Genesis of ground waters in the Coastal Plain of Virginia

Abstract

The Coastal Plain province of Virginia consists of unconsolidated sediments of Cretaceous, Eocene, Miocene and Pleistocene age which dip gently seaward and thicken to more than 2,200 feet in the eastern part of the area. Lower Cretaceous sands are the most prominent water bearers south of James River but north of James River most deep wells end in Eocene strata.In the western part of the province, along the Fall Zone, water from deep wells is soft and has a low total mineralization. Eastward the water gains in mineral content and becomes a hard calcium-bicarbonate water. Still farther to the east it is softened by base exchange and is a soft sodium-bicarbonate water. In the vicinity of Chesapeake Bay this water is somewhat brackish and in places contains more than 1,000 parts per million of chloride.Soft water from the Fall Zone belt contains free carbon dioxide which dissolves calcium carbonate from the sediments as it moves eastward to form the hard calcium-bicarbonate water of the adjacent belt on the east. When the free carbon dioxide found in the Fall Zone water is consumed, gain in bicarbonate does not cease, and further accretions of bicarbonate are accounted for by chemical or biochemical breakdown of sulphate with liberation of carbon dioxide that subsequently forms bicarbonate.South of James River soft sodium-bicarbonate water from the Lower Cretaceous sediments commonly contains from 4 to 6 parts per million of fluoride. North of James River the Eocene sediments characteristically yield water containing from 1 to 3 parts of fluoride.The high chloride in the eastern part of the area is considered to be due to incomplete flushing of the sea water with which the sediments were once saturated. These slightly brackish waters have a somewhat lower hardness and a higher Ca/Mg ratio than would be expected of mixtures of sea water and soft sodium-bicarbonate water. It is thought that base exchange within the system occurs as sea water in the sediments becomes freshened; where saturated with sea water clays will adsorb mostly magnesium and sodium whereas when in contact with dilute solutions calcium and magnesium will be deposited and sodium liberated.