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Salt‐water upconing that occurs in an aquifer overlain by a leaky confining bed is described in terms of an analytical model that assumes the existence of a sharp interface between the fresh water and salt water and the occurrence of a critical rise in the interface, above which only an unstable cone can exist. Drawdown is calculated along the salt‐water/fresh‐water interface due to pumping from a well that partially penetrates the fresh‐water zone, and then the Ghyben‐Herzberg relation is used to calculate the steady‐state rise in the interface, which is assumed to be small compared to the thickness of the aquifer. The interface rise and the critical pumping rate (Qc) are determined in terms of aquifer and confining bed properties and the degree of penetration of the pumping well into the fresh‐water zone. The critical rise is assumed to occur when the interface rise is equal to 0.3 times the distance from the original interface location to the bottom of the well. Based on the analytical model, the nondimensional critical pumping rate increases as the ratio of vertical to horizontal hydraulic conductivity (Kz/ Kr) is decreased, and it decreases as the degree of well penetration (t/b) is increased. As an example application, Qc was calculated using site‐specific data from a test well in Pasco County, Florida. A value of Qc= 4.28 × 105 cubic feet per day was determined by assuming that Kz/Kr= 1.0. If Kz/Kr were subsequently determined to be less than or greater than 1.0, then the estimated value of Qc would be increased or decreased, respectively.

Abstract. In the karst lake district in peninsular Florida in the southeastern United States, as many as 70% of the lakes lack surface outlets, and groundwater outflow is an important part of the water budgets of these lakes. For 11 karst lakes in the Central Lake District, vertical leakage from the lakes to the upper Floridan aquifer averages 0.12 to 4.27 rn yr -•. The vertically averaged vertical conductance Kv/b , a coefficient that represents the average of the vertical conductances of the hydrogeologic units between the bottom of a lake and the top of the upper Floridan aquifer, was determined to range from 0.0394 to 1.00 yr -• for these lakes. For six of the lakes, various hydraulic parameters previously calculated by other investigators are shown to be equivalent to the Kv/b values calculated in this study. If Kv/b is determined for a lake, then vertical leakage can be estimated for other conditions of lake stage and hydraulic head in the upper Floridan aquifer, using Kv/b for the lake and Darcy's equation written for vertical flow. The methodology described in this paper for quantifying Kv/b , which requires only limited data (i.e., vertical leakage, lake stage, and hydraulic head in the upper Floridan aquifer), could be used to investigate the apparent association between relatively large Kv/b values and lake level instabilities at some lakes in the Central Lake District and similar hydrogeologic settings. This methodology for calculating vertical leakage is applicable to the Central Lake District in Florida and to other similar lake and groundwater systems. IntroductionIn water budget calculations for lakes, groundwater outflow can be a significant component of the water budget. In seepage lakes, from which losses occur by seepage into the groundwater [Wetzel, 1983], groundwater outflow can be quite large. Even in drainage lakes, from which losses also occur by surface water flow from an outlet, the groundwater outflow component can be relatively large [Deevey, 1988]

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