What is Long-Term Sustainable Well Yield and how should we calculate it using the Modified Moell Method? Joel Defoe EnviroTech 2018 April 5, 2018

Transcription

1 What is Long-Term Sustainable Well Yield and how should we calculate it using the Modified Moell Method? Joel Defoe EnviroTech 2018 April 5, 2018

2 Alberta Regulatory Environment Alberta Environment and Parks (AEP) requires an assessment of the theoretical long-term yield

3 Safe Yield Various definitions of what safe yield is: Avoiding dangerous depletion of storage reserves (Lee, 1915) Maintaining economic feasibility (Meinzer, 1923) Avoiding undesired results (Todd, 1959) Quantifying the desirable development of a groundwater basin (Alley and Leake, 2004)

4 How much groundwater is available? Available Head: H A 66.6% Available Head: H A Confined Unconfined

5 70% Safety Factor 70% H A 70% of H A Confined Unconfined

6 A History of Q 20 in Alberta Back in 1959

7 Calculating Safe Yield Farvolden identified two possible outcomes when you pump an aquifer: Case 1: During pumping a stable groundwater level is established indicates recharge balances discharge Case 2: During pumping the groundwater level continues to decline indicates the well is drawing water from storage

8 Case 1: Stable Drawdown

9 Stable Drawdown: 3.45 m Pumping Rate (Q): 350 m 3 /day 70% H A : m

10 Safe Yield = Cs * H A * S f Safe Yield = 350 m 3 /day Safe Yield = 1,900 m 3 /day m 3.45 m Pumping Rate Cs = Stable Drawdown H A = Available Drawdown S f = 70%

11 Case 2: Drawing Water from Storage 70% H A : m

12 2 months ~2 years ~20 years ~200 years 70% H A : m

13 Drawdown per log cycle ( s p ): 0.18 m Pumping Rate (Q): 350 m 3 /day 70% H A : m 2 months ~2 years ~20 years ~200 years

14 Safe Yield = Q * H A 8 sp * S f Safe Yield = 350 m 3 /day Safe Yield = 4,600 m 3 /day m m Q = Pumping Rate s p = drawdown per log cycle H A = Available Drawdown S f = 70% 2 months ~2 years ~20 years ~200 years

15 2 months ~2 years ~20 years ~200 years The Farvolden Calculation does not account for well losses!

16 Flashforward to 1975

17 Moell Method Drawdown at 10 mins (s 10 ): 3.25 m Moell (1975) recognized the need for considering well losses. Proposed anchoring the future drawdown from a measured value 2 months ~2 years ~20 years ~200 years

18 Moell Method Safe Yield = Q * H A s sp * S f Safe Yield = 350 m 3 /day Safe Yield = 1,500 m 3 /day m 3.25 m +( m) Q = Pumping Rate s 10 = drawdown at 10 mins s p = drawdown per log cycle H A = Available Drawdown S f = 70% 2 months ~2 years ~20 years ~200 years

19 So Each calculation is a variation of the same formula: Safe Yield = Pumping Rate * 70% Available Head Drawdown at 20 years

20 Modified Moell Method Q 20 = Where 0.7 Q H a S 100min +(S 20yrs S 100min ) theor Q = Pumping Rate during Aquifer Test H a = Available Head; S 100min = measured drawdown at 100 minutes of pumping at Q S 100min = calculated theoretical at 100 minutes of pumping at Q; and S 20yrs = calculated theoretical drawdown at 20 years at Q Farvolden Q 20 = 0.68*T*H a *0.7 Where T = Transmissivity H a = Available Head; and 0.68 = constant coefficient

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22 T e2 T Lots of potential different paths that the water level might take over time. T e1 Which path you choose depends on how long the well was pumped.

23 Farvolden Method

24 Modified Moell Method

25 Visual Q 20 Assessment Safe Yield = 350 m 3 /day m 14.5 Safe Yield = 460 m 3 /day S 20 = ~14.5 m

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27 Head on over to Aqtesolv

28 To get this fit: Added two parallel boundaries (buried channel) Added skin effects on the pumping well

29 Safe Yield = 350 m 3 /day m m Safe Yield = 160 m 3 /day S20 = m!

30 Aquifer has infinite length Fixed Width No Flow Boundary Aquifer No Flow Boundary

31 Complete Fantasy

32 Aquifer has infinite length Constant Head Boundary Fixed Width No Flow Boundary Aquifer No Flow Boundary

33 Safe Yield = 350 m 3 /day m 8.87 m Safe Yield = 740 m 3 /day 8.87 m

34 The only way to know which of these is closer to reality is to perform longer aquifer tests. Or Issue a short term licence and monitor the well while it is being used.

35 Conclusions Q 20 calculations under the Guide for Groundwater Authorization are simplified. In some case they make sense. They completely break down in complex systems. Sometimes the fancy tools we use in complex systems end up providing unreasonable numbers when predicting 20 years in to the future.

36 Conclusions The level of understanding of groundwater conditions in the local and regional scales represents the critical aspect of the assessment of Q20. Long-term monitoring, extended above and beyond of the aquifer testing period is key information with respect to long-term performance. Remember: Hydrogeology is not a perfect science. Every solution includes a subjective component, which will be a function of experience and level of knowledge.