A DENSITY-DEPENDENT FLOW AND TRANSPORT ANALYSIS OF THE EFFECTS OF GROUNDWATER DEVELOPMENT IN A FRESH-WATER LENS OF LIMITED AREAL EXTENT - THE GENEVA AREA (FLORIDA, USA) CASE-STUDY

The Geneva freshwater lens is an isolated flow system within the upper Floridan aquifer surrounded and underlain by brackish water. The lens is sustained entirely by recharge from a surficial aquifer and sits a top a regional flow system which discharges around the fringes of the lens along the St. Johns River. Continual development of groundwater s upply in the Geneva area has raised the concern of how much additional pumping can be allowed from various sites within the lens without adv ersely impacting water quality by inducing the invasion or upconing of salty water. A numerical modeling study was conducted to address thes e water, management issues. A density-dependent, finite-element flow a nd transport code, DSTRAM, was used for cross-sectional and three-dime nsional (3-D) analyses of the Geneva lens system. The model incorporat es an enhanced upstream weighted technique for the transport equation, improved Picard iterations over the nonlinearities, and robust precon ditioned conjugate gradient (PCG) and ORTHOMIN techniques for solving the matrix equations. A steady-state model calibration was performed f or existing conditions at the site. The cross-sectional (2-D) analysis was inadequate in accurately representing the system, since the 3-D e ffects were considerable. However, 2-D cross-sectional simulations are useful for preliminary assessments of certain scenarios, and for guid ance in developing the 3-D model. A comprehensive sensitivity analysis was performed on a number of key parameters. Natural groundwater disc harge rates along the St. Johns River seem to be the most critical unk nown and require better estimates for increased confidence in the conc eptual model. Transient pumping scenarios were imposed on this system to observe the response of the lens and to determine well breakthrough for chlorides, if any. Pumping rates and distribution of pumping were significant factors in determining the quality of the water. Upconing of saline water contributes to high chloride concentrations in water being pumped from the lens center. A localized pumping at the lens cen ter does not affect the lens significantly till a critical value of wi thdrawal rate is reached, beyond which upconing of saline water is rap id. Lateral invasion is more prominent for pumping from locations near the edge of the lens. Maximum withdrawals of fresh water from the len s can be attained by evenly distributing the pumping throughout the fr eshwater zone.