Watershed‐scale mapping of on‐site sanitation‐related environmental issue zones—Case study of Lys River (France and Belgium)

Abstract

In application of the European Water Framework Directive, the water development and management plan, Schéma d'aménagement et de gestion des eaux (Sage) for the Lys (Lily) watershed area sets out general objectives for the use, development, and quantitative and qualitative protection of water resources and aquatic environments. The Lys Sage issues include the pollution of aquatic media and a provision for reducing the impact of discharges from on‐site sanitation devices. The prioritization of public authority action on on‐site sanitation requires the definition and identification of zones with environmental issues. The objective is to identify priority areas for the rehabilitation of noncompliant on‐site sanitation devices in case of a proven risk of pollution. These zones have been identified and mapped following the development and application of a methodology based on current regulations, feedback, and an innovative approach. It is based on the risk assessment of on‐site sanitation. To do this, a hazard (source hydraulic flow) was crossed with the vulnerability of the environment (target flow impact on stream quality). In addition to vulnerability assessed by the chemical quality of watercourses, the concept of zones of ecological interest was introduced.