Numerical modelling in support of a conceptual model for groundwater flow and geochemical evolution in the southern Outaouais Region, Quebec, Canada

A two-dimensional vertical-section numerical model for groundwater flow and transport using age, tritium and chloride was used to help validate a conceptual model of geochemical evolution within a representative regional-scale hydrogeological system in the Outaouais Region, Quebec, Canada. The flow system includes up to 30 m of Quaternary sediments and marine clays overlying fractured silicate rock of the Canadian Shield. Calibration of the regional flow model using observed piezometric levels and tritium concentrations showed that shallow groundwater flow is dominated by local flow systems limited to 30–40 m depth, 1–5 km long, and with groundwater residence times of 10–50 years. Intermediate systems, on the order of 5–15 km long, are less extensive than initially thought and are characterised by maximum depths of about 100 m and residence times of 200–6000 years. A model-calibrated hydraulic conductivity of 8 × 10−5 m.s−1 was required in the upper 50 m of the fractured bedrock. The active flow zone was inferred to extend to depths of about 100–150 m, with any deeper regional flow essentially negligible. Differences between tritium-based ages and simulated mean residence times were attributed to mixing of groundwater in open boreholes. Concentrations of 4He could be explained by diffusive transport from deeper and older groundwater, exacerbated by sampling. With new insight from the numerical modelling, the conceptual flow model has been updated to now include only a weak component of regional flow combined with significant local- and intermediate-scale flow systems connected to the upper fractured bedrock. The simulated flow system is also consistent with the geochemical evolution of the region, which is dominated by young Ca-HCO3-type waters in the unconfined aquifer and by older Cl− signatures from the remnant Champlain Sea seawater

Atmospheric carbon sequestration in ultramafic mining residues and impacts on leachate water chemistry at the Dumont Nickel Project, Quebec, Canada

Passive carbon mineralization in ultramafic mining residues, which allows the sequestration of CO2 through carbonate precipitation, is one of the options being considered to limit the accumulation of anthropogenic CO2 in the atmosphere. The Dumont Nickel Project (DNP) will generate approximately 1.7 Gt of utramafic mining residues over 33 years of production and the mine will release about 127,700 tonnes of CO2 each year. Using two experimental cells filled with ultramafic waste rock (EC-1) and milling residues (EC-2), the impacts of carbon mineralization on leachate water quality were studied and the quantity of sequestered carbon was estimated.Hydrotalcite supergroup minerals, aragonite, artinite, nesquehonite, dypingite and hydromagnesite precipitated through atmospheric weathering, while the inorganic carbon content of the weathered mining waste increased from 0.1 wt% to 4.0 wt% which indicate active CO2 sequestration. The leachate water, sampled at the bottom of the experimental cells, is characterized by an alkaline pH (~9.5), a high alkalinity (~90 to ~750 mg/L) and a high concentration of magnesium (~50 - ~750 mg/L), which is typical from weathering of ultramafic rocks in a system open to CO2. Since 2012, the chemical composition of the leachate water has evolved seasonally. These seasonal variations are best explained by: (1) climatic variations over the year and, (2) increased carbonate precipitation between May and July. Increased carbonate precipitation decreased the alkalinity and magnesium concentrations in the leachate water and produced pore waters which were undersaturated with respect to carbonate minerals such as artinite and hydromagnesite. *Revised manuscript with no changes marked Click here to view linked References Carbonate precipitation thus self-limits carbon sequestration through a negative feed-back loop. The carbon sequestration potential of the DNP residues is also influenced by the hydrogeological properties of the residues. In cell EC-2, a high liquid/solid ratio, which limits carbonate precipitation, was maintained by the hydrogeological properties. Since 2011, an estimate of 13 kg of atmospheric CO2 has been sequestered in the milling residues (EC-2), which corresponds to a mean rate of 1.4 (+/- 0.3) kgCO2/tonne/year. Using this mean rate, the 15 Mt of tailings produced each year, during the planned 33 years of mining operation, could potentially sequester 21,000 tonne of CO2 per year by passive carbon mineralization, about 16% of the 127,700 tonnes of CO2 annually emitted by the planned mining operation

