: Hazard assessment and reduction of health risks from arsenic in private well waters of the northeastern United States and Atlantic Canada

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