1. A water budget perspective of leaky aquifer response to pumping David Scott 1 and Bruce Hunt 2 1 Environment Canterbury 2 University of Canterbury (Retired)

2. Leaky aquifer behaviour

3. Characterizing leaky aquifers Standard measures –Leakance - units of length –K’/B’ - units of 1/time Alternative measure –t 50 - units of time More general applicability

4. The two-aquifer model From Hunt & Scott (2007)

5. The problem

6. The solution Excel macro W_11 from Hunt’s Function.xls

7. Hypothetical example Q = 50 L/s

8. Pumped and water-table aquifers Time = 0.1 day 3.2 m 5 mm

9. Pumped and water-table aquifers Time = 0.177 day

10. Pumped and water-table aquifers Time = 0.316 day

11. Pumped and water-table aquifers Time = 0.562 day

12. Pumped and water-table aquifers Time = 1 day

13. Pumped and water-table aquifers Time = 1.77 days

14. Pumped and water-table aquifers Time = 3.16 days

15. Pumped and water-table aquifers Time = 5.62 days

16. Pumped and water-table aquifers Time = 10 days

17. Pumped and water-table aquifers Time = 11.77 days

18. Pumped and water-table aquifers Time = 31.6 days

19. Pumped and water-table aquifers Time = 56.2 days

20. Pumped and water-table aquifers Time = 100 days

21. Pumped and water-table aquifers Time = 177 days

22. Pumped and water-table aquifers Time = 316 days

23. Pumped and water-table aquifers Time = 562 days

24. Pumped and water-table aquifers Time = 1000 days 3.8 m 0.9 m

25. Vertical drainage after 0.1 days 1 mm drawdown, 1 km radius circle Specific yield 0.1 Volume = 314 m 3, Q avg = 36 L/s Q avg = 72% of pumping rate

26. Drainage rate

28. Drainage rate & drawdown

29. Sensitivity of t 50 to S y, T 0 & K’/B’

30. Modflow solution ● Independent check ● Allows consideration of less idealised cases ● Underlying layer ● Sloping aquifer ● Alternative boundary conditions ● Zone budget ● Modpath - visualisation

34. Effect of an underlying layer?

35. Pumping from the lower layer…

36. Effects of adjacent boundary

37. Sloping aquifer (1:1000)

38. Conclusions Vertical drainage can develop to become a significant proportion of the pumping rate in a relatively short time t 50 value provides a useful measure of aquifer leakiness Water budget perspective provides a parallel to Theis’s analysis of the “source of water derived from wells” Leaky aquifers leak!

39. Aquitard storativity

40. Empirical expression for t 50

41. “... the term aquitard has been coined to describe the less-permeable beds in a stratigraphic sequence. These beds may be permeable enough to transmit water in quantities that are significant in the study of regional ground-water flow, but their permeability is not sufficient to allow the completion of production wells within them.” —Freeze and Cherry,1979