Electrical Resistivity Dynamics beneath a Fractured Sedimentary Bedrock Riverbed in Response to Temperature and Groundwater / Surface Water Exchange

Bedrock rivers occur where surface water flows along an exposed rock surface. Fractured sedimentary 1 bedrock can exhibit variable groundwater residence times, anisotropic flow paths, heterogeneity, along with 2 diffusive exchange between fractures and rock matrix. These properties of the rock will affect thermal transients in 3 the riverbed and groundwater-surface water exchange. In this study, surface electrical methods were used as a non4 invasive technique to assess the scale and temporal variability of riverbed temperature and groundwater-surface 5 water interaction beneath a sedimentary bedrock riverbed. Conditions were monitored on a semi-daily to semi6 weekly interval over a full annual period that included a seasonal freeze-thaw cycle. Surface electromagnetic 7 induction (EMI) and electrical resistivity tomography (ERT) methods captured conditions beneath the riverbed 8 along a pool-riffle sequence of the Eramosa River in Canada. Geophysical datasets were accompanied by 9 continuous measurements of aqueous specific conductance, temperature and river stage. Time-lapse vertical 10 temperature trolling within a lined borehole adjacent to the river revealed active groundwater flow zones along 11 fracture networks within the upper 10 m of rock. EMI measurements collected during cooler high-flow and warmer 12 low-flow periods identified a spatiotemporal riverbed response that was largely dependent upon riverbed 13 morphology and seasonal groundwater temperature. Time-lapse ERT profiles across the pool and riffle sequence 14 identified seasonal transients within the upper 2 m and 3 m of rock, respectively, with spatial variations controlled 15 by riverbed morphology (pool verses riffle) and dominant surficial rock properties (competent verses weathered 16 rock rubble surface). While the pool and riffle both exhibited a dynamic resistivity through seasonal cooling and 17 warming cycles, conditions beneath the pool were more variable largely due to the formation of river ice during the 18 winter season. We show that surface electrical resistivity methods have the capacity to detect and resolve electrical 19 resistivity transience beneath a fractured bedrock riverbed in response to porewater temperature and specific 20 conductance fluctuations over a complete annual cycle. 21 22


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Rainfall was recorded at the University of Guelph Turfgrass Institute, located 2 km northwest of the site, while 218 snowfall accumulation was obtained from the Region of Waterloo Airport roughly 18 km south west of the site.

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Hourly mean river flux was recorded 900 m upstream at the Watson Road gauge operated by the Grand River 220 Conservation Authority. A summary of the weather and river flux data are provided in Fig. 4.

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Riverbed electrical resistivity distribution was initially measured using a Geonics EM-31 ground conductivity meter 224 (Geonics, Mississauga, Canada) during a seasonally cool and warm period: early-spring/high-stage conditions on 3-225 Apr-2013 and mid-summer/low-stage conditions on 7-Jul-2014. Measurements were collected at a rate of 3 226 readings per second with the device operated in vertical dipole mode held ~1 m above the riverbed. The effective 227 sensing depth of this instrument in vertical dipole mode is approximately 6 m, and is minimally sensitive to 228 conditions above the ground surface (McNeil, 1980). Data was recorded along roughly parallel lines spaced ~1.75 229 m apart orthogonal to the river orientation, with the coils aligned parallel to surface water flow direction. Water

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Hydrol. Earth Syst. Sci. Discuss., doi:10.5194/hess-2016-559, 2016 Manuscript under review for journal Hydrol. Earth Syst. Sci. Published: 1 November 2016 c Author(s) 2016. CC-BY 3.0 License. depths over the investigated reach varied from <0.1 m in the riffle during low-flow to nearly 1 m in the pool during 231 high-flow conditions. Data sets were filtered for anomalous outliers prior to minimum curvature gridding.

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For this study, resistivity cables were constructed using a pair of 25 multicore cables (22 gauge strained wire, 600V 239 rating) wound within a PVC jacket. The PVC jacket was split open every meter to expose and cut out a single wire 240 that was connected to an audio-style banana plug. Spliced sections of outer PVC jacket were resealed using heat 241 shrink tubing and silicon. This process resulted in two 24 channel cables each connected to a single multi-pin 242 connector for direct data logger communication. Electrodes were constructed from half-inch diameter stainless steel 243 rod cut to 6 inch lengths. A hole was drilled on one end of the electrode to receive the banana plug connector.
244 Given the exposed bedrock across the site, a half-inch hole was drilled into the rock at 1 m intervals along the 245 ground surface. In some cases, electrodes were buried beneath a rubble zone of the riverbed, or were pushed into a 246 thin layer of sediment. On the shorelines electrodes were fully implanted into the rock along with a few teaspoons 247 of bentonite clay to minimize contact resistance. Each monitoring line was instrumented with dedicated electrodes 248 and cables that remained in place for the duration of the study.

