Evaluating submarine groundwater discharge to the Mediterranean Sea by using

Abstract

Submarine groundwater discharge (SGD) is defined as any flow of water through the continental margin from the seabed to the coastal ocean, including fresh meteoric groundwater and seawater recirculating through coastal aquifers. SGD has been recognized as a major component of the hydrological cycle and a significant source of various dissolved terrestrial compounds (e.g. nutrients, trace metal, carbon, contaminants) to the coastal ocean. These fluxes of chemical elements via SGD may have a profound impact on the biogeochemical cycles of the receiving water bodies. This can be especially relevant in oligotrophic and semi-arid regions, such as the Mediterranean Sea. However, and despite the potential importance of SGD in regulating coastal biogeochemical cycles of the Mediterranean Sea, there is still a lack of detailed assessments on the relevance of SGD as a source of chemical constituents into this basin. Indeed, the magnitude of SGD to the entire Mediterranean basin and its associated fluxes of dissolved compounds have never been evaluated. The main objective of this PhD Thesis is to evaluate the importance of SGD in the Mediterranean Sea by using radium (Ra) isotopes, paying attention to the role that SGD plays as a source of dissolved chemical compounds to the sea and to the use of Ra isotopes as SGD tracers. To this aim, contrasting Mediterranean coastal environments were selected, including: i) a coastal wetland nourished by groundwater inflowing from several aquifers (Peníscola marsh, Castelló); ii) a semi-enclosed embayment highly influenced by bottom sediments (Port of Maó, Minorca, Balearic Islands); and iii) a detrital bay open to the sea (Palma Bay, Majorca, Balearic Islands). Aside from these three specific sites, the first appraisal of the magnitude of SGD into the entire Mediterranean Sea was also conducted, demonstrating its significance as a source of dissolved compounds in a basin-wide scale. Results from these studies provide new insights into the use of Ra isotopes as tracers to quantify SGD and underline their suitability in a wide range of Mediterranean hydrogeological settings. We successfully applied them to estimate SGD-driven fluxes of dissolved nutrients and, for the first time, trace metals to a coastal Mediterranean area, stressing the role SGD may play as a source of these constituents to the marine environment. We show that SGD is a volumetrically important process in the Mediterranean Sea, contributing up to (0.2–4.3)·1012 m3·yr-1, a magnitude that is significantly larger than riverine discharge. SGD also represents a major source of dissolved nutrients to the basin, rivaling the conventional external sources (i.e. atmospheric deposition and river discharge). This new understanding of the magnitude of SGD and its associated chemical fluxes demonstrates the profound implications of SGD in the biogeochemical cycles of the Mediterranean Sea, emphazising the need for its consideration in coastal and basin-wide studies

Date of Award	28 Apr 2014
Original language	English
Awarding Institution	Universitat Autònoma de Barcelona (UAB)
Supervisor	Jordi Garcia Orellana (Director) & Pere Masque Barri (Director)