Figure 2-1 are not specific to environmental media, the models that fit into each category tend to be further subdivided by media. For example, the generic category of environmental fate and transport models can be subdivided further into various types of subsurface containment transport models, surface-water quality models, and air quality models (Schnoor 1996; Ramasawami et al. 2005).
Table 2-2 contains short descriptions of some of EPA’s regulatory activities that rely on modeling. These environmental regulatory modeling activities typically occur as a subset of the full system summarized in Figure 2-1. The underlying statutory requirements, the regulations implementing the statutory requirements, and the importance of the activity dictate the nature of the modeling analysis. For example, assessing the toxicity of new pesticides and other chemicals in the environment may focus on just the fate and transport or toxicity portion of the system. Assessing the risks from leaking underground petroleum storage tanks,