WIDESPREAD PFC CONTAMINATION BY AEROSOL DEPOSITION IN BENNINGTON, VERMONT: A LONG-TERM PROBLEM DUE TO RETENTION IN VADOSE ZONE SOILS

A large (>50 km² area) PFC contaminant plume exists in unconsolidated and fractured rock aquifers in the Bennington, VT area. Most of the perfluorooctanoic acid (PFOA) likely emerged as aerosols from a chemical manufacturer in North Bennington, and was deposited over an area extending at least 8 km downwind (~east) of the smoke stacks. PFOA was eluviated through till and gravel, and entered the fractured rock aquifer. The manufacturer ceased operations in 2002, but PFOA is still present in soils throughout the affected area at concentrations >5 ppb. Vadose zone soil samples from monitoring well borings ~1 km downwind (~east) of the manufacturer exhibit a strong positive correlation between total organic carbon (TOC) and PFOA concentration. This may indicate that organic carbon helps bind PFOA to soil particles. There is no correlation between PFOA concentration and the concentrations of various cations within the same soil samples. Below the water table, PFOA concentrations are generally much lower and a correlation between soil PFOA and TOC does not exist. It is possible that PFOA soil retention is predominantly an issue within the vadose zone. The mass of PFOA retained within vadose zone soils around Bennington will act as a reservoir for continued groundwater contamination for many years.

PFOA concentrations in Bennington’s groundwater are highly variable over small areas. Highest concentrations in private wells, in the range of 2,000 ppt (100 times Vermont’s drinking water standard), are predominantly located within 1 km of the manufacturing facility. However, non-detect wells are also found within this same zone. Generally, shallower fractured-rock wells have higher PFOA concentrations than deeper wells. Natural groundwater discharge from glacial aquifers and monitoring wells set in glacial sediments generally contains several 10’s to several 100’s ppt PFOA. This is likely representative of shallow groundwater that is currently migrating into the fractured rock aquifer.