Water quality of the Ria Formosa at the vicinity of almargem urban wastewater treatment plant and shellfish beds

Open Access

Abstract

Coastal lagoons are very productive and economically important ecosystems that suffer several anthropogenic pressures, including from urban wastewater treatment plants (UWWTP) discharges. This last pressure can induce nutrient enrichment and development of primary producers (toxic or not), microbiological contamination of the water or even eutrophication that ultimately will have impact on the organisms living nearby and on their salubrity. Ria Formosa lagoon is a coastal lagoon on the Portuguese south coast responsible for over 90% of the Portuguese shellfish production, however along its extension it suffers influence from several UWWTP discharging in this system, some of them in in the vicinity of the production areas. Since in Tavira there is an important area of shellfish production close to the Gilão estuary it is important to determine the impact of the Almargem UWWTP upon the water quality of the receiving waters and to evaluate if there is any influence of it on the shell production area. The main goals of this work were: i) to determine the spatial extent of the influence of the Almargem UWWTP along the gradient of dispersal of the effluents in the receiving waters, potential impact on the bivalves production area, and the compliance of data with the applicable legislation; ii) to determine the dilution effect within the study area associated with the daily tidal cycles; iii) to determine the effect of different tidal ranges upon the water quality; iv) to determine the seasonal influence of the sewage discharge impact upon the water quality; v) to assess the temporal evolution of the chemical and microbiologic contamination of the water quality in the study area and vi) to calculate a trophic index (TRIX) in the study area, defining a baseline for Almargem UWWTP. The results showed that the Almargem UWWTP clearly influence the water characteristics till 750 m downstream from the UWWTP discharge. In August and February the E. coli concentration was above the limit imposed for the discharge license, both ...