Diagnosis and quantification of groundwater – stream flow : hydrodynamic and geochemical modelling of the upstream Yvette watershed (France)

Karine Lefebvre – 2015

In the peri-urban context with large farm practices of the Paris region, anthropogenic pressures on streams and rivers impact both their flow and quality. Located in the southwestern of Paris, the Yvette stream drains a watershed of 202 km², in a homogeneous geological context. The stream is sustained by waste water treatment plant (WWTP) and by the Fontainebleau sands aquifer which represents the main source of water.In this context, the sustainable management of the stream and its riparian zones needs the assessment of groundwater discharge locations and chemistry, and their impact on the stream water quality.Water fluxes in the stream and in its main tributaries have been recorded at 11 stations. Dissolved elements distributions have been studied by field works in low-flows conditions. A lumped model, calibrated with stream flow hydrographs, has allowed (i) the estimation of groundwater recharge rates (60 – 160 mm.y-1) and (ii) the distribution of stream flow contributions between 2001 and 2014 (i.e. 55 % from groundwater, 38 % from runoff and 8 % from WWTP). Moreover, the use of geochemical and isotopic tracers (e.g. Cl^–, NO₃^–, SO₄^2-, ²²²Rn, δ¹⁸O_water, δ²H_water) confirmed the large impact of groundwater on stream flow and quality.The hydrological dynamics of the Yvette stream and its tributaries are similar because of the homogeneity of the geomorphological context of the catchment. The flood events are really fast (on the order of hours) and low-flow conditions can be observed in every season. The discretization of the Yvette catchment in 11 sub-watersheds highlighted a divergence between the topographical and hydrogeological catchments. This generates a water deficit in some tributaries heads (e.g. Mérantaise, Ru des Vaux) in favor of some others (e.g. Rhodon). The matching limits of these catchments can only be made on the Yvette channel. During the 2001 – 2014 period, considering a fairly constant WWTP contribution, the groundwater discharge provides more than 90 % of the stream flow during low-flow conditions. This highlights a stream quality dominated by groundwater discharge. However, groundwater chemistry largely varies over the catchment. To study this phenomenon, a small-scale field work has been directed on the Rhodon stream, a main tributary of the Yvette River. At this scale, the groundwater discharge appeared to have two main pathways: 15 % come from the streambed while 85 % transit in wetlands before going to the stream. Within these wetlands, a large part of groundwater flows at the surface and has high levels of nutrients, whereas a small part (6 %) goes through sediments and is nutrients free. reduces The purifying power of these wetlands, significantly reduced by this distribution, is likely related to historical drainage and reinforces the direct connection between the stream and groundwater quality. On the Yvette catchment, the stratification of groundwater chemistry is due to several recharge processes (e.g. pond water infiltration, WWTP effluents infiltration, wetlands water infiltration). Small sub-watersheds (< 50 km²) seem to be dominated by (i) this stratification, and (ii) the WWTP discharges which can contribute from 30 % to 50 % to dissolved fluxes in the streams. The main channel of the Yvette stream primarily depends of the mixing of its tributaries waters. The methods developed here allowed highlighting (i) differences of catchment functioning according to the scale used, and (ii) the utmost importance of the small-scale assessment to understand/decipher and manage streams at larger scales.