CHARACTERIZATION OF A COMPLEX FRACTURED CARBONATE AQUIFER IN THE TOWN OF HINESBURG, WEST- CENTRAL VERMONT II: HYDROGEOLOGY

Hinesburg, Vermont is located along the Hinesburg Synclinorium. It is between the St. George normal fault to the west and the Hinesburg thrust to the east. The bedrock consists of the Cambrian Winooski dolostone, Cambrian Danby dolomitic sandstone, Cambrian Clarendon Springs dolostone, and the Ordovician Shelburne marble. Bedrock is exposed along several hills rising above the surficial deposits. Much of the bedrock is overlain by lacustrine sediments (silts and clays with boulders). North of town, several test wells were drilled to develop groundwater resources for municipal water supplies.

As part of a 120 hr pump test to determine aquifer characteristics, numerous domestic and public wells were monitored. Hydraulic head data from these well were used to develop a potentiometric map under ambient conditions of the area. The potentiometric map shows that the water table mimics the topography. Groundwater flow is generally to the south parallel to the topographic gradient and to a dominant fracture set. The bedrock hills are areas of groundwater recharge.

A step test consisting of six one-hour pumping intervals was completed. Pumping rates between 685 and 1,200 L/minute resulted in a 70 m drawdown in the pumped well. A 120 hr pump test (pumping rates about 900 L/minute) resulted in a 60 m drawdown in the pumped well. The cone of depression did not extend past 850 m. Calculated transmissivities were 17 m²/day. Calculated storativities·radius² (Sr²) were 0.16.

During the 120 hr pump test, the cone of depression that developed around the pumped well was asymmetrical. The long axis of the cone of depression was roughly parallel to an E-W fracture zone mapped in the field and with EMI surveys. The asymmetry suggests that transmissivities are anisotropic, with greater transmissivities parallel to the long axis of the cone of depression. The cone of depression was asymmetrical with respect to the pumping well. The cone extended farther from the pumping well towards the St. George fault. This suggests that the fault may be acting as a barrier to groundwater flow, as suggested by borehole camera and acoustic televiewer logs resulting in a restricted water supply and greater drawdown. Possible refraction of the potentiometric lines across the fault during pump tests further supports that the St. George Fault may affect groundwater flow.