What does a slug test measure?

• A. Porosity
• B. Hydraulic head
• C. Localized values of permeability
• D. Groundwater velocity

Answer: (C)

Slug tests are used to gauge how easily water can move through the sediments immediately around a well. By suddenly changing the water level in the well and watching how quickly it returns to equilibrium, you’re measuring the ease of flow in the near-well zone, which is basically the hydraulic conductivity (permeability) of those sediments. Because this test reflects conditions right at the well screen, it provides localized values rather than properties averaged over the entire aquifer. Porosity is a storage property and doesn’t control the recovery rate in the same way. Hydraulic head is about pressure potential you’d measure in a piezometer, not the rate of recovery after a perturbation. Groundwater velocity isn’t directly measured by a slug test; you could estimate it if you know the gradient and permeability, but the test itself yields permeability information. So the test best corresponds to obtaining localized values of permeability.