Bertrand Aunay

Coping with Urban & Agriculture Water Demand Uncertainty in Water Management Plan Design: the Interest of Participatory Scenario Analysis

Managing water scarcity is a major challenge for regions all over the world. In the European Union, robust methodologies are needed to establish effective programmes of measures aimed at achieving the “good status” of water bodies according to the Water Framework Directive (WFD). These programmes often target the current gap between the actual status of water bodies and the “good” status without accounting for uncertainty in water demand. We develop a new methodological framework that enable to account for uncertainty in future water demand and design programmes in order to increase their likelihood of attaining the good quantitative status. The foresight approach enables to construct and quantify future water demand scenarios hand-in-hand with stakeholders during workshops. They consist in identifying drivers, debating pre-constructed scenarios, reconstructing scenarios and estimating water demand. The impact of the co-constructed scenarios is simulated with a resource-demand balance model for all water resources and a cost-effectiveness analysis makes it possible to construct programmes that target the estimated future water deficits at least cost. The methodology is illustrated with an application to Reunion Island (Indian Ocean, France) considering agriculture (Ag) and urban water (Uw) demand. Three combinations of sector scenarios (Uw, Ag) were produced and coherence was eventually ensured by fitting the land use parameter. This solution can accommodate case studies faced with a binding land constraint for housing and agriculture. As each scenario implies significantly different programmes of measure in terms of intensity and spatial distribution, results demonstrate the importance of taking uncertainty on water demand into account.

Coastal groundwater salinization: Focus on the vertical variability in a multi-layered aquifer through a multi-isotope fingerprinting (Roussillon Basin, France)

The Roussillon sedimentary Basin (South France) is a complex multi-layered aquifer, close to the Mediterranean Sea facing seasonally increases of water abstraction and salinization issues. We report geochemical and isotopic vertical variability in this aquifer using groundwater sampled with a Westbay System® at two coastal monitoring sites: Barcarès and Canet. The Westbay sampling allows pointing out and explaining the variation of water quality along vertical profiles, both in productive layers and in the less permeable ones where most of the chemical processes are susceptible to take place. The aquifer layers are not equally impacted by salinization, with electrical conductivity ranging from 460 to 43,000μS·cm(-1). The δ(2)H-δ(18)O signatures show mixing between seawater and freshwater components with long water residence time as evidenced by the lack of contribution from modern water using (3)H, (14)C and CFCs/SF6. S(SO4) isotopes also evidence seawater contribution but some signatures can be related to oxidation of pyrite and/or organically bounded S. In the upper layers (87)Sr/(86)Sr ratios are close to that of seawater and then increase with depth, reflecting water-rock interaction with argillaceous formations while punctual low values reflect interaction with carbonate. Boron isotopes highlight secondary processes such as adsorption/desorption onto clays in addition to mixings. At the Barcarès site (120m deep), the high salinity in some layers appear to be related neither to present day seawater intrusion, nor to Salses-Leucate lagoonwater intrusion. Groundwater chemical composition thus highlights binary mixing between fresh groundwater and inherited salty water together with cation exchange processes, water-rock interactions and, locally, sedimentary organic matter mineralisation probably enhanced by pyrite oxidation. Finally, combining the results of this study and those of Caballero and Ladouche (2015), we discuss the possible future evolution of this aquifer system under global change, as well as the potential management strategies needed to preserve quantitatively and qualitatively this water resource.