1. Naples, Florida, June 23-27 2008 Tidal Effects on Transient Dispersion of Simulated Contaminant Concentrations in Coastal Aquifers Ivana La Licata Christian Langevin Alyssa Dausmann Luca Alberti Cover SWIM 20th Salt Water Intrusion Meeting

2. SWIM, 20th Salt Water Intrusion Meeting, Naples, Florida, June 23-27 2008 I. La Licata, C. Langevin, A. Dausman, L. Alberti Tidal Effects on Transient Dispersion of Simulated Contaminant Concentrations in Coastal Aquifers 2 Introduction The extent of saltwater intrusion is affected by a large number of physical and hydraulic parameters (dispersivity) Moreover contaminant migration in groundwater is affected by the characteristics of the transition zone The effect of tides necessitates the use of a short simulation time step, resulting in substantial computational effort Simulation of contaminant transport in coastal aquifers is intrinsically complex and computationally expensive because:

3. SWIM, 20th Salt Water Intrusion Meeting, Naples, Florida, June 23-27 2008 I. La Licata, C. Langevin, A. Dausman, L. Alberti Tidal Effects on Transient Dispersion of Simulated Contaminant Concentrations in Coastal Aquifers 3 Purpose To investigate the influence of transient dispersion on the concentration distribution in a variable-density flow and transport model Comparison between No Tide No Tide - Simulations that neglected tidally fluctuating ocean boundary Tide Tide - Simulations that included tidally fluctuating ocean boundary simulations are simple two-dimensional cross-sectional models computer program:SEAWAT (Langevin & Guo, 2006)

4. SWIM, 20th Salt Water Intrusion Meeting, Naples, Florida, June 23-27 2008 I. La Licata, C. Langevin, A. Dausman, L. Alberti Tidal Effects on Transient Dispersion of Simulated Contaminant Concentrations in Coastal Aquifers Problem Description 4

5. SWIM, 20th Salt Water Intrusion Meeting, Naples, Florida, June 23-27 2008 I. La Licata, C. Langevin, A. Dausman, L. Alberti Tidal Effects on Transient Dispersion of Simulated Contaminant Concentrations in Coastal Aquifers Problem Description 5

6. SWIM, 20th Salt Water Intrusion Meeting, Naples, Florida, June 23-27 2008 I. La Licata, C. Langevin, A. Dausman, L. Alberti Tidal Effects on Transient Dispersion of Simulated Contaminant Concentrations in Coastal Aquifers Problem Description

7. SWIM, 20th Salt Water Intrusion Meeting, Naples, Florida, June 23-27 2008 I. La Licata, C. Langevin, A. Dausman, L. Alberti Tidal Effects on Transient Dispersion of Simulated Contaminant Concentrations in Coastal Aquifers 7 Problem Description (6.283 d -1 ; period = 1 day) Tide constant longitudinal and transverse dispersivities: 8 stress periods per tidal cycle 500 m

8. SWIM, 20th Salt Water Intrusion Meeting, Naples, Florida, June 23-27 2008 I. La Licata, C. Langevin, A. Dausman, L. Alberti Tidal Effects on Transient Dispersion of Simulated Contaminant Concentrations in Coastal Aquifers 8 The plume moves toward the freshwater–seawater interface and rises as it reaches the transition zone At the end of the 2-year simulation, the contaminant is at steady state and has reached the ocean Simulation and Results

9. SWIM, 20th Salt Water Intrusion Meeting, Naples, Florida, June 23-27 2008 I. La Licata, C. Langevin, A. Dausman, L. Alberti Tidal Effects on Transient Dispersion of Simulated Contaminant Concentrations in Coastal Aquifers 9 TideNo Tide Contaminant concentrations were compared for Tide and No Tide simulations to analyze the influence of tidal variations on the contaminant and salinity distributions Simulation and Results The tidally driven hydraulic transients in Tide increase the overall mixing resulting in a relatively broader saltwater/freshwater transition zone The increased mixing in Tide also caused contaminant concentrations near the ocean to be lower than for the No Tide simulation salt water contamination source

10. SWIM, 20th Salt Water Intrusion Meeting, Naples, Florida, June 23-27 2008 I. La Licata, C. Langevin, A. Dausman, L. Alberti Tidal Effects on Transient Dispersion of Simulated Contaminant Concentrations in Coastal Aquifers 10 “…those values that yield the best match or calibration of the solute transport model under steady state flow conditions to a plume that developed under transient flow conditions.” (Goode & Konikow, 1990) Apparent Dispersivity Percent difference in contaminant concentration between steady-state (No Tide) and transient (Tide) simulation Simulations with spatially varying dispersivity values were performed with the No Tide model to determine if the mixing due to tidally driven hydraulic transients could be approximated with an increase in dispersivity Simulation and Results - spatially varying dispersivity

11. SWIM, 20th Salt Water Intrusion Meeting, Naples, Florida, June 23-27 2008 I. La Licata, C. Langevin, A. Dausman, L. Alberti Tidal Effects on Transient Dispersion of Simulated Contaminant Concentrations in Coastal Aquifers 11 (Kinzelbach & Ackerer, 1986) Distribution of Apparent Dispersivity Simulation and Results - dispersivity distribution Percent difference in contaminant concentration between steady-state (No Tide) and transient (Tide) simulation Transverse Apparent Dispersivity [m]

12. SWIM, 20th Salt Water Intrusion Meeting, Naples, Florida, June 23-27 2008 I. La Licata, C. Langevin, A. Dausman, L. Alberti Tidal Effects on Transient Dispersion of Simulated Contaminant Concentrations in Coastal Aquifers 12 Conclusion This study presents a comparison of numerical results between contaminant transport simulations (Tide)(No Tide) with (Tide) and without (No Tide) tidal effects Difference in the contaminant and salinity concentration distributions occurs when tides are represented Without tide, the error that occurs in the estimation of concentration near the ocean is a function of the dispersivity in correspondence of the transition zone the mixing from tides results in a more mixed contaminant and salinity concentration distribution it may be possible to replace tidal effects with a change in the dispersivity value in this zone when calibrating a model

13. SWIM, 20th Salt Water Intrusion Meeting, Naples, Florida, June 23-27 2008 I. La Licata, C. Langevin, A. Dausman, L. Alberti Tidal Effects on Transient Dispersion of Simulated Contaminant Concentrations in Coastal Aquifers 13 A more rigorous approach based on the calculation and use of apparent dispersivity distribution, created using the velocities from a transient model, provides a better method for including the effects of tidal mixing in a steady-state model the calculated apparent dispersivity correction appears to be a practical way to replace tidal effects when calibrating a model It’s possible to save time and money in model calibration because the steady-state model does not require multiple stress periods; the model simulations require less time to run Conclusion

14. SWIM, 20th Salt Water Intrusion Meeting, Naples, Florida, June 23-27 2008 I. La Licata, C. Langevin, A. Dausman, L. Alberti Tidal Effects on Transient Dispersion of Simulated Contaminant Concentrations in Coastal Aquifers Conclusion

15. SWIM, 20th Salt Water Intrusion Meeting, Naples, Florida, June 23-27 2008 I. La Licata, C. Langevin, A. Dausman, L. Alberti Tidal Effects on Transient Dispersion of Simulated Contaminant Concentrations in Coastal Aquifers 15 THANK YOU