SIMPLIFIED METHOD OF PUSH-PULL TEST DATA-ANALYSIS FOR DETERMINING IN-SITU REACTION-RATE COEFFICIENTS

The single-well, ''push-pull'' test method is useful for obtaining inf ormation on a wide variety of aquifer physical, chemical, and microbio logical characteristics. A push-pull test consists of the pulse-type i njection of a prepared test solution into a single monitoring well fol lowed by the extraction of the test solution/ground water mixture from the same well. The test solution contains a conservative tracer and o ne or more reactants selected to investigate a particular process. Dur ing 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 anal ysis that can be used to estimate a first-order reaction rate coeffici ent from these breakthrough curves. Rate coefficients are obtained by fitting a regression line to a plot of normalized concentrations versu s elapsed time, requiring no knowledge of aquifer porosity, dispersivi ty, or hydraulic conductivity. A semi-analytical solution to the advec tive-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, conf ined aquifer with a fully-penetrating injection/extraction well and va rying porosity, dispersivity, test duration, and reaction rate. A nume rical flow and transport code (SUTRA) is used to evaluate the ability of the simplified method to estimate reaction rate coefficients in sim ulated push-pull tests in a heterogeneous, unconfined aquifer with a p artially penetrating well. In all cases the simplified method provides accurate estimates of reaction rate coefficients; estimation errors r anged from 0.1 to 8.9% with most errors less than 5%. In an example ap plication, the method is used to estimate reaction rate coefficients f or a microbiological process (denitrification) using data from field p ush-pull tests performed in a petroleum-contaminated, unconfined alluv ial aquifer.