GEOLOGIC FRAMEWORK FOR EVALUATING GROUND WATER RESOURCES IN THE SOUTHERN WORCESTER MOUNTAINS, CENTRAL VERMONT

The identification and protection of ground water resources is an important issue in Vermont. We assembled a multi-disciplinary geologic framework to evaluate ground water resources in the watersheds surrounding the southern Worcester Mountains in central Vermont. These watersheds are underlain by Cambrian-Ordovician bedrock and Pleistocene and Holocene surficial deposits. The data layers for this framework include: 1) bedrock geologic map, 2) surficial material map, 3) photolineament map with structural control, and 4) water well data. Through integration of these data sets, we will assess the factors that affect well yields in the bedrock and surficial “aquifers” in this area. This study will be a prototype for further ground water investigations.

The Worcester Mts are the dominant topographic feature in the study area - a NNE trending, south-plunging anticlinorial ridge cored by resistant schists; this lithology forms the steepest slopes. The flanks and surrounding valleys are composed of generally less resistant amphibolite, phyllites, and granofels. Based on photolineament and structural analysis, the overall topographic grain is parallel to ductile structures, however, specific domains in quartz-rich lithologies are dominated by fractures orthogonal to ductile structures. Drainage patterns in recharge areas are fracture controlled.

Surficial deposits include till, esker and other ice-contact deposits, lacustrine deposits ranging from silty clay to pebbly sand, alluvial fans and fan-terraces, stream terraces, and alluvium. Former lake shorelines range in elevation from 1230 feet down to 650 feet. An esker buried under lake deposits in the Winooski River valley bottom may be an important aquifer. Relatively impermeable ice contact and lacustrine deposits that directly overlie bedrock may serve as aquitards to locally reduce bedrock aquifer recharge and produce artesian conditions in nearby bedrock wells.

Our analyses seek to identify the relationship(s) between well yield and l) lithologic and surficial units 2) proximity to topographic lineaments, 3) surficial material thickness and permeability, 4) surface water proximity, 5) major bedrock structures, 6) slope and other topographic indices, 7) drainage area size.