Qualité des eaux d’une rivière urbaine : Suivi réglementaire versus Perception des riverains Le cas du río Liberia (Costa Rica)

Abstract

The impact of human activities on rivers status is most often assessed through the monitoring of regulatory water quality indicators. The definition of regulatory action networks is constrained by technical or practical aspects. As a result, diagnoses are made on the basis of limited knowledge. Can studying the relationship between residents and rivers be used to improve scientific knowledge on the pressures and state of the river, and therefore in the medium term to improve the definition of control and restoration actions? The thesis is based on the study of the Liberia River (30 km long, 46 km2 catchment area) in Costa Rica - North Pacific Region. The river flows downstream through the urban area of the city of Liberia (39 000 inhabitants). The thesis is based on the coupling of two approaches: (a) the regulatory diagnosis of water quality and the characterization of anthropogenic pressures on the river; (b) the study of the perception and practice of the river by residents. Firstly, fifteen water quality measurement campaigns were carried out in accordance with Costa Rica's regulations, on a network of seven stations along the Liberia River between 2013 and 2015. The physico-chemical, biological and bacteriological indicators show an upstream-downstream gradient of the river state, from slightly to highly polluted. These indicators are not very variable according to hydrological conditions - as seen by a previous rainfall index - but are influenced by anthropogenic pressures, as seen by land use and riparian condition indices. The quality of the river is severely degraded when crossing the urban area. Secondly, a survey was carried out in 2016, among 100 inhabitants and 32 professionals. A map of the river, in terms of use and perception, was carried out. In general, the inhabitants cross the river every day via the various bridges, while the professionals occasionally participate in collective restoration actions. The river is perceived as a degraded and dangerous space by the majority of residents. However, the upstream part of the river, and some urban sites, are appreciated for their good quality. Thirdly, knowledge from the monitoring network and results of the perception survey were analysed jointly. The level of degradation from monitoring stations is consistent with the point of view of residents that identify all the locations of the stations. In addition, inhabitants point new hot spots of degradation that makes it possible to strengthen the monitoring network. Thus, new stations were implemented during a campaign carried out in 2017. The spatial variability of water quality and riparian zones was refined. As a conclusion, residents’ perceptions can be used to improve the assessment of the quality of an urban river. This thesis is a first step in diagnosing the quality of an urban river, combining hydrological approaches, through regulatory indicators, and socio-geographic indicators, through survey of residents. However, it is necessary to better understand both dynamics of water quality, in relation to hydrology, and those of anthropogenic pressures, as well as the temporal variability of the perception of the river by residents. Finally, conditions for involving residents in monitoring and restoring the quality of the river remain to be analyzed, in order to develop a real participatory approach.