A Comparison of Thermal Imaging and In-Situ Thermistor/Tensionmeter Data to Characterize Groundwater Seepage in a Fen

Groundwater flow and its dissolved mineral transport plays a fundamental role in the ecology of many wetlands. Installation of thermistors and tensiometers to map groundwater seepage, however, is invasive and may damage vegetation and potentially affect biodiversity. By mapping surface temperature remotely in the late summer, when the differential between warm soil and cold groundwater is the greatest, we hypothesize that the temperature patterns will reveal areas of greatest upward gradient and flow.

To test the hypothesis, we monitored the effect that hydraulic gradient has on surface temperatures in a fen located at the north end of the Cherry Lake Aquifer, Eddy County, ND (47.73, -98.66). On-the-ground thermal imaging was used to map seepage, with results compared to conventional method of installing shallow ceramic cup tensiometers to measure hydraulic gradient, and estimate flux using Darcy’s law. Shallow temperature loggers (thermistors) were installed to characterize soil temperatures at the same sites. The approach was applied at contrasting two locations: a sedge-cattail covered (Sedge site) and a nearby shady willow- cordgrass covered (Willow site).

The Sedge site showed strong upward gradient whereas the Willow site showed variable gradients, perhaps related to greater transpiration. Temperature observations and trends determined from the thermal imagery and thermistors did not show a relationship to hydraulic gradients measured at either site, suggesting variability due to heterogeneity of hydraulic conductivity (K). Thus, application of thermal imaging to map groundwater discharge requires data on soil stratigraphy.

We used both forward and inverse modeling of temperature profiles, which is based on a one-dimensional solution to the advection-conduction equation (Kurylyk et al. 2017), to more thoroughly characterize the shallow variation of hydraulic conductivity. Coupled with additional field data on temperature distribution, gradient, and conductivity, we were able to map the fen seepage face.

The gradients are affected at some depth because of the varying soil stratigraphy. The groundwater is simply fanned out along the bed rather than mixing to the surface water, which defines the reason why the seepage faces cannot be mapped completely using thermal imaging. Conventional method could measure vertical flux of the location because it is calculated based on depth of the soil distribution than on the spatial playout.