Understanding solute transport in extremely heterogeneous media: What has been learned from 20 years of research at the MADE site?

Since the early 1980s, three large-scale natural-gradient tracer tests and other types of field studies have been conducted at the Macrodispersion Experiment (MADE) site within the Columbus Air Force Base in Mississippi. The field data from the MADE site have been used extensively by researchers around the world to investigate and understand contaminant transport processes in highly heterogeneous aquifers. The MADE site has directly or indirectly inspired the development of a number of new and improved theories and computer models aimed at more accurate representation and prediction of contaminant transport affected by extreme aquifer heterogeneity. A great deal of recent and on-going research on contaminant transport in heterogeneous media has been motivated by the findings from the MADE site that suggest the existence of preferential flow paths resulting from small-scale heterogeneities. In this presentation, we provide an overview of the research activities at the MADE site over the last 20 years and discuss how the findings from field studies have motivated various theories and models purported to accommodate so-called non-ideal transport behavior observed at the MADE site. We focus on our recent field campaigns to further test the hypothesis that solute transport at the MADE site is dominated by small-scale preferential flow paths. Our field data include a forced-gradient dye tracer test, soil coring, and high-resolution resistivity and GPR surveys. The on-going efforts might lead to stronger theoretical underpinnings and useful practical tools for modeling solute transport in extremely heterogeneous aquifers such as that encountered at the MADE site.