Abstract: The city of Santander ceased the discharge of sewage effluents into the bay of Santander in June, 2001 and began discharging at a site 2.4 km offshore in the nearby coastal area (Virgen del Mar, Bay of Biscay) at a water depth of about 40 m. The present study investigates the effects of the new outfall discharges on the water quality of the high-energy coastal area and the recovery of the perturbed temperate estuarine area now only affected by combined sewer overflows (CSOs). Nutrients, phytoplankton biomass and urban pollution indicators were analysed. No significant spatial or temporal change in water quality variables was found in the coastal area around the outfall. No signs of nutrification or increases in chlorophyll-a were observed throughout the study period, although a slight increase in phosphates, suspended solids and turbidity were observed two years after the relocation of the discharge. These changes were not attributed to outfall discharge but to a regional increase also observed at control stations and nearby coastal areas. Considerable reductions in indicators of urban discharges were observed in the estuary after the relocation of discharges, even at stations located around CSOs. Results from this study support the efficiency of ecological quality-driven designs of sanitation systems, which are used as management tools for sensitive and environmentally valuable coastal ecosystems in the north-east Atlantic.

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