Using Heat to Trace Vertical Water Fluxes in Sediment Experiencing Concurrent Tidal Pumping and Groundwater Discharge

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.

Vertical Saltwater Intrusion in Coastal Aquifers Driven by Episodic Flooding: A Review

“…Advancements enable the upscaling of these methods to spatial scales relevant for management (Garcia‐Orellana et al., 2021). There are also opportunities to develop and employ other environmental tracers, such as heat, to monitor the fluxes between ponded surface water and groundwater, to estimate fresh or saline groundwater exfiltration rates along a coastline, or to assess the effects of morphologic change (Gilfedder et al., 2021; LeRoux et al., 2021; Wilson et al., 2016). There is a particular need for detailed maps of nearshore bathymetry and coastal topography to inform groundwater model development, monitor coastal change, and predict flooding incidents (Almar et al., 2021).…”

Drone‐based characterization of intertidal spring cold‐water plume dynamics

“…Prior studies also only present a single snapshot in time for the thermal plumes or consider temporal changes at lower frequencies than in the present study. Our thermal imagery shows that tidal oscillations influence the hydraulic and thermal exchanges associated with groundwater‐surface water interactions in coastal settings (e.g., LeRoux et al, 2021). Our results further highlight the complex dynamics of the orientation and size of cold‐water plumes sourced from intertidal springs.…”

“…Only a few river reaches have been thermally mapped to delineate cold‐water plumes, and, with rare exceptions (Dugdale et al, 2013), such river aerial thermal surveys have only been recorded at a single point in time. These single snapshots may obfuscate important temporal cold‐water plume dynamics, particularly in coastal reaches where hydraulic and thermal interactions between groundwater and surface water are tidally influenced (Lee et al, 2016; LeRoux et al, 2021). The lack of temporal data for cold‐water plumes has limited our understanding of the mechanisms that control their morphology and dynamics and how these conditions may impact cold‐water species.…”

Shallow and local or deep and regional? Inferring source groundwater characteristics across mainstem riverbank discharge faces

“…And while vertical heat tracing methods are more conducive to installation in saturated sediments along exposed riverbanks, common methods of analysis based on diurnal temperature signals alone do not well‐capture subdaily and abrupt changes in flux rate (Lautz, 2012). Numerical methods can accurately capture rapid and variable changes in flux rate, but typically only when paired with vertical pressure data (LeRoux et al, 2021) or via manual calibration of multiple model stress periods, and therefore the analysis is difficult to scale to many measurement locations at seasonal timescales. Recent application of spectral and recursive estimation Kalman Filter‐based approaches may allow accurate tracking of rapid vertical flux changes due to storms, dam operation, and other subdaily to seasonal dynamics (McAliley et al, 2022; Sohn & Harris, 2021).…”