Using Heat to Trace Vertical Water Fluxes in Sediment Experiencing Concurrent Tidal Pumping and Groundwater Discharge. Issue 2 (12th February 2021)

• Record Type: Journal Article
• Authors: LeRoux, N. K.; Kurylyk, B. L.; Briggs, M. A.; Irvine, D. J.; Tamborski, J. J.; Bense, V. F.
• Is Part Of: Water resources research. Volume 57:Issue 2(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-02-12
• Subjects: analytical solutions -- benthic exchange -- groundwater‐surface water interactions -- heat as a groundwater tracer -- numerical modeling -- submarine groundwater discharge; Hydrology -- Periodicals; 333.91
• DOI: 10.1029/2020WR027904
• Languages: English
• ISSNs: 0043-1397

Abstract

Abstract: Heat has been widely applied to trace groundwater‐surface water exchanges in inland environments, but it is infrequently applied in coastal sediment where head oscillations induce periodicity in water flux magnitude/direction and heat advection. This complicates interpretation of temperatures to estimate water fluxes. We investigate the convolution of thermal and hydraulic signals to assess the viability of using heat as a tracer in environments with tidal head oscillations superimposed on submarine groundwater discharge. We first generate sediment temperature and head time series for conditions ranging from no tide to mega‐tidal using a numerical model (SUTRA) forced with periodic temperature and tidal head signals. We then analyze these synthetic temperature time series using heat tracing software (VFLUX2 and 1DTempPro) to evaluate if conventional terrestrial approaches to infer fluxes from temperatures are applicable for coastal settings. We consider high‐frequency water flux variability within a tidal signal and averaged over tidal signals. Results show that VFLUX2 analytical methods reasonably estimated the mean discharge fluxes in most cases but could not reproduce the flux variability within tidal cycles. The model results further reveal that high‐frequency time series of water fluxes varying in magnitude and direction can be accurately estimated if paired temperatures and hydraulic heads are analyzed using numerical models (e.g., 1DTempPro) that consider both dynamic hydraulic gradients and thermal signals. These results point to the opportunity to incorporate pressure sensors within heat tracing instrumentation to better assess sub‐daily flux oscillations and associated reactive processes. Plain Language Summary: Coastal water bodies are hydrologically connected to underlying aquifers or sediment, which allows for exchanges of water, heat, and chemicals between these domains. These interactions can influence the temperatures and chemistry of coastal water bodies and impact ecosystems. Thus, it is important for us to measure the variability of these exchanges in space and time to better understand their impacts on coastal water quality. Presently, it is challenging to accurately measure how these exchanges vary in time due to tidal variability. In this study, we propose that sediment temperature and groundwater pressure data can be measured and analyzed to estimate these vertical exchanges in coastal settings. We use model results to reveal that if both the temperature and water pressure in coastal sediments are recorded, models can be applied to interpret measured data and quantify exchange between coastal sediment and overlying coastal water bodies. Key Points: Temperature and hydraulic head exhibit multi‐frequency periodicity in coastal sediment due to diurnal and semi‐diurnal forcing Multi‐frequency signal convolution complicates application of methods using diurnal temperature signals to trace vertical water exchange These limitations can be overcome by using dynamic sediment porewater head and temperature in conjunction to estimate water fluxes