David Vinson

Presentation given at N.C. Water Resources Research Institute annual conference, March 18-19, 2015, Raleigh, NC (co-authors: Gary Dwyer and Avner Vengosh, Duke University)

Abstract:

As a natural trace element that is enriched in specific materials (e.g. wastewater, coal, marine clays), boron (B) has attracted interest in diverse natural and engineered environments. Boron has distinctive concentration in seawater (4.6 mg/L), imparting boron to marine-derived aquifer material such as the Atlantic Coastal Plain (ACP) of NC. In groundwater, expected behavior of B can yield (1) low B concentration and relatively high B-11/B-10 isotope ratio (adsorption) during salinization; or (2) high B concentration and relatively low B-11/B-10 (desorption) during freshening.

Naturally-occurring boron was examined in groundwater of the confined Cretaceous Upper and Lower Cape Fear aquifer of the ACP in NC. 14 monitoring wells were sampled from fresh, Na-bicarbonate to brackish Na-Cl water (Cl concentration 5-4680 mg/L). Several wells exhibited elevated boron (overall 0.4-6.6 mg/L). A large range of B-11/B-10 was observed (8.5‑51.8‰, as δ¹¹B relative to NBS951 standard). Observed B/Cl ratios (0.003-0.255 mol/mol) are ~10‑1000 higher than seawater, implying B desorption from clays. However, observed groundwater δ¹¹B >39‰ is consistent with the residual, unreacted B after adsorption occurs.

This pattern differs from the overlying Pliocene Yorktown aquifer, in which groundwater δ¹¹B (20.9‑34.7‰) is consistent with desorption of seawater-derived B having δ¹¹B of about 39‰ (Vinson et al. 2011, Hydrogeology Journal, v. 19 p. 981). In the Cape Fear, the pattern of B concentration vs. δ¹¹B could indicate: (1) episodes of partial adsorption and desorption have disrupted the boron isotope mass balance; and/or (2) the boron originally adsorbed to clays had δ¹¹B >39‰ (e.g. marine brine).