RELATIONSHIP BETWEEN FRACTURES, VERTICAL WATER FLOW AND GEOLOGIC STRUCTURES ON THE DISTRIBUTION OF PFOA IN DOMESTIC WELLS, BENNINGTON, VT

The distribution of PFOA contamination in Bennington, VT suggests many different vectors for transport of the contaminant. One vector we are studying is bedrock fractures. We are using borehole geophysical tools to characterize different lithologies intersected by the bore-hole using gamma logging. Acoustical bore hole imaging and caliper logging are used to determine the orientation and aperture of fractures and other planar features intersecting the well. The vertical flow of water in bore holes and groundwater fluxes at fractures are identified using a heat-pulse flow meter accompanied by bore-hole fluid temperature and specific conductivity profiles. A bore-hole video camera is used to further aid in our interpretation of geophysical well logs.

Bore-hole geophysical logs have been completed for 12 wells. Depths of wells range between 42 and 152 meters. The distribution of wells we have logged is affected by the availability of wells. We are using wells that are in use as domestic water supplies. In order to log a well, it is necessary that the pump and other hardware are removed. So, we depend on home owners who are willing to be without water for day while their well is logged.

Lithological interpretations from geophysical logs are consistent with our understanding of the geologic setting. Two wells intersect thrust faults at depth. Thrust faults are clearly identified by extrapolation of bedrock geology maps and identified fault structures in the well logs coincident with changes in gamma logs.

Vertical flow in the wells varies throughout the study area with no discernible pattern in spatial distribution. This suggests that flow in wells is determined largely by which fractures are intersected. Some wells are dominated by upward flow suggesting fractures with high hydraulic heads at depth relative to the heads in more shallow fractures. Other wells exhibit downward flow. A couple of wells have bi-directional flow where water from deeper fractures flows upward while flow from shallow fractures is downward.

Preliminary analysis shows no clear linkages between the bore-hole hydrology and the overall distribution of PFOA in wells. However, another consideration is the effect of airborne distribution of PFOA. We are studying the role of airborne distribution of PFOA and its relationship to groundwater recharge areas.