Seminar Sophie Guillon

Abstract

In highly urbanized areas such as the Seine basin, the hydrographic network and agriculture contribute relatively little to CO2 emissions (around 10%) compared to emissions from transportation, residential development, and industry. However, the role of continental hydrosystems in carbon balances remains a key research topic, with uncertainties regarding lateral flows and river degassing. The Orgeval watershed, located in a carbon-rich sedimentary context, is dominated by intensive agriculture on drained soils. This critical zone observatory benefits from a history of monitoring and research, and is therefore a prime site for improving understanding of carbon transfers, both organic and inorganic, from soils to groundwater and surface water at the headwaters of basins. Monitoring water geochemistry and stable carbon isotopes in the basin’s various compartments allows us to constrain a mass balance model that includes the main processes controlling the evolution of dissolved inorganic carbon from soils and aquifers to the river. The CO2 produced by the mineralization of soil organic carbon is mostly degassed locally. The dissolved fraction that infiltrates is largely transferred and then degassed into the river network. During this transfer in the aquifers, the alteration of carbonates by soil CO2, as well as NH4 introduced by agricultural activities and fertilizers, contributes to changes in pH and alkalinity. In riparian zones rich in organic matter, metabolic activity is high, with C fluxes associated with respiration and denitrification. In the river network, the temporal variability of dissolved element levels is controlled by the mixing of waters that have passed through the aquifers and more superficial, less mineralized flows. Spatial variability is also observed between the different tributaries, which appears to be impacted not only by land use but also by the geological structure of the subsoil, particularly surface silts. The intensification of carbon form monitoring, with in situ monitoring in the different tributaries and at the basin outlet, is underway and will enable progress towards the development of a basin-wide biogeochemical model.