A Numerical Groundwater Flow Model of the Upper and Middle Trinity Aquifer, Hill Country Area __________________________________________ Open-file report 00-02 Robert E. Mace1 Ali H. Chowdhury Roberto Anaya Shao-Chih (Ted) Way Texas Water Development Board P.O. Box 13231 Austin, Texas 78711-3231 1(512) 936-0861, [email protected] February 2000 Table of Contents Abstract.............................................................................................................................. 1 Introduction ....................................................................................................................... 2 Study Area ......................................................................................................................... 3 Physiography and Climate..................................................................................... 5 Geology ................................................................................................................... 8 Previous Work................................................................................................................. 10 Hydrogeologic Setting..................................................................................................... 12 Hydrostratigraphy ............................................................................................... 13 Structure .............................................................................................................. 14 Water levels and Regional Groundwater Flow .................................................. 15 Recharge .............................................................................................................. 21 Rivers, Streams, and Springs .............................................................................. 24 Hydraulic properties ............................................................................................ 25 Discharge ............................................................................................................. 28 Conceptual Model of Groundwater Flow in the Aquifer ............................................ 31 Model Design ................................................................................................................... 33 Code and Processor ............................................................................................. 33 Layers and Grid ................................................................................................... 34 Model Parameters ............................................................................................... 38 Modeling Approach ........................................................................................................ 42 Steady-State Model ......................................................................................................... 43 Calibration ........................................................................................................... 43 Sensitivity Analysis .............................................................................................. 47 Transient Model .............................................................................................................. 48 Calibration and Verification ............................................................................... 48 Limitations of the Model................................................................................................. 49 Input data ............................................................................................................. 51 Assumptions ......................................................................................................... 52 Scale of Application ............................................................................................ 53 Future Work .................................................................................................................... 54 Conclusions ...................................................................................................................... 56 Acknowledgments............................................................................................................ 57 Acronyms ......................................................................................................................... 58 References ........................................................................................................................ 58 ii List of Figures Figure 1. Location of the study area .............................................................................. 4 Figure 2. Location of the major aquifers ....................................................................... 6 Figure 3. Location of the Regional Water Planning Groups ......................................... 7 Figure 4. Stratigraphic and hydrostratigraphic section.................................................. 9 Figure 5. Surface geology............................................................................................ 11 Figure 6. Elevation of the top of the Upper Trinity aquifer......................................... 16 Figure 7. Elevation of the top of the Middle Trinity aquifer....................................... 17 Figure 8. Elevation of the top of the Hammett Shale and the Lower Trinity.............. 18 Figure 9. Water-level elevations in the Middle Trinity for fall, 1975 ......................... 20 Figure 10. Distribution of hydraulic conductivity in the Middle Trinity ...................... 29 Figure 11. Active cells in Layer 1 ................................................................................. 35 Figure 12. Active cells in Layer 2 ................................................................................. 36 Figure 13. Active cells in Layer 3 ................................................................................. 37 Figure 14. Comparison of simulated and measured water levels .................................. 45 Figure 15. Crossplot of simulated and measured water levels ...................................... 46 Figure 16. Comparison of simulated and measured water-level fluctuations ............... 50 iii A Numerical Groundwater Flow Model of the Upper and Middle Trinity Aquifer, Hill Country Area __________________________________________ Abstract _______________________________________________________________________ We developed a three-dimensional, numerical groundwater flow model of the Upper and Middle Trinity aquifer in the Hill Country area to help estimate groundwater availability and water levels in response to pumping and potential future droughts. The model includes historical information on the aquifer and incorporates results of new studies on water levels, structure, hydraulic properties, and recharge rates. We calibrated a steady-state model for 1975 hydrologic conditions when water levels in the aquifer were near equilibrium and a transient model for 1996 through 1997 when the climate transitioned from a dry to a wet period. Using the model, we calibrated values of vertical hydraulic conductivity, specific storage, and specific yield for the aquifer and adequately matched measured water levels. Water levels in the model are most sensitive to recharge, the horizontal hydraulic conductivity of the Middle Trinity aquifer, and the vertical hydraulic conductivity of the Upper Trinity aquifer. Water-level changes are most sensitive to the specific yield of each layer. Model calibrated recharge is four percent of mean annual precipitation. Model results suggest that 20 percent of the recharge moves from the Trinity aquifer to the south towards the Edwards (Balcones Fault Zone) aquifer. Future work includes (1) developing predictive datasets for pumping and recharge including the drought of record, (2) enhancing the recharge, structure, hydraulic property 1 datasets, (3) using the model to predict water levels in the aquifer for various climatic scenarios, and (4) estimating groundwater availability in the area. ________________________________________________________________________ Introduction ________________________________________________________________________ The Trinity aquifer in south-central Texas is an important source of groundwater to municipalities and individuals in the Hill Country area (fig. 1). Although the Trinity aquifer is recognized by the State as a major aquifer (Ashworth and Hopkins, 1995), yields in the aquifer can be comparatively lower than other major aquifers. For example, average yields in the Trinity aquifer in the Hill Country are about 250 times lower than average yields in the Edwards (Balcones Fault Zone [BFZ]) aquifer immediately to the south. New development and recent droughts have increased interest in the Trinity aquifer and have heightened concerns about groundwater availability in the aquifer. Many want to know how current and future pumping and future droughts will affect water levels over the long term and impact groundwater resources and the environment. We developed a three-dimensional finite-difference groundwater flow model for the Trinity aquifer in the Hill Country as a tool to (1) evaluate groundwater availability, (2) improve our conceptual understanding of groundwater flow in the region, and (3) develop a management tool to support Senate Bill 1 regional water planning efforts for the Plateau, Lower Colorado, and South Central Texas Regional Water Planning Groups. This interim report describes