1. Central Texas Trinity Aquifer – Science and Management Marshall Jennings and Richard Earl Texas State University – San Marcos Presented at Texas Groundwater 2004, November 19, 2004; material from Mace, TWDB originally presented to Hays County Commissioners Court in 2003

2. Purpose of Talk To overview aquifer science of the Central Texas Trinity Aquifer that includes 10 counties and 8 GCDs in Central Texas; the material is largely drawn from Mace et al (2000) or the Trinity Aquifer GAM To discuss aquifer management by GCDs and the Hill Country Alliance of GCDs

6. Cross-section

9. The Water Cycle in the Hill Country

10. Groundwater Flowpaths

11. Recharge Previously published values: –Muller and Price (1979) 1.5 % –Ashworth (1983) 4. % –Kuniansky (1989)11. % –Kuniansky and Holligan (1994) 7. % –Bluntzer (1992) (calc.) 6.7 % –Bluntzer (1992) (est.) 5. % –our analysis 6.6 % –our model 4.0 % Conclusions: - Percentage likely varies with rainfall amount - For average conditions, 4 to 5 %

12. Past and Predicted (dry demand) Pumping

13. County-by-County Pumping

14. Groundwater Management Ten counties are in the PGMA; however Comal and Travis Counties do not have a local GCD Eight counties in the Trinity PGMA do have GCDs and by mid-year all of these will have management plans in place The GCDs have formed an Alliance which meets six times per year and coordinates activities Individual GCDs vary in their activities and focus

15. Groundwater Management All GCDs have a water-level monitoring program and some have QW program Several recording GW monitors are in operation including one in HTGCD that has GOES telemetry Data bases are not uniform—all should report to TWDB Aquifer test information could be better captured in common files

16. Pumping Distribution

17. Essentially all the science information from many years of careful hydrogeology work is entrusted to a “groundwater geek” such as Rob Mace….O boy!! The Trinity Aquifer is modeled with a USGS-developed model called MODFLOW; the same model is used in several other Texas GAMs

18. Study Area and Grid

19. GAM Simulation Mace et al (2000) describe a dramatic but realistic simulation for the Trinity GAM A power point “video” for the 50 year simulation is available The following slides show 1997 vs 2050 My well in Hays County is projected to fall 100 – 150 feet by 2050!

20. Click on image to start animation

21. Predicted Water-Level Declines

22. Water Budgets Using the GAM 1975 was a typical year for the Trinity GAM region and a steady-state budget is available The following results for Hays County are similar for all ten counties but with varying quantities Wells in Hays County were only 700 AF/yr in 1975 and in the round-off error

23. Model water budget for the Trinity aquifer in Hays County values in acre-ft/yr

24. Water Budgets Using the GAM Large volumes of water are moving through each County each year but only a fraction is available to wells ---- if rivers & springs are to continuing flowing; Hays has about 52,000 AF/yr available. Groundwater pumpage can come from a fraction of new recharge and from net water moving through Hays County --- perhaps a maximum of 8,000 – 10,000 AF/yr is available

25. Congratulations to the Texas Water Development Board for successfully completing Phase I of the groundwater availability modeling program!