IMPLICATIONS FOR AQUIFER ANALYSES FROM DISCRETE, ANISOTROPIC WATER LEVEL IMPACTS OBSERVED DURING PUMPING TESTS PERFORMED IN FRACTURE-BEDROCK AQUIFERS

The ability to document the hydrogeologic characteristics of fractured bedrock aquifers through long-term pumping tests has important consequences for the calculation of safe yields of high-yielding wells, the identification of other users that are likely to be impacted by pumping, and the delineation of the wellhead protection areas around such wells.

The characterization of fractured-bedrock aquifers is often complicated by the presence of discrete water-bearing fractures, fracture-systems, and/or faults (structural discontinuities). Prediction of water level responses to pumping based on the presumed anisotropic conditions can lead to misleading results. Monitoring of large numbers of available wells (i.e., domestic wells) has shown that water level responses in bedrock observation wells are often unpredictable.

Examples of anisotropic water level impacts observed during hydrogeologic studies conducted in New England and the mid-Atlantic are presented. These studies show that anisotropic water level impacts observed during pumping tests can often be related to changes in rock type, the orientation of bedding and foliation, or the dominant fracture family orientation within the bedrock. In other cases, the impacts reflect a relatively unpredictable and complex interplay of a variety of hydrogeologic parameters.

These findings indicate that large numbers of bedrock monitoring wells are necessary to evaluate bedrock aquifer responses to pumping and that they cannot be selected based solely on presumed anisotropic conditions. The results also support the need for long-term monitoring of bedrock aquifers after high-yield production wells have been placed into permanent service.