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GEOCHEMICAL PATTERNS OF ARSENIC-ENRICHED GROUND WATER IN FRACTURED, CRYSTALLINE BEDROCK, NORTHPORT, MAINE, USA
Citation:
LIPFERT, G., A. S. REEVE, W. C. SIDLE, AND R. MARVINNEY. GEOCHEMICAL PATTERNS OF ARSENIC-ENRICHED GROUND WATER IN FRACTURED, CRYSTALLINE BEDROCK, NORTHPORT, MAINE, USA. R. Fuge (ed.), APPLIED GEOCHEMISTRY. Elsevier BV, AMSTERDAM, Netherlands, 21(3):528-545, (2006).
Impact/Purpose:
to present information
Description:
High arsenic concentrations of up 26.6 µmol/L (1990 µg/L) occur in ground water collected from a fractured-bedrock system composed of sulfidic schist with granitic to dioritic intrusions. The bedrock is the source of the As within the ground water, but the presence of arsenopyrite in rock core retrieved from a borehole with barely detectable As concentrations shows that there are complicating factors. Chemical analysis of water from 35 bedrock wells throughout a small watershed reveals spatial clustering of wells with high arsenic concentrations. Stiff diagrams and box plots distinguish three distinct types; calcium bicarbonate-dominated water with low As concentrations (CaHCO3 type), bicarbonate-dominated water with moderately high As concentrations (NaHCO3 type), and calcium bicarbonate-dominated water with very high As concentrations (High-As type). We propose that flow conditions and bedrock composition are responsible for the chemical distinctions within the watershed. Lack of correlation of As concentrations with pH indicates that desorption controls are insignificant. Correlations of As concentration with Fe and redox parameters indicates that reductive dissolution of ferric oxyhydroxides may play a role in the occurrence of high As concentrations in the NaHCO3 and High-As type water. The oxidation of sulfide minerals occurs within the ground-water system and is ultimately responsible for the existence of As in the ground water, but there is no correlation between As and SO4 concentrations, probably due to precipitation of ferric oxyhydroxides and adsorption of As under oxidizing conditions.
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GEOCHEMICAL PATTERNS OF ARSENIC-ENRICHED GROUND WATER IN FRACTURED, CRYSTALLINE BEDROCK, NORTHPORT, MAINE, USA