GROUND WATER AVAILABILITY IN FRACTURED CRYSTALLINE BEDROCK: EXAMPLES FROM THE SEACOAST OF NEW HAMPSHIRE

Because of the increasing demand for ground water in New England, exploration has begun to focus on the underlying crystalline bedrock aquifers as “new” sources of ground water. The use of crystalline bedrock aquifers in New England has traditionally been for residential wells. These wells typically yield 5 to 10 gallons per minute and are installed on building lots of 1 to 5 acres in size. There are minimal permitting requirements for the installation and development of these types of wells.

The ground water withdrawal needs for these “new” sources are in the hundreds of gallons per minute. As ground water flow in crystalline bedrock aquifers is largely fracture-controlled, understanding the geologic and hydrogeologic characteristics of the crystalline bedrock as well as the regional tectonic setting are critical to identifying favorable areas to site large ground water withdrawal wells. Geologic methods traditionally used to identify favorable drilling targets include fracture trace analysis in conjunction with the field mapping of structural features in bedrock outcrops, especially fracture sets, and the use of various surface geophysical methods. These remote sensing and field mapping techniques are important to understanding the geology and tectonics of the area. However, the performance of an aquifer-pumping test and long-term ecosystem monitoring provide the critical information required to determine if sufficient ground water exists to support the safe and long-term withdrawal of large quantities of ground water from a crystalline bedrock aquifer.

The results of several aquifer-pumping tests performed in crystalline bedrock aquifers along with multi-year monitoring of the overlying ecosystems will be presented for the Seacoast area of New Hampshire.