Superfund Research Program

Arsenic, Iron, Sulfur and Organic Carbon Speciation and Their Impact on Groundwater Arsenic

Project Leader: Benjamin C. Bostick
Co-Investigators: Brian J. Mailloux (Barnard College), Steven N. Chillrud, Alexander F. van Geen
Grant Number: P42ES010349
Funding Period: 2000-2017

Project-Specific Links

Final Progress Reports

Year:   2016  2010  2005 

In 2010, Dr. Van Geen and his research group focused on two themes: (1) Arsenic adsorption and its implication for transport of arsenic in shallow aquifers of Bangladesh and (2) processes regulating As levels in groundwater in Maine.

Kinetics and Equilibrium of As mobilization in Bangladesh

Kathleen Radloff conducted a study along a flow path that leads from a sandy recharge area in a village where groundwater As levels are as low as 10 μg/L to a nearby stream where As concentrations exceed 500 μg/L. She documented that the distribution coefficient for As describing the partitioning of As between the sands and groundwater was remarkably constant (~2 L/kg) along the transect despite relatively constant As levels released from the solid phase by a 1 M phosphate extraction. Arsenic speciation data obtained by X-ray absorption spectroscopy suggest that the discrepancy may reflect the precipitation of As-sulfides that contribute to keeping groundwater As levels low in the recharge area (Aziz et al., 2010a). Subsequent column experiments supported this interpretation.

³H-³He dating of the samples along the same transect in Bangladesh indicates a broad increase in As concentrations with groundwater age that is consistent with results previously reported by the research team from elsewhere in Araihazar. This relationship can be reproduced by a simple groundwater flow and As transport model constrained by the new data from the site. The key implication is that there is significant As adsorption even in aquifers composed of reduced (grey) sands of Bangladesh which means that changes in As concentrations will be retarded by at least a factor of ten due to a large pool of exchangeable As present in the solid phase (Aziz et al., 2010b).

Arsenic During Groundwater Discharge

During groundwater discharge along the Meghna River in Bangladesh and at Waquoit Bay, MA, the partitioning of As between groundwater and sediment is also controlled by sorptive equilibrium. High spatial density piezometers show that groundwater Fe and As showed systematic attenuation from well water to riverbank or bayside porewater, indicating trapping of As in a reactive barrier consisting of amorphous Fe-oxides (Jung and Zheng, submitted-a; Jung et al., submitted-b).

Bedrock Geology and Arsenic Distribution in Greater Augusta, Maine

A geostatistical analysis demonstrates that the bedrock geology controls the spatial pattern of As distribution at intermediate scale (1-10 km) in the study area in Maine. A logistic regression model showed that bedrock geology, soil arsenic content, groundwater pH, dissolved oxygen, nitrate and sulfate played important roles in controlling groundwater arsenic concentrations. The logistic regression model was verified by additional sampling in 2010 of well water (n=307) in 5 additional towns where 23% of wells were found to contain arsenic greater than 10 μg/L. New results confirm a strong spatial association between groundwater arsenic and bedrock geology, pH, dissolved oxygen and nitrate concentrations. This detailed hydrogeochemical study has focused the attention of researchers on three wells to illustrate the effects of different flow paths on As concentrations in fractures using geophysical logging combined with fracture-specific water sampling in 2010.