Causes, effects, and management of river salinization in Spain with special attention to wastewater treatment

[eng] Freshwater salinization (FS) represents a pressing global environmental challenge, significantly impacting aquatic ecosystems. Despite extensive research, critical gaps remain in understanding the full extent and implications of FS. This thesis aims to address and contribute to filling some of these gaps by providing global information necessary to explore FS patterns, temporal variability, and the interaction of FS with other pollutants, particularly nutrients. One of the key focuses of this thesis is the evaluation of the salinity effects on urban wastewater treatment plants (WWTPs), as well as WWTP contributions to FS. The objectives of this thesis are fourfold. First, it aims to compile a global database integrating river salinity parameters and both natural and anthropogenic drivers of freshwater salinization to explore patterns and temporal variability across various regions. Second, it seeks to examine the seasonal dynamics of primary salinization sources such as mining, transportation, agriculture, and urban wastewater discharges, and their effects on the temporal variability of river salinity in Spanish rivers. Third, the thesis investigates the interactive effects of nutrient and salt pollution on river ecosystems through controlled experiments in artificial channels. Fourth, it assesses how increasing salinity impacts microbial communities and pollutant removal efficiency in WWTPs. Chapter 1 presents a global dataset, GlobSalt, which includes 13 parameters related to river salinity and associated ions, with approximately 15 million records. This dataset reveals substantial spatial variability in salinity and serves as a crucial resource for understanding FS dynamics and identifying high-risk regions. Chapter 2 analyzes temporal variations in electrical conductivity (EC) across 91 Spanish rivers, demonstrating how human activities disrupt EC patterns and influence aquatic biodiversity. Chapter 3 explores the combined effects of nutrient and salt pollution on stream microcosms, highlighting significant declines in macroinvertebrate diversity and the complex interactions between pollutants. Chapter 4 assesses the impact of salinity on WWTP performance, revealing how high EC levels affect microbial communities and pollutant removal efficiency. Moreover, each chapter presents an application of the GlobSalt data: (i) the temporal variation of river EC influenced by WWTP discharges and seasonal patterns, (ii) the concentration of phytoplankton in rivers in relation to EC levels for pristine and impacted sites, and (iii) the increase in river EC associated with wastewater discharges in different seasons. In summary, this thesis not only advances scientific understanding of freshwater salinization and its drivers but also offers practical insights for managing and mitigating FS impacts. By integrating global and local data, addressing seasonal and interactive effects, and evaluating WWTP performance and contributions, this research provides valuable information for developing effective management strategies and guiding future studies in the field of freshwater ecology.