Detection and quantification of low submarine groundwater discharge flows by radionuclides to support conceptual hydrogeological model of porous aquifers

International audienceSubmarine groundwater discharge (SGD) is considered as a good indicator of the natural behavior of coastal aquifers especially in the case of intense groundwater withdrawal or when the aquifer recharge conditions are limited. Indeed, a decrease in SGD flows would result in saline intrusion compromising the groundwater reservoir use. The need for SGD studies in Mediterranean and semi-arid environments have long been recognized and little is known on low and diffuse flows. For this reason, the detection and quantification of low and diffuse SGD flows have been attempted in a Mediterranean porous aquifer with low potential of recharge and high touristic pressure from using a multi-tracing approach combining aerial thermal infrared images (TIR), temperature (T), electric conductivity (E.C.), major ions, selected trace elements, H-2, O-18, Rn-222, Ra-223 and Ra-224. The TIR images allowed the detection of slight temperature variations at the seawater surface. Contrasted lithology and strong water-rock interactions due to groundwater residence time of many decades favor the Rn-222, Ra-223 and Ra-224 contents in groundwater that reach the seashores. Low temperature and E.C, as well as high radon and radium activities, evidenced SGD flows in agreement with the known flow conditions within the aquifer. The quantification of SGD flows revealed in very good agreement with the known recharge rates of the aquifer that validates its conceptual hydrogeological model. For the first time in the Mediterranean or in semi-arid context, a natural tracing experiment based on geochemistry and isotope hydrology tools allowed the quantification of SGD flows to validate the hydrogeological conceptual model and to solve the water balance at the whole aquifer scale. Such a methodology could be applied worldwide to other similar coastal aquifers with low and diffuse SGD flow conditions