Fractured aquifers are heterogeneous due to the variable frequency, orientation, and intersections of rock discontinuities. A 100-m-thick Silurian dolostone sequence provides a bedrock aquifer supplying the city of Guelph, Canada. Here, fracture network characteristics and associated influences on hydraulic head were examined using several data types obtained from 24 cored holes in a study that is novel for the quantity and quality of data. High (50–90°) angle joint orientations, heights, and terminations relative to bedding features were determined from acoustic televiewer logs and outcrop scanlines. These data were compared to high-resolution hydraulic head profiles showing head loss over depth-discrete intervals identifying zones with lower vertical hydraulic conductivity. This study reveals that the marl-rich Vinemount Member, traditionally considered the principal aquitard, corresponds to head loss in only 62

Buried bedrock valleys are common erosional features in northern mid-latitude environments that form through glaciofluvial or paleoalluvial processes and are typically infilled by Quaternary-aged sediments. The erosional extent and geometry of the valley including a weathered interface, along with sediment infill that can contain complex sequences of unconsolidated aquifer and aquitard sediments, mean these features may act as preferential pathways to deeper bedrock aquifers. Noninvasive geophysical tools can provide rapid, high-resolution subsurface characterization of these features. This study evaluates the application of electrical resistivity and seismic refraction tomography along two transects centred over a buried bedrock valley in Elora, Ontario, Canada. Geophysical measurements were combined with existing continuous core records and an electrofacies model based on downhole geophysical logs to constrain the morphology and infilled lithostratigraphic architecture of the valley. Bedrock competency associated with lithology may act as a control on depth and width of valley incision during erosion, with resistivity measurements of the bedrock revealing a potential association between interpreted mechanical properties and variations in the resolved valley morphology. Seismic velocity corroborated these contrasting valley widths but could not assess bedrock competency variability below the bedrock interface. This study reveals the sequence of events depositing sediments in the valley, yielding a revised architectural mapping that improves on previous regional scale lithostratigraphic interpretations. Results will be of use to groundwater practitioners requiring detailed conceptualization of this buried bedrock valley and its role on preferential zones of groundwater flow. Similar approaches can be used for delineation of these common and hydrogeologically significant features.

Predicting contaminant transport in groundwater requires an accurate representation of the subsurface geology controlling the spatial distribution of hydrogeologic parameters. Developing accurate geological models for sedimentary systems relies on quality sedimentological data collected from cores. Standard logging forms used to collect data from cores create a persistent data gap in hydrogeology because they hinder efficient collection of high-quality sedimentological data. These logging forms require time-consuming text descriptions of sedimentological characteristics and often result in inconsistent, poorly resolved data insufficient to support realistic geological models. We describe a graphical approach to core logging, the graphical shading log, that facilitates rapid, accurate capture of sedimentological data and a complementary database to store the raw data and interpretations. The visual format of the graphical shading log provides a roadmap of the parameters to log and their possible values, helping to ensure accurate and consistent data collection by loggers with a range of experience. Examples from sites with contaminated groundwater in glaciogenic sediments and siliciclastic and carbonate bedrock show how data from the graphical shading logs improved geological interpretations, supported the design of high-resolution multilevel systems needed to collect minimally blended hydrogeologic data, and helped to more accurately delineate hydrogeologic units. The format of the graphical shading log and complementary database are designed to be customizable and transferable between hydrogeologic settings providing a new tool to advance geological data collection and management. Improved sedimentological data and insight are critical inputs for process-based conceptual site models needed to effectively manage contaminant plumes in the subsurface.

