Groundwater discharge to coastal water bodies is increasingly recognised to contribute significantly to coastal water and solute budgets. In order to evaluate the discharge of low-salinity groundwater of karstic origin and of recirculation of seawater through sediments into Mediterranean lagoons, a study was carried out in La Palme, Salses-Leucate and Thau lagoons on the French Mediterranean coastline in the dry summer months 2009, using radon and salinity as tracers. Whereas Salses-Leucate lagoon receives significant fluxes of karstic low-salinity groundwater, in La Palme and Thau lagoons, karstic groundwater fluxes are small, and have little effect on the lagoons' radon balance. A simultaneous water, salt and radon budget of the small La Palme lagoon (ca 50. ha surface area and 0.6. m mean depth) was used to simulate average salt and radon concentration over a one month period. The results indicate that despite its small flux (0.8-1.2% of lagoon volume per day) the discharge of low-salinity karstic groundwater contributes significantly to maintaining salinity lower than seawater in the seasonally closed lagoon, but makes only a minor contribution (7-18%) to the radon budget.Wind-driven seawater recirculation through sediments on the other hand is a major contributor, estimated at 22-60% of total radon fluxes to the lagoon, equivalent to a water flux of 1.4-7.0% of lagoon volume day-1. The remaining balance of Radon is supplied by diffusion and in-water production from decay of its parent nuclide. Using a stand-alone radon model without considering a water and salt balance would have considerably overestimated the flux of groundwater of karstic origin. Radon can be regarded as a proxy for other dissolved solutes such as nutrients and contaminants transported with groundwater and seawater recirculation fluxes. Nutrient and contaminant enrichment of sediments in Mediterranean lagoons is well documented. Wind-driven seawater recirculation through these sediments as documented in this study may remobilise these nutrients and contaminants. It may thus play a considerable role in lagoonal biogeochemical budgets, and may require consideration in water quality management in Mediterranean coastal lagoons.