Saline ground-water resources of Lee County, Florida

Abstract

Lee County, an area of 786 square miles (1260 square kilometres) on the southwest coast of Florida, is underlain at depths greater than 400 feet (120 metres) by formations containing saline water. Two saline water-bearing zones occur within the depth interval 400 to 1,200 feet (120 to 370 metres); the upper zone is termed the lower Hawthorn aquifer and the lower zone is termed the Suwannee aquifer. Fresh water infiltrates into the aquifers in the central highlands region of Florida where water levels are as much as 130 feet (40 metres) above sea level. The water subsequently moves southwestward into the report area becoming progressively more saline toward the coast. Both aquifers are under sufficient artesian pressure to cause wells tapping them to flow at land surface. Water levels range from more than 50 feet (15 metres) above sea level in the northeastern part of the county to about 30 feet (9 metres) or less along the coast. Artesian pressure in the aquifers has been reduced along the Caloosahatchee River by heavy withdrawal and this reduction in pressure is a major factor in deterioration of water quality in the McGregor Isles area, south of Fort Myers. The average depth of wells tapping the lower Hawthorn aquifer is 572 feet (174 metres); the average depth of wells tapping both the lower Hawthorn and Suwannee aquifers is 858 feet (262 metres). Few wells tap only the Suwannee aquifer. The average length of casing in wells tapping the lower Hawthorn aquifer is 170 feet (52 metres), whereas the average length in wells tapping both aquifers is only 179 feet (55 metres). Flow rates range from 20 to 750 gallons per minute (1 to 47 litres per second) and average 233 gallons per minute (15 litres per second). The saline water from the lower Hawthorn and Suwannee aquifers is hard and sulfurous. The dissolved solids, as determined from analyses of 31 samples,ranged from 700 to 3,300 mg/1 (milligrams per litre) and averaged 1,936 mg/1. The chloride concentrations ranged from 160 to 1,600 mg/1 and averaged 869 mg/l. The calcium magnesium hardness ranged from 110 to 1,000 mg/1 and averaged 625 mg/l. Sulfate concentrations ranged from 92 to 800 mg/1 and averaged 289 mg/l. Although the mineral content is high, water from the lower Hawthorn and Suwannee aquifers has been used in the past for many purposes, particularly for irrigation. Currently, water from these aquifers may be used to supplement the dwindling supplies of better quality from aquifers at shallow depths.