The Liard Basin, in northwestern Canada, contains one of Canada’s largest natural gas reserves. There are concerns about degradation of Dunvegan Formation groundwater quality, which is the basin's main shallow freshwater aquifer, as a consequence of oil and gas development. Previous lithostratigraphic studies interpreted the Dunvegan as conglomeratic alluvial fans. In this study, newly collected cores and surface resistivity surveys are integrated with legacy hydrocarbon well data and measured sections for improved sedimentary bedrock characterization. Rather than alluvial fans, we interpret the upper Dunvegan as a coarse-sand to gravel bed fluvial system that was likely tributive to incised valleys associated with linked Alberta Basin base-level falls. Correlation of well logs and measured sections suggest the lower Dunvegan comprises a prograding delta complex with off-lapping clinoforms fed by smaller sand-bed rivers that grade into the upper Dunvegan conglomeratic system, which built southeast towards the Cretaceous Western Interior Seaway. This sequence stratigraphic and facies architectural approach is employed to inform the aquifer–aquitard flow system and aid in understanding the 3D groundwater flow system, once hydrogeologic characterisation confirms position and thickness of important hydrogeologic units. Resistivity surveys suggest upper Dunvegan channel-belt facies may be well-connected and laterally extensive, potentially acting as interconnected aquifer units; whereas lower Dunvegan channel belts may be discontinuous, indicating smaller-scale flow units separated by floodplain and marine shale. This revised geologic understanding provides opportunities for determining impacts or resilience from shallow and deep sources of contamination associated with human activities, including oil and gas development, land-use, and (or) climate change.

This study applies innovative methods to characterize and quantify the magnitude of groundwater flow in a fractured and variably cemented sandstone aquifer to inform an in-situ remediation strategy for trichloroethene (TCE) contamination. A modified active-distributed temperature sensing (A-DTS) approach in which fiber optic cables were permanently grouted in the borehole was used to quantify groundwater flow rates. Two additional tracer tests were conducted: 1) fluorescein tracer injection followed by rock coring and sampling for visual mapping and porewater analysis, and 2) deployment of passive flux meters in conventional monitoring wells to evaluate groundwater velocity and mass flux distributions. Forced gradient injection of fluorescein tracer sug-gests a dual porosity flow system wherein higher rates of groundwater flow occur within discrete features including highly permeable bedding planes and fractures, with slower flow occurring within the rock matrix. Tracer was observed and detected in the unfractured matrix porewater >1.5 m away from the injection well. Beyond this distance, >6 m radially away from the injection hole, tracer was primarily detected within and adjacent to high transmissivity fractures serving as preferential flow paths. The Darcy flux calculated using active distributed temperature sensing (A-DTS) shows depth-discrete values ranging from 7 to 60 cm/day, with average and median values of 23 and 17 cm/day, respectively. Passive Flux Meters (PFMs) deployed in three conven-tional monitoring wells with slotted screens and sand filter packs showed groundwater flux values ranging from 2 to 11 cm/day, with an overall average of 4 cm/day and are likely biased low due to spreading in the sand pack. The study results were used to inform an in-situ remediation system design including the proposed injection well spacing and the amendment delivery approach. In addition, the results were used to build confidence in the viability of delivering an oxidant to the rock matrix via advective processes. This is important because 1) the matrix is where the majority of the TCE mass occurs, and 2) it provides insights on processes that directly affect remedial performance expectations given advective delivery to preferential pathways and the matrix overcomes diffusion only conditions.

Efficiently measuring groundwater flow in bedrock aquifers is inherently challenging due to the irregular distribution and fine scale of fractures. Recent advances in Active Distributed Temperature Sensing (A-DTS) in boreholes temporarily sealed with liners have made it possible to quantify flow rates in such aquifers at many different depths using heat as a tracer, but until now only data collected under a single hydraulic condition have been published. This paper presents the first field data from multiple A-DTS field tests conducted under different hydraulic conditions to quantify groundwater flow redistribution within a bedrock aquifer. Three separate quasi steady state A-DTS tests were collected in a sealed borehole: (1) natural gradient condition where all boreholes were sealed with flexible and impermeable liners, (2) cross-connected condition where a nearby borehole was open allowing vertical flow within the borehole, and (3) forced gradient condition where the nearby open borehole was pumped at a constant rate of 54 L/min. The depth-discrete hydraulic head responses were also measured during the three tests using a string of transducers in a sealed borehole. Results provide quantifiable insights as to how the bedrock aquifer responds, including A-DTS-derived measurements of flow changes in fractures at multiple depths driven by changes in gradients. The results confirm that a single open borehole or long-screened well can significantly alter the site hydraulics and demonstrate that not all large or transmissive fractures show evidence of active flow and thus, transmissivity and aperture should not be used alone to infer active flow zones.

