Elevated Manganese Concentrations in United States Groundwater, Role of Land Surface–Soil–Aquifer Connections

Chemical data from
43 334 wells were used to examine the
role of land surface–soil–aquifer connections in producing
elevated manganese concentrations (>300 μg/L) in United States
(U.S.) groundwater. Elevated concentrations of manganese and dissolved
organic carbon (DOC) in groundwater are associated with shallow, anoxic
water tables and soils enriched in organic carbon, suggesting soil-derived
DOC supports manganese reduction and mobilization in shallow groundwater.
Manganese and DOC concentrations are higher near rivers than farther
from rivers, suggesting river-derived DOC also supports manganese
mobilization. Anthropogenic nitrogen may also affect manganese concentrations
in groundwater. In parts of the northeastern U.S. containing poorly
buffered soils, ∼40% of the samples with elevated manganese
concentrations have pH values < 6 and elevated concentrations of
nitrate relative to samples with pH ≥ 6, suggesting acidic
recharge produced by the oxidation of ammonium in fertilizer helps
mobilize manganese. An estimated 2.6 million people potentially consume
groundwater with elevated manganese concentrations, the highest densities
of which occur near rivers and in areas with organic carbon rich soil.
Results from this study indicate land surface–soil–aquifer
connections play an important role in producing elevated manganese
concentrations in groundwater used for human consumption.