Slug test

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A slug test is a particular type of aquifer test where water is quickly added or removed from a groundwater well, and the change in hydraulic head is monitored through time, to determine the near-well aquifer characteristics. It is a method used by hydrogeologists and civil engineers to determine the transmissivity and storativity of the material the well is completed in.

This is in contrast to standard aquifer tests, which typically involve pumping a well at a constant flowrate, and monitoring the response of the aquifer in nearby monitoring wells. Often slug tests are performed instead of a constant rate test, because:

time constraints (quick results, or results for a large number of wells, are needed),
the transmissivity of the material the well is completed in is too low to realistically perform a proper pumping test (common for aquitards or some bedrock monitoring wells), or
the general size (order of magnitude) of the aquifer parameters is all the accuracy that is required.

Because the flow rate into or out of the well is not constant, as is the case in a typical aquifer test, the standard Theis solution does not work.

Mathematically, the Theis equation is the solution of the groundwater flow equation for a step change in discharge rate at the pumping well; a slug test is a pulse change in discharge rate. This means that a superposition (or more precisely a convolution) of many slug tests through time, would effectively be a "standard" aquifer test.

There are several known solutions to the slug test problem, a common engineering approximation is the Hvorslev method, which approximates the more rigorous solution to transient flow in a slug test with a simple decaying exponential function.