GEOLOGICAL CONTROL, GROUNDWATER REDOX EVOLUTION AND NATURALLY OCCURRING TRACE ELEMENTS (AS, CR, V, MN) IN THE NORTH CAROLINA PIEDMONT GROUNDWATER SYSTEM

In the Piedmont and Blue Ridge of North Carolina, crystalline rock aquifers provide drinking water to over 2.4 million residents, where geogenic arsenic, chromium, vanadium, and manganese exceed health advisory levels for safe drinking water. These trace elements tend to co-occur with groundwater, mostly redox-sensitive elements with similar chemical and thermodynamic properties, such as affinity for adsorption and nearby pe-pH boundaries in aqueous environments. While a single contaminant in drinking water can be toxic to public health, having multiple contaminants in the same water increases the health risks multiple times. This dissertation characterizes the geochemical and hydrological controls on the release of trace metals from parent rock to moving groundwater, including the co-occurrence of (i) arsenic (As) and manganese (Mn), and (ii) chromium (Cr) and vanadium (V).
In Research Study 1, two proposed redox frameworks (V1 and V2) were applied to the NCWELL water quality data of approximately 53,000 private wells without DO but containing partial redox parameters of 8 crystalline rock terranes in the Piedmont and Blue Ridge of NC. Redox parameters pH, NO3-, Mn, and Fe were used to classify water samples. Nitrate was used as the reliable oxic state indicator, Mn as the reliable anoxic state indicator, and pH to categorize water samples into four groups—probable oxic, anoxic, oxic/anoxic, and no pH categories—before applying the V2 framework. In both frameworks, approximately 89% of the samples were successfully classified into I-oxic, II-suboxic/low NO3-, III-mixed, IVA-Mn reducing, and IVB-Fe reducing categories. Across all terranes, 55 to 61% of the water samples were either oxic or low-nitrate oxic, whereas 21% were anoxic, and 18 to 24% were in the mixed redox category. However, significant differences were found between individual terranes. Charlotte Terrane was characterized by the prevalence of oxic conditions ( 69% in V1; 65% in V2), while Carolina Terrane had the most anoxic conditions (30% in V1 and V2). These differences could be attributed to geological variations, with aquifer mineralogy serving as the primary factor and redox conditions acting as the secondary factor. The redox classification frameworks are valuable for categorizing water samples lacking DO and addressing missing data, a common issue in large water quality datasets. Additionally, the insights gained from the redox distribution and framework development in this study can be applied to other datasets for a deeper understanding of redox conditions.
Research Study 2 examines the geochemical evolution of groundwater along representative flow paths in the Charlotte and Carolina Terranes to quantify the dominant reactions influencing pH and redox conditions, which are the master variables of trace element behavior from initial to final water masses. Field-collected water chemistry data were integrated with expected mineral assemblages, inverse models generated by PHREEQC, and calculated pe values. A representative flow path in the Carolina Terrane is more geochemically evolved overall (–2.45E-04 mmol/L), with higher net redox mole transfers compared to a representative flow path in the Charlotte Terrane (- 4.53E-04 mmol/L). Reducing conditions are more dominant in the Carolina Terrane, and oxidizing conditions in Charlotte, consistent with the broad regional trends observed in Research Study 2.
Research Study 3 examines redox-sensitive elements (As, Cr, V, and Mn) and their co-occurrence at two research sites in the Charlotte (LGMR) and Carolina (NCZGMR) Terranes, advancing DEQ/USGS studies by introducing (i) trace element speciation and (ii) discrete depth sampling. In both sites, the proportion of water samples with As (> 0.1 µg/L) and Mn (> 50 µg/L) increases with pH, alkalinity, but decreasing DO, with As remaining as As(III) and Mn as Mn(II) in the near-neutral to slightly alkaline pH range (6.5 to 8). Statistical analysis of the Carolina Terrane samples showed a strong co-occurrence (ρ = 0.8) between As and Mn, with elevated As levels frequently exceeding the EPA MCL. Despite high Cr (median, 0.3 µg/L) and V (median, 1.3 µg/L) concentrations in the mafic aquifers of the Charlotte Terrane, no consistent co-occurrence of these elements is observed. Most Cr occurred as Cr (VI) (median, 0.3 µg/L), exceeding the NC Health Advisory Limit of 0.07 µg/L, and V as V(V). Overall, Cr and V do not co-occur in the Piedmont groundwater, with both elements largely existing in their most soluble and toxic forms (Cr(VI) and V(V)).
These contributions enhance the understanding of groundwater chemistry, geochemical evolution, and redox processes of redox-sensitive elements, while also providing a valuable tool for detecting trace metals that may exceed recommended levels, thereby helping to prevent public health risks. These hydrochemical drives can be utilized to identify areas of higher risk and predict the occurrence and co-occurrence of trace metals in the Piedmont Groundwater System. On a broader scale, this knowledge can support the management and protection of sustainable water supplies, ensuring their long-term viability for current and future residents.