By Karl E. Butler, Ph.D., P.Geo., P.Eng.
Department of Earth Sciences
University of New Brunswick
n the spring of 2021, Earth Science departments all over the world grappled with how to offer field schools during the Covid-19 pandemic. At the University of New Brunswick, in Fredericton, NB, Canada, we were ultimately permitted to proceed in-person with special measures in place. For our geoenvironmental field school, this included changing the site from a closed coal mine 70 km away to a river valley aquifer just steps from campus – eliminating the need to transport students in vehicles. While health concerns were top of mind, the change also conveniently offered much better geology for teaching students how to use ground penetrating radar (GPR).
With support from water supply staff at the City of Fredericton, a geochemist colleague and I tasked students with investigating the impact of urban runoff on groundwater within one of the City’s well fields. The field work involved three components: (i) measuring water levels in wells to infer groundwater flow directions, (ii) hydrogeochemical sampling of the wells to seek evidence of urban runoff including road salt and nitrate, and (iii) hydrogeophysical surveying to better define ‘windows’ through a discontinuous clay-silt aquitard that could allow surface runoff to reach the underlying sand and gravel aquifer. While the study did not reveal any concerns related to runoff, the exercise was a success for giving students the opportunity to learn field work skills. This article highlights some of the hydrogeophysical activities.
Hydrogeological Setting
The Fredericton Aquifer is a sinuous esker-like deposit of sand and gravel, buried within glacial and post-glacial sediments up to 60 m thick, that partially fill the modern Saint John River Valley. The aquifer is semi-confined, overlain by a clay-silt aquitard that has been eroded by past meanderings of the Saint John River, exposing high points along the buried esker ridge to create so-called ‘windows’. Where these windows underlie the river, they are beneficial – permitting recharge of water pumped by the municipal well fields. On the other hand, windows into the aquifer that underlie the city present vulnerabilities from the perspective of possible contamination.
Hydrogeophysical Surveys
The choice of geophysical survey methods was influenced by what had worked well previously for delineating windows below the river. In that case, waterborne apparent conductivity mapping had been augmented by electrical resistivity imaging (ERI) along the shore, and acoustic sub-bottom profiling on the water. In this field school, operating on land, GPR profiling (Figure 1) was substituted for acoustic sub-bottom profiling – providing similar high resolution stratigraphic information that could not be provided by the other two conductivity/resistivity-based methods. Here, I focus on showing the complementarity of apparent conductivity and GPR surveys.
Figure 2 shows a map of apparent conductivity, acquired to better define the limits of a window known to underlie a park and rugby field in the vicinity one of the City’s well fields. The data were acquired using an EM31 conductivity meter having an effective depth of exploration of approximately 6 m, making it useful to search for the presence or absence of the electrically conductive (~ 35 – 50 Ωm) clay-silt unit which is typically (where present) buried below 4 – 5 m of much more resistive fluvial sands. The abrupt change in apparent conductivity near the north end of the rugby field is interpreted to represent the northern edge of a clay-silt window in this area, as suggested by nearby boreholes.
Students were asked to verify this interpretation by acquiring a series of GPR and ERI survey lines along the length of the rugby field. The GPR data were acquired using our Sensors & Software pulseEKKO® system with the recently released Ultra receiver (Figure 1).
Figure 3 shows sample GPR profiles acquired with 100 MHz (top) and 50 MHz (bottom) antennas along line RF5. On the left (northern) end of the line, there is a prominent reflector at ~ 3 m depth corresponding to the erosionally defined top of the clay-silt aquitard – or perhaps, more likely, the top of the perched water table in the fluvial sands above that unit. Also on the left, there is second reflector which rises from ~ 7 m depth towards the south until it is truncated at the top of the clay-silt. We interpret this as a layer within the silt, exhibiting drape on the adjoining, underlying esker – something also seen in acoustic sub-bottom profiles beneath the Saint John River. On the south (right) side of the line, at positions greater than 50 metres, the clay-silt layer is absent, revealing a prominent flat water table reflection at 7 m depth within the sand and gravel esker aquifer.
Farther west in the municipal park, apparent conductivity mapping gave a rough idea of the position of the southern edge of the aquitard window, but the change in conductivity was much more gradational (Figure 2). In this area, the depth information, and the geometry of subsurface reflectors evident in GPR data (Figure 4) were especially helpful for delineating the position of the window edge. GPR surveys at this location included profiles collected using shielded 250 MHz center frequency bistatic antennas in addition to the 100 MHz center frequency unshielded antennas. While the higher center frequency, 250 MHz, shielded antennas were convenient when working in the vicinity of the metal fences and bleachers around the edges of the baseball field, the reflectors of most interest are more coherent in the lower center frequency, deeper penetrating 100 MHz data.
Students learned to operate the GPR system quickly and enjoyed seeing these clear profiles scroll across the display unit. By interpreting their own data, students gained appreciation for the importance of good field practices. They also learned how one method can be very useful to resolve ambiguity in the interpretation of another method. More generally, the hands-on experience will give them more confidence to apply geophysical methods to help resolve geological questions in their professional careers.
Also see:
Butler, K.E., Nadeau, J.-C., Parrott, R., and Daigle, A., 2004, Delineating recharge to a river valley aquifer by riverine seismic and EM methods. J. Environmental and Engineering Geophysics, 9, 95-109.
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