Lutte contre les changements climatiques du Québec (MDDELCC). The work was funded through the eco... more Lutte contre les changements climatiques du Québec (MDDELCC). The work was funded through the ecoEnergy Innovation Initiative (ecoEII) and the Program for Energy Research and Development (PERD) of the Energy Sector, and through regular funds of the Environmental Geoscience Program of the Lands and Minerals Sector, both of Natural Resources Canada. This is Geological Survey of Canada contribution 20170284.
Environmental concerns have been raised with respect to shale gas exploration and production, esp... more Environmental concerns have been raised with respect to shale gas exploration and production, especially in eastern Canada and northeastern United States. One of the major public concerns has been the contamination of fresh water resources. This paper focuses on the investigation of possible fluid upward migration through structural features in the intermediate zone (IZ), located between a deep shale gas reservoir and shallow aquifers. The approach provides insights into how such an investigation can be done when few data are available at depth. The study area is located in the shale dominated succession of the St. Lawrence Platform (eastern Canada), where the Utica Shale was explored for natural gas between 2006 and 2010. Detailed analyses were carried out on both shallow and deep geophysical log datasets providing the structural attributes and preliminary estimates of the hydraulic properties of faults and fractures. Results show that the active groundwater flow zone is located within the upper m of bedrock, where fractures are well interconnected. Fractures from one set were found to be frequently open in the IZ and reservoir, providing a poorly connected network. The fault zones are here described as combined conduitbarrier systems with sealed cores and some open fractures in the damage zones. Although no direct hydraulic data were available at depth, the possibility that the fracture network or fault zones act as large-scale flow pathways seems very unlikely. A conceptual model of the fluid flow patterns summarizing the current understanding of the system hydrodynamics is also presented.
Nitrate transport in heterogeneous bedrock aquifers is influenced by mechanisms that operate at 2... more Nitrate transport in heterogeneous bedrock aquifers is influenced by mechanisms that operate at 20 different spatial and temporal scales. To understand these mechanisms in a fractured sandstone aquifer with high porosity, a groundwater-flow and nitrate transport model-reproducing multiple hydraulic and chemical targets-was developed to explain the actual nitrate contamination observed in groundwater and surface water in a study area on Prince Edward Island, Canada. 24 Simulations show that nitrate is leached to the aquifer year-round, with 61% coming from 25 untransformed and transformed organic sources originating from fertilizers and manure. This 26 nitrate reaches the more permeable shallow aquifer through fractures in weathered sandstone that 27 represent only 1% of the total porosity (17%). Some of the nitrate reaches the underlying aquifer, 28 which is less active in terms of groundwater flow, but most of it is drained to the main river. The river water quality is controlled by the nitrate input from the shallow aquifer. Groundwater in the underlying aquifer, which has long residence times, is also largely influenced by the diffusion of 31 nitrate in the porous sandstone matrix. Consequently, following a change of fertilizer application practices, water quality in domestic wells and the river would change rapidly due to the level of nitrate found in fractures, but a lag time of up to 20 years would be necessary to reach a steady level due to diffusion. This demonstrates the importance of understanding nitrate transport mechanisms when designing effective agricultural and water management plans to improve water quality. 37
<p>Transport of solutes in aquifers is controlled by the heterogeneous spatial dist... more <p>Transport of solutes in aquifers is controlled by the heterogeneous spatial distribution of hydraulic properties, but the characterization of aquifer heterogeneity is quite challenging with conventional methods. Hydraulic tomography (HT) was developed to define the heterogeneous distribution of hydraulic conductivity (<em>K</em>) and specific storage (S<sub>s</sub>). HT involves the emission of a series of hydraulic head perturbations in a stressed well and the recording of this signal at several levels in the stressed and observation wells. All recorded hydraulic head responses are simultaneously analyzed through numerical inversion, which provides the spatial distribution of hydraulic properties at a scale relevant for local site investigations.</p> <p>This communication reports on a tomographic experiment carried out in a heterogeneous and highly anisotropic granular aquifer at the Saint-Lambert research site near Quebec City, Canada. This site has already been the object of detailed characterizations with multiple hydraulic methods: pumping tests, packer slug tests, flowmeter profiles, vertical interference tests, and slug test tomography. A relatively new approach named oscillatory hydraulic tomography (OHT) was tested, in which multi-frequency oscillatory head perturbations are induced in an interval isolated by packers of the stressed well by a submerged rod that is electronically controlled by a winch system. Hydraulic responses are measured in the stressed intervals and in multiple intervals of an observation well.