The duration of a slug test is a function of both formation and well-construction parameters. The drillstem test (DST) is used by hydrogeologists to estimate the hydraulic conductivity of low-permeability formations, especially in the case of formations that are at a considerable depth below land surface. The major advantage of a DST over a conventional slug test analyzed with the Ferris–Knowles method is that the well is shut-in at a relatively large normalized head, resulting in the Ferris–Knowles assumptions being applicable at a much larger normalized head than in a conventional slug test. A slug test was performed in a monitoring well in Lincoln County, Kansas that was screened in a confined deltaic sequence consisting of mudstone interbedded with very fine sandstone. The shut-in slug test is based on isolating the screened interval of the well from the portion of the casing open to the atmosphere.