In order to address the environmental impact produced for decades to the water bodies, the authorities of the major coastal municipalities and counties in North Spain (Gulf of Biskay) initiated, during the 90's, the design and construction of new sanitation systems, according to the legal requirements imposed by the 91/271/EC Directive. Site planning, environmental risk assessment of discharges and objective-oriented monitoring were the three main tasks supporting the decision-making process related to the validation of the new sewerage projects. This process consists of four consecutive stages: 1) Design and selection of alternatives (strategic assessment), 2) Risk evaluation for different scenarios (impact assessment), 3) Verification of adverse effects (objective-oriented monitoring) and 4) Action plan for mitigation of registered impacts (management of pollution events). Regarding impact assessment, mathematical and statistical studies for the selection of appropriate points of discharge of sewage outfalls have been an important time and labor-consuming task for the pre-commissioning stage, taking into account the random nature of the factors that influence the dispersion and reaction of different contaminants in the sea, Thus, depending on the stage of the analysis (planning vs. project design) and, consequently, the precision level needed to infer from the respective results, this procedure might support either the preliminary selection of alternatives (SEA) or the final design of the projected alternative (EIA). On the other hand, monitoring programs were based on modeling predictions and assumptions about likely responses of a great variety of environmental compartments to disturbances, through the implementation of a program of continuous measurements, that support both the definition of the specific objectives to be addressed and the more appropriate technical designs, in order to produce predefined management information useful for decision making.
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