USING HEAT TO TRACE VERTICAL WATER FLUXES IN SEDIMENT EXPERIENCING CONCURRENT TIDAL PUMPING AND GROUNDWATER DISCHARGE

Heat has rarely been applied as a quantitative groundwater tracer in coastal sediment due to challenges caused by tidal head oscillations producing sub-daily periodicity in thermal signals and vertical water fluxes. The resultant oscillations in heat advection are not incorporated into any standard analytical approaches. The thermal signal interference between the diel radiation and semi-diurnal tidal signals complicates the interpretation of temperatures to estimate porewater fluxes. We investigate the mixing of these signals in coastal sediments to assess the viability of using heat as a tracer in environments with tidal head oscillations superimposed on submarine groundwater discharge.

We use a numerical model (SUTRA) to produce synthetic sediment temperature and head time series for conditions ranging from no tide to mega-tidal. We then process the modeled temperatures with the widely used tools VFLUX2 and 1DTempPro, to evaluate if common thermal approaches are applicable for tracing porewater fluxes in sediment experiencing tidal head oscillations. We consider the porewater flux variability within a tidal signal and the flux averaged over tidal signals. Results show that while the analytical models included in VFLUX2 reasonably estimated the mean discharge flux for lower tidal amplitudes, these methods could not produce the flux variability for higher tidal amplitudes. Results further reveal that temporally variable water flux can be accurately estimated in 1DTempPro for a range of tidal scenarios providing both temperature and hydraulic head time series data are included in the modeling process. These results highlight the opportunity to incorporate pressure sensors within heat tracing instrumentation for environments with sub-daily head oscillations to better assess mass fluxes and biogeochemical processes.