Chapter 5 Uranium- and Thorium-Series Nuclides as Tracers of Submarine Groundwater Discharge

Abstract

A number of U- and Th-series isotopes have become popular tools for quantifying submarine groundwater discharge (SGD). These isotopic techniques enable large-scale estimates of various components of SGD, allowing detailed studies on the processes involved. Radium isotopes have proven to be useful tracers of total SGD in many environments on both small- and large scales. The existence of four naturally occurring radium isotopes makes Ra particularly useful for quantifying multiple sources of SGD. The utility of 222Rn as a tracer of SGD has been demonstrated in a wide range of environments from coastal embayments to the coastal ocean. The approach for quantifying SGD using 222Rn is similar to radium (226Ra), except for a few key differences. The 222Rn signature of SGD is best observed near the source because of its short half-life. In contrast, 226Ra has the ability to integrate the SGD signal over much wider spatial scales. Finally, of the intercomparison studies conducted till date, there appears to be no systematic difference in the two techniques based on the coastal hydrogeologic setting, though a wider range of estimates has been typically observed in areas with fractured crystalline rock aquifers and where springs have been a major conduit for SGD. More recently, the geochemical budget for uranium has been shown to be impacted by SGD. As uranium is enriched in seawater relative to most coastal groundwater, the observed depletion of uranium in estuarine and coastal waters may provide an estimate of the seawater component of SGD that recharges coastal aquifers, or submarine groundwater recharge (SGR). Additionally, new isotope techniques for in situ study are under development including underwater g-spectrometry and continuous radon monitoring. These techniques may be more useful for location of SGD discharges rather than estimation of their magnitudes.