GROUNDWATER AVAILABILITY IN SURFICIAL AND BEDROCK UNITS IN THE PLAINFIELD QUADRANGLE, NORTH-CENTRAL VERMONT

Bedrock and surficial mapping is integrated with well logs and borings to evaluate the availability of groundwater. Water from 20 bedrock wells has been analyzed for major and trace elements.

Thick surficial deposits occur in the valley bottoms of the Winooski River (WR), Kingsbury Branch (KB), and Nasmith Brook. Uplands are underlain by dense till, with abundant bedrock outcrops on the hilltops. Lake deposits are widespread in the valleys in the quadrangle. Most appear to have formed in glacial Lake Winooski (GLW), which had a present-day shoreline elevation of about 294 m at Plainfield village.

Surficial deposits in the WR valley are commonly >30 m thick and consist of a sequence of stream deposits overlying the GLW deposits, which, in turn, overly bedrock, till, or ice-contact sand and gravel. Surface and subsurface data suggest that the gravel and sand below the lake deposits may be discontinuous esker deposits. This buried sand and gravel may have significant aquifer potential. A thick gravel deposit located 0.1 km east of the confluence of the KB and the WR may be a northern extension of the WR esker deposits. However, no other indication of coarse-grained deposits is seen in the KB valley south of East Calais village in the N part of the area, where there are again thick buried gravels.

The buried paleochannel in the Nasmith Brook valley is roughly parallel to the present Nasmith Brook and was a tributary of the paleo-Winooski. It is buried under >80 m of sediment, with sandy till at the surface and stratified sand and gravel below.

Bedrock lithologies consist of interlayered phyllites and impure marbles (Waits River Fm), interstratified phyllites and phyllitic quartzites (Gile Mt Fm), and granites of the New Hampshire Series. The dominant foliation in the metasedimentary rocks strikes NNE and dips moderately westward whereas the most abundant fracture attitude trends ~E-W and dips steeply. Secondary porosity can be associated with marble beds and/or ductile or brittle structures.

Significant groundwater resources appear to exist in buried ice-contact deposits below GLW deposits in the main WR valley and at least parts of the KB valley, in buried ice-contact deposits below till in the Nasmith Brook valley, in bedrock where thick surficial deposits overlie bedrock, and in bedrock areas with significant secondary porosity.