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Download fileSoil Weathering as an Engine for Manganese Contamination of Well Water
journal contribution
posted on 2016-08-29, 00:00 authored by Elizabeth
C. Gillispie, Robert E. Austin, Nelson A. Rivera, Rick Bolich, Owen W. Duckworth, Phil Bradley, Aziz Amoozegar, Dean Hesterberg, Matthew L. PolizzottoManganese (Mn) contamination
of well water is recognized as an
environmental health concern. In the southeastern Piedmont region
of the United States, well water Mn concentrations can be >2 orders
of magnitude above health limits, but the specific sources and causes
of elevated Mn in groundwater are generally unknown. Here, using field,
laboratory, spectroscopic, and geospatial analyses, we propose that
natural pedogenetic and hydrogeochemical processes couple to export
Mn from the near-surface to fractured-bedrock aquifers within the
Piedmont. Dissolved Mn concentrations are greatest just below the
water table and decrease with depth. Solid-phase concentration, chemical
extraction, and X-ray absorption spectroscopy data show that secondary
Mn oxides accumulate near the water table within the chemically weathering
saprolite, whereas less-reactive, primary Mn-bearing minerals dominate
Mn speciation within the physically weathered transition zone and
bedrock. Mass-balance calculations indicate soil weathering has depleted
over 40% of the original solid-phase Mn from the near-surface, and
hydrologic gradients provide a driving force for downward delivery
of Mn. Overall, we estimate that >1 million people in the southeastern
Piedmont consume well water containing Mn at concentrations exceeding
recommended standards, and collectively, these results suggest that
integrated soil-bedrock-system analyses are needed to predict and
manage Mn in drinking-water wells.