Considerations in the study of ground-water-flow systems in fractured crystalline rock of the New England Uplands

Bulk-fluid flow processes in crystalline rock of the New England Uplands were evaluated by use of cross-sectional, finite-difference, steady-state models of ground-water flow. These models are based on a generalized landscape within the study area; sensitivity tests were done to identify factors controlling flow, such as variations in horizontal and vertical hydraulic conductivity of the bedrock and overlying glacial drift and small topographic alterations of the generalized landscape. These changes cause large variations in patterns of bedrock recharge from the drift. Applied recharge was automatically adjusted when heads rose near land surface. Crystalline-rock aquifers in hilly and mountainous terrains of New England are characterized by ground-water-flow systems with steep hydraulic gradients and shallow water-table depths under hilltops and summits because of the low bulk permeability of the rock. Flow systems in these setting are highly sensitive to topographic irregularities, which alter the water-table slope. The low bulk permeability of the rock results in flow patterns that are insensitive to variations in saturated thickness because hydraulic conductivity and, hence, transmissivity generally are low. Bedrock heterogeneity can cause significant variations in patterns of bedrock recharge. Trends in bulk rock permeability may be detected by mapping regional inflection points between bedrock recharge andmore » discharge areas. Differences in permeability between the bedrock and overlying glacial drift of 1:100 can affect rates of bedrock recharge and discharge; thus, it is important to identify these conditions.« less