Influences of Dam Operations and groundwater pumping in the control of Groundwater-SurfaceWater Interactions

International audienceLocalizing and quantifying groundwater/surface water exchanges remains challenging since exchange processesvary both in time and space, leading to an extreme variability in the distribution of fluid exchanges. One difficultycomes from the varying boundaries conditions that are often unknown or not well defined in the field. Here, westudied groundwater – surface water interactions on the Sélune river (20-meter width), a dam-regulated riverinteracting with an alluvial aquifer used for water supply. As for many rivers in the world, this dam was builtfor hydroelectric power generation. The fluctuations in river stage control interactions between the river andthe alluvial aquifer, where groundwater levels are also controlled by groundwater pumping. Since groundwaterand surface water exchanges should be well controlled by dam operations and groundwater pumping, thedam-regulated Sélune river can be used as a field laboratory to quantify these interactions and test innovativemethods such as active-Distributed Temperature Sensing method. Moreover, since bed sediments, an alternationof sand and gravels with the local presence of boulders, are associated with a succession of riffles and pools, itallows studying the relationship between hydromorphology and groundwater exchanges. Therefore, the presentstudy aims to i) estimate the distribution of groundwater flow velocities, ii) characterize the effect of the riverbedmorphology on flow distribution and river morphology iii) defines the interests and limitations of the differentmethods to quantify SW-GW interactions at high spatial resolution.To characterize exchanges between the river and the alluvial aquifer, piezometers were installed in the bank ofthe river to monitor piezometer levels and groundwater temperature fluctuations. Additional temperature sensorswere deployed at different depths within the streambed sediments providing vertical temperature profiles andallowing calculating vertical water fluxes and recording heat transport during river stage variations. To go further,we developed an active-DTS method allowing measuring groundwater flow velocities in streambed sediments.The experimental setup consists in heating a 600 m fiber-optic cable, previously deployed along the streambedwithin the sediments. The temperature increase along the cable depends on the sediment thermal properties andon groundwater flow velocities that partly dissipate heat through advection. The application of the active-DTSmethod provides the variability and distribution of groundwater flow velocities within the streambed sediments.The results obtained are compared to estimate groundwater/surface water interactions obtained from othersclassical methods. These estimates allow investigating the effect of the riverbed morphology on groundwatersurface water interactions and demonstrating the effect of dam operations and groundwater pumping. Since threeparallel sections of fiber-optic cables have been deployed along the stream from the bank to the middle of theriver, we also aim to characterize the lateral variability of exchanges. Lastly, we discuss the temporal variability ofexchanges by comparing results obtained at different times of the yea