Flow and sorption controls of groundwater arsenic in individual boreholes from bedrock aquifers in central Maine, USA.
To understand the hydrogeochemical processes regulating well water arsenic (As) evolution in fractured bedrock aquifers, three domestic wells with [As] up to 478 μg/L are investigated in central Maine. Geophysical logging reveals that fractures near the borehole bottom contribute 70-100% of flow. Borehole and fracture water samples from various depths show significant proportions of As (up to 69%) and Fe (93-99%) in particulates (>0.45 μm). These particulates and those settled after a 16-day batch experiment contain 560-13,000 mg/kg of As and 14-35% weight/weight of Fe. As/Fe ratios (2.5-20 mmol/mol) and As partitioning ratios (adsorbed/dissolved [As], 20,000-100,000 L/kg) suggest that As is sorbed onto amorphous hydrous ferric oxides. Newly drilled cores also show enrichment of As (up to 1300 mg/kg) sorbed onto secondary iron minerals on the fracture surfaces. Pumping at high flow rates induces large decreases in particulate As and Fe, a moderate increase in dissolved [As] and As(III)/As ratio, while little change in major ion chemistry. The δD and δ(18)O are similar for the borehole and fracture waters, suggesting a same source of recharge from atmospheric precipitation. Results support a conceptual model invoking flow and sorption controls on groundwater [As] in fractured bedrock aquifers whereby oxygen infiltration promotes the oxidation of As-bearing sulfides at shallower depths in the oxic portion of the flow path releasing As and Fe; followed by Fe oxidation to form Fe oxyhydroxide particulates, which are transported in fractures and sorb As along the flow path until intercepted by boreholes. In the anoxic portions of the flow path, reductive dissolution of As-sorbed iron particulates could re-mobilize As. For exposure assessment, we recommend sampling of groundwater without filtration to obtain total As concentration in groundwater.
Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964, USA; School of Earth and Environmental Sciences, Queens College and Graduate Center, City University of New York, 65-30 Kissena Blvd., Flushing, NY 11367, USA.
Qiang
Q
Yang
U.S. Geological Survey, Maine Water Science Center, 196 Whitten Road, Augusta, ME 04330, USA.
Charles W
CW
Culbertson
U.S. Geological Survey, Maine Water Science Center, 196 Whitten Road, Augusta, ME 04330, USA.
Martha G
MG
Nielsen
U.S. Geological Survey, Maine Water Science Center, 196 Whitten Road, Augusta, ME 04330, USA.
Charles W
CW
Schalk
U.S. Geological Survey, Branch of Geophysics, 11 Sherman Place, Unit 5015, University of Connecticut, Storrs, CT 06269, USA.
Carole D
CD
Johnson
Maine Geological Survey, 93 State House Station, Augusta, ME 04333, USA.
Robert G
RG
Marvinney
Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964, USA.
Martin
M
Stute
Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964, USA; School of Earth and Environmental Sciences, Queens College and Graduate Center, City University of New York, 65-30 Kissena Blvd., Flushing, NY 11367, USA. Electronic address: yan.zheng@qc.cuny.edu.
Yan
Y
Zheng
D000327:Adsorption; D001151:Arsenic / Q000032:analysis*; D004784:Environmental Monitoring; D019015:Geologic Sediments / Q000737:chemistry; D060587:Groundwater / Q000737:chemistry*; D008281:Maine; D014872:Water Movements; D014874:Water Pollutants, Chemical / Q000032:analysis*; D014881:Water Supply
D014874:Water Pollutants, Chemical; D001151:Arsenic
Arsenic; Domestic well; Fractured bedrock aquifer; Geophysical logging; Pumping test; Sorption