Relationship Between Pumping‐Test and Slug‐Test Parameters: Scale Effect or Artifact?

Abstract

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.

Structural geology and geophysics as a support to build a hydrogeologic model of granite rock

“…Multiple explanations exist for scale effects. Illman (2004) provides a thorough discussion on scale effects and their origin, pointing to other issues such as poorly developed wells (Butler and Healey, 1998) and turbulence in the boreholes (Lee and Lee, 1999). Guimerà et al (1995) argue that long-term pumping tests are purposefully performed at the most conductive intervals, implying that they are not representative.…”

Integration of hydrogeophysics and remote sensing with coupled hydrological models

“…Another possible explanation of substantially lower values of K ST as compared to K P T was provided by Butler and Healey (1998) who attributed it to insufficient borehole-development prior slug test. That could be particularly relevant at typical low yield of the saprolite in our case.…”

“…Due to the generally low borehole yield, in all 5 boreholes, we performed slug tests instead of pumping tests (Butler and Healey, 1998). Slug tests allow determining efficiently the hydraulic conductivity but are generally considered to be only representative of a small volume of aquifer material around the well and do not provide storage properties.…”

Upscaling of water flow and mass transport in a tropical soil: numerical, laboratory and field studies

“…In L. P. Wilding, In common practice, these parameters are determined in the field or in the laboratory, and then they are used in models to conduct predictions, with no concern about the scale (support) at which they were measured ( , 1984; Khan & Jury, 1990; Pang & Hunt, 2001; Silliman & Simpson, 1987; Vik, Bastesen, & Skauge, 2013b). Regarding hydraulic conductivity, some authors suggest that there is no scale effect and that the differences in value at different scales are primarily due to problems during the measurements and not due to its measurement support (Butler & Healey, 1998a, 1998b. However, many studies have shown that hydraulic conductivities computed in the laboratory tend to have a smaller mean and a larger variance than conductivities observed in the field over larger scales (Chapuis et al, 2005; Clauser, 1992; Parker & Albrecht, 1987; Rovey & Niemann, 1998;Sobieraj, Elsenbeer, & Cameron, 2004; Yang et al, 2017).…”