Assessment of Spanish Rivers Current and Future Ecological Status Using Urban Wastewater Dilution Factor

The EU Water Framework Directive (WFD) ecological status classification of surface water bodies (SWBs) provides a synthetic assessment of how anthropogenic pressures impact on aquatic environments. In the evaluation of River Basins Management Plans of the second cycle (2015–2021), the EU commission reported the need for Spain: (i) to improve apportionment of pressures among individual sectors to be able to target appropriate measures, (ii) to take operational measures to mitigate climate change effects on the different sources of pressure affecting surface water quality status. Urban wastewater treatment plants (WWTPs) discharges to river SWBs represent an impactful point source of pressure. This study postulates that the dilution factor (DF) of WWTPs discharged to river flow can be estimated for EU Member states and can be used as suitable proxy to predict the EU SWBs ecological status. The implemented approach uses the available datasets centralized by the Environmental European Agency, and a focus on Spain’s situation is provided. Climate change effects on SWBs river flows are considered using a set of scenarios based on IPCC regionalized estimates. Results confirm that cumulative WWTP discharge and DF can be estimated and mapped for EU member states. The study confirms that it is statistically sound to use DF to assess the probability for a SWB to reach or not a good ecological status. 21% and 33.9% of the SWBs in Spain show a decreasing DF for the projections based on the high representative concentration pathway emission scenarios (RCPs 4.5; 8.5), respectively for the early (2011–2040) and midcentury (2041–2070) periods. Using the DF as an indicator, results show that in the span of these two periods, 55% of the 1046 SWBs with a current good ecological status have a higher than 70% probability to lose their good ecological status classification.