RETHINKING THE USE OF OCEAN SEDIMENT TEMPERATURE-DEPTH PROFILES TO INFER SUBMARINE GROUNDWATER DISCHARGE

Heat can be applied to trace groundwater flow because thermal advection due to mobile groundwater causes thermal regimes to deviate from conduction-dominated conditions. Hydrogeologists began proposing approaches to apply heat as a groundwater tracer in terrestrial environments in the mid 1960s, and these techniques were transferred to ocean environments in the 1970s. The standard approach is a steady-state solution that can be applied to infer fluxes from the curvature of a temperature-depth profile. Early analyses yielded groundwater flux estimates from submarine borehole thermal profiles that were too high based on the ocean bed permeability. Seminal works on heat tracing in ocean sediments demonstrated that temperature profile curvature can also be caused by processes other than heat advection, including temperature change at the ocean bed surface, changes in the thermal properties with depth, and removal/deposition of sediment. Presumably, not accounting for these perturbations in the standard, steady-state heat tracing approach can cause unrealistically high groundwater fluxes.

There has been intensified interest in the oceanography community to study submarine groundwater discharge rates as subsurface pathways provide conduits for land-based contaminants to be transported to ocean environments and impact local ecosystems. Recent studies using heat to trace groundwater have developed innovative methods that account for many of the additional factors noted above that can cause further curvature in subsurface temperature profiles, but these new techniques have generally not yet been applied in ocean environments. In this study, we highlight the potential strength of these new heat-tracing techniques to study aquifer-ocean interactions in complicated marine settings. Illustrative examples are developed using a subset of ~ 50 ocean sediment temperature-depth profiles collected from the Nova Scotia slope (eastern Canada) under up to 4.5 km of ocean water and to sediment depths of 6 m. The results suggest groundwater fluxes vary spatially both in terms of their magnitude and direction and that the magnitude of the vertical flux typically decreases with depth. This study also highlights the difficulties in applying these new techniques given the lack of long term ocean bed temperature data.