Date of Award
Master of Science (MS)
Philip J. Gerla
Groundwater flow and its dissolved mineral transport plays a fundamental role in the ecology of many wetlands. Installation of equipment 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, the temperature patterns may reveal areas of greatest upward gradient and flow.
To test the hypothesis, the effect that hydraulic gradient has on surface temperature in a fen located at the north end of the Cherry Lake Aquifer, Eddy County, ND (47.73, -98.66) was monitored and measured. Thermal imaging was used to characterize groundwater seepage, the results were compared to conventional method of installing shallow ceramic cup tensiometers to measure hydraulic gradient, and estimate flux using Darcy’s law. Shallow temperature loggers were installed to characterize soil temperature at the same sites. The approach was applied at contrasting two locations: a sedge-cattail covered site (Sedge) and a nearby site with cordgrass and closed-canopy shrubs and trees (Willow).
Both sites showed variable hydraulic gradients between the shallow and deep tensiometer, perhaps related to variation in transpiration. The temperature trend determined from the thermal imaging showed a closer relationship to hydraulic gradients measured at the Sedge site more than at the Willow site. The hydraulic conductivity, K ranged from 6 × 10-6 to 2 × 10-4 m/s for both sites, which falls within values typical for fen sediments. The flux calculated for the Willow site ranged from 1.4 × 10-5 to 1.1 × 10-4 m/s and that of the Sedge site ranged from 4.5 × 10-6 to 1.1 × 10-5 m/s. Willow site thermal imaging did not show similar trend with the hydraulic gradient, suggesting tree cover can affect thermal signature at the surface. Temperature profile observations from the thermal aerial imagery and the FLIR C2 camera showed a similar trend.
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), were used to more thoroughly characterize the shallow variation of flux compared to thermal imaging, coupled with additional field data on temperature distribution, thermal conductivity, depth, and layer thickness. Accounting for soil layer properties plays a role in characterizing groundwater seepage direction and rate.
The gradients are affected at some depth because of the varying soil stratigraphy, which explains the reason why the seepage faces cannot be mapped completely using thermal imaging at these sites.
Ozotta, Ogochukwu, "Using Thermal Imaging To Characterize Groundwater Seepage In A North Dakota Fen" (2018). Theses and Dissertations. 2424.