CESAM has recently submitted the scientific report of the Environmental Impact Assessment Study following the accidental discharge of effluents into the Lis River, which occurred on 12 August 2025. The study was carried out by a multidisciplinary team coordinated by Nelson Abrantes, researcher at CESAM/DBIO.
The report was presented within the framework of the Monitoring Committee for discharges into the Lis River, composed of representatives from the Secretary of State for the Environment, the Portuguese Environment Agency (APA), the municipalities of Leiria and Marinha Grande, the Intermunicipal Community of the Leiria Region (CIMRL), the irrigators’ association, local parish councils, and Águas do Centro Litoral (AdCL), the entity that commissioned the study.
The research combined physico-chemical, microbiological, biological/ecological and ecotoxicological data collected at multiple locations upstream and downstream of the pumping station where the discharge occurred. This approach made it possible to distinguish the acute effects of the incident from natural variability and from existing environmental pressures in the area.
According to the CESAM document, the integrated analysis indicates that the environmental impact of the incident was limited, localized and short-lived, with clear signs of recovery in aquatic communities and in the ecological quality of the Lis River, despite chronic pressures already present in the watershed.
The physico-chemical and biological results classified the assessed stretch of the Lis River as having an overall “Moderate” ecological status, consistent with the historical classification assigned in the RH4 River Basin Management Plans. Biological groups such as phytobenthos and macroinvertebrates showed evolutionary trends pointing to a gradual recovery of favourable ecological conditions in the river.
Ecotoxicological tests with microalgae and aquatic invertebrates also suggest the absence of negative effects in water and sediment samples collected downstream of the accidental discharge point, when compared with samples taken upstream.
However, the results clearly show that the current environmental status of the Lis River is strongly influenced by pre-existing chronic pressures, of diffuse and/or point-source origin, with variable expression throughout the year and a marked seasonal worsening during the autumn period. These pressures include persistent microbiological contamination, widespread nutrient enrichment (nitrogen and phosphorus), and recurrent episodes of dissolved oxygen depletion.
In addition to confirming a positive trend after the incident, the CESAM report also includes recommendations and an integrated set of mitigation and ecological restoration measures, such as reducing diffuse and point-source pressures, improving the efficiency of drainage and pumping infrastructure, promoting hydromorphological and ecological rehabilitation of the river, and implementing a continuous monitoring plan.
AdCL also reaffirmed its commitment to implement actions to strengthen the system’s resilience, highlighting investments already made, such as a new ultraviolet disinfection system and the refurbishment of the solid treatment stage at the Coimbrão WWTP (ETAR do Coimbrão), considered important steps to protect the Lis ecosystem.
The CESAM report will be used to guide continuous monitoring plans, ecological recovery measures and future environmental management actions, in collaboration with AdCL and other involved entities.