Analysis of convergent flow tracer tests in a heterogeneous sandy box with connected gravel channels

We analyzed the behavior of convergent flow tracer tests performed in a 3-D heterogeneoussandbox in presence of connected gravel channels under laboratory-controlled conditions. We focused onthe evaluation of connectivity metrics based on characteristic times calculated from experimental breakthroughcurves (BTCs), and the selection of upscaling model parameters related to connectivity. A conservativecompound was injected from several piezometers in the box, and depth-integrated BTCs weremeasured at the central pumping well. Results show that transport was largely affected by the presence ofgravel channels, which generate anomalous transport behavior such as BTC tailing and double peaks. Connectivityindicators based on BTC peak times provided better information about the presence of connectedgravel channels in the box. One of these indicators, b, was defined as the relative temporal separation of theBTCs peaks from the BTCs centers of mass. The mathematical equivalence between b and the capacity coefficientadopted in mass transfer-based formulations suggests how connectivity metrics could be directlyembedded in mass transfer formulations. This finding is in line with previous theoretical studies and wascorroborated by reproducing a few representative experimental BTCs using a 1-D semianalytical bimodalsolution embedding a mass transfer term. Model results show a good agreement with experimental BTCswhen the capacity coefficient was constrained by measured b. Models that do not embed adequate connectivitymetrics or do not adequately reproduce connectivity showed poor matching with observed BTCs.