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Chester County is an area about 760 square miles (1,970 square kilometers) in the Piedmont of southeastern Pennsylvania. Rocks of the county include: (1) Precambrian metamorphic and igneous rocks; (2) upper Precambrian to lower Paleozoic metamorphic rocks, predominantly metamorphosed sedimentary rocks; (3) upper Precambrian and lower Paleozoic sedimentary rocks that are moderately metamorphosed; and (4) Upper Triassic sedimentary and igneous rocks. Overlying the bedrock is a generally thick zone of consolidated weathered rock.
Most wells derive water from fractured rock. Intergranular permeability is present only in the weathered zone and in some of the sedimentary rock. Solution of the carbonate rock and weatherine increase the permeability.
Reported yields range from 0 to 1,800 gallons per minute (114 liters per second). Yields from carbonate rock, particularly the Cockeysville Marble and Ledger Formation, are generally higher than yields from other units. The difference in yield between nearby wells in the same unit, however, may be greater than the difference in yield between wells in different units. Exceptionally high yields of 100 gallons per minute (6 liters per second) or more were reported in almost all parts of the county and from almost all units, as were exceptionally low yields of 5 gallons per minute (0.3 liters per second) or less.
The estimated number of wells that would be needed in an area to yield 1 million gallons per day (0.04 cubic meters per second) continuously are indicated on a map. The estimates are based on evaluation of data on reported yields of more than 1,900 wells, on specific capacity for almost 900 wells, and on estimates of long-term specific capacity from 97 pumping tests. Yields that might be obtained by intensive exploration of an aquifer were estimated. These yields were then reduced by half to approximate the long-term average yield.
Aerial imagery of various kinds was evaluated as a potential aid in the study. The imagery included: side-scanning radar, infrared scanning, Skylab photography landsat imagery, and aerial photographs from various altitudes. These provided images at scales of 1:1,000,000; 1:250,000; 1:100,00; 1:60,000; and 1:24,000. The Landsat and Skylab images give an overview of the geology and structure of the area that is not available from other sources.
Straight or gently curved linear features are visible on all scales of aerial imagery that were examined, and most have geologic and hydrologic significance. Many linear features indicate fracture zones where fractured rock might be tapped by wells. Linear features were mapped from Landsat imagery and from 1:60,000 aerial photos. The mapped features are generally topographic lows, and most indicate zones of weakness in the rock.
Most ground water moves from where it enters the ground in upland areas and hillsides to the nearby valleys, where it is discharged to streams. Ground-water levels fluctuate in response to precipitation and evapotranspiration. Water levels generally decline during the growing season and recover during the rest of the year.
A water-budeget analysis for the county indicated that an average of about 1,650 million gallons of water per day or 72 cubic meters per second enter and leave the area. The basin yield from the county during a near-average water year (1968) was 620 million gallons per day (27 cubic meters per second). Base flow, ground-water outflow, accounts for about 420 million gallons per day (18 cubic meters per second) and direct (storm) runoff about 200 million gallons per day )9 cubic meters per second).
The dissolved-solids concentration of most ground water in the county is less than 500 milligrams per liter. Chemical-quality problems are predominantly caused by acidity, iron and manganese, or nitrate. Hardness is sometimes a problem.
About 7 billion gallons (26 cubic hectometers) of water was withdra
Additional Publication Details
USGS Numbered Series
Ground-water resources of Chester County, Pennsylvania
Water-Resources Investigations Report
U.S. Geological Survey, Water Resources Division,
vi, 76 p. :ill., maps (some col., 6 fold. in pocket) ; 26 cm.