Wastewaters are often discharged into coastal waters through outfall diffusers that efficiently dilute effluent and usually restrict any environmental impact to a small area. Predicting this impact is however difficult because of the complexity of the hydrodynamic processes that mix the wastewater and also because of the variability in oceanic conditions. Despite great improvements in the understanding of these mixing processes over the years, since models are now available that can make reasonable predictions under steady-state conditions, there remain many aspects unknown and unpredictable. For this reason, much effort has been devoted recently to improving means to monitor and characterize sewage plumes under a variety of oceanographic conditions.

Traditional sampling techniques for underwater observation are generally expensive and do not offer a comprehensive coverage, specially as the requirements of oceanographic and environmental field studies, as those considered here, become more and more demanding. Clearly, in situ platforms, such as Autonomous Underwater Vehicles (AUVs), which are cost effective, mobile, and capable of capturing key phenomena adaptively, which is particularly interesting for oceanographic processes that vary widely and over several time and length scales, as is the case of sewage plumes, may improve environmental monitoring and predictive modeling for decision-making and management (Fletcher, 2001; Robinson et al., 1999; Yu et al., 1994).

In this chapter we describe how AUVs can be used with advantage for environmental impact assessment and management of sewage outfall discharges. The details and results of a monitoring campaign to a Portuguese outfall using an AUV are fully described.

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