The Chicot and Evangeline aquifers are the only hydrologic units bearing fresh (less than 1,000 milligrams per liter dissolved solids) or slightly saline water (1,000-3,000 milligrams per liter dissolved solids in Brazoria County. These aquifers are composed of gravel, sand, silt, and clay of Pliocene, Pleistocene, and Holocene age.