</p> <p>This study was primarily aimed at testing, first on an operational level, if the OHT signal could be generated in the stressed well and propagated to the observation well in a highly anisotropic granular aquifer. Second, the study developed a rigorous workflow for the treatment of the measured hydraulic heads. Third, in terms of characterization efficacy, the study aimed to determine if multiple controlled frequencies would allow the assessment of <em>K</em> spatial distribution.</p> <p>Results show that the field experiment provided clear measured hydraulic responses that could be used to obtain the 2D distribution of hydraulic properties from the inversion of OHT measurements. Comparison was made of inversion results using a single oscillatory frequency and multiple frequencies. Under conditions of realistic field measurement noise and uncertainty, it will be valuable in future work to compare the imaging capabilities of oscillatory hydraulic tomography against other tomographic methods. Further investigation is also needed to examine the information content of oscillatory hydraulic tomographic data for characterizing <em>K</em> and <em>Ss</em> heterogeneities through a sensitivity and resolution analysis. This study demonstrates the practical potential for the implementation of OHT experiments in relatively low permeability and highly anisotropic granular aquifers.</p>
At the former Schlema-Alberoda mining site, located in the south-east of Germany, waste rock dump... more At the former Schlema-Alberoda mining site, located in the south-east of Germany, waste rock dump 38neu was built along a natural slope on the edge of a valley. The dump is 750 m wide, with a maximum thickness of 30 m and a total height of 100 m. Dump remediation involved surface re-sloping and placement of a 1 m thick low-permeability soil cover. A characterisation aimed to verify the soil cover efficiency: its air permeability, radon concentrations under the cover and radon fluxes, as well as differential air pressures across the soil cover. In summer, relatively high radon concentrations and fluxes were found to occur across the cover in the lower slope of the dump. Differential pressures indicate that gas flow is upwards in the dump below the 9·5°C mean atmospheric temperature and downwards otherwise. A numerical model was developed to explain the relation between dump airflow and atmospheric conditions. The results indicate that airflow is controlled by dump gas buoyancy relati...
Potential impacts of shale gas development on shallow aquifers has raised concerns, especially re... more Potential impacts of shale gas development on shallow aquifers has raised concerns, especially regarding groundwater contamination. The intermediate zone separating shallow aquifers from shale gas reservoirs plays a critical role in aquifer vulnerability to fluid upflow, but the assessment of such vulnerability is challenging due to the general paucity of data in this intermediate zone. The ultimate goal of the project reported here was to develop a holistic multi-geoscience methodology to assess potential impacts of unconventional hydrocarbon development on freshwater aquifers related to upward migration through natural pathways. The study area is located in the St. Lawrence Lowlands (southern Quebec, Canada), where limited oil and gas exploration and no shale gas production have taken place. A large set of data was collected over a ~500 km 2 area near a horizontal shale gas exploration well completed and fracked into the Utica Shale at a depth of 2 km. To investigate the intermediate zone integrity, this project integrated research results from multiple sources in order to obtain a better understanding of the system hydrodynamics, including geology, hydrogeology, deep and shallow geophysics, soil, rock and groundwater geochemistry, and geomechanics. The combined interpretation of the multidisciplinary dataset demonstrates that there is no evidence of, and a very limited potential for, upward fluid migration from the Utica Shale reservoir to the shallow aquifer. Microbial and thermogenic methane in groundwater of this region appear to come from the shallow, organic-rich, fractured sedimentary rocks making up the regional aquifer. Nonetheless, diluted brines present in a few shallow wells close to and downstream of a normal fault revealed that some upward groundwater migration occurs, but only over a few hundred meters from the surface based on the isotopic signature of methane. The 3 methodology developed should help support regulations related to shale gas development aiming to protect groundwater.
The copyright of individual parts of the supplement might differ from the CC-BY 3.0 licence. Tabl... more The copyright of individual parts of the supplement might differ from the CC-BY 3.0 licence. Table S1. Classification of nitrate concentrations for each watershed and percentage change compared to 2001 (%/2001) in average nitrate concentration for the entire Prince Edward Island for scenarios 1 to 9. B is for Background, L, M and H stand for Low, Medium and High nitrate concentration class, respectively. Locations of each watershed with associated watershed number are indicated in Figure 1.
