Main drivers of the seasonal and annual changes in phosphorus content in the Guadaira river (South of Spain)

High amounts of nutrients favor the growth of algae that consume oxygen from the aquatic environment causing eutrophication. In the case of phosphorus, it comes mainly from two sources: fertilizers washed from agricultural areas by runoff water and urban and industrial development. In the first case, the phosphorus loads do not have a clear point of entry into the water channels, whereas in the second one, the phosphorus loads can be generated from point sources, such as discharges from the wastewater treatment plants (WWTP) but also from non-point sources, such as urban areas runoff in episodes of intense rainfall. 
The main purpose of this work is to analyze the content of phosphorus in water for more than 40 years and inquiry into the origin of the sources that may have produced the phosphorus loads. For this purpose, the Guadaira river basin (South of Spain), where agricultural land uses converge with numerous human activities resulting in high pressures on water quality, was selected. 
The results highlight that the phosphates threshold value established for good/moderate state (0.32 mg PO4/l) is exceeded by 96% of the measurements during the period 1981-2022 in a water quality control point located downstream of the main WWTP, which threat the wastewater of Seville, and that in addition collects the contributions from the other WWTPs and agricultural lands located in the basin. The episodes of sediment contribution that occurred during the period 1981-2022 were analyzed at this control point, and from the 184 episodes found, 30 episodes may have been due to runoff (which also may have originated from agricultural areas or from the overflow of water collectors) (type 1 episodes) and 79 may have been due to urban spills (type 2). 80% of both types of episodes were found to be higher than 1.5 mg/l being able to reach concentration values of up to 14 mg/l. Most of the episodes of dry months were categorized as type 2, reaching the highest concentration values (8-17 mg/l), while type 1 episodes were mostly present in rainy months.

Finally, despite the increase of the stable population (+0.52% ∼ +1.42% per year between 2000 and 2012) and tourism (average ≈ +3.23% per year), the WWTP improvement has achieved a decrease in the mean phosphorus concentrations of -0,2% per year. Despite the investment in the WWTP of the basin is necessary to improve its operation and efficiency as well as its adaptation to the increase in population and tourism to ensure better water quality of the water resources.

Acknowledgements: This work has been funded by the project TransDMA – Adaptation of the Water Framework Directive to the Andalusian reality: The Guadalquivir estuary as an integrated management model, promoted by the Ministry of Economy, Knowledge, Business and University and co-financed by the operational program FEDER 2014-2020 in Andalusia.