ANALYSIS OF GROUNDWATER RESOURCES IN THE TOWN OF EAST MONTPELIER, CENTRAL VERMONT

During 2011, The Vermont Geological Survey and Norwich University completed bedrock and surficial geologic maps of East Montpelier. Well log data was integrated with these geologic maps to produce derivative maps that bear on groundwater resources. We summarize the geologic maps, highlight findings of the derivative maps, and then compile hydrogeologic data related to local springs.

A faulted Silurian unconformity, that runs through the NW East Montpelier, separates metamorphic rocks of the Pre-Silurian Green Mountain Belt (GMB)(west) from those of the Silurian-Devonian Connecticut Valley Belt (CVB)(east). The GMB is composed of phyllites and quartzites, whereas phyllites, siliceous marbles interlayered with phyllites, and granites comprise the CVB.

Uplands are underlain by dense Pleistocene silt- and fine-sand-matrix glacial till. Thin (<20’) till and abundant bedrock outcrops are found on hilltops. Bedrock striations indicate that the latest ice movement was SSE or S. Winooski River valley deposits consist of Holocene stream deposits overlying Pleistocene lacustrine sand, silt, and silty clay deposits, which, in turn, overlie till or ice-contact sand and gravel. The gravels and sands underlying the lake deposits may be esker deposits. Thick fine-grained deposits are found in the Kingsbury Branch valley.

The isopach map shows that the thickest surficial deposits (> 100’) are located in the Winooski River valley. A generalized potentiometric surface map shows groundwater flow directions toward the North Branch in the NW quarter of town and toward the Winooski River and tributaries in the SE three-quarters of town.

Wells completed in the Silurian-Devonian rocks have twice the average yield as those in Pre-Silurian rocks. Many bedrock wells with yields >20 gpm are found in the vicinity of the Winooski River Valley, where overburden thickness is >25’. We hypothesize that groundwater from the porous and permeable surficial aquifer percolates downward and supplements the yield of the bedrock aquifer.

Locally, productive springs emanate from specific Silurian-Devonian bedrock aquifers. By dividing the spring discharge volume/year by the groundwater recharge/year, we obtained a recharge area that is consistent with the area of the small encompassing watersheds.