MULTI-DISCIPLINARY CHARACTERIZATION OF FRACTURED ROCK AQUIFERS (FRA) TO SUPPORT SOCIETAL NEEDS

The Vermont Geological Survey (VGS) and partners integrate geologic mapping, the spatial analysis (GIS) of well driller reports, and geophysical well logging to construct 3-D conceptual site models (CSM) for characterizing FRA. A CSM can be used to identify groundwater resources for towns and/or for resolving problems in existing wells. Two case studies follow where CSMs were used in decision-making by the Vermont Dept. of Environmental Conservation, town officials, and consultants.

Case Study #1 focuses on East Berkshire, where groundwater is extracted from a grouping of shallow springs and a deep (158.5 m) bedrock well. This array is periodically unable to satisfy town water demands. A GIS analysis of well reports identified a nearby private well purported to produce 80 gpm. Bedrock mapping, geophysical logging, and passive seismic surveys were integrated to produce a CSM showing that this well was initiated in the hanging wall (green phyllite) of a Silurian reverse fault and completed in the footwall (phyllitic quartzite) at 69.2 m. This fault was deformed by tight Devonian folds and the associated cleavage. Heat Pulse Flow Meter and Acoustical Borehole Imager measurements show that groundwater enters the well near the fault (25 m) and exits along an open foliation plane at 67 m. Our CSM is the foundation for plans to site a new public well on an adjacent property.

For Case Study #2, a deep (244 m) bedrock well no longer meets the water needs of a Fairfax housing subdivision. After logging this well and mapping the surrounding bedrock geology, the VGS concluded: 1) An abrupt well deviation from vertical at 120 m, which was caused by a thin conglomeratic quartzite layer, prevented the pump from a deeper placement that would access more storage, 2) The well was blocked at 216 m, isolating the bottom 28 m, and 3) Groundwater emanated from a shallow fracture above the static water level at 21 m. A CSM constructed from these data sets recommends that a shorter pump be installed to get past the well deviation, so that more storage and producing fractures can be accessed between 120 - 216 m, and that the shallow groundwater zone be cased off to mitigate potential nitrate contamination.

In addition to the water quantity issues in East Berkshire, Fairfax, and other towns, VGS also characterizes FRAs with nitrate, PFAS, and radionuclide contamination.