ASSESSING SUBMARINE GROUNDWATER DISCHARGE (SGD) IN HIGHLY HETEROGENEOUS COASTAL KARST AQUIFERS: CHALLENGES AND SOLUTIONS

Almost half of the Mediterranean Sea coastline is comprised of karstic carbonate deposits. Coastal karst aquifers play a key role in regional socioeconomics by providing water to coastal populations and serving as a conduit of nutrients to the sea though submarine groundwater discharge (SGD) that impacts coastal ecosystems. Thus, localizing and quantifying SGD is crucial to assess coastal aquifers water budgets. However, due to its highly heterogeneous spatial and temporal distribution in karst systems, assessing SGD may be challenging, and thus attempts often fail.

We examined the SGD budget of a 16-km shoreline in the Maro-Cerro Gordo Protected Coastal Area (Southern Spain) for almost two years and found that even within this small area, there are at least four different mechanism of discharge to the ocean: diffusive flow through coastal sediments, deep submarine springs, emerged coastal springs, and groundwater-fed creeks.We adopted four different approaches (i.e. salinity mixing models, seepage meters, ²²²Rn and ²²⁴Ra mass balances) to adequately assess each mode of SGD. Additionally, we estimated SGD-derived nitrate fluxes (SGD-N) to coastal waters comparing an intensely populated area with the protected zone.

We found that the total fresh SGD in the study area ranges between 3.6 and 2.4·10⁶ m³ year^-1 during wet and dry periods, which can be taken into account for future water resources management. SGD-N is significantly higher (2.7 – 1.9·10⁵ mol year^-1) in the unprotected highly developed area; while in the protection zone, with nearly inexistent anthropogenic impact, SGD-N only represents 9.3 – 5.6·10⁴ mol year^-1. Extensive nitrate delivery in the unprotected zone occurs in the presence of endemic and endangered species that are rapidly disappearing, therefore, we propose to extend the protection zone as a measure to preserve their remaining population.