Comparison of Slug Test Methodologies for Determination of Hydraulic Conductivity in Fine-Grained Sediments

Abstract

Slug tests were conducted on four lithologic units at a waste disposal site in southwestern Illinois. All tested units were fine-grained glacial tills that had laboratory-determined hydraulic conductivity values between 10-9 and 10-6 cm/s. A total of 37 field tests of hydraulic conductivity were run in 29 open-hole piezometers, with some tests lasting up to a year. The piezometers were oriented both vertically and at a 45° angle to test the effects of possible vertical fractures on hydraulic conductivity. All slug test results were analyzed using three different methods. The methods selected were those of Cooper, Bredehoeft, and Papadopulos; Hvorslev; and Nguyen and Pinder. The geometric mean values for each geologic unit determined by all three methods were within an order of magnitude and well within one standard deviation. All three methods could not be applied to all data with the same degree of reliance. This is to be expected since each method has different underlying assumptions. However, all data did fit at least one theoretical curve very well. The importance of these results is twofold: (1) any of the three methods can produce reliable results if properly applied, and (2) data that appear to be unusable by one method may be usable by changing to another method of analysis.