Combining radon, short-lived radium isotopes and hydrodynamic modeling to assess submarine groundwater discharge from an anthropized semiarid watershed to a Mediterranean lagoon (Mar Menor, SE Spain)

Abstract

Summary In highly anthropized watersheds, surface water tributaries may carry unexpected high quantities of radon and radium to coastal lagoons. Investigating submarine groundwater discharge (SGD) with radionuclide tracers is therefore a complex task. In order to quantify SGD and decipher the influence of the different water sources, we combined a radon ( 222 Rn) and short-lived radium ( 223 Ra, 224 Ra) survey with the hydrodynamic modeling of a lagoon. We applied it to the Mar Menor lagoon (SE Spain) where surface water tributaries and undocumented emissaries carry water from groundwater drainage and brines from groundwater desalinization. We identified the areas of influence of the plume of radionuclides from the river, located major areas of SGD and proposed a location for two submarine emissaries. Porewater, i.e. interstitial water from underlying sediments, was found to be the most representative SGD end member, compared to continental groundwater collected from piezometers. Mass balances in winter and summer seasons provided yearly SGD fluxes of water of 0.4–2.2 ⋅ 10 8 m 3 /y ( 222 Rn), 4.4–19.0 ⋅ 10 8 m 3 /y ( 224 Ra) and 1.3 ⋅ 10 8 m 3 /y ( 223 Ra, measured in winter only). Tidal pumping was identified as a main driver for recirculated saline groundwater, while fresh submarine groundwater discharge from the aquifer ranged between 2% and 23% of total SGD.