Aquifer Test Analysis in Nonradial Flow Regimes: A Case Study

A low-rate aquifer pumping test was conducted in a folded and fractured limestone formation in the Conasauga Group of East Tennessee. An elongate drawdown pattern was observed, suggesting that pumping response was controlled by a highly conductive zone associated with a pair of anticlinal folds that bisect the test site. This drawdown pattern, the nonlinearity of graphs of observation well drawdown versus the log10 of time, and the existence of a linear relationship between drawdown and the square root of pumping time indicate that flow in response to pumping was nonradial. Thus, use of pumping test interpretive techniques based on radial flow models would be invalid. This situation may occur frequently when pumping tests are done in low-permeability media, because fractures and other discontinuities often dominate the hydraulic behavior of low-permeability media. The solution technique used to estimate aquifer parameters (Gringarten and Witherspoon, (1972) treats the fold axes as a highly conductive vertical fracture in an anisotropic matrix of much lower hydraulic conductivity. The complexity of both the site and the solution technique necessitated iterative trial-and-error variable substitution to estimate aquifer parameters. The resulting solution includes estimates of matrix transmissivity in two directions, the storage coefficient of the matrix, and the effective length and location of the master fracture. Estimates of hydraulic parameters lack the apparent precision of values that would be generated by radial flow techniques, but they are consistent with geologic conditions at the site and present a clearer picture of the hydrogeologic regime than could be obtained with other investigative techniques. The solution technique used, whose field application has not been described previously in published literature, makes it possible to interpret the results of aquifer tests in complex hydrogeologic settings. This and related analytical techniques for other nonradial flow situations may prove useful in other hydrogeologic investigations in low-permeability media affected by folds, faults, fractures, or solution channels.