Exercise 2
A pumping test is conducted on a production well located in an extensive, isotropic, homogeneous, 25‑m thick, totally confined, sand and gravel aquifer. The production well has a 15 m screened interval. The well is pumped at a constant rate of 1,200 m3/d for 240 minutes. Time‑drawdown data are collected at three observation wells located 61 m, 122 m, and 244 m from the pumping well as shown in the image below. An Excel® data base of the time‑drawdown data is available on the web page for this book↗.
Information related to Exercise 2. A production well is pumped at a constant rate in a totally confined isotropic and homogeneous aquifer that is infinite in lateral extent. Time‑drawdown data are collected from three observation wells. Configuration of the pumping well location, screen length and location of the observation wells are shown in cross section. Time‑drawdown data sets for the three observation wells are presented (modified from Lohman, 1972).
- Prepare log‑log pots of the time‑drawdown data for each of the observation wells. Using manual or automated curve matching, determine values of transmissivity and storativity for each data set.
- Compare and contrast the values computed. Should they all be the same? If not, how would you present the results to the well owner?
- Analysis of pumping a confined aquifer can also be accomplished using the Cooper‑Jacob straight line method. Plot the time‑drawdown data for the observation well located at 122 m from the pumping well as a semi‑log plot and determine T and S. Compare these results to the results from the part (a) type curve analyses, and comment on their similarity or differences.
- The confined time‑drawdown data can also be interpreted using the distance‑drawdown method. Make a semi‑log plot of the distance‑drawdown data at 100 minutes and calculate T and S. Compare these results to those derived from type curve and time‑drawdown straight line analyses. Discuss why the values are similar or different.
- Using the distance‑drawdown plot examine the efficiency (i.e., measured drawdown divided by theoretical drawdown) of the production well. If the production well diameter is 0.20 m and the drawdown at 100 minutes in the pumping well is 12.34 m, what is the efficiency of the pumping well?
- The well is planned to be used to supplement the city water system. After a seasonal supply evaluation, it was decided to pump the well for 200 days at a constant rate of 1,000 m3/d. There are other wells in the area and a regulatory agency wants to know if other wells would be affected when this well is pumped. Ignoring the effects of pumping in the other wells, what is the predicted drawdown 1000 m from this well at the end of the pumping period?