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 Complex geology
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. Cells “Communicate” Groundwater flow
Cells Can Keep Track of Water by Using Math Equations that Describe Groundwater Flow
Groundwater
flow

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
Permeability
Storage value
Thickness
Water remaining in storage

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. Hensell Sand N

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: , gpd/ft
3, ft2/day
Hydraulic Conductivity: ft/day
Storativity: x10-4

33. Well Capacity
20 gpm
50 gpm

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

50. Simulation Periods Time Period pre-1940: steady state period
Stress Periods
Length
pre-1940: steady state period
prior to major pumping 1 -
1940 – 1990: calibration period 50
1 year
1990 – 2004: calibration period
180
1 month
2005 – 2060: predictive period ?

51. Path to completion Calibrate model Incorporate demands to 2060
Simulate aquifer impacts