Oracle-Orgeval, Long-term Research observatory of the critical zone

The Orgeval Long-term Research Observatory, part of the French critical zone network (OZCAR RI), is a 104 km² agricultural catchment, located 70 km east of Paris, in France. The Orgeval catchment is representative of intensive agriculture (80 % of its total area), the main land use in the Seine river basin. For more than 50 years, both quantity and quality of water are monitored throughout the catchment, from sub-hourly to yearly time scales. This rich dataset allows improving the understanding of critical zone structure and reactivity, in a holistic and interdisciplinary approach. Specific basic and applied research topics relate to extreme hydrological events, agricultural tile drainage, land use planning, and more generally the evolution of agricultural activities facing climate change and urban growth. The Orgeval research observatory is a unique testbed to investigate the functioning and evolution of the Critical Zone. Multidisciplinary approaches are implemented thanks to collaborations between research institutes and universities, combining knowledge and methods from different disciplines, such as hydrology, ecology, biogeochemistry, geophysics and socioeconomics. Created in 1962, the observatory was initially devoted to study floods and weathering research questions. Since then, is evolved towards other societal and environmental topics. Initiated in 1975, lots of monitoring and research was dedicated to diffuse agricultural pollution, especially nitrates, which contributed to a better understanding of the interactions between agricultural activities and surface and groundwater quality. Since the 2000’s, research questions opened to pesticides and biology and biodiversity. The Orgeval observatory is also highly adapted to develop technological innovations, such as in situ biochemical monitoring. A multi-scale observation strategy is implemented in both space and time, ranging from local (with more than 80 monitoring sites) to regional scale, and from time-lapse campaigns to high-frequency measurements (from 1 Hz for geophysics to 1 h or 1 week for chemistry), most often with a long-term approach. The main measurements include: Water level and water discharge : at the outlet of each sub-catchment. groundwater level : in piezometers in the riverbanks and in the aquifers. Precipitation : in addition to Météo-France stations. Main weather variables : air temperature, humidity, and radiation. Soil moisture : from the soil surface to a depth of 1.5 meters. Water quality : dissolved organic and inorganic carbon, dissolved gases (O 2 , CO 2 , Rn), major and trace ions, nutrients, but also water, carbon or strontium stable isotopes. This includes the RiverLab prototype, installed in June 2015 at the outlet of Avenelles sub-catchment for high-frequency measurement (every 30 minutes) of the river's chemical composition. Organic and inorganic contaminants : pesticides but also metals and microplastics in surface and groundwater. Ecotoxicological and ecological indices : ecological assessment. Surface and groundwater temperature : using heat as a tracer of surface-groundwater exchanges. Hydrogeophysics : ERT, GPR, seismic, especially in the riparian areas. Borehole core samples and logging : lithofacies description and characterization. Water level and water discharge : at the outlet of each sub-catchment. groundwater level : in piezometers in the riverbanks and in the aquifers. Precipitation : in addition to Météo-France stations. Main weather variables : air temperature, humidity, and radiation. Soil moisture : from the soil surface to a depth of 1.5 meters. Water quality : dissolved organic and inorganic carbon, dissolved gases (O 2 , CO 2 , Rn), major and trace ions, nutrients, but also water, carbon or strontium stable isotopes. This includes the RiverLab prototype, installed in June 2015 at the outlet of Avenelles sub-catchment for high-frequency measurement (every 30 minutes) of the river's chemical composition. Organic and inorganic contaminants : pesticides but also metals and microplastics in surface and groundwater. Ecotoxicological and ecological indices : ecological assessment. Surface and groundwater temperature : using heat as a tracer of surface-groundwater exchanges. Hydrogeophysics : ERT, GPR, seismic, especially in the riparian areas. Borehole core samples and logging : lithofacies description and characterization. All these data support the development and use of numerical models, for scientific questions but also for environmental impact assessment and territorial management. This catchment is used to develop and/or validate numerical methodologies at the headwater catchment scale, in parallel to larger scale modeling (typically for the Seine river basin)