The primary sources of water in St. Johns County are the surficial and Floridan aquifers. The surficial aquifer is the principal source of public and rural water supplies in most of the county. The Floridan aquifer serves as the major source of water for irrigation. In much of the county, water from the Floridan aquifer does not meet secondary drinking water standards established by the U.S. Environmental Protection Agency. The surficial aquifer is comprised of deposits of clay, sand, shell, and coquina that extended to a maximum depth of about 120 feet below land surface. Yields to 2-inch wells tapping the aquifer range from 1 to 40 gallons per minute and water generally meets drinking water standards. The Floridan aquifer consists of limestone and dolomite of Eocene age. In St. Johns County, the top of the aquifer ranges from about 89 to more than 360 feet below sea level. Yields from 4-inch wells range from 100 to several thousand gallons per minute. The presence of ancient seawater and the dissolution of gypsum and anhydrite are the primary factors governing water quality in the aquifer. In the southwest part of the county, intensive pumpage for irrigation has resulted in substantial increases in chloride concentration.
