A strategy for delineating the area of ground-water contribution to wells completed in fractured bedrock aquifers in Pennsylvania

Abstract

Delineating a contributing area to a well completed in a fractured bedrock aquifer in Pennsylvania is difficult because the hydrogeologic characteristics of fractured rocks are extremely complex. Because of this complexity, a single method or technique to delineate a contributing area will not be applicable for all wells completed in fractured-bedrock aquifers. Therefore, a strategy for refining the understanding of boundary conditions and major heterogeneities that control groundwater flow and sources of water to a supply well is suggested. The strategy is based on developing and refining a conceptual model for the sources of water to the well. Specifically, the strategy begins with an initial conceptual model of the groundwater flow system, then requires the collection of hydrogeologic information to refine the conceptual model in a stepwise manner from one or more of six categories: (1) hydrogeologic mapping, (2) water-level and streamflow measurements, (3) geochemistry, (4) geophysics and borehole flowmetering, (5) aquifer testing, and (6) tracer testing. During the refinement process, the applicability of treating the fractured-rock aquifer as a hydrologic continuum is evaluated, and the contributing area is delineated. Choice of the method used to delineate the contributing area is less important than insuring that the method is consistent with the refined conceptual model. By use of such a strategy, the improved understanding of the ground-waterflow system will lead to a technically defensible delineation of the contributing area. Water Supply and Community Health, is developing a wellhead-protection program for the Commonwealth. In cooperation with PaDER, the U.S. Geological Survey is evaluating approaches that can be used to delineate the contributing area for wells completed in bedrock aquifers throughout Pennsylvania. Purpose and Scope This report describes a strategy to delineate contributing areas to wells completed in bedrock aquifers that can be used by PaDER to assist communities in establishing local wellhead-protection plans for bedrock aquifers. The strategy is based on developing and refining a conceptual model for sources of water contributed to a pumping well. Thus, this report outlines hydrologic investigations in six categories that are useful for refining the understanding of groundwater flow to a well in a fractured-bedrock aquifer. Application of the strategy is illustrated by the use of a hypothetical aquifer setting. Previous Investigations The river is simulated with constant-head cells. All discharge is to river or well.