NATURALLY OCCURRING GROUNDWATER CHROMIUM AND VANADIUM IN PIEDMONT SAPROLITE AND FRACTURED CRYSTALLINE ROCKS: RELATIONSHIP BETWEEN REDOX-SENSITIVE SOLUTES AND TRACE ELEMENT SPECIATION

Naturally-occurring chromium (Cr) and vanadium (V) have emerged as elements of public health concern in groundwater in Piedmont fractured crystalline rocks in North Carolina. Both are oxyanion-forming trace elements, and it is conceivable that similar conditions support their mobilization and co-occurrence in groundwater. However, the relationships of these elements with respect to geological sources and groundwater chemical (redox) conditions are not well understood, limiting prediction of occurrence and the potential for human exposure.

These elements were investigated at the Langtree Peninsula Research Station on the Charlotte Terrane. Bedrock geology is primarily felsic meta-intrusive rocks, with a smaller proportion of finer-grained mafic rocks. Six clusters of wells (saprolite, transitional, and bedrock) were sampled (15 wells total). Water samples were examined for redox-sensitive solutes, trace elements, and Cr speciation. Chromium ranged from undetectable to 4.1 μg/L (median 0.5 μg/L). Hexavalent chromium (Cr(VI)) and total dissolved Cr were positively correlated (Spearman’s ρ=0.97) and exhibited slope ~1, indicating that Cr occurs primarily as the oxidized Cr(VI) form. Chromium was correlated with redox-sensitive solutes, including dissolved oxygen (DO; ρ=0.63), nitrate (ρ=0.63), and Mn (ρ=-0.65). Vanadium in well water was 0.2-14.9 μg/L (median 2.1 μg/L) and was less correlated with DO, nitrate, and Mn than was Cr (ρ=-0.22, 0.57, and -0.34, respectively).

Cr and V are not strongly correlated (ρ=0.40), which implies that they may have only partially analogous source mineralogy, speciation, and/or environmental controls. An initial analysis of solid-phase V speciation was undertaken using X-ray absorption near edge structure (XANES) spectroscopy. Preliminary XANES analysis of 6 intermediate to mafic bedrock samples (230-280 mg/kg V determined by XRF) implies that solid-phase V occurs as a mix of V(III) and V(IV) and not entirely as the reduced form V(III). If the intermediate form V(IV) is environmentally relevant, the behavior of V may be considerably more complex than Cr, as both V(IV) and the oxidized form V(V) are thought to be soluble in fresh water. The results of this study point toward improved models for Cr and V occurrence that link regionally significant groundwater evolution during saprolite-water interaction with trace element concentrations in groundwater, including in bedrock fractures typically utilized by drinking water wells.