In situ evaluation of solute retardation using single-well push-pull tests

More efficient methods are needed for the in situ evaluation of solute sorption to aquifer sediments. The objective of this study was to develop a simplified method for estimating retardation factors for injected solutes from "push-pull" test extraction phase breakthrough curves (BTCs). Sensitivity analyses based on numerical simulations were used to evaluate the method performance for a variety of test conditions. Simulations were conducted for varying retardation factors, aquifer parameters and injection phase durations, for tests performed under nonideal transport conditions such as nonlinear equilibrium and linear nonequilibrium sorption, and for a test performed in a physically heterogeneous aquifer. Predicted retardation factors showed errors less than or equal to 14% in tests performed under ideal transport conditions (physically homogeneous aquifer with spatially uniform dispersivity that does not vary from solute to solute, spatially uniform linear equilibrium sorption). The method performed more poorly for solutes with large retardation factors (R > 20) and for tests conducted under nonideal transport conditions, and is expected to perform poorly in aquifers with highly heterogeneous sorption. In an example application, we used the method to estimate the distribution coefficient for Sr-85 using data from a field test performed by Pickens JF, Jackson RE, Inch KJ, Merritt WF. (Water Resour Res 1981;17:529-44). Reasonable agreement was found between distribution coefficients obtained using the simplified method of estimation and those obtained by Pickens et al. (1981). (C) 2000 Elsevier Science Ltd. All rights reserved.