Analytical solution and computer program ( FAST ) to estimate fluid fluxes from subsurface temperature profiles

Water Resources Research

Mohammed

et al. 2018

Submarine groundwater fluxes across the seafloor facilitate important hydrological and biogeochemical exchanges between oceans and seabed sediment, yet few studies have investigated spatially distributed groundwater fluxes in deep‐ocean environments such as continental slopes. Heat has been previously applied as a submarine groundwater tracer using an analytical solution to a heat flow equation assuming steady state conditions and homogeneous thermal conductivity. These assumptions are often violated in shallow seabeds due to ocean bottom temperature changes or sediment property variations. Here heat tracing analysis techniques recently developed for terrestrial settings are applied in concert to examine the influences of groundwater flow, ocean temperature changes, and seabed thermal conductivity variations on deep‐ocean sediment temperature profiles. Temperature observations from the sediment and bottom ocean water on the Scotian Slope off eastern Canada are used to demonstrate how simple thermal methods for tracing groundwater can be employed if more comprehensive techniques indicate that the simplifying assumptions are valid. The spatial distribution of the inferred groundwater fluxes on the slope suggests a downward groundwater flow system with recharge occurring over the upper‐middle slope and discharge on the lower slope. We speculate that the downward groundwater flow inferred on the Scotian Slope is due to density‐driven processes arising from underlying salt domes, in contrast with upward slope systems driven by geothermal convection. Improvements in the design of future submarine hydrogeological studies are proposed for thermal data collection and groundwater flow analysis, including new equations that quantify the minimum detectable flux magnitude for a given sensor accuracy and profile length.

Section: Methods: Thermal Analysismentioning

confidence: 99%

mentioning

confidence: 99%

Rethinking the Use of Seabed Sediment Temperature Profiles to Trace Submarine Groundwater Flow

Kurylyk

Water Resources Research

Mohammed

et al. 2018

“…However, profiles that are thermally disturbed by climate change must be analyzed with a transient method forced with a boundary condition to represent recent climate change. In contrast, computer programs (e.g., Kurylyk and Irvine 2016;Li et al 2019) specifically designed to automate solutions of transient numerical or analytical solutions for applying heat as a groundwater tracer are easier to implement. Although ground surface temperatures and surface air temperatures are often assumed to be coupled on a mean annual or decadal basis, studies have suggested changes to snowpack insulation or other nearsurface conditions may violate this assumption (Beltrami and Kellman 2003;Mann and Schmidt 2003;Smerdon et al 2006).…”

Section: Conclusion and Path Forwardmentioning

confidence: 99%

Heat: An Overlooked Tool in the Practicing Hydrogeologist's Toolbox

Kurylyk¹,

2019

Groundwater

Self Cite

“…The solution also assumes steady-state conditions and neglects high-frequency and low-frequency temperature transience, which can lead to over-or under-estimates of Darcy fluxes in streambeds (Anibas et al, 2009) and in deep profiles (Ferguson & Woodbury, 2005;Irvine et al, 2016). However, the steady-state approach is attractive as it circumvents complicating issues associated with determining unknown initial conditions for deep profiles (Kurylyk & Irvine, 2016), as well as the need to run transient simulations. The steadystate approach is best suited to discharge zones where both the diel and seasonal temperature envelopes do not extend as deeply as in recharge zones, because the downward conduction is impeded by upward advection (Kurylyk, MacQuarrie, Caissie et al, 2015).…”

Section: Limitationsmentioning

confidence: 99%

Heat as a groundwater tracer in shallow and deep heterogeneous media: Analytical solution, spreadsheet tool, and field applications

Kurylyk

Carey

et al. 2017

Hydrological Processes

Self Cite

Groundwater flow advects heat, and thus, the deviation of subsurface temperatures from an expected conduction‐dominated regime can be analysed to estimate vertical water fluxes. A number of analytical approaches have been proposed for using heat as a groundwater tracer, and these have typically assumed a homogeneous medium. However, heterogeneous thermal properties are ubiquitous in subsurface environments, both at the scale of geologic strata and at finer scales in streambeds. Herein, we apply the analytical solution of Shan and Bodvarsson (), developed for estimating vertical water fluxes in layered systems, in 2 new environments distinct from previous vadose zone applications. The utility of the solution for studying groundwater‐surface water exchange is demonstrated using temperature data collected from an upwelling streambed with sediment layers, and a simple sensitivity analysis using these data indicates the solution is relatively robust. Also, a deeper temperature profile recorded in a borehole in South Australia is analysed to estimate deeper water fluxes. The analytical solution is able to match observed thermal gradients, including the change in slope at sediment interfaces. Results indicate that not accounting for layering can yield errors in the magnitude and even direction of the inferred Darcy fluxes. A simple automated spreadsheet tool (Flux‐LM) is presented to allow users to input temperature and layer data and solve the inverse problem to estimate groundwater flux rates from shallow (e.g., <1 m) or deep (e.g., up to 100 m) profiles. The solution is not transient, and thus, it should be cautiously applied where diel signals propagate or in deeper zones where multi‐decadal surface signals have disturbed subsurface thermal regimes.