ASSESSING LATERAL AND VERTICAL HYDRAULIC CONNECTIVITY IN A MULTI-LAYERED CARBONATE BEDROCK AQUIFER USING DYNAMIC HYDRAULIC HEAD MONITORING

Multiple high-resolution characterization techniques were implemented at a site in Erin, Ontario to parameterize distinct hydrogeologic units and identify the position and thickness of aquitards within the carbonate sequence. Data from continuous core, geophysical/hydrogeophysical logging and removable pore-pressure monitoring were collected in two bedrock boreholes (MW13A-20 and MW14A-20). These data were used to design depth-discrete multilevel monitoring systems (MLS) each completed with seven ports. Select ports from the MLS are a part of a 3D monitoring network about 430 and 380 m away, due west and south, from a permitted water supply well completed in a shallow fractured bedrock aquifer. Prior to MLS completion, the core holes were instrumented with a total of 48 pressure transducers at targeted depth-discrete intervals along the open borehole lengths (n = 26 in MW13A-20, n = 22 in MW14A-20) and sealed behind a FLUTe^TM liner to obtain high-frequency (1 second) hydraulic head and dynamic head responses.

Water levels are sensitive to fluctuations in natural and anthropogenic stresses that create noise in the water level and can make estimating hydraulic properties of the subsurface difficult. Signals from the pumping well, barometric pressure and Earth tides were removed using regression with elastic net regularization in a deconvolution model. Response functions were then generated from the model output and used to estimate aquifer properties. Preliminary results suggest the two bedrock MLS respond differently to the nearby pumping well. Monitoring location MW13A-20 to the west shows a classic Theis confined response in each of the monitoring depths, with distinct response times and a decrease in magnitude with depth below the pumping interval. In contrast, MW14A-20 to the south displays a leaky aquifer response, suggesting lateral variability in the overlying/underlying vertical hydraulic conductivity. Separate studies show no impact to surficial features. Our study shows important insights regarding vertical hydraulic connectivity, the position and thickness of aquitards, and laterally variable confinement in a multilayered carbonate rock system in vertical detail at more than one location. This can in turn improve assessment of pumping influences within spatially complex 3D flow systems.