Computer simulation of the steady-state flow system of the Tertiary limestone (Floridan) aquifer system in east-central Florida

Abstract

The predevelopment steady-state ground-water flow system for the Tertiary limestone (Floridan) aquifer system in east-central Florida is simulated by means of a digital computer model. The modeled area covers about 13,700 square miles. The configuration of the model is that of a three-layered system; the lower Floridan aquifer, the upper Floridan aquifer, and the surficial aquifer. The model results indicate that about 1,900 cubic feet per second recharges the Floridan aquifer as downward leakage from the surficial aquifer. The average recharge rate where recharge actually occurs (approximately 6,550 square miles) is about 4 inches per year. The maximum recharge rate is about 14 inches per year. An additional 20 cubic feet per second is recharged to the modeled area of the Floridan aquifer by means of lateral boundary inflow along the northeast boundary. The Floridan aquifer system, as simulated, discharges about 1,300 cubic feet per second as springflow, 540 cubic feet per second as diffuse upward leakage to the surficial aquifer in an area of approximately 7,150 square miles and 80 cubic feet per second as lateral boundary outflow to the southwest and to the east. The average transmissivity of the upper Floridan aquifer, as simulated, is about 120,000 feet squared per day while that for the lower Floridan is about 60,000 feet squared per day. Inflow and outflow for the upper Floridan can be summarized as follows: 1. The highest rates of recharge to the Floridan take place in four areas; the west flank of the Volusia County high, the PutnamAlachua County high, the potentiometric ridge area of mid-Polk and Highlands Counties, and the northeast flank of the central Florida potentiometric in Lake and Orange Counties and extending into east Marion County. 2. Most discharge from the upper discharge (about 69 percent of the total) leakage to streams located within 25 Floridan is in the form of point to known springs and upward miles of recharge areas. 3. The highest rates of diffuse upward leakage from the upper Floridan (about 28 percent of total discharge) occur near areas where the downward leakage rates are also high. Thus, it is inferred that thin or permeable confining beds are common to the areas of high downward leakage and to the nearby areas of high upward leakage. 4. Lateral boundary outflow from the upper Floridan aquifer (principally occurring along the Atlantic coast) is relatively small and consists of about 4 percent of the total discharge. Lateral flow to the south is about 0.7 percent of the total discharge. 5. Interchange between the upper and lower Floridan aquifer layers is very small compared to the flow within the upper Floridan. 6. The model-derived transmissivities are considered to have more regional significance than transmissivity values derived from individual aquifer tests. In summary, the hydraulics of the Floridan aquifer system under predevelopment conditions involve mostly recharge to the upper Floridan (via leakage from the surficial aquifer), lateral movement through the upper Floridan for short distances, and discharge via springs and upward leakage to streams. Relatively small amounts of discharge occur along the coast and small amounts move into and out of the underlying lower Floridan. This natural hydraulic equilibrium implies a vigorous flow system locally in the upper Floridan and, except in west Orange County and southeast Lake County, a relatively sluggish flow system in the lower Floridan.