The effect of capital ratios on the risk, efficiency and profitability of banks: Evidence from OECD countries

Using a sample of 1,992 banks from 39 OECD countries during the 1999-2013 period, we examine whether the imposition of higher capital ratios is effective in reducing risk and improving the efficiency and profitability of banking institutions. We demonstrate that while risk-and non-risk based capital ratios improve bank efficiency and profitability, risk-based capital ratios fail to decrease bank risk. Our results cast doubts on the validity of the weighting methodologies used for calculating risk-based capital ratios and on the efficacy of regulatory monitoring. The ineffectiveness of risk-based capital ratios with regard to bank risk is likely to be exacerbated by the adoption of the new Basel III capital guidelines. While Basel III requires banks to hold higher liquidity ratios along with higher capital ratios, our findings suggest that imposing higher capital ratios may have a negative effect on the efficiency and profitability of highly liquid banks. Our results hold across different subsamples, alternative risk, efficiency, and profitability measures and a battery of estimation techniques

Factors affecting river turbidity in a degrading permafrost environment : the Tasiapik River, Umiujaq (Nunavik)

This study focuses on spatiotemporal changes in water turbidity in relation to permafrost to document the impact of meteorological conditions and water flow on hydro-sedimentary processes in northern regions. Starting in June of 2019, water turbidity data were collected at six sites along the Tasiapik River (Nunavik). A statistical analysis was completed based on records of water turbidity, precipitation, water flow, and air temperature. Our results show a significant correlation between air temperatures and turbidity, with a correlation of up to r = 0.59. These correlations depend on the location of the site along the river and the time of the study period (June–October 2019). The flow rate was the primary factor that caused variations in the turbidity of the Tasiapik River. Our results showed that following an increase in flow rate, there was an almost simultaneous increase in turbidity due to erosion of the banks. The duration and intensity of precipitation events are also important factors affecting the process of sediment transport. Even though meteorological conditions play an important role in turbidity variation, other characteristics of the site such as the topography and the existence of thermokarst lakes are additional factors that influence the dynamics of sediment transport in the Tasiapik River.Les travaux menés en Arctique et Subarctique démontrent une accélération de la dégradation du pergélisol durant les dernières décennies, provoquant des tassements importants du sol et par le fait même, un accroissement du fluage d’eau chargée de sédiments vers les lacs et les rivières. Cette étude vise à mieux comprendre la variation spatio-temporelle de la turbidité fluviale en contexte périglaciaire dans le but de faire avancer les connaissances sur les impacts des conditions météorologiques et du débit sur les processus hydrosédimentaires des régions nordiques. Des données de turbidité de l’eau de la rivière Tasiapik, située à 5 km à l’est du village d’Umiujaq (Nunavik), ont été enregistrées de juin à octobre 2019 dans six sites distincts. Des analyses statistiques réalisées sur ces enregistrements indiquent qu’il existe une corrélation significative (r = 0,59) entre les températures de l’air et la turbidité de la rivière. Ces relations sont plus ou moins importantes selon l’emplacement du site le long de la rivière et selon le moment de la période d’étude. Le débit était le principal facteur à l’origine des variations de la turbidité de la rivière Tasiapik. Nos résultats ont montré qu’à la suite d’une augmentation du débit, il y a eu une augmentation presque simultanée de la turbidité due à l’érosion des berges et de la quantité des sédiments en suspension. La durée et l’intensité des précipitations sont également des facteurs importants ayant fait varier la turbidité de la rivière. Bien que les conditions météorologiques jouent un rôle important dans la variation de la turbidité, il s’est avéré que les caractéristiques du site telles que la topographie et la présence de lacs thermokarstiques sont des facteurs importants dans la dynamique du transport sédimentaire de la rivière Tasiapik

