Theory, tools, and multidisciplinary applications for tracing groundwater fluxes from temperature profiles

Abstract

Quantifying groundwater fluxes to and from deep aquifers or shallow sediment is a critical task faced by researchers and practitioners from many environmental science disciplines including hydrology, hydrogeology, ecology, climatology, and oceanography. Groundwater discharge to inland and coastal water bodies influences their water budgets, thermal regimes, and biogeochemistry. Conversely, downward water flow from the land surface or from surface water bodies to underlying aquifers represents an important water flux that must be quantified for sustainable groundwater management. Because these vertical subsurface flows are slow and typically diffuse, they cannot be measured directly and must rather be estimated using groundwater tracers. Heat is a naturally occurring groundwater tracer that is ubiquitous in the subsurface and readily measured. Most of the academic literature has focused on groundwater temperature tracing methods capitalizing on the propagation of diel temperature sine waves into sediment beneath surface water bodies. Such methods rely on temperature–time series to infer groundwater fluxes and are typically only viable in the shallow subsurface and in locations with focused groundwater fluxes. Alternative methods that utilize temperature–depth profiles are applicable across a broader range of hydrologic environments, and point‐in‐time measurements can be quickly taken to cover larger spatial scales. Applications of these methods have been impeded due in part to the lack of understanding regarding their potential applications and limitations. Herein, we highlight relevant theory, thermal data collection techniques, and recent diverse field applications to stimulate further multidisciplinary uptake of thermal groundwater tracing methods that rely on temperature–depth profiles. This article is categorized under: Water and Life > Methods Science of Water > Methods Water and Life > Nature of Freshwater Ecosystems

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

“…Thus, while some thermal tracing methods were originally developed to estimate fluxes across aquitards, the methods can be limited in very low permeability environments, even when groundwater flow or solute advection are important (Kurylyk et al 2019). This thermal advection disturbs subsurface temperatures and leaves behind a thermal signature.…”

“…Figure 4 shows temperature data from two recent studies plotted on top of normalized temperature-depth curves to demonstrate the utility of the Bredehoeft and Papadopulos (1965) method across different spatial scales and hydrogeological environments (basin hydrogeology vs. groundwater-surface water interactions). A detailed review is outside the scope of this paper and was recently published elsewhere (Kurylyk et al 2019). The classic approach proposed by Bredehoeft and Papadopulos (1965); see Figures 2a, 2b, 4a, and 4b) assumes steady-state thermal conditions.…”

“…As such, heat can only be applied as a groundwater tracer in hydrogeological systems where heat advection due to groundwater flow discernibly impacts the thermal regime. Thus, while some thermal tracing methods were originally developed to estimate fluxes across aquitards, the methods can be limited in very low permeability environments, even when groundwater flow or solute advection are important (Kurylyk et al 2019). Two major developments in this topic were published in the same year.…”

“…Although they have received less attention in the academic literature, we believe that temperature-depth methods for estimating groundwater fluxes (Figure 2a 2b) have more applicability in conventional hydrogeologic practice than temperature-time series methods (Figure 2c and 2d). The former are useful across a much wider range of hydrogeologic environments and are applicable for lower groundwater flux magnitudes and longer timescales, such as those relevant for water resources planning (Kurylyk et al 2019). For example, the Bredehoeft and Papadopulos (1965) equation can be differentiated and equated to zero to determine the maximum thermal offset between a linear temperature-depth profile and a curved thermal profile due to groundwater flow (Kurylyk et al 2018a, Equation 9).…”

Quantitative guidance for efficient vertical flow measurements at the sediment–water interface using temperature–depth profiles

“…Estimates of upwelling based on Darcy's law are uncertain given the large range and spatially variable nature of sediment hydraulic conductivity (Calver, 2001;Cardenas & Zlotnik, 2003). Heat tracer methods offer several advantages over chemical or hydraulic methods, primarily because temperature data can be measured inexpensively and easily without laboratory analyses (Anderson, 2005;Anibas et al, 2009;Kurylyk, Irvine, & Bense, 2019). In particular, the use of heat as a tracer of groundwater discharge has increased in recent years, following reviews by Anderson (2005), Constantz (2008), and Rau, Andersen, McCallum, Roshan, and Acworth (2014).…”