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Hydrogeological Concept of Vertical Coupling in a Freshwater Lens
Hugh J. Mitchell-Tapping
Vertical coupling of aquifers, irrespective of lithology, is not unusual where there are no competent confining layers. However, its effect on saline intrusion is not yet fully understood. It occurs on oceanic islands where permeability differences between the upper sediments and underlying limestone can lead to freshwater mixing with the underlying saline water and, as a consequence, cause a reduction in the freshwater zone by the formation of a large transition zone. The practical application of this concept was studied in an area of about 60 mi2 (155 km2) surrounding the town of Geneva in northeast Seminole County, Florida. An isolated recharge area of the Floridan aquifer system forms a freshwater lens here, known as the Geneva Lens or Bubble, which is completely surrounded by saline water. The lens underlies all surface areas above 20 ft (6 m) NGVD (national geodetic vertical datum), that is, an area about 17,000 acres (7,000 hectares), and has maximum thickness of 340 ft (104 m). Post-Miocene quartz sand and shell beds compose the surficial aquifer; Miocene quartz sands, clays, and shell beds form a very leaky confining unit; and permeable Eocene limestones are part of the Floridan aquifer system. The highest recharge areas are relict Pleistocene beach ridges and dune deposits, outlined by the 35-ft (11-m) NGVD contour, that are not underlain by the presence of thick clays with very low permeability of the Miocene Hawthorn Formation. Based on well lithology, well water levels, and the absence of low- permeability clays over certain areas of the lens, it is considered that vertical coupling of the surficial and Floridan aquifers is taking place. Chloride concentrations range from less than 20 mg/l within the lens area to about 5,100 mg/l immediately outside the lens. The transition zone delimited by chloride concentrations from 250 mg/l to 1,000 mg/l is 0.5 km wide at the perimeter of the lens and is 10 ft (3 m) thick near the center of the lens. Differences in chloride concentration over lateral distances are thought to be caused by vertical coupling of the aquifers and by changes in lithomicrofacies and fluid densities. As all the freshwater in the lens results entirely from rainfall, the recharge rate is important in estimating freshwater yield of the aquifer in the lens area. Using long-term average values, recharge was calculated as 11.5 in/yr (290 mm/yr) with a safe yield of +0.17 in/yr (+4 mm/yr). Freshwater that recharges the lens is either pumped out or flows through the subsurface to discharge into the St. Johns River at an annual rate of 10 in/yr (264 mm/yr). If this outflow is reduced, or the cumulative impact determination is less than zero, water quality will deteriorate. As future projected population in the year 2010 is expected to be 8,500 persons, the increase in demand of 0.95 in/yr (24 mm/yr) is considered beyond the long-term average sustained capacity of the lens, especially if there is a prolonged drought and if vertical coupling of the aquifers is taking place.
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