HYDROGEOLOGY OF A FRACTURED BEDROCK AQUIFER IN STRONGLY DEFORMED AND METAMORPHOSED ROCKS OF THE ROWE-HAWLEY BELT, CENTRAL VERMONT

Over the past decade, the Harwood Union High School, located in Duxbury, VT, has struggled to meet increasing water demands. During the summer of 2013, two new bedrock public water supply wells (Wells N and O) were drilled to provide additional groundwater capacity. We used modern geophysical well logging techniques within the open boreholes, including temperature, conductivity, gamma, caliper, and acoustic televiewer tools, to better understand the hydrogeology of this well field.

The well field area lies in the Rowe-Hawley Belt, a tectonic assemblage of rocks that were deformed and metamorphosed by the Ordovician Taconian and Devonian Acadian orogenies. Both wells were completed within the Stowe Formation, which is characterized by fine-grained, silvery green quartz-muscovite-chlorite-albite-magnetite schists. The dominant foliation in these rocks strikes ~north-south and dips steeply east. The dominant fracture set dips steeply and trends ~E-W, orthogonal to foliation. Well N is 185 m deep, encountered the top of bedrock at 33 m, and has a driller's yield of 196 liters per minute (lpm). Well O is 215 m deep, the top of bedrock is at 23 m, and has a yield of 76 lpm.

Important findings of this study are: 1) a high gamma zone in both wells that appears to correlate with a water producing zone / fracture zone; this zone of connection may explain the rapid drawdown in Well O when Well N was pumped (and vice-versa), 2) abrupt changes of conductivity and temperature logs usually correlate with planar structures on caliper logs and water-producing zones, and, 3) surface waters of nearby Loyzelle Brook likely provide recharge to the bedrock aquifer, either through fractures or along the surficial sediment / bedrock interface.

We seek to correlate the geophysical logging data from both wells and then integrate this with pumping test data that was completed after the wells were drilled. We believe that our work will aid with other well siting efforts in the Rowe-Hawley Belt, where multiply deformed metamorphic rocks are the only available aquifer type.