A FIELD STUDY OF A BEDROCK RIVER USING TEMPERATURE TO MEASURE GROUNDWATER DISCHARGE ALONG DISCRETE FRACTURES

Bedrock rivers occur when surface water flows along an exposed bedrock streambed. Their channels contain few alluvial sediments and their floodplains often lack the attenuative protection of an overburden. The characteristics of groundwater – surface water interaction (GW – SW) in bedrock rivers have not been addressed in the discrete fracture network (DFN) context, likely due to the lack of methods for measuring parameters at the appropriate spatial and temporal scales necessary without undue disturbance to these ecologically-sensitive environments. Temperature, as a natural, non-invasive groundwater tracer, has been used in a range of borehole and surface applications to measure groundwater discharging into a sedimentary bedrock river in Guelph, ON. A 2400 m²field site was selected along the Eramosa River, where a dolostone pavement streambed extends up on to the adjacent floodplain, displaying mappable vertical joint sets at surface.

In the DFN context, groundwater flow occurs along the fractures, controlled by fracture aperture, orientation and connectivity. Streambed fractures were mapped and transducers were distributed along fracture transects to collect high-resolution spatial and temporal temperature and head data above, within and below the streambed. Small-diameter (<75 mm) portable drills were used to create three corehole pairs drilled in the bedrock floodplain to a depth of 30 mbgs, with each pair consisting of one inclined and one vertical hole. The purpose of the inclined holes, plunging at 60° with azimuths 50°, 195° and 340° was to inform the 3-D fracture network geometry beneath and adjacent to the channel. Temperature logs were collected within the static water column of coreholes sealed with FLUTe™ liners to measure ambient conditions in the bedrock without the effects of borehole cross-connection. These temperature profiles, combined with core logs (lithology and feature), geophysical logs and depth-discrete hydraulic test results were used to build a 3-D fracture network model that will inform the interpretation of GW-SW interaction along discrete fracture pathways and the physical-hydrochemical conditions observed along this section of river channel.