A fundamental assumption when analyzing a pumping test is that the water level measured represents the water level in the aquifer. This is mainly true in observation wells, but not in the pumping well (PW) itself.
The water level measured in a pumping well is not the water level in the aquifer.
In an ideal world, the drawdown is measured in multiple observation wells. However, budgetary or logistic restraints often limit measuring drawdown to inside the pumping well only.
How is this possible?
In and around a pumping well, some “well effects” can occur which lead to a difference between the water level in the aquifer and the pumping well. These can include:
- Turbulent flow
- Storage in the well casing
- Well entry losses
- Filter pack influence
- Clogging
Good news first…
As long as you keep the discharge rate constant, all the factors above will be constant in time – at least after a certain pumping duration. This means that they just sum up to an additional drawdown, which will no longer change. So the rate of change in the water level is caused by aquifer parameters; in the case of a confined aquifer it is the transmissivity.
In a semi-log plot of time vs. drawdown (time on the log scale) the plotted points will form a straight line – at least after a certain pumping time. The slope of that line will tell you the transmissivity of the aquifer.
Transmissivity can be obtained from the drawdown data measured in the pumping well.
The limitation…
In the semi-log plot mentioned above the well effects will lead to a vertical translation of the straight line. Typically the quantity of the well effects is unknown. Since the position of the straight line is used to calculate the storativity (whereas in case of transmissivity only the slope is used) there will always be an error, leading to a wrong number for storativity.
To makes things worse even the radial position of measurement (radius) is unknown. Often the screen radius is used, sometimes also the effective radius (calculated from borehole and screen radius and gravel pack porosity). However, the “real effective radius” is still uncertain.
The value of storativity cannot be determined from the data recorded in the pumping well because it correlates with well effects and radius.
In a simple analysis in AquiferTest Pro (e.g. Theis, Cooper & Jacob), which returns T and S only, there is no option for well effects. So if the results obtained by the program hit the boundaries (e.g. S>0.5) the only option you have is to change the radius.
Your Options in AquiferTest Pro
AquiferTest pro contains a variety of graphing techniques and deriative analyses that allow you to analyze drawdown data from the pumping well. You may want to use the Theis analysis, but in a semi-log view in order to produce the straight line analysis mentioned above. Or, you may use the Papadopulos & Cooper Analysis method, to account for well bore storage in large diameter wells.
Examples
The following are examples from single-well analysis in AquiferTest Pro. These projects can be found in the “Examples” folder following the installation.
References
Kruseman and de Ridder, Analysis and Evaluation of Pumping Test Data. Second Edition.
AquiferTest Pro v.2014.1 Users Manual.