Identifying groundwater discharge to an Atlantic coastal lagoon (Oualidia, Central Morocco) by means of salinity and radium mass balancesKarstic groundwater discharge to the coastal lagoon of Oualidia

Radium isotopes, 223Ra and 224Ra, and salinity measurements were used to assess submarine groundwater discharge from a karstic aquifer system into a coastal lagoon located at Oualidia (Morocco), on the Atlantic shoreline. The Oualidia lagoon is classified as a Ramsar area owing to its ecological significance. Groundwater discharge to the lagoon occurs as intertidal springs, submarine springs and probably diffuse leakage. Groundwater sources and discharge fluxes were determined using mass balance calculations following single and multi-box approaches. Calculated flow rates varied from 0.2 ± 0.2 to 1.2 ± 0.6 m3 s−1. The single-box model can be considered representative enough of water cycling and mixing within the Oualidia lagoon. However, a certain complexity of the hydrological processes should be taken into account as the discharge varies from low to high tide periods. The discharge amount suggests that the Oualidia lagoon may contribute significantly to the regional karstic discharge. Both the regional geological structure and the piezometric map indicate a convergence of groundwater to the area of the lagoon which is located in the bottom of a large geological depression. It is deduced that a close relationship exists between the karst development and the lagoon.