Comparison of ²²²Rn - δ¹³C investigations to calibrate groundwater discharge in rivers

Karine Lefebvre, Florent Barbecot, Marie Larocque, Marina Gillon, Elisabeth Gibert-Brunet, François Hardy

In the proceedings of: GeoMontréal 2013: 66th Canadian Geotechnical Conference; 11th joint with IAH-CNC

Session: Isotope Tracing II

ABSTRACT: Assessing the distribution of groundwater inflow to watersheds is an important step to appreciate the sensitivity of low flows to environmental changes. This distribution depends on the hydrogeological context and on watershed management. It is poorly constrained because the resolution on river flow measurements is generally low. Different environmental tracer experiments have been proposed during the last decade. In this work, we compare the significance of A222Rn, 13CTDIC, 18Owater, 2Hwater coupled with in situ measurements (river discharge, water temperature, electrical conductivity, pH, and alkalinity) on two different sites to better understand and to quantify groundwater inflow to rivers. These tracers have been chosen as they may depict signals significantly different from groundwater to surface water. Here, we focus on how processes and kinetics that sustain tracer evolutions in surface waters could be site dependent. Located in the Paris basin, the two studied watersheds have small sizes (219 and 21 km2), and have similar shapes and rivers lengths. The Hallue River (14.5 km) flows over the fractured Chalk aquifer while the Rhodon River (10.5 km) receives groundwater from porous flows in the Fontainebleau Sands aquifer. Although aquifer geology is highly contrasted for the two sites, hydraulic conductivities are similar (10-4 m/s). Precipitation is high (740 mm on the Hallue River and 709 mm on the Rhodon River) and runoff is limited on the two watersheds. Both watersheds are occupied mostly by agriculture and host a number of riparian wetlands. Field work was performed in April 2011 on the Hallue River and will be performed in July 2013 on the Rhodon River. Field work consists in measuring in situ water temperature, electrical conductivity, pH and alkalinity approximately every kilometers along the length of both rivers. Other parameters (222Rn, 13CTDIC, 18Owater and 2Hwater) were measured in river water samples collected at the same locations. 222Rn is analyzed using a liquid scintillometer, 13CTDIC is analyzed using a mass spectrometer Isoprime, while 18Owater and 2Hwater are analyzed using a mass spectrometer Finnigan Delta +. 18Owater and 2Hwater are performed in the laboratory of the University of Paris Sud (Orsay, France) and 222Rn, 13CTDIC, are performed in the laboratories of the University of Quebec in Montreal (Québec, Canada). On the Hallue River, the results of 18Owater and 2Hwater show that there is no evaporation of the river water. The correlations between 222Rn, electrical conductivity and 18Owater, between 13CTDIC and pH and the anti-correlation between 222Rn and 13CTDIC are strong. This shows that the major processes modifying the chemical signature of the stream are the groundwater inflow and the degassing towards the atmosphere. Tracing groundwater inflow to the river is performed by simulating 222Rn and 13CTDIC evolution, using a diffusion exchange approach discretized along the river. The degassing process is based on two parameters fitting: the thickness of the diffusive layer and the CO2 pressure in the atmosphere above the water surface. The preliminary results for the Hallue River show that the coupling of 222Rn and 13CTDIC has the required sensitivity to quantify groundwater inflow on a small watershed. The model fitting parameters will be compared for the two sites to better understand the dynamics of river-aquifer exchanges on these two apparently similar rivers which flow on contrasted geological media.

Please include this code when submitting a data update: GEO2013_631

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