Historical changes and role of river discharge in water quality: 30 years of environmental monitoring in Uruguay River

Abstract Background Land-use intensification has enhanced impacts on water quality of South American rivers. A long-term database was compiled from public monitoring data to evaluate the temporal variation in water quality in the mid-lower Uruguay River from 1987 to 2019. Forty-two parameters (physicochemical, nutrients, metals, microbiological, pesticides, persistent, bioaccumulative and toxic substances and hydrological) in three sites across 440 km of the river were selected. Bayesian Generalized Linear Models were applied to understand the temporal and discharge-related drivers of each water quality parameter. Finally, a water quality index (CCME-WQI) was calculated to evaluate temporal trends in water quality. Results Most of the parameters (70%) exceeded the regulation values at least once during the 30-year period. Pesticides like aldrin, DDT, dieldrin, heptachlor and lindane, and nutrients (Kjeldahl nitrogen and total nitrogen) showed higher average values in the 1990s. Cadmium, chromium, copper, lead and mercury showed the highest averages in the 2000s, while iron concentration was the highest in the 2010s. The response of water quality parameters to time and discharge varied across sites, with most of the effects observed upstream Salto Grande reservoir. In all three sites, calcium, BOD 5 , and magnesium decreased over time, while iron and nitrite increased. The 30-year CCME-WQI revealed an improvement in water quality (from Poor to Good) over time, concomitant with the ban of persistent organic contaminants towards the 2000s (e.g., dieldrin and lindane). However, a CCME-WQI calculated for a recent period (2018–2019) from a monitoring program with 181 parameters and a different parameter selection setup, showed barely Marginal and Good values, highlighting the sensitivity of the index to the parameters included. Conclusions During the 30-year period, the water quality of the mid-lower Uruguay River was never ranked as excellent, although improvements were observed in specific parameters (i.e. banned pesticides) increasing overall water quality. However, several water quality parameters (i.e. metals and nutrients) remain above the regulations recommended levels with increasing concentrations over time. The results provide valuable information to support environmental management and highlight the increasingly complex challenges of water quality dynamics in large river systems.