Despite being a major vector for the land-ocean transport of materials, the role of groundwater in oceanic elemental mass balances has been poorly characterized. And where groundwater has been explicitly considered, most studies have not quantified the importance of chemical transformations that occur during the mixing of meteoric water with saline pore water (the subterranean estuary or STE) prior to surface water discharge. Here we examine the importance of karstic coastal hydrogeology on the flux of trace elements from STEs. Data from two field sites will be presented: the Yucatan Peninsula and southwest Florida. There exists an interesting contrast with aluminosilicate-dominated STEs in that the freshwater endmember is highly enriched in alkaline earth elements with evidence for removal during mixing with ocean water. These subterranean karstic systems tend to be reducing environments, which appears to be a common thread among many other STEs regardless of the aquifer lithology; the geochemistry of uranium, iron, and other redox sensitive trace metals will also be discussed. Lastly, radium isotopes and radon-222 will be used to quantify the potential oceanic fluxes of these elements that may be occurring due to submarine groundwater discharge.