HEAT AS A GROUNDWATER TRACER

Temperature measurements and heat transport are of interest in many disciplines in Earth Sciences. In geothermics, perturbations in the thermal profile are regarded as anomalies that might be explained by disturbance by groundwater flow. Temperature profiles affected by groundwater advection are either discarded or corrections are applied to remove the effects. Hydrogeologists, on the other hand, are interested in analyzing the anomalies to delineate groundwater recharge and discharge areas, estimate recharge/discharge rates independently of head data and constrain estimates of hydraulic conductivity. Additionally, heat carried by groundwater serves as a tracer to identify surface water infiltration from streams, lakes, and wetlands, and flow through fractures. Temperature measurements are also used to delineate flows in the mixing zone beneath streams (the hyporheic zone) and to estimate submarine groundwater discharge and depth to the salt-water interface in coastal aquifers.

The fundamentals of using heat as a groundwater tracer were published in the 1960s but recent work has significantly expanded the application to a variety of hydrogeological settings including both basin and site scale applications. A powerful application involves using temperature data in solutions of the inverse problem to estimate groundwater flow and hydraulic conductivity with coupled groundwater- and heat-flow models. Much work along these lines has been done using temperature fluctuations in streams as the driver of temperature fluctuations in groundwater. These studies build on the seminal work of Stallman (1965, Journal of Geophysical Research 70, no. 12: 2821–2827) who used a one-dimensional, transient analytical solution of the heat flow equation with sinusoidal variation of temperature at the land surface to estimate vertical groundwater velocity. Recent extensions of this type of analysis to wetland systems show that daily fluctuations in surface temperatures provide sufficient signal to help constrain solutions of the groundwater flow equation.