Investigation of subsurface tracer transport using a high-resolution model of fluvial aquifer heterogeneity

In this work, we consider how small-scale geologic variability affects tracer migration at the Macrodispersion Experiment (MADE) site in Columbus, Mississippi. Most previous research at the MADE site has focused on natural-gradient tracer tests where solute transport occurs over relatively large field distances (tens to hundreds of meters). Here, we consider data from a single-well injection-withdrawal (SWIW) tracer test, which was specifically designed to study the transport sensitivity to small heterogeneities (centimeters to decimeters). The SWIW test was conducted using bromide as a conservative tracer [Liu et al., 2009]. We analyze the migration behavior of this tracer using a 3D heterogeneity model that consists of two key elements: (1) a heterogeneous background hydraulic conductivity field with geostatistical properties determined from previous flowmeter data; and (2) sub-meter scale, highly permeable channels that occupy less than ten percent of the modeled volume. This conceptualization is supported by a variety of site data. In particular, we rely on recent grain-size analyses from extensive core sampling in the SWIW test area [Bianchi et al., 2009]. Using this detailed heterogeneity model, we perform numerical simulations to evaluate groundwater flow and bromide transport during the SWIW test. Our high-resolution model allows for explicit simulation of preferentially fast as well as relatively slow advection processes that lead to complex, non-Gaussian transport behavior at MADE. As a result, although we use a transport formulation with only advection and a small dispersion term, we are still able to reproduce major characteristics of the tracer breakthrough curve, including a delay in the peak concentration arrival. This work offers insights for future research aimed at understanding how small-scale geologic heterogeneities may contribute to complex transport dynamics observed at larger scales.