Condensed from a presentation at NEGSA 2007:
Groundwater Resources of Woodstock, Vermont
by Laurence Becker, David DeSimone, Peter Thompson,
and Marjorie Gale
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 nearly horizontal, 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.
CLICK ON THE IMAGES BELOW TO ENLARGE THEM (PDF FILES).
Paper copy of 12 plates may be purchased for $60.00 or $10.00 per map. Click here to download digital data (Geodatabase)