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Estimated Recoverable StorageWhat it does, doesn’t and might mean for planning
Wade A. Oliver, P.G.Texas Alliance of Groundwater Districts
Quarterly MeetingFebruary 26, 2014
Disclaimer: The materials presented here were developed for informational purposes, are considered preliminary, and should not be used without consulting the author.
The “9 Factors” Districts Shall Consider When Adopting Desired Future Conditions
Paraphrased Factors in Texas Water Code Sec. 36.108(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 impacts5. Impact on subsidence6. Socioeconomic impacts7. Impact on private property rights8. Feasibility of achieving the DFC9. Any other relevant information
Definition
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. 356.10
Estimated Recoverable Storage in the News
From the Austin American StatesmanIn his remarks, [the county judge] cited the Texas Water Development Board’s recent survey indicating the Simsboro Aquifer contains some 50 million acre-feet of water, 25 percent of which, according to the board, is easily rechargeable and available for export without harming the aquifer.“There being more than 11 million exportable acre-feet of water available means the 45,000 acre-feet reserved by Hays County is but a small percentage — 0.04 percent — of the available water in Lee and Bastrop counties,” [he] said.
Estimated Recoverable Storage in the News
GAM Task 13-035
Confining Unit
Confining UnitUnconfined Aquifer
Confined Aquifer
Types of Aquifers
One Aquifer – Two Types
Unconfined Confined
Northwest Southeast
Measured Water Level
Unconfined vs. Confined Storage
Takeaway: Each foot of drawdown yields much more water when an aquifer is unconfined than when it is confined.
>From Heath (1983)
Specific Yield Storativity
Guidance fromTWDB
30 miles x 30 miles900 square miles576,000 acres
500 feet 15%
Typical County Example
Guidance fromTWDB
30 miles x 30 miles900 square miles576,000 acres
500 feet 15%
Typical County Example
43.2 million acre-feet
=
Guidance fromTWDB
30 miles x 30 miles900 square miles576,000 acres
500 feet 15%
Typical County Example
43.2 million acre-feet
=
1000 feet 0.0001 0.06 million acre-feet
=
Unconfined Portion: Specific Yield
Confined Portion: Storativity
Guidance from TWDB
Total Estimated Recoverable Storage and Modeled Available Groundwater, Why They Are Different
Presentation by TWDB available at:http://www.twdb.texas.gov/groundwater/docs/TotalEstimatedRecoverableStorage.pdf
MAGs and Storage by GMA
GMA
Total Estimated Recoverable Storage
(million acre-feet)MAG in 2060
(million acre-feet)
Total MAG over50 Years
(million acre-feet)Total MAG as
Percent of TERS1 588 2.270 150 25%2 968 1.344 94 10%3 476 0.461 23 5%4 160 0.207 10 6%5 Not Defined Not Defined Not Defined Not Defined6 180 0.422 22 12%7 447 0.648 33 7%8 1,628 0.386 19 1%9 33 0.096 5 15%
10 46 0.100 5 11%11 Not Defined 0.543 27 Not Defined12 1,380 0.337 15 1%13 2,756 0.485 24 1%14 Not Defined 0.907 47 Not Defined15 443 0.488 24 6%16 2,205 0.358 18 1%
Total 11,310 7.603 442 4%(excluding GMAs 5,11,14)
Sometimes Caveats Are Important
• 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– Practicality/economics of development
Sometimes Caveats Are Important
• 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– Practicality/economics of development
Example Depletion of Confined Aquifer
Confi
ning
Uni
tAq
uife
r
Partially Penetrating WellFully Penetrating Well
Initial Water Level
500 feet of confined pressure head
500 feet of saturated thicknessand well screen
100-foot screen on partially penetrating well
S = 0.0001
Sy = 0. 1
5
Drawdown vs. Storage Volume in anExample 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 to0.0001 and specific yield set to 0.15.
Confined vs. Unconfined Aquifer Response to Pumping
Source: TWDB
OgallalaUnconfined Aquifer6,000,000 ac-ft pumping50 – 150 feet drawdown
TrinityConfined Aquifer200,000 ac-ft pumping800 – 1000 feet drawdown
Well Yield Decline with Aquifer Depletion
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 to0.0001 and specific yield set to 0.15.
Removal of waterproviding confiningpressure. Depleted“available drawdown”.
Draining of aquiferpore space.
Partially penetrating well
Fully penetrating well
Sometimes Caveats Are Important
• 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– Practicality/economics of development
“Recoverable” is Aquifer Specific
• The range of 25% - 75% is not an appropriate range for all aquifers.– Could be 55 -75% or more for highly productive, unconfined
aquifers at the surface such as the Ogallala and Seymour.– Likely no more than 3 - 15% for most dipping, confined aquifers in
Texas (Trinity, Carrizo-Wilcox, Gulf Coast, etc.). Recovery of anywhere close to 75% is physically impossible given current well depths and impacts to water levels, quality, existing wells, well yields, surface water, and subsidence.
– For karst aquifers, total storage is practically irrelevant to aquifer planning and management long term (Edwards). Total storage is relatively small and fluctuates significantly over time due to recharge events.
Takeaways: The Meaning of Total Estimated Recoverable Storage
• What it does mean:– How much water is in the aquifer
• What it doesn’t mean:– That the water is available for production– That using small fractions of the total volume cannot
seriously harm the aquifer and its users– That it is a useful tool in the planning and management of a
particular aquifer• What it might mean:– That you’ll need to understand it well enough to explain to
your boards, permit applicants, and the public if – and to what extent – it is relevant in your district