Summary The main objective of this study was to test the application limits of a groundwater rech... more Summary The main objective of this study was to test the application limits of a groundwater recharge assessment technique based on the inversion of a vertical one-dimensional numerical model of advective–conductive heat transport, using temperature time series at three different depths (1, 3, 5 m) in the unsaturated zone. For this purpose, several synthetic hourly datasets of subsurface temperatures, representing various weather, ground cover, and soil texture conditions, thus covering a wide range of groundwater recharge values, were produced with the vertical one-dimensional coupled heat and moisture transport simulator SHAW (Simultaneous Heat and Water model). Estimates of the vertical flux of water in the soil were then retrieved from these realistic temperature profiles using a simple one-dimensional numerical simulator of advective and conductive heat transport in the unsaturated zone that was developed as part of this study. The water flux was assumed constant on a weekly, monthly, semiannual, and annual basis. From these vertical water flux estimates, annual (potential) groundwater recharge rates were then computed and results were compared to those calculated previously with SHAW to assess the accuracy of the method. Results showed that, under ideal conditions, it would be possible to estimate annual recharge rates that are above 200 mm/y, with an acceptable error of less than 20%. These “ideal” conditions include the resolution of the water flux on a weekly basis, error-free temperature measurements below the soil freezing zone, and model parameter values (thermal conductivity and heat capacity of the soil) known a priori with no uncertainty. However, this work demonstrates that the accuracy of the method is highly sensitive to the uncertainty of the input model parameters of the numerical model used to carry out the inversion and to measurement errors of temperature time series. For the conditions represented in this study, these findings suggest that, despite the best modeling and field instrumentation practices, heat-based techniques for the assessment of diffuse groundwater recharge rates are likely not well suited for real field conditions, but could still represent a viable approach for applications carried out in engineered materials and under controlled conditions.
Lutte contre les changements climatiques du Québec (MDDELCC). The work was funded through the eco... more Lutte contre les changements climatiques du Québec (MDDELCC). The work was funded through the ecoEnergy Innovation Initiative (ecoEII) and the Program for Energy Research and Development (PERD) of the Energy Sector, and through regular funds of the Environmental Geoscience Program of the Lands and Minerals Sector, both of Natural Resources Canada. This is Geological Survey of Canada contribution 20170284.
Environmental concerns have been raised with respect to shale gas exploration and production, esp... more Environmental concerns have been raised with respect to shale gas exploration and production, especially in eastern Canada and northeastern United States. One of the major public concerns has been the contamination of fresh water resources. This paper focuses on the investigation of possible fluid upward migration through structural features in the intermediate zone (IZ), located between a deep shale gas reservoir and shallow aquifers. The approach provides insights into how such an investigation can be done when few data are available at depth. The study area is located in the shale dominated succession of the St. Lawrence Platform (eastern Canada), where the Utica Shale was explored for natural gas between 2006 and 2010. Detailed analyses were carried out on both shallow and deep geophysical log datasets providing the structural attributes and preliminary estimates of the hydraulic properties of faults and fractures. Results show that the active groundwater flow zone is located within the upper m of bedrock, where fractures are well interconnected. Fractures from one set were found to be frequently open in the IZ and reservoir, providing a poorly connected network. The fault zones are here described as combined conduitbarrier systems with sealed cores and some open fractures in the damage zones. Although no direct hydraulic data were available at depth, the possibility that the fracture network or fault zones act as large-scale flow pathways seems very unlikely. A conceptual model of the fluid flow patterns summarizing the current understanding of the system hydrodynamics is also presented.
