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

Contribution de l'hydrogéochimie à la compréhension des écoulements d'eaux souterraines en Outaouais, Québec, Canada

En Outaouais, les eaux souterraines sont une ressource essentielle pour les besoins domestiques et agricoles (hors ville de Gatineau, qui utilise les eaux de surface). La région d’étude repose sur le Bouclier Canadien, fait de roches silicatées fracturées recouvertes par les sédiments résultants de la dernière glaciation-déglaciation. Dans ce contexte, améliorer les connaissances sur le fonctionnement des aquifères est vital pour une utilisation durable de la ressource. Pour atteindre cet objectif, un modèle conceptuel a été proposé résultant de l’interprétation des données géochimiques couplée à la modélisation numérique. L’altération des silicates, l’intrusion d’eau saline de l’ancienne Mer de Champlain suivie de l’échange cationique dans les argiles marines affectant les aquifères confinés ainsi que les mélanges d’eaux sont les principaux processus identifiés par une analyse statistique multivariée. A échelle locale, une signature des isotopes stables similaire à celle de la pluie actuelle et la présence de tritium indiquent des temps de résidence relativement courts dans les 100 premiers mètres de profondeur, ce qui est confirmé par la modélisation des écoulements et des âges. La présence d’hélium 4 est attribuée à la diffusion depuis des eaux plus profondes et plus âgées, résultant de la diminution de la conductivité hydraulique avec la profondeur. Des mélanges résultent de l’échantillonnage dans des puits ouverts. Dans les aquifères non confinés, les activités en 14C sont principalement le résultat de l’équilibre de l’eau de recharge avec le CO2 du sol dans un système ouvert suivi de l’altération des silicates par du CO2 fossile ou de la dissolution des carbonates en milieu fermé plutôt que de la décroissance radioactive. Finalement, la simulation des chlorures montre que les restes de la Mer de Champlain seraient présents dans les argiles marines et dans les zones moins perméables de l’aquifère confiné par rapport aux eaux caractérisés par l’échange cationique. L’étude a permis de caractériser la qualité des eaux souterraines dans la partie fracturée du bouclier canadien dans les dépôts du Quaternaire à l’échelle régionale. Les systèmes locaux d’écoulement, avec des temps de résidence courts dans la partie supérieure du roc sont essentiels et doivent être pris en compte pour évaluer la vulnérabilité des ressources en eaux souterraines.Throughout most of the Outaouais Region, groundwater is an essential resource for both for domestic and agricultural use (except in the city of Gatineau which uses surface water). The study area lies in the Canadian Shield, and includes a fractured silicate bedrock aquifer which is covered by sediments from the last glacial-deglacial period. In this context, improving our understanding of aquifers is vital for a sustainable use of this resource. To fulfill this objective, a conceptual model was proposed based on the interpretation of geochemical data coupled to numerical modelling. Silicate weathering, seawater intrusion by the former Champlain Sea and subsequent cation exchange in marine clays affecting groundwater quality in confined aquifers, and mixing between waters of different ages are identified by a multivariate statistical analysis as the principal geochemical processes. At the local scale, a stable isotope signature similar to current precipitation and the presence of tritium indicate relatively short residence times in the first 100 m below ground surface, which was confirmed by numerical flow and age modelling. The occurrence of helium 4 is attributed to diffusion from deeper older groundwater within lower hydraulic conductivity zones. Some mixing due to sampling in open boreholes may also have occurred. In the unconfined aquifer, 14C activities appear to be the result of such as equilibration of recharge water with soil CO2 in open conditions coupled to silicate weathering by fossil CO2 or carbonate dissolution under closed conditions, rather than from radioactive decay. Finally, chloride transport simulations show that remnants of the Champlain Sea would still be found in marine clays and in the less permeable zones of the confined aquifer whereas groundwater characterised by cation exchange are found in more permeable zones. The study has helped characterize regional groundwater quality in the upper fractured zone of the Canadian Shield and in the Quaternary sediments of the Outaouais region. It has shown the importance of local scale groundwater flow systems and relatively rapid flow in the upper part of the bedrock which need to be considered when assessing the vulnerability of these regional groundwater resources

Assessing Potential Shale Gas Impacts on Groundwater Resources: Recommendations for Groundwater Monitoring and Definition of Baseline Conditions

Exploitation of shale gas by hydraulic fracturing (fracking) is highly controversial and concerns have been raised regarding induced risks from this extraction technique. The SHEER project, an EU Horizon 2020-funded project, is developing best practice to understand, prevent and mitigate the potential short- and long-term environmental impacts and risks of shale gas exploration and exploitation. Three major potential impacts were identified: groundwater contamination, air pollution and induced seismicity. This presentation will deal with the hydrogeological aspect. As part of the SHEER project, baseline and operational groundwater monitoring was carried out at an extraction site in Wysin, Northern Poland. Baseline monitoring was carried out from December 2015 to June 2016 in four monitoring wells intercepting the main drinking water aquifer located in Quaternary sediments. Fracking operations occurred in two deviated horizontal wells in June and July 2016. Monitoring continued for 1.5 years post-fracking although no significant gas production occurred during this period. Collected data include measurements of groundwater level, electrical conductivity and temperature at 15-min intervals, field measurements of groundwater physico-chemical parameters and frequent sampling for laboratory analyses. Groundwater samples were analysed for a range of constituents including dissolved gases and stable isotopes. This presentation will provide an overview of the monitoring results and the ensuing recommendations for groundwater monitoring in the context of shale gas exploitation. These recommendations relate to: (1) site characterisation prior to any activity, (2) baseline and on-going groundwater monitoring, and (3) relationships between regulators, operators and general public. During the presentation, we will particularly focus on the monitoring methodology and establishing accurate background values for key parameters for baseline monitoring, including suggestions on how to clearly communicate the information to the general public. We will conclude on techniques to identify deviations from baseline values

