Analyzing the Relationships Between Aqueous Manganese and Geological Factors in Groundwater in the Shenandoah Valley (thesis)

Abstract

Manganese (Mn) can be either harmful or beneficial to human health, depending on the phase and concentration that the element is found in. In recent years research has indicated that low level chronic exposure to aqueous Mn concentrations >120 ppb in drinking water can result in developmental complications and birth defects. Naturally occurring in the aqueous phase in groundwater and in the solid phase as Mn ore deposits in the Appalachians, concentrations of geogenic Mn in drinking water are not limited by any rigorously enforced environmental laws in the United States. In the United States, the Environmental Protection Agency (EPA) has set an unenforceable health reference level of 300 ppb for Mn. Mn is of particular concern in the Shenandoah Valley due to numerous Mn ore deposits and mining operations, and the fact that a substantial fraction of the Virginia population relies on ground and spring water for domestic use. This thesis examines seven different sites in order to assess the presence of aqueous Mn and identify any possible trends in where it occurs throughout the Shenandoah Valley. Water samples were collected from springs at these sites to supplement pre-existing groundwater well data from the United States Geological Survey's National Water Information System (NWIS) and the Virginia Household Water Quality Program (VAHWQP). Through ion chromatography (IC) and inductively coupled plasma-mass spectrometry (ICP-MS), water samples were analyzed for cation, anion, major element, and trace metal concentrations. Factors such as aquifer lithology were linked with each location to determine if correlations exist with aqueous Mn concentrations. While carbonate aquifers appear to have a protective effect on spring waters by decreasing soluble Mn, those with primary or secondary rock types bearing sulfur (such as black shales) are correlated with increasing soluble Mn in springs, potentially due to reducing conditions. Analyzing the distribution of Mn in Shenandoah Valley drinking water sources suggests that groundwater wells containing shales and sandstones are more at risk to contain aqueous Mn in high enough concentrations to result in chronic long-term exposure. In addition, one of the trends identified is a strong correlation between Mn concentration and iron (Fe) concentration. The data suggests that Mn will increase proportionally to the amount of Fe also found in a water source. Fe and Mn exhibit similar redox-dependent solubility behaviors, with reducing and acidic conditions promoting aqueous Fe(II) and Mn(II) and oxidizing and basic conditions promoting solid Fe(III) oxide and Mn(III/IV) oxide minerals. Other trace metals, such as Fe, that are heavily affected by redox geochemistry were also found to have trends that could be correlated to Mn. Overall, it appears that surface lithology has a notable effect on the presence of soluble Mn (and other elements) in groundwater and what form it takes as it moves throughout solid and fluid reservoirs.