TRACER TESTS AT THE MADE SITE: CHARACTERIZING SMALL-SCALE PREFERENTIAL FLOW CHANNELS AND THEIR EFFECTS ON SOLUTE TRANSPORT

Previous studies have indicated that small-scale preferential flow channels likely occur at the Macrodispersion Experiment (MADE) site in Columbus, Mississippi. These preferential flow channels are believed to be responsible for the non-Gaussian transport behavior over several hundreds of meters observed during three previous natural-gradient tracer experiments. To provide further field evidence of preferential flow channels and quantify their effects on subsurface solute transport, a couple of tracer tests have recently been conducted at the MADE site. The first test is a single-well injection-withdrawal (SWIW) experiment and bromide is used as the conservative tracer. The bromide breakthrough curve is very asymmetric and shows an early-time high peak followed by extensive low-concentration tailing during the late recovery stage. Initial analyses indicate that the data collected from the SWIW test can be used as the basis to estimate the parameters required by the dual-domain mass transfer model in order to reproduce the observed plume behaviors in previous natural-gradient experiments at the site. In the second test, a dye tracer (Brilliant Blue FCF) is injected into the aquifer and 20 soil cores are subsequently collected in the vicinity of injection well. The soil cores are visually inspected in an attempt to construct a three-dimensional preferential flow channel network by correlating the dye distribution between different soil cores.