Determination of Transmissivity from Specific Capacity Tests in a Karst Aquifer

Abstract

AbstractSpecific capacity tests are useful for estimating transmissivity in aquifers that have few good‐quality pump tests. In karst aquifers, this has been done by (1) correcting specific capacity for turbulent well loss and using analytical relationships between transmissivity and specific capacity, and (2) correcting specific capacity for well loss and deriving an empirical relationship between transmissivity and specific capacity. This study focuses on the uncertainties of estimating well loss and presents an empirical relationship between transmissivity and uncorrected specific capacity for a karst aquifer.Well loss is difficult to estimate without good‐quality step‐drawdown tests. Pipe‐flow theory tends to underestimate well loss, and an empirical relationship between specific capacity and well‐loss constant has a large prediction interval that leads to well loss exceeding measured drawdown. To overcome uncertainties of estimating well loss, transmissivity and uncorrected specific capacity were related for aquifer tests from the Edwards aquifer in Texas. The resulting best‐fit line is T = 0.76(Sc)1.08 for T and Sc in m2 d–1 with a coefficient of determination, R2, of 0.89 and a 95‐percent prediction interval spanning approximately 1.4 log cycles. Though the prediction interval is large, approximate but useful estimates of transmissivity can be determined because the relationship extends over five orders of magnitude from 1 to 100,000 m2 d‐1. The relationship is applicable in at least one other karst aquifer and therefore may be useful for others.