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Resistivity measurements were recorded using a Syscal Junior Switch 48 (Iris Instruments, Orléans, France) 250 resistivity meter. A Wenner array was selected for its higher S/N ratio. A dipole-dipole array was tested, but found 251 to be very susceptible to noise (i.e., excessive number of bad data points due to low measured potentials); this was 252 attributed to the high-contact resistances with rock combined with the instruments moderate power capability (max 253 400 V, 1.3 A). Although the Wenner array geometry results in a stronger signal (i.e., potentials are measured across 254 a pair of electrodes located between the current electrodes with an equal inter-electrode spacing), it will be less 255 sensitive to lateral variations across the riverbed compared to the dipole-dipole array, and thus, less sensitive to the 256 presence of a single or package of vertical fractures between adjacent electrodes. Surface resistivity data were 257 recorded on a semi-daily to semi-weekly interval from 18-Jul-2014 to 3-Jul-2015 covering a complete annual cycle, 258 which included a seasonal freeze-thaw cycle, and numerous wetting-drying events accompanied by large river stage 259 fluctuations. The timing of resistivity measurement events are shown with corresponding river flow rates and 260 atmospheric data in Fig. 4. Resistivity measurements were generally recorded between 8 AM and 1 PM.

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Measured apparent resistivity data was manually filtered to remove erroneous data points prior to being inverted  fractures terminate at surface as vertical joint sets along two regional orientations: 10° to 20° NNE and 280° to 290° 290 SNW (Fig. 3b, ii). Matrix porosities from the corehole were relatively low, ranging from 0.5 % to 5 %, with the 291 lowest porosities observed along the highly weathered riverbed surface and lower portion of the Eramosa Formation.

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Hydraulic head data collected in the river and at the base of SCV6 (10.5 m bgs) suggest a seasonally sustained 293 upward vertical gradient (i.e., groundwater discharge zone) at the pool.

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Spatial electrical resistivity data were highly variable across the pool (Fig. 11a-h). The highest resistivities were 361 observed along the south shoreline, which coincided with the presence of competent bedrock (Fig. 3b, i)

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Although seasonal geoelectrical dynamics were observed at both the pool and riffle, the pool was more transient and 523 exhibited a broader range, yet spatially more uniform distribution in resistivity. Conversely, the riffle exhibited more 524 lateral variability in resistivity along across the riverbed. Seasonal cooling was accompanied by a higher-resistivity 525 zone emanating from the south shore to north shore in both the pool and riffle. This resistivity trend reversed during 526 the seasonal warming cycle, becoming less-resistive toward the south shoreline as seasonal temperatures increased 527 and river flow decreased. The formation of ground frost and basal ice along the riverbed had a strong and 528 sometimes negative impact on the seasonal resistivity profiles during the winter months. Intraseasonal geoelectrical 529 transience associated with major precipitation events, which were accompanied by short-period perturbations in 530 surface water temperature and specific conductance had a relatively small impact on riverbed resistivity. This may 531 be explained by the presence of a strong groundwater discharge zone across this reach of the river, which may have 532 limited or moderated the electrical resistivity changes within the suspected groundwater-surface water mixing zone.

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Time-lapse temperature profiling within the angled borehole underlying the river revealed active groundwater flow 534 zones. While our resistivity measurements captured geoelectrical dynamics within the upper few meters of riverbed, 535 these data are indirect evidence of a groundwater surface water mixing zone; whether the observed geoelectrical 536 transience are primarily a function of seasonal temperature fluctuations or transience in ionic concentration, in 537 response to precipitation events, will require further investigation. Nevertheless, our study demonstrates that surface 538 electrical resistivity has the capacity to detect and resolve changes in electrical resistivity within a bedrock riverbed.

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Given the complex fracture distribution, geometry and dissolution features inherent to sedimentary rock, surface 540 resistivity methods may be most effective in the initial design and placement of more direct measurement methods, 541 thereby reducing instrumentation costs and impacts to ecosensitive environments.

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The data used in this study are presented in the figures. Complete monitoring data sets (Figures 10 and 11) and can 544 be made available upon request from the corresponding author.