Groundwater Modeling of the Woodville Karst Plain: A Planning Tool

1. Groundwater Modeling of the Woodville Karst Plain: A Planning Tool Timothy J. Hazlett Hazlett-Kincaid, Inc.

2. Presentation Overview • Finite element modeling using quantitative dye tracing data • KARSTMOD approach

3. Measured Heads After Davis, 1996. Hydrogeologic Investigation and Simulation of Ground-Water Flow in the Upper Floridan Aquifer of North-Central Florida and Southwestern Georgia and Delineation of Contributing Areas for Selected City of Tallahassee, Florida, Water-Supply Wells. USGS WRIR 95-4296. • Large trough of convergent flow • MODFLOW model used T ≈ 10,000,000 ft2/d to compensate for cave systems and yet did not calibrate Approximate model area

4. Finite Elements with Discrete Karst Conduits • FEFLOW (www.wasy.de) • Cave systems included in model discretely • Darcy • Manning-Strickler • Remainder of problem is represented as a porous media • Steady state • 3-D • Unconfined • Dye tracing results used in modeling • Inferred conduits • Dye travel times (velocities) • Dispersivity (DL ) • ~ 80,000 triangular elements

5. Boundary Conditions 6m (~20 ft) constant head 0 m (sea level) constant head No flow on the sides Constant head 1m (~3 ft) at spring

6. Discrete Conduit Meshing Leon SinksCave System • Indian • Sally Ward • Wakulla • Loops • Branching • Porous media “islands”

7. Caves Only: UFA Heads

8. Caves Only: Particle Tracks

9. Caves Only: Velocity Magnitude

10. Caves + Wakulla: Heads

11. Caves + Wakulla: Particle Tracks

12. Caves + Wakulla + St. Marks: Heads

13. Caves + Wakulla + St. Marks: Particle Tracks

14. Modeled Heads – Most Recent Model

15. Particle Tracks – Most Recent Model • If we follow a parcel of water, where does it go? • How well does even a simple model with conduits predict pathways? • Black Creek • Ames Sink Ames Sink Black Creek Inferred conduit placed in model

16. Particle Tracks • Leon Sinks to Wakulla • North in the Wakulla cave, against the regional flow gradient Southern end of mapped Leon Sinks cave Wakulla Spring Southern end of mapped Wakulla cave

17. Calibration Using Dye Tracing Parameters Assume that well tests are probably wrong Assign “conductivities” to conduits and matrix Match head patterns - This fixes the ratio between conduit-matrix “K” Use dye trace data If travel times are fast, assume conduit flow and fix actual conduit K Use previously defined ratio to back out effective matrix K Use dye trace DL for transport and further model refinement

18. Hybrid (KARSTMOD) Approach What approach do you use when you have few or no mapped conduits? KARSTMOD Hybrid modeling approach Discrete features Probabilistic features Where features are known or inferred, put them in the model Where they are not known, apply the probabilistic conduit approach Input: Distributed recharge Output: Discharge at a spring

19. “Karst Elements” node n : 1  N connection c : 1  C element e : 1  E • Seed the basin with nodes • Mesh it using Delaunay triangulation • Record nodes, elements, and connections • Recharge over elemental areas • Diffuse flow distributed to probable conduits • Flow through conduits to spring (Darcy-Weisbach) • Sinking streams/sinkholes (to be added)

20. KARSTMOD – Comparison Plot Flux [L3/t] Time [t]

21. Future Research • Karst basin model development using FEFLOW supported by quantitative tracing data • Groundwater flow • Nitrate transport • Code development of KARSTMOD • Addition of sinking streams and slow sinks • Automatic mesh generation (Delaunay) • Multiple springs