Oka estuarioko ur-kalitatearen hobekuntzaren lehen zantzuak saneamenduaren ondorioz

Abstract

In the Basque Country, urban wastewater discharges, especially in the inner part of the Oka estuary, have posed a risk in terms of eutrophication and bathing water uses. Thus, in 2015 a new wastewater treatment plant with higher capacity was implemented, and in 2021 almost all wastewater in the catchment were connected to the sewerage system. The aim of this study was to determine the spatial and temporal trends of water quality in relation to the implementation of the different phases of the sewerage system. Nutrient concentrations and main oceanographic variables were sampled quarterly at three stations along the estuary from 1995 to 2021. Additionally, faecal coliforms were analysed fortnightly at four beaches during the bathing season from 2008 to 2021. A gradient from the innermost station to the outermost station has been found in the mean concentration values of all nutrients, as well as oxygen and chlorophyll. On the other hand, as the clean-up progressed, the improvement in the physico-chemical quality of the water throughout the estuary has been evidenced, especially in recent years. As for the bathing waters, the inner beaches were removed from the official census in 2016 for having been classified as ‘poor’ quality for the last four years, but in 2021 one of the beaches was reinstated on the list of bathing areas. These early signs of improvement indicate that remedial measures are proving effective in managing nutrient inputs and in restoring and protecting coastal services.