A mini slug test method for determination of a local hydraulic conductivity of an unconfined sandy aquifer

Abstract

A new and efficient mini slug test method for the determination of local hydraulic conductivities in unconfined sandy aquifers is developed. The slug test is performed in a small-diameter (1 inch) driven well with a 0.25 m screen just above the drive point. The screened drive point can be driven from level to level and thereby establish vertical profiles of the hydraulic conductivity. The head data from the test well are recorded with a 10 mm pressure transducer, and the initial head difference required is established by a small vacuum pump. The method described has provided 274 spatially distributed measurements of a local hydraulic conductivity at a tracer test site at Vejen, Denmark. The mini slug test results calculated by a modified Dax slug test analysing method, applying the elastic storativity in the Dax equations instead of the specific yield, are in good accordance with the results from two natural gradient tracer experiments performed at the test site. The original Dax, the Bouwer and Rice, and the Chirlin analysing methods all led to an underestimation of the effective hydraulic conductivity by a factor of more than 2, when compared with the tracer tests. In contrast the spherical flow model of Karasaki et al. overestimated the results of the tracer tests by approximately a factor 1.4. The Dax and the Cooper et al. methods, assuming only radial flow to the partially screened well, yielded a better approximation of the horizontal hydraulic conductivity, than the Chirlin method, which also considers axial flow. This fact is suggested to be a result of aquifer anisotropy, as a significant higher horizontal than vertical hydraulic conductivity may suppress the significance of the axial flow component.