FRACTURED BEDROCK HYDROGEOLOGY OF A WELL FIELD IN THE COMPLEXLY DEFORMED CONNECTICUT VALLEY TROUGH OF CENTRAL VERMONT

In 2009, the Town of Berlin drilled three bedrock wells in a triangular pattern to serve as future public water supplies as development in Berlin continues. In order to supplement the data (lithologic logs and pumping tests) that had already been acquired by well drillers and hydrogeologists, we logged the open boreholes of these wells using the following geophysical tools: 1) temperature, 2) conductivity, 3) gamma, 4) caliper, and 5) acoustic televiewer. Using all data sets, this study aims to better understand the hydrogeology of the Berlin well field by constructing a series of detailed geologic and hydrogeologic cross sections between these wells.

The well field area lies in the Connecticut Valley Trough (CVT), a Silurian-Devonian basin that was deformed and metamorphosed during the Devonian Acadian Orogeny. The Berlin wells were completed within the Waits River Formation, which is comprised of interlayered marbles and phyllites of varying thicknesses. The dominant foliation in these rocks strikes north-south and dips steeply west. The map pattern in the area is controlled by two asymmetric fold sets. The dominant fracture set dips steeply and trends ~E-W, orthogonal to foliation. The well depths range from 183-184 m and the long-term tested yields range from 182-189 liters/minute.

To date, important findings of this study are: 1) gamma logs were used to construct detailed stratigraphic columns of each well that served as a framework for interpreting the other two logs, 2) groundwater flow into the borehole was indicated by abrupt changes on temperature and conductivity logs, which corresponded with lithologic changes/bedding surfaces, 3) correlation of lithologies was controlled by gentle fold plunges in the N-S dimension, and by the steep westward dip of the dominant foliation in the E-W direction, and, 4) thick marble horizons (>15 m) that were found at depths >135 m are of particular interest as groundwater source zones.

Our ongoing work will correlate the geophysical logging data between all three wells and integrate this analysis with pumping test data. We feel that the hydrogeological models that are developed for the Berlin well field will also be broadly applicable to other wells situated in the CVT, a lithotectonic structure that encompasses ~25% of Vermont, and major parts of New England and southern Quebec.