A world-leader in groundwater age-dating, USGS was tasked with sampling and analyzing environmental tracers in Bennington’s groundwater to estimate when the water infiltrated to, or recharged, the aquifer.

“The groundwater recharge-age data provided by the USGS gave valuable context for the other ages that followed in this study and will continue to be used as a recharge-age framework for Vermont,” said lead study author Jon Kim, a geologist for the Vermont Department of Environmental Conservation and the Vermont Geological Survey.

Consuming drinking water contaminated with PFOA is linked to adverse health outcomes including cancer and thyroid disease. In 2016, hundreds of private wells around a plastics-plant in Bennington had PFOA levels that exceeded the maximum contaminant level limit set by the U.S. Environmental Protection Agency (EPA). Therefore, Vermont launched a multidisciplinary effort to characterize the physical and chemical components of the polluted aquifer, now published in Frontiers in Water.  The study outlines the groundwater flow in Bennington’s fractured bedrock based on collaborative hydrogeological research undertaken from 2016 to 2019, which included USGS science.

“The age dating really helped to constrain where and how PFOA was moving through the groundwater system, clarified how the geologic structure affected its movement, and provided a timescale,” said USGS New England Water Science Center hydrologist James Shanley, who, together with Hank Johnson of the USGS Oregon Water Science Center, applied the model used in the study.

Shanley and USGS colleagues Tom Mack and Joe Levitt sampled groundwater from eight wells with a range of PFOA contamination in Bennington. The USGS Groundwater Dating Laboratory in Reston, VA, analyzed the samples for certain environmental tracers whose concentrations (or absence) help identify groundwater age.

“Very few laboratories in the U.S. have the expertise and analytical equipment to conduct recharge-age dating of groundwater using the Chlorinated Fluorocarbons (CFC-11, -12, and -113), Sulfur Hexafluoride (SF_⁶), and Tritium [tracers],” said Kim.

Groundwater age is used to predict whether contaminants may be present, and pollutants from anthropogenic sources, like PFOA, are more common in younger groundwater. The geologic make-up of an area influences how fast or slow groundwater moves and, therefore, how old or young the groundwater is. Importantly, the water age differences among the sampled wells enable researchers to delineate groundwater pathways and flow dynamics.

Groundwater in the Bennington aquifer may be part of a younger or older age group, with PFOA contamination found in the younger groundwater and none detected in the older groundwater taken from wells near the fault boundaries.

“The individual ages made a lot of sense with respect to the geology and PFOA concentrations,” said Shanley. Knowing the ages of the groundwater at specific locations defines how the physical characteristics of the bedrock influence when and where recharge occurs. The comprehensive characterization created in this study can be applied to other fractured-rock aquifers in the future.

USGS participated in the study as part of a science team led by the Vermont Geological Survey, including collaborators from Middlebury College, Bennington College, State University of New York at Plattsburgh, University of Massachusetts at Amherst and EPA Region 1.

PFOA is a synthetic chemical that resists heat and chemical reactions and is used in products that repel oil, grease, stains and water. The chemical can persist in the environment and the human body for a long time. To learn more about our work on PFOA and other Perfluoroalkyl and polyfluoroalkyl substances (PFAS) contaminants, click here.