Predicting tracer and contaminant transport with the stratified aquifer approach

Abstract

The assumption of perfect stratification in an aquifer has been widely used in solute-transport modeling studies. This assumption is especially useful for applied studies where limited site characterization data are available, but geologic well logs indicate significant layering. Chapter 3 investigates the issue of vertical sampling density via a sensitivity analysis of the number of aquifer layers used in a model of tracer transport through a heterogeneous synthetic aquifer. Tracer breakthrough in the synthetic aquifer is predicted by layered models. Given a variance of ln K of 2 and an exponential covariance function, sampling the synthetic aquifer at more than 12 elevations did not produce any significant improvement in the predictions. Even six sampling points, however, produced more accurate predictions of transport compared to a full-aquifer, homogeneous approach employing a local-scale dispersivity. Chapter 4 presents and interprets data from a dual-well, forced-gradient tracer experiment conducted in a confined aquifer underlying a contaminant source zone of a Superfund site. Tracer breakthrough was monitored at an extraction well and at four levels of a centerline monitoring well. A perfectly stratified numerical transport model based on multi-level data successfully predicted tracer breakthrough at the extraction well. Given the added vertical resolution associated with the layered model, it was possible to use dispersivity values more than an order of magnitude lower than the value used in a vertically integrated model. It is expected that the multi-layer model would allow for more robust analyses of solute transport at the site. In Chapter 5, TCE elution during the same dual-well experiment is predicted with a stratified numerical model incorporating rate-limited desorption, rate-limited diffusion, and rate-limited dissolution of nonaqueous phase liquid (NAPL). Based on model results, initial mass calculations, and other indirect lines of evidence, it is concluded that NAPL is the primary cause of rate limitations for TCE transport at the site. NAPL presence is the primary reason a large pump-and-treat system at the site has failed to reduce contaminant concentrations to federal drinking water standards. Alternative remediation technologies are thus necessary for restoring the aquifer, especially in the contaminant source zone.