Three-dimensional numerical simulations of methane gas migration from decommissioned hydrocarbon production wells into shallow aquifers

Three-dimensional numerical simulations are used to provide insight into the behavior of methane as it migrates from a leaky decommissioned hydrocarbon well into a shallow aquifer. The conceptual model includes gas-phase migration from a leaky well, dissolution into groundwater, advective-dispersive transport and biodegradation of the dissolved methane plume. Gas-phase migration is simulated using the DuMux multiphase simulator, while transport and fate of the dissolved phase is simulated using the BIONAPL/3D reactive transport model. Methane behavior is simulated for two conceptual models: first in a shallow confined aquifer containing a decommissioned leaky well based on a monitored field site near Lindbergh, Alberta, Canada, and secondly on a representative unconfined aquifer based loosely on the Borden, Ontario, field site. The simulations show that the Lindbergh site confined aquifer data are generally consistent with a 2 year methane leak of 2–20 m3/d, assuming anaerobic (sulfate-reducing) methane oxidation and with maximum oxidation rates of 1 × 10−5 to 1 × 10−3 kg/m3/d. Under the highest oxidation rate, dissolved methane decreased from solubility (110 mg/L) to the threshold concentration of 10 mg/L within 5 years. In the unconfined case with the same leakage rate, including both aerobic and anaerobic methane oxidation, the methane plume was less extensive compared to the confined aquifer scenarios. Unconfined aquifers may therefore be less vulnerable to impacts from methane leaks along decommissioned wells. At other potential leakage sites, site-specific data on the natural background geochemistry would be necessary to make reliable predictions on the fate of methane in groundwater

Mass fluxes of xenobiotics below cities: challenges in urban hydrogeology

Urban areas are the focus of major ecological, social and economical activity. They are thus also prime locations of increasing conflict with regard to water use and water protection. As a direct and/or indirect consequence of urban land use and human activity, urban water systems are frequently polluted with organic contaminants including waste water-born xenobiotics such as pharmaceuticals, personal care products (collectively known as PPCPs) and endocrine-active substances. This study reviews new integrated methodologies including flux calculations as well as chemical investigations for determining the impact of human activities on urban water systems and on processes within the urban watershed. The use of indicator substances, representing different contaminant sources and pathways, integral pumping tests and mass balance approaches are suitable alternatives within these environments. The issues are explored using contaminant mass balance examples from Halle/Saale and Leipzig, German

Regional groundwater flow and the future of hydrogeology: evolving concepts and communication

Consideration of regional groundwater flow in aquifer systems allows for solving groundwater issues on a larger scale than single aquifers and contributes to all practical aspects of the UN’s Sustainable Development Goals for water. The approach has been extended to a wide range of hydrogeological environments. However, it suffers from poorly constrained terminology and conceptualisation, compounded by the difficulties of interpreting complex groundwater flow systems. This essay aims to initiate a discussion on improving the application of regional groundwater flow approaches

Numerical evaluation of grouting scenarios for reducing water inflows in underground excavations – Goldcorp’s Éléonore mine study case

Water inflows through fracture networks are one of the many challenges that the Éléonore mine has to face. Although pregrouting of pilot holes during mine development has been proven to efficiently reduce water inflows into mine excavations, the actual design methods are empirical and can be optimized to increase grouting efficiency and decrease the associated costs. Optimization of the amount of cement needed for pre-grouting is achieved by designing the grouting approach based on the location of major faults around the excavations. Here, a base case finite-element numerical model and associated sensitivity analyses are used to simulate groundwater inflows into a stope, based on the Éléonore mining site characteristics. Simulations are conducted for testing various grout injection scenarios for various major fault locations around the stope. Sensitivity analyses have shown that for a fault located above the stope, the inflow reduction is greater when the zone between the fault and the stope is grouted instead of directly grouting the fault itself. Also, in the case of a fault intersecting a stope, the results have demonstrated that the fault itself should be grouted as widely as possible, instead of sealing only the immediate surroundings of the stope