Geological modeling of submeter scale heterogeneity and its influence on tracer transport in a fluvial aquifer

Abstract

We developed a new model of aquifer heterogeneity to analyze data from a single‐well injection‐withdrawal tracer test conducted at the Macrodispersion Experiment (MADE) site on the Columbus Air Force Base in Mississippi (USA). The physical heterogeneity model is a hybrid that combines 3‐D lithofacies to represent submeter scale, highly connected channels within a background matrix based on a correlated multivariate Gaussian hydraulic conductivity field. The modeled aquifer architecture is informed by a variety of field data, including geologic core sampling. Geostatistical properties of this hybrid heterogeneity model are consistent with the statistics of the hydraulic conductivity data set based on extensive borehole flowmeter testing at the MADE site. The representation of detailed, small‐scale geologic heterogeneity allows for explicit simulation of local preferential flow and slow advection, processes that explain the complex tracer response from the injection‐withdrawal test. Based on the new heterogeneity model, advective‐dispersive transport reproduces key characteristics of the observed tracer recovery curve, including a delayed concentration peak and a low‐concentration tail. Importantly, our results suggest that intrafacies heterogeneity is responsible for local‐scale mass transfer.