ASSESSING MANGANESE CONCENTRATIONS IN GROUNDWATER ACROSS THE SHENANDOAH VALLEY, VA

Elevated concentrations of aqueous manganese (Mn) in drinking water cause adverse human health effects and undesirable aesthetic consequences. Chronic exposure to aqueous Mn ≥ 100 parts per billion (ppb) may cause increased cancer mortality rates, Parkinson’s disease, and developmental complications in children. Private drinking water sources are particularly vulnerable to Mn contamination because their water quality is not routinely monitored. Mn²⁺ can be mobilized into anoxic groundwater during reductive dissolution of Mn oxides This study uses groundwater data collected by the Virginia Household Water Quality Program (VAHWQP) from 915 private wells in the Shenandoah Valley, VA to assess the spatial distribution and mechanisms of aqueous Mn contamination. Samples were collected from the tap inside residences and analyzed for cations and major anions. The reported type of water treatment system present (if applicable) was recorded; Wilcoxon signed-rank tests determined that UV light and water softener treatments decreased Mn concentrations in a statistically significant way. Geospatial and statistical analyses including concentration mapping, regression analysis, hierarchical cluster analysis, and principal component analysis identified spatial and geochemical characteristics most inclined to contribute to elevated Mn²⁺. Results show that 63% of samples with Mn ≥ 100 ppb occur in shale lithologies near the valley center. This is consistent with groundwater flow modeling that evinces upwelling of old groundwater (≥ 20 years) in the same region. To evaluate mineralogic origins of Mn in groundwater, we performed a ternary analysis of Fe²⁺, Mg²⁺, and Si to link Mn mobilization to reduction of oxides, dissolution of carbonates, and weathering of silicates, respectively. Elevated Mn²⁺ in groundwater is predominantly controlled by redox conditions and is associated with elevated Fe²⁺. When Fe > 1 mole% of the Fe/Mg/Si sum, average Mn concentration was 214 ppb; when Fe < 1 mole%, the average Mn concentration was 17.9 ppb. Consideration of these factors may help reduce risk of Mn exposure among the Shenandoah Valley’s population, ≥ 36% of which utilize private wells for drinking water.