Determining groundwater Ra end-member values for the estimation of the magnitude of submarine groundwater discharge using Ra isotope tracers

Radium isotopes (228Ra and 226Ra) are excellent tracers of submarine groundwater discharge (SGD). To estimate SGD magnitudes, information on the end-member values of Ra concentrations in groundwater is critical; however, the distribution characteristics of Ra in coastal aquifers are poorly understood. In this study, we show that Ra concentrations in coastal groundwater are primarily dependent on salinity based on the data (n > 500) obtained from global coastal aquifers, although previous end-member calculations averaged all Ra concentrations without considering salinity. If we assume that SGD is composed mainly of seawater infiltrating the aquifer, previous estimates of SGD for the Atlantic Ocean and the global ocean were overestimated twofold to threefold. This may be similar for other applications using different Ra isotopes. Our study highlights that the end-members of Ra isotopes in groundwater should be carefully considered when estimating SGD using Ra isotope mass balances in the ocean.