Implications of poor-regulated wastewater treatment systems in the water quality and nutrient fluxes of a subtropical coastal lagoon

Abstract

Sewage treatment is a way of controlling coastal eutrophication worldwide. Yet is it enough to ensure water quality and avoid the effects of eutrophication? In this study, we hypothesize that sewered and non-sewered regions of Conceição lagoon (Southern Brazil) have similar water quality, due to the poorly designed sewerage system and the lack of effective public policies. The LOICZ (Land–Ocean Interactions in the Coastal Zone) biogeochemical model was used to understand how the lagoon responds to nutrient inputs from the watershed. There was no significant difference in the water quality between the urbanized rivers. Both regions (sewered and non-sewered) showed high concentrations of phosphate, ammonium and fecal coliform. The percentage of urbanization was directly correlated with deoxygenation, nutrients and coliform. Lower concentrations of nutrients and oxic waters were found in the non-urbanized rivers and in the lagoon. As such, biogeochemical and physical processes counterbalance the nutrient inputs. However, the sewage runoff might be fueling the dead zone of the lagoon, since the water residence time and nutrient retention are high. Our results indicate that the sewage treatments are inefficient in preventing the input of organic matter and nutrients to the system. Additionally, new management strategies are required to revert eutrophication, such as the expansion of the serviced area by the sewage system and the improvement of its technology.