Summary of the ground-water resources of the James River basin, Virginia

Abstract

The James River basin, in central Virginia, encompasses an area of 10,000 square miles. The river flows eastward through varied geologic settings including the Valley and Ridge, Blue Ridge, Piedmont, and Coastal Plain provinces. Springs are most abundant in the Paleozoic rocks of the Valley and Ridge province. Most of the large springs, some of which yield as much as 7,000 gallons per minute, emanate from solution channels and fractures in carbonate rock or sandstone. Well yields range from less than 10 to more than 1,000 gallons per minute; the larger yields usually are found in formations that also contain the larger springs. Wells located at the intersection of fractures also usually have large yields. Discharge of ground water to streams is estimated to be 100,000 to 300,000 gallons per day per square mile. Rocks of the Blue Ridge and Piedmont provinces are metamorphic and intrusive rocks largely of Precambrian age. They are highly fractured but in general have lower porosity and permeability than rocks of the Valley and Ridge province. Small to moderate yields (1 to 100 gallons per minute) are usually obtained from the lower part of the saprolite (weathered rock residuum) and the upper fractured part of the unweathered crystalline rock. Most wells in the Blue Ridge and Piedmont provinces obtain most of their water within a depth of 200 feet. Wells drilled indraws usually have larger yields than wells drilled on hilltops. Rocks of Triassic age are present in small isolated basins in the Piedmont. Moderate yields are obtained from sandstone or conglomerate layers. Coastal Plain sediments consist of gravel, sand, silt, and clay beds of Holocene to Early Cretaceous age. Sediments of Pleistocene age, which are usually near the surface, contain aquifers that are about 10 to 30 feet thick and that usually yield 10 gallons per minute or less of soft water. Sediments of Miocene age usually are confining beds, but in some areas they contain thin sand or shell aquifers that yield small to moderate amounts of water that is generally hard. Sediments of Eocene, Paleocene, and Late Cretaceous age are characterizedby abundant glauconitic sand that yields moderate amounts ofwater. However, the glauconitic sand often clogs wells and yields waterwith a disagreeable taste and odor. Quartz sand beds of Late Cretaceousage yield moderate amounts of water. Sediments of Early Cretaceous age consist of thick alternating bedsof sand and clay of continental origin. Quartz sand beds more than30 feet thick are common, and some are more than 100 feet thick. Somemultiscreen wells yield more than 2,000 gallons per minute. Large withdrawalsof ground water have lowered potentiometric levels as much as 200 feet near Franklin, Va. Total pumpage from aquifers within the James River basin is estimatedto have been more than 68 million gallons per day in 1971. Recharge takesplace at outcrop areas near the Fall Line and throughout the area byseepage through the overlying confining beds. The quality of the water in Lower Cretaceous sediments ranges from soft water of low mineral and bicarbonate content near the Fall Line through hard calcium bicarbonate water of moderate mineral content in the west-central part of the area to soft sodium bicarbonate water of moderate to high mineral content in the eastern parts. Salty water occurs in coastal areas and a considerable distance inland. The horizontal and vertical extent of the salty water is not known, but, becausethe hydraulic gradient is from areas of salty water toward the areas ofheavy pumping, the water must be moving toward areas of heavy pumpage.The rate of movement is unknown.