Modeling attenuation of volatile organic mixtures in the unsaturated zone: codes and usage

Typical contaminants found in the unsaturated zone (diesel oil, unleaded gasoline, etc.) are mixtures of several volatile organic compounds (e.g., toluene, xylene, etc.) having variable properties. A large number of models and codes dealing with the description of contaminant attenuation in the subsurface have appeared in the literature. In the present work, several representative codes capable of unsaturated zone modeling are presented. Only a limited number of these codes consider vapor phase diffusion, despite its significance in practical applications. These codes are discussed here in more detail. Three of these codes treat contaminants as mixtures of more than one compound with variable properties. However, only two of them include degradation, which is also an important attenuation mechanism. New approaches trying to overcome code limitations—in terms of compound number they can handle—are also discussed.