GROUNDWATER RESOURCES OF WOODSTOCK, VERMONT

The Town of Woodstock requested surficial and aquifer mapping from the Vermont Geological Survey to aid in groundwater protection and planning, which are priorities in the Woodstock Municipal Plan and the Two Rivers-Ottauquechee Regional Plan. Bedrock mapping was included since many residential wells penetrate bedrock.

The Waits River Formation consists of interbedded garnet schist and punky-weathering sandy marble and underlies much of the town. This bedrock was easily abraded by glacier ice and hills were rounded and veneered with till. Marble layers are more permeable than schists. Where the layers are gently to moderately inclined, bedrock wells penetrate alternating schist and marble and thus are likely to yield more water. Where the layers are more vertical, wells may show exceptionally high or low yields. A few high-yield wells in the SW corner of town tap well jointed, dense Barnard Volcanics. Two dominant joint sets, oriented roughly east-west and northeast, may explain the course of the Ottauquechee River and may also control the location of glacially scoured pockets in bedrock. Lineaments that are coincident with joint sets measured in the field may help locate future productive wells.

As part of the surficial mapping project, data from well logs were consulted to evaluate yield, recharge potential to aquifers, delineate the extent of overburden aquifers, and to determine the 3-D distribution of glacial deposits. Thin till predominates in the upland region. Ice contact deposits, chiefly sand and gravel, occur primarily as isolated kamic deposits including minor kame terraces along the major valleys. Valleys contain thick overburden, primarily till, with glacially over deepened valley pockets. Beneath this till is a gravel unit yielding water to wells from a confined aquifer. The stratigraphy of these gravel wells is commonly capped by a fluvial terrace or flood plain unit of sand and gravel representing an unconfined aquifer.

Maps that combine information from the bedrock and surficial studies help to identify areas where thin till with relatively high recharge potential coincide with areas underlain by more permeable, moderately inclined marble layers. Derivative groundwater resource maps and recharge area maps are the result of these studies.