Thermal evidence of water exchange through a coastal aquifer: Implications for nutrient fluxes

We report the discovery of a semi‐confined, high permeability zone (HPZ) 2 meters below the sea bed where exchanges between coastal aquifers and the ocean occur. A temperature probe placed in the HPZ recorded a 1°C semidiurnal cycle. The cycle was in phase with the tide, indicating tidal pumping was driving water exchange through the HPZ. We use these temperature variations to quantify water exchange between the HPZ and the ocean. Water in the HPZ is enriched in nutrients and radium; both total dissolved nitrogen and phosphorus correlate strongly with dissolved 226Ra. We use these correlations and previously quantified subterranean fluxes of 226Ra to demonstrate that this system may be a significant source of nutrients to the coastal ocean.