Slug test

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

1 Slug test methodThe slug of water can either be added to or removed fromthe well the only requirement is that it be done asquickly as possible (the interpretation typically assumesinstantaneously), then the water level or pressure is mon-itored. Depending on the properties of the aquifer andthe size of the slug, the water level may return to pre-testlevels very quickly (thus complicating accurate collectionof water level data).A slug can be added by either quickly adding a measuredamount of water to the well or something which displacesa measured volume (e.g., a long heavy pipe with the endscapped o). An alternative object is a solid polyvinylchloride (PVC) rod, with sucient weight to sink intothe groundwater. The objective here is to displace water,not merely be heavy. A slug of water can be removedusing a bailer or pump, but this is more dicult to dosince it must be done very quickly and the equipment forremoving the water (pump or bailer) will likely be in theway of getting water level measurements.

2 Performing a slug testA slug test is in contrast to standard aquifer tests, whichtypically involve pumping a well at a constant owrate,andmonitoring the response of the aquifer in nearbymon-itoring wells. Often slug tests are performed instead of aconstant rate test, because:

time constraints (quick results, or results for a largenumber of wells, are needed),

the well does not or cannot have a pump installed onit (slug tests do not require pumping),

the transmissivity of the material the well is cased inis too low to realistically perform a proper pumpingtest (common for aquitards or some bedrock moni-toring wells), or

the general size (order of magnitude) of the aquiferparameters is all the accuracy that is required.

The size of the slug required is determined by the aquiferproperties, the size of the well and the amount of timewhich is available for the test. For very permeableaquifers, the pulse will dissipate very quickly. If thewell has a large diameter, a large volume of water mustbe added to increase the level in the well a measurableamount.

3 Slug test interpretationBecause the ow rate into or out of the well is not con-stant, as is the case in a typical aquifer test, the standardTheis solution does not work.Mathematically, the Theis equation is the solution of thegroundwater ow equation for a step increase in dischargerate at the pumping well; a slug test is instead an instan-taneous pulse at the pumping well. This means that asuperposition (or more precisely a convolution) of an in-nite number of sequential slug tests through time wouldeectively be a standard Theis aquifer test.There are several known solutions to the slug testproblem; a common engineering approximation is theHvorslev method, which approximates the more rigoroussolution to transient aquifer ow with a simple decayingexponential function.The aquifer parameters obtained from a slug test are typ-ically less representative of the aquifer surrounding thewell than an aquifer test which involves pumping in onewell andmonitoring in another. Complications arise fromnear-well eects (i.e., well skin and wellbore storage),which may make it dicult to get accurate results fromslug test interpretation.

4 See also Aquifer test Well test

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