Abstract

Continued population growth in the Borough of Carroll Valley (Borough) coupled with the drought of 2001 have increased the demand for ground water in the Borough. This demand has led Borough officials to undertake an effort to evaluate the capability of the crystalline-bedrock aquifers to meet future, projected growth and to establish a drought-monitor well within and for the use of the Borough. As part of this effort, this report summarizes ground-water data available from selected sections within the Borough and provides geohydrologic information needed to evaluate ground-water availability and recharge sources within part of the Borough.

The availability of ground water in the Borough is limited by the physical characteristics of the underlying bedrock, and its upland topographic setting. The crystalline rocks (metabasalt, metarhyolite, greenstone schist) that underlie most of the study area are among the lowest yielding aquifers in the Commonwealth. More than 25 percent of the wells drilled in the metabasalt, the largest bedrock aquifer in the study area, have driller reported yields less than 1.25 gallons per minute. Driller reports indicate also that water-producing zones are shallow and few in number. In general, 50 percent of the water-producing zones reported by drillers are penetrated at depths of 200 feet or less and 90 percent at depths of 370 feet or less. Borehole geophysical data indicate that most of the water-producing zones are at lithologic contacts, but such contacts are penetrated infrequently and commonly do not intersect areas of ground-water recharge. Single-well aquifer tests and slug tests indicate that the bedrock aquifers also do not readily transmit large amounts of water. The median hydraulic conductivity and transmissivity of the bedrock aquifers are 0.01 foot per dayand 2.75 feet squared per day, respectively.

The crystalline and siliciclastic (Weverton and Loudoun Formations) bedrock aquifers are moderately to highly resistant to weathering, resulting in topographic highs coupled with steep, narrow valleys. This rugged topography results in extensive surface runoff, which limits infiltration and hence recharge to the shallow and deep ground-water systems. Streams that flow through the study area generally are small and ephemeral. Where perennial, the streams represent areas of ground-water discharge.

Thickness of the overlying mantle (regolith or depth to bedrock) varies from 0 to more than 65 feet over short distances. In general, a thick regolith will store and transmit large quantities of water to the underlying bedrock aquifers. In the study area, however, there is no correlation between thick regolith and greater reported yields. Thus, it appears that the hydraulic connection between water-bearing fractures at depth and ground water stored in the regolith is poor, which further limits ground-water availability.

Recharge to the bedrock aquifers from the approximately 46 inches of annual precipitation aver-ages about 13 inches per year, or 975 gallons per day per acre. During drought years, however, this recharge rate may average only 9 inches per year [675 gallons per day per acre]. Decreased recharge to the bedrock aquifers results in declining water levels and possibly dry wells, as well as reduced flows to streams and other surface-water bodies. Although the consumptive use of ground water by homeowners is minor (about 14 percent), the pumping of a well will change the natural flow paths of ground water and reduce the amount of water stored (at least temporarily) in the bedrock aquifers.