Balancing Total Estimated Recoverable Storage and. Wade A. Oliver, P.G. Texas Alliance of Groundwater Districts Groundwater Summit August 27, 2015

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1 Balancing Total Estimated Recoverable Storage and Sustainability Wade A. Oliver, P.G. Texas Alliance of Groundwater Districts Groundwater Summit August 27, 2015

2 The 9 Factors Districts Shall Consider When Adopting Desired Future Conditions Paraphrased Factors in Texas Water Code Sec (d) : 1. Aquifer uses or conditions 2. Water supply needs and management strategies 3. Hydrological conditions, including for each aquifer in the management area the total estimated recoverable storage as provided by the executive administrator [of TWDB] 4. Other environmental impacts 5. Impact on subsidence 6. Socioeconomic impacts 7. Impact on private property rights 8. Feasibility of achieving the DFC 9. Any other relevant information

3 Definitions Total Estimated Recoverable Storage The estimated amount of groundwater within an aquifer that accounts for recovery scenarios that range between 25% and 75% of the porosity-adjusted aquifer volume Texas Administrative Code Sec

4 Types of Aquifers Unconfined Confined Northwest Southeast Measured Water Level

5 Guidance from TWDB Typical County Example 30 miles x 30 miles 900 square miles 576,000 acres 500 feet 15%

6 Guidance from TWDB Typical County Example 30 miles x 30 miles 900 square miles 576,000 acres 500 feet 15% = 43.2 million acre-feet

7 Guidance from TWDB Unconfined Portion: Specific Yield 30 miles x 30 miles 900 square miles 576,000 acres Typical County Example Confined Portion: Storativity 500 feet 0.15 = 43.2 million acre-feet 500 feet = 0.03 million acre-feet

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11 GMA MAGs and Storage by GMA Total Estimated Storage (million acre-feet) Total MAG over 50 Years (million acre-feet) MAG in 2060 (million acre-feet) Total MAG as Percent of Storage % % % % 5 NA NA NA NA % % 8 1, % % % 11 2, % 12 1, % 13 2, % 14 3, % % 16 2, % Total 16, %

12 Implications Water demand in Texas 20 million acre-feet between 2020 and 2070 At 25% recoverability, do we have enough groundwater to meet all demands for 200 years even if it doesn t rain again? If it sounds too good to be true No consideration given to: Aquifer water quality Water levels dropping below pumps Land surface subsidence Degradation of water quality Changes to surface water-groundwater interaction Recharge Practicality/economics of development

13 Unconfined vs. Confined Storage Specific Yield > Storativity Takeaway: In theory, each foot of drawdown yields much more water when an aquifer is unconfined than when it is confined. From Heath (1983)

14 Aquifer Lithology

15 Drawdown vs. Storage Volume in an Example Confined Aquifer Exact shape of curve is aquifer specific and depends on initial water levels, aquifer thickness, and storage properties (storativity and specific yield). Idealized curve developed using a 500 ft thick aquifer with 500 feet of confined head. Storativity set to and specific yield (Sy) is varied.

16 Balancing Test Paraphrased Factors in Texas Water Code Sec (d-2) : The desired future condition proposed under Subsection (d) must provide a balance between the highest practicable level of groundwater production and the conservation, preservation, protection, recharging, and prevention of waste of groundwater and control of subsidence in the management area

17 Less Pumping Balancing Test More Pumping Conservation, Preservation, etc. Highest Practicable Modeled Available Groundwater Must Fall in this Range Total Estimated Recoverable Storage Highest Practicable Total Estimated Recoverable Storage

18 Conservation, Preservation, etc. Balancing Test Highest Practicable Total Estimated Recoverable Storage Environmental Impacts Subsidence Impacts Socioeconomic Impacts Private Property Rights Hydrological Conditions Aquifer uses or conditions Water supply needs and management strategies Socioeconomic Impacts Feasibility of achievement Private Property Rights Hydrological Conditions

19 In Conclusion As calculated, TERS represents the approximate fraction of total storage in the aquifer that is in the water-producing zones (e.g. sands), not what is recoverable. TERS is a simple volumetric calculation that does not account for many important factors that limit groundwater production With few exceptions, TERS is far greater than the highest practicable level of groundwater production and is not a useful tool for the planning and management of aquifers.

20 Contact: Wade A. Oliver, P.G. INTERA Inc