Groundwater Resources of Williston, Vermont - a poster presented at NEGSA, 2008 by
Laurence Becker, Jonathan Kim, David DeSimone, Marjorie Gale and George Springston

ABSTRACT
Bedrock and surficial geologic maps of Williston serve as an integrated framework for addressing groundwater quantity and quality (radionuclide) issues. The town is underlain by metamorphosed Late Proterozoic-Cambrian rift to drift stage clastic rocks of the Green Mountains structurally overlying Lower Cambrian- Middle Ordovician carbonate and clastic continental shelf rocks of the Champlain Valley. These sections were juxtaposed along the west-directed Hinesburg Thrust (HBT) during the Taconian Orogeny. Pleistocene- Holocene surficial deposits overlie this bedrock. Gently sloping terraces underlain variably by sand, silt-clay or deformed till protrude through rounded hills in the north part of town and step down to the Winooski River. Terraces were coeval with Fort Ann Upper, Fort Ann Lower and Champlain Sea levels. The south consists of structurally controlled stream valleys draining glacially scoured hills and ridges. Bedrock lithologies and structures and glacial deposits strongly influence the hydrogeology.

Recently drilled domestic wells indicate that the HBT is a high-yielding structural aquifer with yields averaging ~50 GPM. In the southern part of town, the Lake Iroquois Thrust (LIT) is east of the HBT, but these faults merge together to the north. Fractures in the upper plate of the combined HBT and LIT are dominantly E-W trending whereas lower plate fractures trend more NW. Localized elongate topographic basins in the bedrock formed along the leading edge of the HBT and LIT and also along E-W and NW-SE fracture zones and were filled with thicker surficial deposits; higher yielding wells in these basins may locally benefit from thick permeable overburden. A prominent E-W fracture set controlled the preglacial development of Old Creamery and Butternut tributary valleys. Both valleys were in-filled with a wedge of till and Old Creamery also accumulated considerable melt water sediment. Allen Brook flows north and follows prominent ductile structures, but also likely excavated a preglacial valley just east of the trace of the LIT. Several producing gravel wells confirm the presence of a minor aquifer in the bottom of the bedrock trough beneath thick till. We also hypothesize a possible northward and/or eastward extension of this Allen Brook trough; this would represent an overburden aquifer prospect.

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