Determination of Ellenburger Aquifer Sustainability

1. Determination of Ellenburger Aquifer Sustainability Southeast Gillespie County James Beach LBG-Guyton Associates Paul Tybor HCUWCD

2. Minor Aquifers

3. Ellenburger Aquifer

4. Facts • Ellenburger is the primary water supply • Fredericksburg • Irrigation • Rural • Complex geology • HCUWCD (1987) • Aquifer has limits

5. Objectives • Better understand the Ellenburger aquifer (SE Gillespie County) • Update and refine model • Assess long-term availability • Evaluate aquifer impact • Provide predictive tool for appropriate planning, management and regulation

6. 1.Data Evaluation • Hydrostratigraphy and Structure • Water Levels • Hydraulic Properties • Well Production • Streamflow • Water Chemistry • Precipitation

7. 2.Model Refinement • Model Extent and Boundaries • Hydrostratigraphy and Structure • Calibration Targets • Recharge • Hydraulic Properties • Pumping Allocation • Sensitivity Analysis

8. 3. Groundwater Availability • Use updated model to assess: • Long term availability • Impact of increased demands • Optimize production

9. Modeling Basics

10. Groundwater Flow Modeling Model “Cell” or “Gridblock”

11. Groundwater flow Cells “Communicate”

12. What Goes on In A Gridblock? Groundwater flow

13. Gridblock Accounting Natural recharge Irrigation return flow Water removed from storage by pumping Exchange of water with neighboring cells Water remaining in storage • Permeability • Storage value • Thickness

14. Geology

15. Faults

16. Stratigraphy Glen Rose Layer 1 Hensell Sand Ellen-burger Layer 2

17. Model Area Hensell Sand Ellenburger

18. Surface Geology

19. Extent of Model Gillespie County New Model Old Model

20. Topography

21. Model Area All Wells Gillespie County New Model Old Model

22. Model Area High Capacity Wells

23. Hensell Thickness

24. Hensell Sand

25. N Hensell Sand

26. Ellenburger • Wells generally <350 feet deep • Very deep wells have not encountered significant permeability-porosity • Assume Ellenburger thickness of 200 feet

30. Groundwater Flow

31. Fault

32. Hydraulic Properties • 3 pump tests Specific capacity: 13 – 79 gpm/ft Transmissivity: 25,000-38000 gpd/ft 3,500-5000 ft2/day Hydraulic Conductivity: 22-33 ft/day Storativity: 1x10-4

33. 20 gpm 50 gpm Well Capacity

34. Well Capacity

35. Fredericksburg Pumping

36. Monthly Demand

37. Precipitation

38. Ellenburger Potentiometric Surface Source: HCUWCD

39. Hensell Potentiometric Surface Source: HCUWCD

40. Ellenburger Hydrographs:confined and unconfined responses

41. Hydrologic Summary Near Old Wellfield • Water Levels in Ellenburger near old wells historically trend downward until new wells come online in the 1990s • Both Hensell and Ellenburger water levels are typically below river levels • Recent HCUWCD gain-loss studies and historical TWDB gain-loss studies in the Pedernales near the old wells show losses

42. Long-Term Ellenburger Water Level Trends Near Old Wells

43. Levels of Hensell and Ellenburger Near Old Wells are Typically Below River Elevation

44. PEDERNALES FLOW AT FREDERICKSBURG VS. MEASURED GAIN/LOSS AT BROWN LOCATION (near old wells)

45. Intermediate-Term Ellenburger Water Level Trends Near New Wellfield

46. Hydrologic Summary Near New Wellfield • New Wellfield is near the Ellenburger outcrop on the Pedernales • Ellenburger water level elevations at this location appear to be buffered near the river elevation • Recent HCUWCD gain-loss studies at the Goehmann location suggest that at Ellenburger water levels at or above the river elevation the Pedernales gains, below the river elevation the Pedernales loses

47. Hydrograph of Ellenburger Well Near Outcrop

48. Gain-Loss Data Near Outcrop

49. Modeling Periods Pre-Development Time Calibration- Verification Prediction Steady-state period Water Elevation in Well 1940 1990 2004 2060 Observed Water LevelModel Water Level

50. Simulation Periods