Improving the Use of Radium Isotopes and Radon as Tracers of Submarine Groundwater Discharge

Abstract

Natural tracers – any substance present in the environment in small but measurable amounts – are a central tool in many scientific disciplines, providing information about processes and systems. In hydrology and oceanography, these scientific objects have become fundamental for understanding processes that are invisible or have disparate temporal scales, some of them differing greatly from the human time scale. One of the research topics that has been most closely linked to the use of natural tracers is the assessment of submarine groundwater discharge (SGD). This process, which involves the discharge of terrestrial and marine groundwater from coastal aquifers to the coastal ocean, has been recognized as an important process modulating the chemical budgets of the coastal ocean, controlling coastal ecosystems, and providing significant ecosystem services to society. The radioactive tracers of radium isotopes and radon represent the most extensive and widespread tool for investigating the magnitude and implications of this process in a wide variety of environments, from small coves to the entire ocean. However, reporting SGD estimates by means of these tracers is complex and requires profound knowledge regarding fundamental steps in the process of quantifying SGD using these radionuclides, from the tracer measurement techniques to the estimation of groundwater and solute fluxes. This Thesis explores the use of radium isotopes and radon as tracers of SGD by addressing a set of research gaps dealing with (1) the analytical techniques for measuring and quantifying these radionuclides, (2) their geochemical behavior in groundwater systems, and (3) their applications as tracers for both groundwater systems and the coastal ocean. Regarding the analytical techniques, this Thesis includes an assessment of the quantification systematics of the RaDeCC system, the most widely used counter for quantifying short-lived Ra isotopes, providing quantification limits and guidelines. The work represents a significant advance in pursuing better and more precise SGD estimates, as well as for any hydrological and oceanographic application of these tracers. Additionally, the Thesis presents a comprehensive analysis of the behavior of Ra isotopes and Rn in groundwater through a novel transport model of radionuclides. This model enables the use of these tracers for identifying SGD pathways, constraining the tracer concentration in the discharging groundwater, and evaluating the groundwater flow characteristics. Finally, this dissertation presents one of the first works evaluating the variations of SGD and associated nutrient fluxes induced by extreme precipitation events using Ra isotopes as tracers. The work emphasizes the relevance that these episodic events may have for coastal ecosystems, their relative significance for annual SGD estimates, and their implications in future climate change scenarios. Overall, the works presented in this Thesis contribute to improving the current knowledge both about the use of radium isotopes and radon as tracers of environmental processes and about the magnitude and implications of submarine groundwater discharge.

Date of Award	19 May 2022
Original language	English
Supervisor	Garcia Orellana, Jordi (Director) & Valenti Rodellas Vila (Director)