Geothermal energy resources in Ethiopia : status review and insights from hydrochemistry of surface and groundwaters

Ethiopia has an estimated >10,000 MW of geothermal energy potential, more than double its current power generating capacity (4,400 MW). Electricity access stands at 44% of the total population, with 31% in rural areas, so effective development of this low-carbon resource could make a significant impact to equitable delivery of electricity. However, geothermal energy exploitation must be done responsibly to protect valuable water resources under stress from climate-change driven drought conditions and competing uses across agricultural, domestic, and industrial sectors. Our review provides progress updates on geothermal developments—which soon aim to deliver more than 1,000 MW of electricity—and performs a high-level assessment of hydrochemical data for ground and surface waters across Ethiopia. A water quality database was built using publicly available information and three quality control criteria: well-defined sample location, cation-anion balance (CAB) of ±10%, and clear fluid type definition. Ethiopia hosts two major geothermal water types, sodium-alkalinity dominated in the Main Ethiopian Rift and sodium-chloride dominated in the Afar Depression, separated by sodium-mixed waters between Dofan-Fantale and Meteka. H and O stable isotopes suggest a largely meteoric source for geothermal waters, with δ 18O enrichment adding to evidence of a high enthalpy resource at Tendaho. Hydrochemical investigations provide critical information for successful delivery of sustainable geothermal energy developments. However, the current lack of data available for Ethiopia poses a significant challenge for completion of predevelopment baselines and ongoing environmental impact assessment. We encourage the release of unpublished findings from private companies and government agencies to build upon our database and demonstrate social and environmental responsibility in the development of Ethiopian geothermal resources. This article is categorized under: Engineering Water > Methods

Geothermal Energy and Water Resources in Ethiopia

No abstract available

Geothermal Energy and Water Resources in Ethiopia

No abstract available

Shale Gas Impacts on Groundwater Resources: Understanding the Behavior of a Shallow Aquifer Around a Fracking Site in Poland

Exploitation of shale gas by hydraulic fracturing (fracking) is highly controversial and concerns have been raised regarding induced risks from this technique. As part of the EU-funded SHEER Project, a shallow aquifer used for drinking water, overlying a zone of active shale-gas fracking, has been monitored for more than a year. Early results reveal the functioning of the shallow aquifer and hydrochemistry, focusing on the identification of potential impacts from the shale gas operation. This stage is an essential precursor to modeling impact scenarios of contamination and to predict changes in the aquifer

Addressing essential hydrogeological and environmental constraints for geothermal development in East Africa

Geothermal energy is vastly under-utilized and represents an exciting means of addressing energy challenges, alleviating poverty, and promoting economic development in the nations of the East African Rift System (EARS). The countries that straddle the rift system are home to a combined population of more than 400 million, a significant proportion of whom do not have access to power or safe drinking water resources. These coexisting water and energy issues have traditionally been tackled as separate challenges. The Combined Power and Freshwater Generation (Combi-Gen) project aims to initiate a disruptive shift in the approach to the twin challenges of energy shortage and water-scarcity through development of a novel thermal chimney driven air-cooled condenser that will capture a substantial portion of the post-flash and reaction turbine geothermal vapour and convert it into potable water, without creating a parasitic power load. In order to enhance the design of such systems, a robust understanding of the geothermal resource and the wider hydrogeological systematics must be obtained. This is essential for assessment of fluid composition, analysis of scaling and corrosive species, flow rate and pressure control, and ultimately optimization of engineering performance. In addition, it is also imperative to gauge the wider hydrological connectivity of geothermal ground waters in order to establish potential impacts on the environment and existing essential water resources

SHEER “smart” database: technical note

The SHEER database brings together a large amount of data of various types: interdisciplinary site data from seven independent episodes, research data and those for the project results dissemination process. This concerns mainly shale gas exploitation test sites, processing procedures, results of data interpretation and recommendations. The smart SHEER database harmonizes data from different fields (geophysical, geochemical, geological, technological, etc.), creates and provides access to an advanced database of case studies of environmental impact indicators associated with shale gas exploitation and exploration, which previously did not exist. A unique component of the SHEER database comes from the monitoring activity performed during the project in one active shale gas exploration and exploitation site at Wysin, Poland, which started from the pre-operational phase. The SHEER database is capable of the adoption of new data such as results of other Work Packages and has developed an over-arching structure for higher-level integration

Induced seismicity response of hydraulic fracturing: results of a multidisciplinary monitoring at the Wysin site, Poland

Shale oil and gas exploitation by hydraulic fracturing experienced a strong development worldwide over the last years, accompanied by a substantial increase of related induced seismicity, either consequence of fracturing or wastewater injection. In Europe, unconventional hydrocarbon resources remain underdeveloped and their exploitation controversial. In UK, fracturing operations were stopped after the Mw 2.3 Blackpool induced earthquake; in Poland, operations were halted in 2017 due to adverse oil market conditions. One of the last operated well at Wysin, Poland, was monitored independently in the framework of the EU project SHEER, through a multidisciplinary system including seismic, water and air quality monitoring. The hybrid seismic network combines surface mini-arrays, broadband and shallow borehole sensors. This paper summarizes the outcomes of the seismological analysis of these data. Shallow artificial seismic noise sources were detected and located at the wellhead active during the fracturing stages. Local microseismicity was also detected, located and characterised, culminating in two events of Mw 1.0 and 0.5, occurring days after the stimulation in the vicinity of the operational well, but at very shallow depths. A sharp methane peak was detected ~19 hours after the Mw 0.5 event. No correlation was observed between injected volumes, seismicity and groundwater parameters