Ron E. Jessup

Flow interruption: A method for investigating sorption nonequilibrium

Abstract A flow-interruption method for investigating sorption nonequilibrium is presented. The technique has a greater sensitivity to nonequilibrium than to traditional column experiments and hence provides a greater capacity to investigate nonequilibrium. The technique may provide more reliable determinations of kinetic parameter values. In some situations, the technique may be used to assist in the delineation of mechanism(s) responsible for sorption nonequilibrium. Use of the technique is demonstrated with miscible displacement of the herbicide 2,4-D through saturated soil columns. The sorption nonequilibrium exhibited by 2,4-D is suggested to be a result of intraorganic matter diffusion. Experimentally determined values of the sorption rate constant show an inverse relationship to organic matter content.

Modeling solute transport influenced by multiprocess nonequilibrium and transformation reactions

We present an advective-dispersive solute transport model that explicitly accounts for multiple sources of nonequilibrium and transformation reactions during steady state flow in porous media. The multiprocess nonequilibrium with transformation (MPNET) model is formulated for cases where nonequilibrium, caused by a combination of transport-related and sorption-related processes, and abiotic/biotic transformations can be described as first-order processes. The impact of the coupling of nonequilibrium and transformation reactions on solute transport is examined using selected illustrative examples. The performance of the model is evaluated by comparing predictions obtained with the model, where values for all model parameters are obtained independently, to a data set obtained from the literature. The prediction obtained with the MPNET model matched the data very well, much better than did the predictions obtained with the MPNET model assuming no degradation and with a model that does not account for rate-limited sorption.

CRITIQUE OF: COMPUTER SIMULATION MODELING FOR NITROGEN IN IRRIGATED CROP LANDS

Models for simulation of plant uptake of water and nitrogen are critically examined. Inherent complexity of the soil-plant system as well as a general unavailability of the input parameters required in the microscopic-level models precludes model validation at the present time. Thus, macroscopic-level models that provide a gross description of nitrogen and water uptake by plants may be of more immediate practical use. A simple model describing plant uptake of nitrogen during a crop growing season is presented and validated using field-measured data. The need for conducting careful experiments to develop an extensive data base for validation of the simulation models is discussed.