GROUND-WATER AVAILABILITY AND DROUGHT SUSCEPTIBILITY OF CRYSTALLINE ROCKS IN THE BLUE RIDGE OF PENNSYLVANIA

The crystalline rocks of the Blue Ridge Physiographic Province in south-central Pennsylvania receive approximately 46 inches of precipitation per year. Based on work done in the Piedmont Physiographic Province, the Blue Ridge probably receives 9- (drought) to 13-inches or more (normal to wet) in annual ground-water recharge. Yet, despite ample precipitation, the crystalline rocks in the Blue Ridge have low ground-water availability and are highly susceptible to drought.

Ground-water availability and drought susceptibility of the crystalline rocks in the Blue Ridge of Pennsylvania are controlled by the physical characteristics of the underlying bedrock and its upland topographic setting. The metabasalt, metarhyolite, and greenstone schist that underlie most of the study area are among the lowest yielding aquifers in the Commonwealth; median driller-reported yields are 3.0, 4.5, and 5.0 gallons per minute, respectively. Specific-capacity tests indicate borehole storage [specific capacities of 0.04 gallons per minute per foot of drawdown or less] is a critical source of water for more than half the wells in the study area. Driller records indicate water-producing zones are shallow and few in number, with 90 percent penetrated by depths of 370 feet or less. Borehole geophysical data indicate most water-producing zones are at lithologic contacts and there is little vertical borehole flow between fractures. Single-well aquifer and slug tests indicate median hydraulic conductivity and transmissivity are 0.01 foot per day and 2.75 feet squared per day, respectively. The overlying regolith does not appear to act as a reservoir to the underlying bedrock aquifers. However, the presence of water-quality indicators (chloride, nitrate, bacteria, wastewater compounds) in the water from a small number of sampled wells suggests some type of connection between secondary openings in the bedrock aquifers and the overlying regolith. The rugged topography and dense residential development on 0.5 acre lots further enhances drought susceptibility by increasing surface runoff.