Assessing the Effects of Wastewater Treatment Plant Effluents on the Ecological Quality Status in a Mediterranean River Basin

Abstract: Many Mediterranean rivers are characterized by an inter-annual climatic variability, additionally intensified by climate change. Moreover, these rivers have limited extensions and a reduced dilution capacity which make them vulnerable to human pressures, especially to water pollution. During the 1970s, the Besòs river basin had historically suffered the continuous discharge of untreated residual water, leading to be considered as the most polluted river in Europe. During the 1990s and 2000s, environmental laws such as the Catalan Treatment Plan and the Water Framework Directive (2000/60/EC) were implemented to resolve the problem. Consequently, the number of wastewater treatment plants (WWTPs) and long-term monitoring programs to assess the ecological status increased in the territory. In order to evaluate these changes, this study assesses the biological quality status (IBMWP, QBR and IPS) in the Besòs river water at three spatial scales (headwater, middle and lower sections). Our results clearly show that water quality across the basin has been improved. This improvement has become more significant after the implementation of new treatment systems. The trends in ammonia concentrations (NH₄⁺, in mg/L) and conductivity (EC, in μS/cm) discharged from 13 selected WWTPs during the period 2000–2018 displays a clear decreasing tendency. Finally, we have applied a thorough statistical analysis to the IBMWP dataset to demonstrate the positive influence of the treatment systems on the ecological quality status across the river basin. We conclude that the implementation and enforcement of environmental laws are valid and effective tools to improve water quality in Mediterranean rivers. Highlights: • Wastewater treatment plant upgrading has positive effects on biological quality status. • High NH₄⁺ and conductivity levels provoke alterations on biological quality status. • Correct quality status (IBMWP) is reached when NH₄⁺ < 1 mg/L and EC is within 500–2000 μS/cm.