The slug test is one of the most common techniques for the in situ estimation of hydraulic conductivity in unconfined flow systems. Recently, Bouwer and Rice model describing the flow of ground water in response to a slug test in an unconfirmed flow system has been proposed. For slug tests performed in homogeneous, isotropic formations that would be classified as aquifiers, the Bouwer and Rice model provides estimates within 30% of actual field values. In less-permeable, clay-rich formations, however, estimates may overpredict formation conductivity by more than 100%. The Bouwer and Rice model introduces the largest error in the presence of low permeability skin. Uncertainty about anisotropy can also be the source of considerable error.