In the event of a release of toxic contaminant, either accidental or intentional, it would be useful to have an evolvable sensor network for tracking the toxic plume. In such an event, Homeland Security or DoD personnel are responsible for modeling the transport and dispersion of the plume. To do this requires specific source and meteorological data. Such data may not be available; however, it might be recoverable from concentration data monitored by a mobile sensor network. To be useful for assimilating the monitored data into a dispersion model, the sensor network must be sited strategically and should be evolvable to follow the plume of toxic contaminant. This paper discusses the requirements of such a network from the point of view of data needs for assimilating the sensor data into the transport and dispersion models.