What Is a Slug Test?

A slug test is designed to measure the hydraulic conductivity, or flow, of ground water near an aquifer by removing or displacing a slug of water or soil from below the ground and measuring the flow or pressure at which the ground water table changes. Civil and environmental engineers, as well as hydrologists and hydrogeologists, will use a slug test to determine the characteristics of an aquifer. A slug test differs from a common aquifer test in that it usually measures only one well while an aquifer test changes one well and then measures the changes that result in a nearby well.

In a typical slug test, a heavy, long PVC rod is inserted into a ground water well to collect or displace the water in the well. Once it is removed, the hydraulic conductivity of the water near an aquifer can be measured. According to the US Environmental Protection Agency (EPA) Standard Operating Procedure #2046, “the hydraulic conductivity is an important parameter for modeling the flow of groundwater in an aquifer.”

A person performing a slug test in the US will need to follow both the protocol set up by the US EPA, the protocol established by the state’s environmental governing agency, and his or her internal company protocol. As with any scientific experiment or test, data needs to be recorded before, during and after the test in order to be able to accurately measure the hydraulic conductivity of the area and the aquifer. Data acquired during a slug test includes the site or location identification number, the person performing the test, the date the data is collected, the slug volume, the test method, elapsed time of the test, the depth to the water and any additional comments.

There are two main advantages to slug tests. First, the results are measured in-situ, which means onsite. This is different from off-site tests which might produce errors depending on how the samples were handled. Second, slug tests are usually less expensive to perform than pumping and they do not require additional wells for measurement.

A limitation of a slug test is that it only measures the area immediately surrounding the well. This is in contrast to most aquifer tests, which simultaneously measure the results of a slug test in many wells in a particular area. So a slug test might be representative of just a small section of the geography, while a standard aquifer test might have more comprehensive and accurate results.