Detection and quantification of groundwater flow in fractures is challenging due to its irregular distribution and fine scale, requiring intensive and depth‐discrete field data collection along boreholes. This study presents a new method using fiber optic active distributed temperature sensing (A‐DTS) in sealed boreholes to efficiently quantify depth‐discrete flow rates along the full length of a bedrock borehole. The method combines field data and numerical modeling to quantify groundwater flow rates under natural gradient conditions, which is important for assessing groundwater flow and contaminant transport. An empirical relationship between enhanced heat dissipation and groundwater flow rates is determined using a numerical model of groundwater flow and heat transport for a system of idealized parallel plate fractures in a homogeneous porous rock with negligible flow through the rock matrix. The empirical relationship is applied to a detailed profile of apparent thermal conductivity measured using A‐DTS that combines the effect of rock thermal properties and groundwater flow. In zones with no flow, the A‐DTS‐derived apparent thermal conductivity matches the laboratory effective rock thermal conductivity values measured independently. Local increases of A‐DTS apparent thermal conductivity relative to the rock matrix thermal conductivity can be used to estimate groundwater flow rates using the empirical relationship. The results are in reasonable agreement with straddle pacer tracer dilution tests in the same borehole, which helps to validate the approach. This new approach allows identification of active flow zones and quantification of flow rates and can be efficiently applied in single or multiple boreholes.

Vertical seismic profiles (VSPs) collected using fiber optic distributed acoustic sensors (DAS) are becoming increasingly common; yet, ensuring good cable coupling with the borehole wall remains a persistent challenge. Traditional cable deployment techniques used in the petroleum industry are either not possible or do not provide data of sufficient quality for shallow applications. Additionally, no direct field comparison of coupling techniques in the same borehole exists to determine the impacts of poor coupling on DAS VSP data quality. This paper addresses these issues by: (1) presenting a novel cable coupling solution using a removable and relatively inexpensive FLUTe™ flexible borehole liner; and (2) presenting field examples of DAS VSPs under different coupling conditions. The proposed coupling technique is analogous to a fully cemented deployment in that the cable is continuously coupled directly to the formation. Field experiments conducted to assess and validate the technique demonstrate a marked improvement in VSP data quality when the cable is coupled with a flexible borehole liner. Without the liner, seismic profiles are dominated by a high-amplitude cable wave and the p-wave arrival is not observed; however, with cable coupling provided by a borehole liner inflated using hydrostatic pressure, the cable wave is suppressed and clear p-wave arrivals are visible. Additional tests examining the influence of fiber optic cable structure on seismic responses demonstrate that tight buffered fibers are more sensitive to dynamic strain than loose tube fibers making them potentially better suited for certain DAS applications.

Understanding agricultural contamination in bedrock aquifers is challenging due to complexity of fracture networks and limitations in data acquisition imposed by instrumentation and drilling costs. Engineered assemblages known as multilevel monitoring systems (MLSs) maximize data from each borehole by providing numerous, depth-discrete monitoring intervals for profiles of hydraulic head and hydrochemistry. This article describes a hybrid MLS that uses key components of the Waterloo MLS, with extra piezometers of sufficient diameter to accommodate removable transducers for continuous pressure monitoring, attached to the outside using custom clamps. Monitoring intervals are created with sand packs separated by bentonite seals, either via backfilling from surface or tremie placement. The hybrid MLS is best suited for use in rotary drilled boreholes 12 to 15 cm diameter, smaller holes have insufficient space for the MLS and added piezometer(s) while larger holes have excessive backfill material and purge requirements. Variations were installed to 60 m depth in sandstone in Prince Edward Island, and to 150 m in dolostone in southwestern Ontario. Transducers in the external piezometers provided temporal head data under ambient and stressed conditions in key intervals, and manual measurements in all ports provided detailed vertical hydraulic snapshots. Combined with hydrochemistry profiles from groundwater sampling, the hybrid MLS provided detailed composite datasets for interpreting flow and transport in these bedrock aquifers. The hybrid MLS offers promise as a versatile low-cost option for groundwater studies in agricultural areas providing improved insights on groundwater flow systems and vertical distribution of nitrate and other contaminants, allowing more informed management decisions.