Nonequilibrium and dispersion during transport of contaminants in groundwater: field-scale processes

Several processes can cause nonideal behavior, including physical nonequilibrium, chemical nonequilibrium, intrasorbent diffusion, spatial variations in sorption coefficient, and spatial variability in hydraulic conductivity. Nonideality can be investigated with an analysis of data gathered from a recent natural gradient field experiment at Borden, Ontario. The nonideal behavior exhibited by the field data appears to result from spatial variability in hydraulic conductivity and sorption coefficients. The relative impact of the nonideality mechanisms can be qualitatively evaluated by comparing the effective dispersivity values associated with each process. The relative importance of the processes are scale dependent. Given the typical heterogeneous nature of most aquifer systems, and the manner in which water samples are usually taken (vertically-integrated), macrodispersion is most likely to be the predominant process influencing contaminant dispersion at the field scale. -from Authors