Evaluation of Water Vapor Sorption and Electrical Conductivity Methods to Determine Bentonite Content of a Soil-Bentonite Barrier

Two alternative methods were evaluated to measure bentonite content in soil-bentonite mixtures: (1) the water vapor sorption (WVS) method; and (2) the electrical conductivity (EC) method. The WVS method is a new technique developed for this study; the EC method was developed previously and modified in this study. Soil-bentonite mixtures blended with a pug-mill to have 5–6% bentonite content were sampled from three different locations of an engineered barrier. Samples of the base soil used for the barrier and the bentonite were also obtained. Calibration curves were created in the laboratory using base soil and bentonite collected from the field to determine the bentonite contents of samples of the field mixture. Bentonite contents determined from the field-mixed samples were compared to bentonite contents computed from scale records maintained during construction of the engineered barrier. Mixtures prepared in the laboratory and mixtures sampled in the field were treated chemically and physically to eliminate the influence of the initial distribution of bound cations on the test outcome and to amplify the sensitivity of the testing methods to bentonite content. The WVS and EC methods have linear correlations with bentonite content and can be used to determine the bentonite content of field mixtures accurately. However, physical and chemical treatments must be applied for accurate determination of bentonite content.