The monitoring of the fish fauna in the Lis, following the fault at a lifting station in Leiria that forced the discharge of untreated effluent into the river, confirms the ecological recovery of the watercourse, as announced today.
New results within the scope of the Environmental Impact Assessment Study, carried out following the accidental discharge, indicated that twelve species of fish were identified, eight native and four non-native, including the European eel, classified as ‘endangered’, and the rudd, classified as ‘vulnerable’.
“The data reveal differences in the composition of fish communities along the river, greater richness and diversity at the intermediate point and an increase in the proportion of non-native species in the upstream-to-downstream direction.”
“The document was today presented at the final meeting of the monitoring committee created to assess the fault, in August 2025, at the Monte Real lifting station (Leiria), its impacts and the need for investments, which took place at the Coimbrão wastewater treatment plant, in the municipality of Leiria.”
“On 12 August 2025, a fault at that lifting station, managed by Águas do Centro Litoral (AdCL), led to the discharge of thousands of cubic metres of untreated effluent into the Lis River, which originates in the municipality of Leiria and flows into Praia da Vieira (Marinha Grande) and into irrigation ditches.”
The Portuguese Environment Agency (APA) estimated that between 20,000 and 25,000 cubic metres of raw sewage were discharged into the river.
At the time, swimming at Praia da Vieira was prohibited and the Municipality of Leiria advised against activities on the stretch of the Lis River downstream of Monte Real, such as water withdrawals for irrigation, swimming and fishing.
The results of the monitoring of the fish fauna carried out in the Lis River are part of the Technical Study for the Assessment of Environmental Damages and Mitigation Measures, awarded by AdCL to the Centre for the Study of the Environment and the Sea of the University of Aveiro, whose final report was presented in January.
The report noted that the impact of the accident at the Monte Real lifting station was “acute, localized and of short duration”, in a broader context characterized by chronic environmental pressures that have historically affected the Lis River basin, mainly of agricultural, livestock, urban and hydromorphological origin.
And despite the impacts at the time of the accidental discharge, the water quality of the Lis River presents an overall ecological status classified as ‘reasonable’, with the main limiting factors related to “high concentrations of nutrients, namely nitrogen and phosphorus, and persistent microbiological contamination, reflecting diffuse and chronic pressures existing in the basin”.
This report also suggested mitigation and rehabilitation measures, such as reducing diffuse and point pressures on water quality, improving the efficiency and redundancy of drainage and lifting infrastructures, hydromorphological and ecological rehabilitation of the river, and the implementation of a monitoring plan.
The monitoring committee includes the municipalities of Leiria and Marinha Grande, the Intermunicipal Community (CIM) of the Leiria Region, the Association of Irrigators and Beneficiaries of the Lis Valley, the Parish Council of Vieira de Leiria, the Union of Parishes of Monte Real and Carvide, AdCL and the Secretariat of the Environment, APA. Representatives of the last two were not present at the meeting.
The president of the CIM, Jorge Vala, stated that the final document of the study will be forwarded to the authorities and APA, and that the committee’s dissolution has been decided and the creation of a working group to monitor the Lis and Lena rivers, the latter rising in Porto de Mós and a tributary of the former, “with potential monitoring”.
“There was also talk of the possibility of having continuous monitors along, notably, the Lis River,” declared Jorge Vala, also the mayor of Porto de Mós, a suggestion that will be forwarded to the APA, to which there will be insistence on the “urgent need to implement the maintenance of the Lis bed.”
The new working group, possibly with the same entities and bringing in others, aims to “have a more systematic monitoring of the issues facing the Lis and Lena rivers,” added Jorge Vala.
