Assessment of the pollution in an enclosed gulf in Greece before and after the operation of WWTPs: an integrated approach

Patraikos Gulf, a closed bay adjacent to the third largest city of Greece, is the recipient of treated municipal and industrial wastewaters, coming from three Wastewater Treatment Plants (WWTPs). Before 2001, wastewater was discharged untreated into the sea. The present work aims at evaluating the pollution status of the recipient before the construction and operation of the WWTPs (in 1993) and after 10 years of operation, from 2010 to 2023. Sediment, water and bivalve tissues were collected (n = 207, 618 and 58, respectively) and analyzed for physicochemical parameters and toxic metals. Sediment toxicity was assessed through ERM (Effect Range Median) and PEL (Probable Effects Level) Sediment Quality Guidelines and the Contamination Degree. The pollution status of marine sediments was classified as “Medium–High” in 1993 and “Medium–Low” in 2010, mainly due to the decreased concentrations of anthropogenic metals, and remained almost stable until the end of the study. Based on the EC bathing water quality directive, the seawater was classified as "excellent". In bivalve tissues, metals were observed at similar or lower levels than those reported in other Mediterranean countries. In 2011, a new port began operating and high-speed vessels began sailing closer to the coast. In marine sediments metals of anthropogenic origin were homogenized with those of natural background, after 2012, as factor analysis revealed. Undoubtedly, when top priorities in the waste hierarchy (e.g. reuse) are not an alternative option, tertiary or higher treatment should be applied prior to wastewater disposal. In addition, other risk factors, such as the navigation path and speed of ships must be controlled under a strict operational framework to avoid disturbance and achieve a sustainable marine environment, especially in enclosed gulfs.