1. Comparison of Estimated Areas Contributing Recharge to Selected Springs in North-Central Florida by Using Multiple Ground-Water Flow Models Prepared in cooperation with the St. Johns River Water Management District Florida Department of Environmental Protection University of Florida Andrew M. O’Reilly March 30, 2005

2. Blue Spring (Volusia) Silver Springs Alexander Springs Silver Glen Springs

3. Objective and Methodology Delineate areas contributing recharge using particle tracking results from multiple regional ground-water flow models. Areas contributing recharge were defined as the land- surface area wherein water entering the ground-water system at the water table eventually discharges to a spring. Areas contributing recharge were based on 100-year travel time.

4. Blue Spring 54% of composite area jointly delineated by 2 or 3 models

5. Blue Spring 100-year travel time (Volusia County model)

6. Blue Spring 500-year travel time (Volusia County model)

7. Blue Spring

8. Silver Springs 74% of composite area jointly delineated by 2 or 3 models

9. NCF Alexander Springs 44% of composite area jointly delineated by 2 or 3 models

10. Silver Glen Springs 47% of composite area jointly delineated by 2 or 3 models

11. Conclusions As expected, different models predicted different areas contributing recharge. In general, the differences were due to different hydrologic stresses, subsurface permeability properties, and boundary conditions that were used to calibrate each model. Each model is an alternative way of representing the ground- water system. In complex, natural systems, selecting only one model as superior to all others often is difficult, especially where only a limited amount of data describing the system is available. It may be necessary to accept with equal confidence the range of results provided by the various models. Therefore, the composite areas contributing recharge to springs may be the best estimates available.