Assessing submarine groundwater discharge (SGD) and nitrate fluxes in highly heterogeneous coastal karst aquifers: Challenges and solutions

Groundwater discharge in coastal karst aquifers worldwide represents a substantial part of the water budget and is a main pathway for nutrient transport to the sea. Groundwater discharge to the sea manifests under different forms, making its assessment very challenging particularly in highly heterogeneous coastal systems karst systems. In this study, we present a methodology approach to identify and quantify four forms of groundwater discharge in a mixed lithology system in southern Spain (Maro-Cerro Gordo) that includes an ecologically protected coastal area comprised of karstic marble. We found that groundwater discharge to the sea occurs via: (1) groundwater-fed creeks, (2) coastal springs, (3) diffuse groundwater seepage through seabed sediments, and (4) submarine springs. We used a multi-method approach combining tracer techniques (salinity, 224Ra, and 222Rn) and direct measurements (seepage meters and flowmeters) to evaluate the discharge. Groundwater discharge via submarine springs was the most difficult to assess due to their depth (up to 15 m) and extensive development of the springs conduits. We determined that the total groundwater discharge over the 16 km of shoreline of the study area was at least 11 ± 3 × 103 m3 d−1 for the four types of discharge assessed. Groundwater-derived nitrate (NO3−) fluxes to coastal waters over ∼3 km (or 20%) in a highly populated and farmed section of Maro-Cerro Gordo was 641 ± 166 mol d−1, or ∼75% of the total NO3− loading in the study area. We demonstrate in this study that a multi-method approach must be applied to assess all forms of SGD and derived nutrient fluxes to the sea in highly heterogeneous karst aquifer systems