For the most part investments in restricting the propagation of pollutants have focused on managing a steady, invariant, average condition of the aquatic environment. In this there has been success. But the activities of society, in all its forms of land use (urban, agricultural, and silvicultural), have presumably still the capacity to generate as much potential contamination of the environment as previously. It is simply that we have now placed effective barriers – our wastewater control infrastructures – between these activities of society and the surrounding environment. And just as there would be a concern for the long-term reliability of a dam structure for a water reservoir, so there must now be an increasing concern for the reliability of our wastewater control infrastructures. Such concern is generic: transient perturbations about an equilibrium are as relevant to agricultural and silvicultural control infrastructures as they are to our systems of urban sewerage and wastewater treatment. The paper assembles the diverse features of transient pollution events, their monitoring, modelling and criteria for management, in order to make a start on providing a more coherent framework for their analysis. The notion of the frequency spectrum of system perturbations is used for this purpose. In this, succinctness is achieved, so that a better appreciation of the relationships between long-term trends and high-frequency disturbances can be obtained. In particular, the problems of managing transient pollution events can be seen loosely against the backdrop of a project's life cycle, in a manner that illuminates a tension in our attitudes towards the passive and active paradigms of operating the control structures that protect the environment from pollution.
Gradients of column CO<sub>2</sub> across North America from the NOAA Global Greenhouse Gas Reference Network