Groundwater Age-Tracing Data and Models in a PFOA-Contaminated Bedrock Aquifer, North Bennington, Vermont

We computed mean groundwater ages and age distributions at eight wells by calibrating lumped parameter models (LPMs) to concentrations of tracers (3H, CFCs, SF6) in samples using the computer program TracerLPM (Jurgens and others, 2012). We also measured 14C but all samples contained ancient carbonate from the limestone bedrock, and we were unable to resolve the atmospherically-derived 14C sufficiently to use it as a tracer. Tritium was below the detection level in two of the eight well-water samples, indicating the water was recharged prior to about 1950. Groundwater from the other six wells could be fitted to LPMs that were either piston flow models (PFM), exponential ? piston flow models (EPM), or binary mixing combinations of PFM and EPM (see Jurgens and others (2012) for model descriptions). Tracer concentrations in samples from three wells were best described using simple models (one PFM and two EPMs), and had mean ages ranging from 22 to 28 years. Tracer concentrations in one well-water sample were fit using a binary EPM-PFM model, and had a mean age of 22 years; water older than 70 years contributed 11% of the water. Tracer concentrations in groundwater from the two remaining wells were fit using binary PFM-PFM models, each with a very young fraction (2.9 and 6.6 years) and a >70 year fraction. In both cases the older component represented about 30% of the water. The modeled groundwater ages and transport mechanisms suggested by the models were consistent with our conceptual model of the hydrogeologic framework and helped to constrain the timescale of movement of PFOA contaminant within the aquifer. Jurgens, B.C., B�hlke, J.K., and Eberts, S.M., 2012, TracerLPM (Version 1): An Excel� workbook for interpreting groundwater age distributions from environmental tracer data: U.S. Geological Survey Techniques and Methods Report 4-F3, 60 p., https://pubs.usgs.gov/tm/4-f3/.