Radium Isotope Tracers to Evaluate Coastal Ocean Mixing and Residence Times

The identification and provenance of unique coastal water masses is essential in near-shore biogeochemical studies. Water mass mixing and residence times impact water quality and can play a role in the evolution of algal blooms. Such information is thus critical for resource managers who have an interest in understanding the source and fate of contaminants and their eventual fate in the coastal ocean. If mixing is important for quantitatively assessing the amount of exchange, the water residence time or water age is important to assess the rate of this exchange. An understanding of water mass residence times is useful to examine time scales of contaminant discharge and to evaluate transport phenomena. This review summarizes the scientific significance, measurement approaches, and models to evaluate coastal water mixing and residence times using radium isotopes. Each method or model described here is valid, although each has its own advantages and disadvantages. Examples of mixing among different end-members are given as case studies. All approaches presented here demonstrate the utility of radium isotopes for the evaluation of water mass mixing and residence times.