COUPLED EFFECTS OF NONLINEAR, RATE-LIMITED SORPTION AND BIODEGRADATION ON TRANSPORT OF 2.4-DICHLOROPHENOXYACETIC ACID IN SOIL

The transport and fate of many contaminants in subsurface systems can be influenced by several coupled processes, such as nonlinear, rate-li mited sorption and biodegradation. We investigated the transport in so il of a model organic compound, 2,4-dichlorophenoxyacetic acid (2,4-D) , influenced by nonlinear, rate-limited sorption and biodegradation. M iscible displacement and batch sorption experiments were conducted usi ng a wide range of solute concentrations to investigate the impact of concentration-dependent behavior on transport. The sorption isotherm w as approximately linear at a low concentration, and nonlinear over the extended range of concentrations. Results from the transport experime nts, with the fitted N values approaching 1 at low input concentration s, were consistent with the batch sol prion study. Nonlinear sorption significantly influenced the position of the breakthrough curves becau se of concentration-dependent retardation. However, although both nonl inear and rare-limited sorption influenced the shape of the breakthrou gh curves, the impact of rate-limited sorption was greater. The effect ive travel time of 2,4-D transport is influenced by synergistic intera ctions between sorption and biodegradation. For example, the sequentia l rightward shift of the breakthrough curves with decreasing input con centration, due to nonlinear sorption of 2,4-D, is enhanced by biodegr adation.