Integrated Soil-Bedrock System Analyses Reveal Near-Surface Manganese Repartitioning and Release to Groundwater in North Carolina.

Manganese (Mn) contamination of well water is a widespread problem and an increasing concern in North Carolina’s (NC’s) drinking water supplies.  Roughly 50% of wells in NC have Mn concentrations exceeding the state standard of 50 μg L^-1.  Despite Mn being naturally derived, specific sources of Mn to groundwater are generally unknown, and concentrations are spatially variable, ranging from ~0 to >2000 μg L^-1.  Therefore, it is often difficult to predict risks to exposure. The primary objective of this research is to identify environmental factors that regulate dissolved Mn concentrations in groundwater of the NC Piedmont.  To accomplish this objective, chemical analyses of Mn in regolith (soil and saprolite), bedrock, and well-water samples from ten NC Division of Water Resources groundwater research stations are being integrated with existing statewide well-water data, soil maps, and geology maps. In general, a zone of solid-phase Mn-oxide accumulation persists near the water table (~4.6-9.1 m) in saprolite, and solid-phase Mn speciation – as determined by sequential extraction and X-ray absorption spectroscopy – is dominated by primary, less-reactive Mn-bearing minerals at deeper depths. Across the region, dissolved Mn concentrations in wells are generally highest just below the zone of solid-phase accumulation and decrease with depth.  Based on results from adsorption isotherm experiments, Mn in groundwater is ~30 times more likely to adsorb onto saprolite than the transition zone or bedrock. Collectively, these results suggest that near-surface cycling has led to Mn repartitioning and delivery to groundwater. Following accumulation near the water table, Mn is reductively mobilized and transported downward through a network of bedrock fractures, implying that integrated soil-bedrock system analyses are needed for effective Mn prediction and management.