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Abstract: Hydrogeological Concept of Vertical Coupling in a Freshwater Lens

Hugh J. Mitchell-Tapping

Vertical coupling of the aquifers is not unusual where there are no competent confining layers. It is aided by differences between the permeability of the upper sediments and that of the underlying limestone, which can lead to freshwater mixing with the underlying saline water and, as a consequence, reduction in the freshwater zone by the formation of a transition zone. A study area of about 60 mi2 surrounding the town of Geneva in northeast Seminole County, Florida, was used to study the practical application of this concept. This area contains an isolated recharge area of the Floridan aquifer system that forms a freshwater lens, known as the Geneva Lens or Bubble, completely surrounded by saline water, and has been defined in this study as all surface areas with an elevat on above 20 ft NGVD, an area about 17,000 ac (26 mi2) having a maximum lens thickness of 350 ft. The surface post-Miocene quartz sand and shell beds comprise 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. Based on the absence of the impermeable class over certain areas of the lens, that vertical coupling of the surficial and Floridan aquifers is taking place. These areas are relict Pleistocene beach ridges and dune deposits, outlined by the 35-ft NGVD contour, that are not underlaid by the presence of the very low permeable clays of the Miocene Hawthorn Formation. As the population has increased so has the demand for freshwater from this lens for local development and for public supply to areas outside the lens. Chloride concentrations range from less than 20 mg/l at the center of the recharge area to about 5100 mg/l immediately outside the lens. Because all the freshwater in the lens results entirely from local recharge, the recharge rate is important in estimating freshwater yield of the aquifer in the lens area. Recharge was estimated to be about 11.5 in./yr, and a water budget using 30-yr average values was calculated to be about +0.17 in./yr. Freshwater that recharges the lens is either pumped out or flows through the subsurface to discharge in the St. Johns River. The average subsurface annual outflow is about 10 in. If this outflow is reduced or the cumulative impact determination is less than zero, deterioration of water qualit will occur. However, detectable movement of the saltwater-freshwater interface will not be immediately apparent because of the large volumes of water that must be displaced before a noticeable change in water quality can be observed. As future projected population for the year 2010 is expected to be 8500 persons, the increase demand will be 0.95 in./yr. This demand is considered beyond the long-term average sustained capacity of the lens, especially if there is a prolonged drought.

AAPG Search and Discovery Article #90983©1994 GCAGS and Gulf Coast SEPM 44th Annual Meeting, Austin, Texas, October 6-7, 1994