Using heat as a tracer to estimate the depth of rapid porewater advection below the sediment-water interface

•Flow processes at the sediment-water interface are similar in streams and the ocean.•The MATTSI computer program detects porewater exchange using heat as a tracer.•Flushing is approximated in a 1-D framework using an effective thermal dispersivity.•The model detected episodic sub-seafloor flushing at a continental shelf site.•Flushing occurred during periods with high current velocity and thermal instability.

SummaryRapid exchange of surface waters and porewaters in shallow sediments has important biogeochemical implications for streams and marine systems alike, but mapping these important reaction zones has been difficult. As a means of bridging the gap between the stream and submarine groundwater discharge communities we suggest that the rapid, transient mixing in this zone be called “hydrodynamic exchange”. We then present a new model, MATTSI, which was developed to estimate the timing, depth and magnitude of hydrodynamic exchange below the sediment-water interface by inverting thermal time-series observations. The model uses an effective thermal dispersion term to emulate 3-D hydrodynamic exchange in a 1-D model. The effective dispersion is assumed to decline exponentially below the sediment water interface. Application of the model to a synthetic dataset and two field datasets from 50 km offshore in the South Atlantic Bight shows that exchange events can be clearly identified from thermal data. The model is relatively insensitive to realistic errors in sensor depth and thermal conductivity. Although the datasets tested here were too shallow to fully span the depth of flushing, we were able to estimate the depth of hydrodynamic exchange via sensitivity studies.

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