GROUNDWATER AGE-TRACERS SHED LIGHT ON THE NATURE OF PFOA TRANSPORT IN THE N. BENNINGTON, VERMONT BEDROCK AQUIFER

In September 2016, the U.S. Geological Survey sampled eight homeowner wells within a PFOA contaminated area in N. Bennington, Vermont. The objective was to provide insight on the pattern of PFOA concentrations and movement by modeling groundwater age distributions using recharge tracers across variations in bedrock lithology and a gradient in PFOA concentrations. We purged wells until field parameters (pH, specific conductance, temperature, dissolved oxygen) stabilized, then measured alkalinity, total and dissolved Fe, sulfide, nitrate and the above field parameters on site. We then sampled for CFCs (CFC-11, CFC-12, and CFC-113), SF₆, tritium, tritium/helium ratios, ¹⁴C, ¹³C, noble gases, and common dissolved gases. The multiple tracers cover a broad range of groundwater ages, and allow modeling of age distributions using a lumped parameter model (TracerLPM) for best-fit interpretation of non-unique solutions. We will present model results and discuss their implications toward understanding PFOA contamination and transport in the bedrock aquifer. Our interpretation and model selection will be aided by geologic mapping and borehole characteristics provided by the Vermont Geological Survey.