GEOLOGIC FRAMEWORK FOR EVALUATING GROUNDWATER RESOURCES, CHARLOTTE, VERMONT

Well logs are integrated with surficial and bedrock maps to produce a groundwater resource analysis of Charlotte, VT. Derived maps include hydrogeologic units, isopach, flow lines, yields, well depths, and fractures. Charlotte is located in the Champlain Valley in western VT. The Champlain Thrust (CT) transects the town. The upper plate (UP) is Cambrian quartzites, dolostones, and limestone; the lower plate (LP) is Cambrian to Ordovician carbonates and black shales. The gently dipping CT is overprinted by upright N-S open folds (F2) and upright, undulating, open, E-W folds (F3). These interfere to form a dome and basin map pattern. Two high angle faults are inferred in the LP. Some of the dominant fractures are actually fracture cleavages that are axial planar to F2 and F3. There are also fractures parallel to the high angle normal faults of the LP. We are investigating the role that the cleavages play in groundwater flow.

Bedrock is commonly overlain by dense silt-to-fine-sand diamict (lodgement till). The thickness varies widely. Below about 600 feet elev. the till is blanketed with thick lake bottom and shoreline deposits formed in glacial Lake Vermont or the Champlain Sea. Some well logs indicate lenses of gravel and sand deposited directly on bedrock. These are interpreted to be esker channel deposits or subaqueous fans and are overlain by lacustrine silt and clay and younger sediments. In some locations, a younger deformation till overlies the LV deposits. Because the source of this till was largely the lake sediments, it has a silt/clay-rich matrix. The CS deposits postdate this younger till.

Water from 27 bedrock wells was analyzed for 26 metals, 4 non-metals, hardness, and alkalinity. Preliminary interpretation: 1) VT Dept. of Health (VDH) primary (health) standards for U or As were exceeded at only 2 wells, 2) Secondary VDH standards (taste, color, odor) were exceeded for Mn (7.4% of wells), Fe (22% of wells), Cl (3.7% of wells), 3) 74% of all wells had hard water (>150 ppm), and 4) the highest Na level was from a well completed in shale.

Median yield in LP shales is 1.2 gpm, whereas median yield in LP carbonates is 6.0 gpm. In the UP quartzites and cabonates, median yield is 4.5 gpm. The low yields in the LP shales indicate that they are relatively impermeable. The thick silt/clay lacustrine and marine deposits are similarly impermeable.