SOURCES AND CONTROLS ON NATURALLY-OCCURRING BORON IN NORTH CAROLINA COASTAL PLAIN GROUNDWATER: EVIDENCE FROM BORON ISOTOPES

Boron (B) has attracted recent attention for its occurrence in materials such as coal, brines, sewage, and seawater. Boron is distinctive in coastal waters because of its high concentration in seawater (4.6 mg/L) and ratio of boron-11 to boron-10 (δ¹¹B 39‰ relative to NBS951 standard). As seawater intrudes, or as saline aquifers are freshened, B adsorption and desorption occurs. Therefore, B occurrence is connected to the availability of fresh groundwater. Boron isotopes record B adsorption-desorption events because of equilibrium fractionation among aqueous B species. The charged species B(OH)₄^- has lower δ¹¹B and is preferentially adsorbed to marine clays. The expected occurrence of B in Coastal Plain groundwater might include (1) low B concentration and elevated δ¹¹B during salinization (boron adsorption); or (2) high B concentration and lower δ¹¹B during freshening (boron desorption).

Elevated B has been documented in confined Coastal Plain aquifers - up to 8.6 mg/L in SC (Lee 1984, USGS Open-File Report 84-247). Here we present boron concentrations and δ¹¹B from 14 wells in the confined Cretaceous Upper and Lower Cape Fear aquifer in NC exhibiting Na-bicarbonate to Na-chloride composition. In the Cape Fear, we observed a large range of salinity (Cl^- concentration 5-4680 mg/L), boron (overall 0.4-6.6 mg/L), B/Cl^- ratios (0.003-0.255 mol/mol), and δ¹¹B (8.5‑51.8‰).

In the overlying Pliocene Yorktown aquifer, B concentrations are lower (0.1-2.6 mg/L) and δ¹¹B (20.9‑34.7‰) is consistent with seawater-derived B desorbing into groundwater (Vinson et al. 2011, Hydrogeol. J., v. 19 p. 981). In both aquifers, B/Cl^- ratios are higher than seawater. Also, B/Cl^- ratios and δ¹¹B are negatively correlated in both aquifers (high δ¹¹B associated with low B/Cl^-), indicating that boron levels are controlled by dilution and desorption.

The combination of high B/Cl^- ratio and δ¹¹B >39‰, observed in the Cape Fear aquifer, warrants special attention because this combination is inconsistent with simple desorption of seawater-derived boron. In the Cape Fear, the pattern of B concentration vs. δ¹¹B could indicate: (1) that boron is seawater-derived, but the B mass balance was disrupted by partial flushing events in the past; and/or (2) boron is from a brine rather than seawater source.