The slug of water can either be added to or removed from the well — the only requirement is that it be done as quickly as possible, then the water level or pressure is monitored. Depending on the properties of the aquifer and the size of the slug, the water level may return to pre-test levels very quickly. A slug can be added by either quickly adding a measured amount of water to the well or something which displaces a measured volume. An alternative object is a solid polyvinyl chloride rod, with sufficient weight to sink into the groundwater. The objective here is to displace water, not merely be "heavy". A slug of water can be removed using a bailer or pump, but this is more difficult to do since it must be done very quickly and the equipment for removing the water will likely be in the way of getting water level measurements.
Performing a slug test
A slug test 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,
the well does not or cannot have a pump installed on it,
the general size of the aquifer parameters is all the accuracy that is required.
The size of the slug required is determined by the aquifer properties, the size of the well and the amount of time which is available for the test. For very permeable aquifers, the pulse will dissipate very quickly. If the well has a large diameter, a large volume of water must be added to increase the level in the well a measurable amount.
Slug test interpretation
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 increase in discharge rate at the pumping well; a slug test is instead an instantaneous pulse at the pumping well. This means that a superposition of an infinite number of sequential slug tests through time would effectively be a "standard" Theis 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 aquifer flow with a simple decaying exponential function. The aquifer parameters obtained from a slug test are typically less representative of the aquifer surrounding the well than an aquifer test which involves pumping in one well and monitoring in another. Complications arise from near-well effects, which may make it difficult to get accurate results from slug test interpretation.
