The proposed deepening of the navigation channel in the St. Johns River to about 46-48 feet below sea level (45 feet below mean low water) may breach up to 11 feet of limestone unit of the lower part of the surficial aquifer along a 25-mile channel. The limestone supplies water to numerous domestic wells along the river in the Jacksonville area. Recent channel improvements to about 39 feet below sea level have already breached the limestone at many locations. Where breaching has not yet occurred, less than 1 to 6 feet of undifferentiated sediments overlie the limestone. These sediments, consisting predominantly of sand, are generally too permeable to form an effective confining layer. Chloride concentrations determined from water in wells tapping the limestone unit range from 8 to 6,600 milligrams per liter. However, chloride concentrations in water from wells located more than 500 feet from the river, were generally less than 30 milligrams per liter. The proposed dredging operation is not expected to alter significantly the present hydrologic system. The current position of the interface most likely represents conditions that will be present after future channel improvements. (USGS)
