Submarine Groundwater Discharge of Artificial Releases? Using a Hydrodynamic Model to Assess the Origin of 222Rn Activities in a Coastal Lagoon Connected to a Highly Anthropized Aquifer (Mar Menor, SE Spain).

English Abstract:

In highly anthropized watersheds, surface water tributaries may carry unexpectedly high quantities of radon (²²²Rn) to surface water masses. Assessing the impact of these additional sources of radionuclides on the spatial repartition of radionuclides is a hard task for submarine groundwater discharge (SGD) investigations. A methodology was tested to the Campo de Cartagena aquifer - Mar Menor lagoon hydrosystem in SE Spain. In this area, intensive irrigation and subsequent recharge caused a severe water table rise, from which groundwater started to discharge to the main surface water stream. In addition, the uncontrolled release of brines from private groundwater desalination potentially induced artificial surface water discharges through the main stream or directly to the Mar Menor lagoon. In order to decipher the influence of the different water sources on the spatial repartition of radionuclides and to strengthen the quantification of submarine groundwater discharge fluxes, we combined a ²²²Rn survey with the hydrodynamic modeling of the Mar Menor lagoon. Once demonstrated that reverse osmosis desalination does not affect significantly ²²²Rn activities, it was shown that brine discharge exerted a main control on the total radionuclide discharge of the surface stream. Hydrodynamic modeling then demonstrated that high values measured along the discharge coastline were, unexpectedly, not indicators of localized SGD fluxes. Indeed, the observed high values were rather explained by the propagation of the river plume (central area), by artificial releases to the lagoon (northern area) and by agricultural groundwater drainage (southern area). These findings helped quantifying the impact of artificial releases and by then, strengthened the SGD quantification.