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Wells: Pumping Tests
The hydraulic characteristics of wells and aquifers are determined by measuring changes in water levels in pumping and observation wells. Analysis of these data will define the long-term, or sustainable, yield of the well and the radius that it will influence at various pumping rates and over different periods of time. A complete pumping test includes a variable dishcarge rate test (step test) of about one day duration, followed by a constant rate discharge test of three, five, or more days duration. Also included is sampling of the pumped water to determine its chemical quality characteristics.
Pumping tests for municipal and other high-yield wells usually include an array of observation wells at different distances and in different directions from the pumping wells as suggested by Figure 28. Data collected during the test include well discharge, drawdown (difference between pumping level and static level), time of a particular drawdown, and radial direction and distance from the pumping well to the observation wells. In a variable discharge rate test, the well is pumped for short intervals of time (say 100 minutes) at increasingly greater rates (steps), while drawdown is measured in the pumping and observation wells. This information defines the operating characteristics of the well under different pumping rates and provides an early indication of the well's efficiency and sustainable pumping rate.
Following the completion of the step test and recovery of the local ground water levels (typically 24 hours), a constant-rate pumping test is run at a fixed discharge that is continuously maintained for three or more days. A typical arithmetic plot of drawdown in the pumping well (or in any one observation well) against time is illustrated in Figure 29. Just as important as the drawdown measurements during the pumping period are recovery measurements after the pump is shut down. These data are free of any irregularities caused by variations in pump operation. The recovery mirrors the drawdown because the aquifer releases water from storage at nearly the same rate it takes water into storage. The "time-drawdown" data are analyzed by a variety of techniques to define the hydraulic characteristics of the aquifer, such as transmissivity and storativity, and to define how the well and aquifer will respond in the future to various pumping rates that may be imposed by the operator.
Also very useful are the maximum drawdowns in the observation wells at different distances from the pumping well. The so-called "distance-drawdown" data also are used to define aquifer hydraulic characteristics, but more importantly are used to define the radius of influence of the well and the magnitude of aquifer drawdown under different pumping rates and at different future times.
A common value determined from a pumping test is the specific capacity of the well, which is its yield in gallons per minute per foot of drawdown. A good bedrock well in Maine will yield 10 gal/minute/foot of drawdown, while a good gravel well will yield 100 gal/minute/foot of drawdown. For comparison, the typical domestic bedrock well in Maine produces only around 0.1 gal/minute/foot of drawdown.
It is beyond the scope of this handbook to describe the various methods of analyzing pump-test data. For additional information, the reader is referred to Chapter 9 of Ground Water and Wells, (Second Edition) S. G. Driscoll; E. E. Johnson Company, 1986.
Last updated on March 25, 2009
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