Investigating new approaches for mapping groundwater systems in karstic carbonate bedrock: A case study in the Early Silurian formations of the Niagara Escarpment cuesta, southern Ontario, Canada
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The Ontario Geological Survey conducts geoscience mapping that is designed to support groundwater resource investigations across the province. Recent geological mapping in the Early Silurian carbonate bedrock in southern Ontario has revealed significant geological complexity. Such complexity and the resultant heterogeneity prompted the Ontario Geological Survey to fund this research, which comprises the development of new approaches for improving groundwater characterization in these settings. This research is motivated by the desire to more successfully identify groundwater resource exploration targets and to more effectively map large, regional groundwater flow systems in karstic carbonate bedrock. This thesis is organized in two research themes. The first theme focusses on understanding the spatial relationships between hydraulic conductivity and geology, while the second theme comprises an investigation into geochemical tools and modeling for regional-scale flow systems characterization. The first challenge that presented itself upon initiation of this research was the poor spatial coverage of hydraulic conductivity values (K) relative to the geological heterogeneity of the karstic, carbonate bedrock. Spatial coverage of K values for large-scale groundwater investigations is often poor because of the high costs associated with hydraulic testing and the large areas under investigation. Domestic water wells are ubiquitous and their well logs represent an untapped resource of information that includes mandatory specific capacity tests, from which K can be estimated. These specific capacity tests are routinely conducted at such low pumping rates that well losses are normally insignificant. In this study, a simple and practical approach to augmenting high-quality K values with reconnaissance-level K values from water well specific capacity tests was assessed. This assessment was conducted by making comparisons at two different scales: study area-wide (600 km2) and in a single geological formation within a portion of the study area (200 km2). Results of the comparisons demonstrate that reconnaissance-level K estimates from specific capacity tests approximate the ranges and distributions of the high-quality K values. Sufficient detail about the physical basis and assumptions that are invoked in the development of the approach are presented so that it can be applied with confidence by practitioners seeking to enhance their spatial coverage of K values. The large set of varied-quality, yet vetted, K values were then integrated with the geologic characterization of the carbonate bedrock to assess the relative influence of specific geologic features on hydraulic conductivity. Three geologic controls were investigated: i) proximity to bedrock valleys, ii) carbonate rock texture and iii) sequence stratigraphic breaks. Results demonstrate that high K values do not correlate with a single geological feature, but that they are associated with various features that have been enhanced by carbonate dissolution. Predicting the spatial distribution of K in carbonate rocks requires a regional understanding of the geological history with a conceptualization of where and when waters have interacted with the bedrock to dissolve and enhance porosity through geological time. This investigation concludes with a map identifying the area with the greatest probability of encountering high K in the main hydrostratigraphic unit, the Gasport Formation. The map supports the selection of groundwater resource exploration targets for a local municipality. Beyond the local benefits of this work, this investigation offers an approach that can be adopted by practitioners exploring for bedrock groundwater resources or characterizing contaminant transport pathways in complex, karstic carbonate bedrock groundwater systems. The second research theme of this thesis represents a shift towards exploring the use of geochemical tracers and modeling tools for mapping regional groundwater flow systems in the same karstic carbonate bedrock setting investigated in the first research theme. Recharge timing and controls were investigated using several isotopic and geochemical indicators of recent recharge to groundwater. Spatial trends of higher tritium are consistent with aerobic redox chemistry in the carbonate groundwater systems underlying areas of thin or permeable sediment cover. Groundwater chemical evolution beyond recharge areas was assessed with general chemistry, redox characteristics and an investigation of water-rock interaction. A comparison of strontium isotope ratios (87Sr/86Sr) in bedrock and groundwater shows that long residence times can sometimes result in the isotopic signature of the rock imprinting on the groundwater. Observed increasing Sr to Ca ratios along the groundwater flow path are likely resulting from incongruent dissolution of dolomite and the precipitation of calcite with evolution. Sulphur isotopic composition of sulphate (δ34SSO4 and δ18OSO4) in groundwater shows isotopic evidence of pyrite oxidation in recharge areas, and a Silurian sulphur isotopic signature where bedrock aquifers are covered by thick and low permeability sediments, downgradient of identified recharge areas. For this investigation, isotopic and hydrochemical tools provided essential lines of evidence towards the development of a conceptual model of recharge and groundwater evolution in this complex geologic setting. The main geochemical, hydrochemical and isotopic processes that form the conceptual model were then simulated with reactive transport modeling. The reactive transport model simulates chemical changes along a hypothetical 50 km flowpath, from recharge to closed-system evolution in the carbonate bedrock aquifer system. Disagreement between simulated and observed values provided insight into possible flaws in the conceptual model. Model calibration was done manually, without the commonly applied statistical improvements of fit and sensitivity analyses. This approach provided an appropriate level of scrutiny, given that the model objective was to quantify and assess the major processes rather than perfectly simulate the field dataset. Magnesium was the only parameter for which the model results gave a very poor approximation of the field data in both concentration levels and general trend direction. Further investigation into the controls on dissolved magnesium is required. Despite this, the poor approximation of one component is considered a positive outcome, given that model contains 16 components that are involved in many coupled hydrochemical, geochemical and isotopic processes. The simulation results indicate that the geochemical, hydrochemical and isotopic processes that form the conceptual model are reasonably well understood throughout the flow system.
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Elizabeth Priebe (2019). Investigating new approaches for mapping groundwater systems in karstic carbonate bedrock: A case study in the Early Silurian formations of the Niagara Escarpment cuesta, southern Ontario, Canada. UWSpace. http://hdl.handle.net/10012/14500