Simplified Method of “Push‐Pull” Test Data Analysis for Determining In Situ Reaction Rate Coefficients

Abstract

AbstractThe single‐well, “push‐pull” test method is useful for obtaining information on a wide variety of aquifer physical, chemical, and microbiological characteristics. A push‐pull test consists of the pulse‐type injection of a prepared test solution into a single monitoring well followed by the extraction of the test solution/ground water mixture from the same well. The test solution contains a conservative tracer and one or more reactants selected to investigate a particular process. During the extraction phase, the concentrations of tracer, reactants, and possible reaction products are measured to obtain breakthrough curves for all solutes. This paper presents a simplified method of data analysis that can be used to estimate a first‐order reaction rate coefficient from these breakthrough curves. Rate coefficients are obtained by fitting a regression line to a plot of normalized concentrations versus elapsed time, requiring no knowledge of aquifer porosity, dispersivity, or hydraulic conductivity. A semi‐analytical solution to the advective‐dispersion equation is derived and used in a sensitivity analysis to evaluate the ability of the simplified method to estimate reaction rate coefficients in simulated push‐pull tests in a homogeneous, confined aquifer with a fully‐penetrating injection/extraction well and varying porosity, dispersivity, test duration, and reaction rate. A numerical flow and transport code (SUTRA) is used to evaluate the ability of the simplified method to estimate reaction rate coefficients in simulated push‐pull tests in a heterogeneous, unconfined aquifer with a partially penetrating well. In all cases the simplified method provides accurate estimates of reaction rate coefficients; estimation errors ranged from 0.1 to 8.9% with most errors less than 5%. In an example application, the method is used to estimate reaction rate coefficients for a microbiological process (denitrification) using data from field push‐pull tests performed in a petroleum‐contaminated, unconfined alluvial aquifer.