Occurrence and distribution of hexavalent chromium in groundwater from North Carolina, USA.

Hexavalent chromium (Cr(VI)) is a groundwater contaminant that is potentially harmful to human health. Understanding the occurrence of Cr(VI) in groundwater resources is critical for evaluating its risks to human health. Here we report a large dataset (n = 1362) of Cr(VI) and total chromium (CrT) concentrations in public, private, and monitoring wells from different aquifers across North Carolina. These water quality data come from new and previous measurements conducted at Duke University, as well as data reported by the U.S. Environmental Protection Agency, the N.C. Department of Environmental Quality, and the U.S. Geological Survey. The data confirm that Cr(VI) is the predominant species of dissolved Cr and that groundwater from aquifers in the Piedmont region contain significantly higher concentrations than groundwater from the coastal plain. Though there is only one exceedance of the U.S. EPA Maximum Contaminant Level (100 µg/L for CrT) in the dataset, over half of all wells measured for Cr(VI) (470 out of 865) in the dataset exceeded the N.C. Health Advisory Level of 0.07 µg/L. Using information from this dataset, we explore three different approaches to predicting Cr(VI) in groundwater: (1) CrT concentrations as a proxy for Cr(VI); (2) Exceedance probabilities of health goals for groundwater from aquifers located in specific geologic areas; and (3) Censored linear regression using commonly measured field parameters (pH, electrical conductivity, dissolved oxygen) with relationships to Cr(VI) as regressors. Combining these approaches, we have identified several areas in the Piedmont region where Cr(VI) in drinking water wells is expected to be higher than the advisory level, which coincide with large population groundwater reliant populations. While this study focuses on N.C., the wide-spread occurrence of Cr(VI) in groundwater at concentrations above health guidelines in aquifers of the Piedmont region could pose high human health risks to large populations in the eastern U.S.

Role of Calcium in the Coagulation of NOM with Ferric Chloride.

Natural organic matter (NOM) is capable of interfering with Fe hydrolysis and influencing the size, morphology, and identity of Fe precipitates. Conversely, Ca2+ raises surface potential and increases the size and aggregation of Fe precipitates, leading to more effective coagulation and widening the pH range of water treatment. Experiments and modeling were conducted to investigate the significance of the Fe/NOM ratio and the presence of Ca2+ in coagulation. At the high Fe/NOM ratio, sufficient or excess Fe was available for NOM removal, and coagulation proceeded according to expectations based upon the literature. At the low Fe/NOM ratio, however, NOM inhibited Fe hydrolysis, reduced zeta potential, and suppressed the formation of filterable Fe flocs, thereby interfering with NOM removal. In these dose-limited systems without Ca2+, complexation of Fe species by NOM appears to be the mechanism by which coagulation is disrupted. Equilibrating NOM with 1 mM Ca2+ in dose-limited systems prior to dosing with FeCl3 increased Fe hydrolysis and zeta potential, decreased the fraction of colloidal Fe, and improved NOM removal. In systems with Ca2+, data and modeling indicate that Ca2+ complexation by NOM neutralizes some of the negative organic charge and minimizes Fe complexation, making Fe species available for hydrolysis and effective coagulation. This finding represents an important advance in understanding not only how Ca2+ may improve coagulation outcomes, but also in predicting the conditions under which Ca2+ may prove beneficial.

Modeling silica sorption to iron hydroxide.

Experiments were conducted to investigate the fundamentals of silica sorption onto preformed ferric hydroxide at pH 5.0-9.5 and silica concentrations of 0-200 mg/L as SiO2. At all pHs studied, sorption densities exceeding monolayer sorption were observed at silica levels typical of natural waters. Under some circumstances, sorption equaled or exceeded a monolayer while the particle zeta potential remained positive, a phenomenon that is completely inconsistent with available surface complexation models. To address this deficiency, an extended surface complexation model was formulated in which soluble dimeric silica (i.e., Si2O2(OH)5-) sorbs directly to iron surface sites. This new model fit sorption density data up to 0.40 mol SiO2/mol Fe and accurately predicted trends in zeta potential and observed H+ release during silica sorption to ferric hydroxide.