Soil-water adaptive management process: the case of On-site Wastewater Treatment Systems in peri-urban areas in France

Résumé En

This paper proposes a contribution to tackle urban sanitation issues giving some hindsight on a place-based science-practice collaborative project. Insights from a French public service in charge of On-site Wastewater Treatment Systems (OWTS) highlighted the need to deepen the understanding of soil infiltration assessments and to clarify their relation to some misunderstandings between actors. The initial aim was to explore how a knowledge based approach can offer an original perspective to historical “septic tanks” and overcome technical and organizational difficulties. In this work we consider OWTS as hybrid infrastructures necessary to collect, transport, treat and dispose of domestic wastewater on the plot where it is generated. We suppose that OWTS adaptive management (sludge not included) offer many opportunities regarding water circulation (ecological function), infrastructure diversity (redundancy), and soil-based decision making (spatial planning). The main objective of this paper is to document the French sociotechnical configuration through the interplay between soil and water actors, OWTS technics and local institutions. The originality is to look at OWTS through the lens systemic perspective of Nature-Based Solutions (NBS). We first contextualize OWTS implementation and planning by presenting technical design studies (water-soil interactions, indicators assessment) and the diversity and capacity of actors. Second, we build on the application of a soil-based methodology in a peri-urban district to provide a place-based outlook on the influence of soil infiltration rates variability in day-to-day management. While the current consideration given to soil infiltration rate in OWTS design studies created a situation of conflicts and contestations between actors, a sociotechnical transition is taking place with new regulations and innovative energy-driven device. Moreover, collective infrastructure for domestic wastewater is currently challenged by densification constraints in metropolitan areas (urban sprawl regulation). As a consequence, resolving binding measures for indicators assessment would support OWTS integration into mainstream urban development. Careful investigation of soil hydraulic functions is thus a steppingstone in the search for an adaptive strategy both at the plot and the neighborhood scales. Not only OWTS are opportunities to support the adaptive management process in the water sector, but OWTS have also the potential to improve soil-based decision-making in urban areas. Finally, valuating OWTS as useful NBS for wastewater management in urban areas supports the evaluation of soil capacities to deliver ecosystem services and contributes to justify land-use changes on a functional knowledge basis.