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Integrated modelling for water quality management in a eutrophic reservoir in south-eastern Portugal

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Abstract

The Enxoé Reservoir was built in 1998. Since 2000, it has exhibited frequent high chlorophyll-a concentrations, reaching a geometric mean three times higher than the national limit for eutrophication, presenting the reservoir with the highest eutrophic state in Portugal. Toxic algal blooms have also often been observed, which pose serious challenges to water managers, as the reservoir is used for potable water production (25,000 inhabitants). The objective of this study was to implement a reservoir model (CE-QUAL-W2), with inputs from a watershed model (SWAT), in order to represent the actual reservoir state and to test management measures to reduce its trophic level and algal bloom concentration peaks. The integrated model was used to depict the origin of its trophic status. Simulations were also compared to measured data at the reservoir surface (water level, nitrate, orthophosphate, suspended solids, and oxygen) and in water profiles (temperature, oxygen). The model was able to represent stratification and thermocline depths, as well as the actual chlorophyll-a and dissolved oxygen concentrations. The model results showed that internal phosphorus load from deposited sediments was an important factor in fuelling the algal blooms. This process occurs predominantly in summer, when stratification takes place and aeration is reduced, promoting anoxic conditions in the bottom waters. Since the reservoir is relatively shallow (average 5 m), released phosphorus is then easily able to reach the photic zone in most parts of the reservoir, where it is consumed. Different management scenarios were tested, suggesting that a mesotrophic level could barely be reached and maintained simply by reducing the nutrient loads (both external and internal). It is suggested that only an increase in the reservoir dam height could produce a mesotrophic level, averting anoxia by blocking the release of phosphorus from sediments to the photic zone. Future work should focus on a cost–benefit analysis to test the feasibility of each of the proposed scenarios, taking advantage of the integration strategy to assess where in the watershed load reductions would be most effective.

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Notes

  1. The Portuguese trophic level classification was chosen, adapted from Organisation for Economic Co-operation and Development (OECD). The geometric average is used to reduce the weight of extreme values.

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Acknowledgements

This work was supported within the framework of the EU Interreg SUDOE IVB program (SOE1/P2/F146 AguaFlash project), the Project Mirage (EU-FP7), and the Project EUTROPHOS (PTDC/AGR-AAM/098100/2008) of the Fundação para a Ciência e a Tecnologia (FCT). MARETEC acknowledges the national funds from the FCT (Project UID/EEA/50009/2013). TBR was supported by FCT grant SFRH/BPD/110655/2015.

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Authors and Affiliations

  1. MARETEC, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001, Lisbon, Portugal

    David Brito, Tiago B. Ramos & Ramiro Neves

  2. Instituto Nacional de Investigação Agrária e Veterinária, Quinta do Marquês, Av. República, 2784-505, Oeiras, Portugal

    Maria C. Gonçalves

  3. Laboratório da Água, Universidade de Évora, Rua da Barbarrala, 1, Parque Industrial e Tecnológico, 7000, Évora, Portugal

    Manuela Morais

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  1. David Brito
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  2. Tiago B. Ramos
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  4. Manuela Morais
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  5. Ramiro Neves
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Correspondence to David Brito.

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Brito, D., Ramos, T.B., Gonçalves, M.C. et al. Integrated modelling for water quality management in a eutrophic reservoir in south-eastern Portugal. Environ Earth Sci 77, 40 (2018). https://doi.org/10.1007/s12665-017-7221-5

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