Nonequilibrium sorption of organic contaminants during flow through columns of aquifer materials

Batch equilibrium and miscible displacement studies were conducted to investigate sorption and transport of trichloroethylene (TCE) and p‐xylene in two sand aquifer materials low in organic carbon content. The utility of a bicontinuum (two‐site) sorption model to describe the extent of sorption nonequilibrium during transport in saturated columns was evaluated. Model parameters estimated from the breakthrough curves (BTC) obtained at a high pore‐water velocity (v = 24 cm/h) were used successfully to describe the BTC measured at a lower pore‐water velocity (v = 6 cm/h). The assumption of local sorption equilibrium was valid at the lower velocity, and the measured BTC could be described by an equilibrium sorption model coupled to a convective‐dispersive transport model. Values for column retardation factors estimated by fitting the two‐site model to the BTC were comparable with those estimated from equilibrium sorption isotherms. BTC measured for displacement of binary mixtures (TCE plus p‐xylene) and single‐solute (TCE or p‐xylene alone) were identical, suggesting that there is no competitive sorption between these two solutes. These column observations were consistent with sorption isotherm data.