Contamination sources were originated by a point spill or leak in the soil or in the aquifer. Nonpoint sources were originated and generated by a point source in an unknown distant past in the soil or in the aquifer. Chemical reactions between the contaminant and the soil particles are governed by a linear kinetic adsorption isotherm. Biological, radioactive or chemical decay are governed by a first-order degradation process. Contaminant volatilization is negligible compared to advection, dispersion, adsorption and decay.
Contaminant propagation is essentially a three-dimensional process in the unsaturated zone.
This implies that the vertical component of flow is neglected in the aquifer and that the concentration values reported by the model are average over the vertical concentration magnitudes. The vast majority of spills do not offer the detailed data required by numerical models.