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Oil and gas well leakage is of public concern primarily due to the perceived risks of aquifer contamination and greenhouse gas (GHG) emissions. This study examined well leakage data from the British Columbia Oil and Gas Commission (BC OGC) to identify leakage pathways and initially quantify incident rates of leakage and GHG emissions from leaking wells. Three types of leakage are distinguished: “surface casing vent flow” (SCVF), “outside the surface casing leakage” (OSCL), and “cap leakage” (CL). In British Columbia (BC), the majority of reported incidents involve SCVF of gases, which does not pose a risk of aquifer contamination but does contribute to GHG emissions. Reported liquid leakage of brines and hydrocarbons is rarer. OSCL and CL of gas are more serious problems due to the risk of long-term leakage from abandoned wells; some were reported to be leaking gas several decades after they were permanently abandoned. According to the requirements of provincial regulation, 21,525 have been tested for leakage. In total, 2,329 wells in BC have had reported leakage during the lifetime of the well. This represents 10.8% of all wells in the assumed test population. However, it seems likely that wells drilled and/or abandoned before 2010 have unreported leakage. In BC, the total GHG emission from gas SCVF is estimated to reach about 75,000 t/y based on the existing inventory calculation; however, this number is likely higher due to underreporting.

a b s t r a c tStudy region: The study of groundwater chemistry of the Charlevoix/HauteCôte-Nord (CHCN) region in the province of Quebec in Canada is part of a regional hydrogeological characterization project. Study focus: Groundwater was sampled in 113 wells over the 4500 km 2 study area and analyzed for 39 parameters including major, minor, trace and inorganic constituents, plus stable isotopes 2H and 18O. Two multivariate statistical methods, hierarchical cluster analysis (HCA) and R-mode factor analysis (RFA) were combined with graphic methods to classify the samples according to plausible levels of groundwater evolution in that region. New hydrological insights for the region: Four sample clusters were identified. Cluster 1 is composed of low-salinity Ca-HCO 3 groundwater corresponding to recently infiltrated water in surface granular aquifers in recharge areas. Cluster 4 Na-(HCO 3 -Cl) groundwater is more saline and corresponds to more evolved groundwater probably from confined bedrock aquifers. Cluster 2 and Cluster 3 (Ca-Na)-HCO 3 and Ca-HCO 3 groundwater, respectively, correspond to mixed or intermediate water between Cluster 1 and Cluster 4 from possibly interconnected granular and bedrock aquifers. This study identifies groundwater recharge, water-rock interactions, ion exchange, solute diffusion from marine clay aquitards, saltwater intrusion and also hydraulic connections between the Canadian Shield and the granular deposits, as the main processes affecting the hydrogeochemical evolution of groundwater in the CHCN region.

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An exact, closed-form analytical solution is developed for calculating ground water transit times within Dupuit-type flow systems. The solution applies to steady-state, saturated flow through an unconfined, horizontal aquifer recharged by surface infiltration and discharging to a downgradient fixed-head boundary. The upgradient boundary can represent, using the same equation, a no-flow boundary or a fixed head. The approach is unique for calculating travel times because it makes no a priori assumptions regarding the limit of the water table rise with respect to the minimum saturated aquifer thickness. The computed travel times are verified against a numerical model, and examples are provided, which show that the predicted travel times can be on the order of nine times longer relative to existing analytical solutions.

An approach is developed to simulate leaching of a dissolved chemical constituent in the vadose zone of an aquifer. Specifically, nitrate loading at the water table for different water table depths, for a range of aquifer permeability values, and for different cases of heterogeneity of the aquifer, are considered. Models from the literature are first used to derive soil-water characteristic curves (water retention and hydraulic conductivity) from a grain size distribution curve for unsaturated conditions. Given infiltration from the surface, the initial conditions for the chemical concentration, and the water content profile, leaching of the chemical in the vadose zone is simulated as a function of both time and depth. The methodology is illustrated for a permeable aquifer. Simulations are undertaken using a finite element code for saturated and unsaturated flow. Different scenarios are simulated depending on the heterogeneity of the aquifer and the depth of the water table. Modeling results show that in the example case studied, nitrate concentration loading at the water table does not depend strongly on the position of the water table, but rather on the material properties of the aquifer. The contribution of this endeavor resides in the methodology which allows a prediction of nitrate leaching using only the grain size property of the aquifer. It allows practitioners to obtain a first assessment of leaching with limited data.

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