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CE 473/573 Groundwater Fall 2008 Comments on homework 6 37. I asked for the four curves on a single graph because the curves are easier to compare that way. Part b can be discussed with conservation of mass and Darcy’s law. For larger values of K ′ , more of the flow to the well can be supplied by leakage, and the drawdown in the confined aquifer is less. 38. Several groups varied both K and K ′ , but a more efficient approach is to divide the two expressions for drawdown to get s1 K0 (r1 /λ) = s2 K0 (r2 /λ) and solve for λ using Goal Seek or another iterative method. 39. One common problem was to add the drawdown from each well. I should have emphasized to you that while that approach works for confined aquifers, it does not work for unconfined aquifers. The flow equation for unconfined aquifers is linear in terms of h2 . Therefore, for N wells pumping at the same flow rate Q0 , the water table elevation is given by ! N X Q R 0 ln , h2 = H 2 − 1/2 πK j=1 [(x − xj )2 + (y − yj )2 ] where the jth well is at (xj , yj ). I placed my wells along both of the long sides of the site with equal spacing. Then the minimum drawdown occurs on the end of the centerline. With 12 total wells, the maximum water table elevation was 22.2 m, which is lower than the required 23 m. 41. The easiest approach is to use the spreadsheet I gave you. The Jacob method, which is an approximate method that applies for large time (or small u), gives T = 0.0125 m2 /s and S = 4.4 × 10−4 . Using the spreadsheet with Solver to improve the estimates gives T = 0.01 m2 /s and S = 9.9 × 10−4 .