There is often a fraction of contaminants in the subsurface which is observed to persist for long periods of time and resist removal. The phenomenon of incomplete or slow desorption has been shown in the laboratory for many different soil and sediment types and chemicals and a model was developed known as irreversible sorption. Hydrocarbon data from several field sites are compared to the linear and irreversible sorption isotherms and it appears that field data are described by a combination of the linear and irreversible models. A one-dimensional transport model which includes a linear or irreversible term for sorption generally predicts significantly lower concentrations with the linear term than the irreversible term. However, estimates of risk through groundwater and air exposure pathways decrease when sorption is described by the irreversible model. The combination of these factors with the uncertainties that remain presents a complicated problem for site management.