Nonideal transport of reactive solutes in heterogeneous porous media 2. Quantitative analysis of the Borden natural-gradient field experiment

Field experiments constitute an integral component of research on transport and fate of contaminants in the subsurface. One of the most well known of the few field experiments performed with reactive solutes is the natural-gradient experiment conducted at the Borden site during 1982 to 1984. A major finding of the experiment was that the transport of the reactive, organic compounds was nonideal. First, the velocities of the centers of mass of the plumes decreased with time, which was reflected in a temporal increase in effective retardation. Second, the longitudinal spreading observed for the organic solutes was about three times larger than that of the nonreactive tracers for an equivalent travel distance. Third, the breakthrough curves measured at selected monitoring points exhibited greater asymmetry compared to the nonreactive tracers. The cause(s) of the nonideal transport observed for the organic solutes has remained unexplained, despite a number of attempts. We have used a multi-scale, multi-factor mathematical model to successfully predict the displacement and spreading behavior of the tetrachloroethene and tetrachloromethane plumes. Based on our analyses, we conclude that a near-field trend of increasing sorption capacity was a primary cause of the deceleration of the centers of mass of the organic- solute plumes. The coupled effects of nonlinear sorption and enhanced spreading caused by spatially variable hydraulic conductivity and spatially variable sorption also influenced plume displacement. In addition, it is possible that the combination of spatially variable hydraulic conductivity and sorption contributed directly to plume deceleration. However, a magnitude of sorption variability larger than has been measured to date is required for this contribution to be significant. The combined spatial variability of hydraulic conductivity and sorption, and a potential negative cross correlation between them, appears to have been the major cause of the enhanced longitudinal spreading observed for the organic-solute plumes in comparison to the nonreactive-solute plumes. However, nonlinear sorption, the spatial trend of increasing sorption capacity, and rate-limited sorption/mass transfer also influenced spreading behavior. In total, it is evident that the transport of the organic compounds during the Borden natural-gradient field experiment was influenced by several interacting factors and coupled processes, and that accurate prediction of the observed behavior requires the use of a mathematical model that accounts for this complexity.