Ground-water resources of St Johns County, Florida

TLDR: The two primary sources of ground water in St. Johns County are the surficial and Floridan aquifers as mentioned in this paper, which are the principal sources of water supply for drinking and domestic purposes.

Abstract: The two primary sources of ground water in St. Johns County are the surficial and Floridan aquifers. The surficial aquifer is the principal source of water supply for drinking and domestic purposes in most of the county. The Floridan aquifer is the major source of water supply for irrigation use. In most of the county, water from the Floridan aquifer does not meet drinking water standards for some characteristics. Available supplies of potable water from the surficial aquifet are adequate to meet short-term increases in demand, but future growth will require additional sources of potable water. The surficial aquifer is present throughout the county and extends from land surface to a maximum depth of about 120 feet. The aquifer is recharged primarily by rainfall and in some areas by infiltrating irrigation water withdrawn from the Floridan aquifer. The Floridan aquifer underlies the county at a depth of about 90 to more than 360 feet below sea level. Recharge to the aquifer is almost entirely from the lake region in Alachua, southwestern Clay, eastern Bradford, and western Putnam Counties. Discharge is from springs, from wells used for irrigation, and by upward leakage. Withdrawal of large quantities of water from the Floridan aquifer has caused declines in the potentiometric surface. During the spring, when withdrawal is heaviest and rainfall is lightest, the potentiometric surface is lowered an average of about 5 feet throughout the county and a maximum of about 15 feet in the agricultural areas. Comparison of predevelopment water levels with 1980 water levels indicates that withdrawals of water Afor irrigation and public supply have lowered the potentiometric surface about 10 to 20 feet throughout most of the county. Water from the surficial aquifer generally meets most of the secondary drinking-water quality standards in most of the county and is satisfactory for most uses. Concentrations of sulfate and chloride generally do not exceed 250 milligrams per liter in St. Johns County, and only in a small part of the county do dissolved-solids concentrations exceed 500 milligrams per liter. Iron concentrations commonly exceed 0.3 milligrams per liter. In most of St. Johns County, water from the Floridan aquifer is more saline than water from the surficial aquifer. Variations in the concentrations of chemical constituents occur both areally and with depth. Highly saline water is present in the upper part of the aquifer in most of the southern part of the county. The presence of saline water and the dissolution of gypsum and anhydrite are the primary factors that govern water quality in the aquifer. In the southwest part of the county* intensive pumpage for irrigation has resulted in substantial increases in chloride concentration caused primarily from the upconing of more saline water.