Wednesday, November 13, 2013

Intermittent Pumping

One of the many applications for the variable pumping rate option in AQTESOLV is to simulate drawdown resulting from intermittent pumping in one or more pumping wells. Use this feature for pumping test interpretation or drawdown modeling.

In the following figure, AQTESOLV predicts drawdown due to an irrigation well extracting water from a nonleaky confined aquifer with daily on/off cycles extending over a growing season of 245 days. During each pumping cycle, the well withdraws groundwater at 220 gallons-per-minute (gpm) for 12 hours followed by 12 hours of recovery. At the end of the growing season, all pumping stops and sustained recovery begins. Drawdown in the pumping well (blue line) clearly shows the cycles of intermittent pumping; whereas the two observation wells (red lines) located 500 and 2000 ft from the extraction well demonstrate how the cyclic drawdown response dissipates with distance.
Drawdown due to intermittent (cyclic) pumping in a nonleaky confined aquifer.
Entering a lot of pumping cycles by hand can be tedious, so AQTESOLV makes the task easy for you. One method is to prepare the cyclic rate history in Excel and copy/paste the rates from Excel into AQTESOLV. Alternatively, you may enter just one cycle into AQTESOLV and have the software repeat the cycle a specified number of times (find details in the AQTESOLV manual).

If your primary interest is the average drawdown at relatively distant observation wells over long periods of intermittent pumping, it is often sufficient to assume a constant average pumping rate. The following figure shows the predicted drawdown for a well extracting at 110 gpm over the same 245-day growing season as the previous intermittent pumping scenario.
Drawdown assuming average pumping rate in a nonleaky confined aquifer.
The two preceding examples applied the Theis (1935) nonequilibrium solution to predict drawdown under intermittent pumping conditions in a nonleaky confined aquifer of infinite extent; however, you may use virtually any of the pumping test solutions in AQTESOLV to forecast drawdown for constant- or variable-rate pumping in nonleaky confined, leaky confined, unconfined or fractured aquifers. Plus, AQTESOLV allows you to add no-flow or constant-head boundaries to model surface water boundaries and limited aquifers as required.

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