Relationship between pumping-test and slug-test parameters: Scale effect or artifact?

In most field investigations, information about hydraulic conductivity (K) is obtained through pumping or slug tests. A considerable body of data has been amassed that indicates that the K estimate from a pumping test is, on average, considerably larger than the estimate obtained from a series of slug tests in the same formation. Although these data could be interpreted as indicating a natural underlying scale dependence in K, an alternate explanation is that the slug-test K is artificially low as a result of incomplete well development and, to a much lesser extent, failure to account for vertical anisotropy. Incomplete well development will often result in only the most permeable zones being cleared of drilling debris, with much of the screened interval remaining undeveloped. More cursory development can leave a low-K skin along the entire screened interval. Failure to recognize such conditions can result in a K estimate from a slug test that is much lower than the average K of the formation in the vicinity of the well. By contrast, neither a skin nor vertical anisotropy will have a significant impact on K estimates from pumping tests when semi-log analyses and/or observation wells are used. However, a reasonable estimate of aquifer thickness is required to convert the transmissivity calculated from a pumping test into an average K for the aquifer. Prior to invoking a natural scale dependence to explain the results of different types of hydraulic tests, head data should be closely examined and serious consideration given to alternate explanations.