Nitrate transport in heterogeneous bedrock aquifers is influenced by mechanisms that operate at 2... more Nitrate transport in heterogeneous bedrock aquifers is influenced by mechanisms that operate at 20 different spatial and temporal scales. To understand these mechanisms in a fractured sandstone aquifer with high porosity, a groundwater-flow and nitrate transport model-reproducing multiple hydraulic and chemical targets-was developed to explain the actual nitrate contamination observed in groundwater and surface water in a study area on Prince Edward Island, Canada. 24 Simulations show that nitrate is leached to the aquifer year-round, with 61% coming from 25 untransformed and transformed organic sources originating from fertilizers and manure. This 26 nitrate reaches the more permeable shallow aquifer through fractures in weathered sandstone that 27 represent only 1% of the total porosity (17%). Some of the nitrate reaches the underlying aquifer, 28 which is less active in terms of groundwater flow, but most of it is drained to the main river. The river water quality is controlled by the nitrate input from the shallow aquifer. Groundwater in the underlying aquifer, which has long residence times, is also largely influenced by the diffusion of 31 nitrate in the porous sandstone matrix. Consequently, following a change of fertilizer application practices, water quality in domestic wells and the river would change rapidly due to the level of nitrate found in fractures, but a lag time of up to 20 years would be necessary to reach a steady level due to diffusion. This demonstrates the importance of understanding nitrate transport mechanisms when designing effective agricultural and water management plans to improve water quality. 37
<p>Transport of solutes in aquifers is controlled by the heterogeneous spatial dist... more <p>Transport of solutes in aquifers is controlled by the heterogeneous spatial distribution of hydraulic properties, but the characterization of aquifer heterogeneity is quite challenging with conventional methods. Hydraulic tomography (HT) was developed to define the heterogeneous distribution of hydraulic conductivity (<em>K</em>) and specific storage (S<sub>s</sub>). HT involves the emission of a series of hydraulic head perturbations in a stressed well and the recording of this signal at several levels in the stressed and observation wells. All recorded hydraulic head responses are simultaneously analyzed through numerical inversion, which provides the spatial distribution of hydraulic properties at a scale relevant for local site investigations.</p> <p>This communication reports on a tomographic experiment carried out in a heterogeneous and highly anisotropic granular aquifer at the Saint-Lambert research site near Quebec City, Canada. This site has already been the object of detailed characterizations with multiple hydraulic methods: pumping tests, packer slug tests, flowmeter profiles, vertical interference tests, and slug test tomography. A relatively new approach named oscillatory hydraulic tomography (OHT) was tested, in which multi-frequency oscillatory head perturbations are induced in an interval isolated by packers of the stressed well by a submerged rod that is electronically controlled by a winch system. Hydraulic responses are measured in the stressed intervals and in multiple intervals of an observation well.</p> <p>This study was primarily aimed at testing, first on an operational level, if the OHT signal could be generated in the stressed well and propagated to the observation well in a highly anisotropic granular aquifer. Second, the study developed a rigorous workflow for the treatment of the measured hydraulic heads. Third, in terms of characterization efficacy, the study aimed to determine if multiple controlled frequencies would allow the assessment of <em>K</em> spatial distribution.</p> <p>Results show that the field experiment provided clear measured hydraulic responses that could be used to obtain the 2D distribution of hydraulic properties from the inversion of OHT measurements. Comparison was made of inversion results using a single oscillatory frequency and multiple frequencies. Under conditions of realistic field measurement noise and uncertainty, it will be valuable in future work to compare the imaging capabilities of oscillatory hydraulic tomography against other tomographic methods. Further investigation is also needed to examine the information content of oscillatory hydraulic tomographic data for characterizing <em>K</em> and <em>Ss</em> heterogeneities through a sensitivity and resolution analysis. This study demonstrates the practical potential for the implementation of OHT experiments in relatively low permeability and highly anisotropic granular aquifers.</p>
At the former Schlema-Alberoda mining site, located in the south-east of Germany, waste rock dump... more At the former Schlema-Alberoda mining site, located in the south-east of Germany, waste rock dump 38neu was built along a natural slope on the edge of a valley. The dump is 750 m wide, with a maximum thickness of 30 m and a total height of 100 m. Dump remediation involved surface re-sloping and placement of a 1 m thick low-permeability soil cover. A characterisation aimed to verify the soil cover efficiency: its air permeability, radon concentrations under the cover and radon fluxes, as well as differential air pressures across the soil cover. In summer, relatively high radon concentrations and fluxes were found to occur across the cover in the lower slope of the dump. Differential pressures indicate that gas flow is upwards in the dump below the 9·5°C mean atmospheric temperature and downwards otherwise. A numerical model was developed to explain the relation between dump airflow and atmospheric conditions. The results indicate that airflow is controlled by dump gas buoyancy relati...
Potential impacts of shale gas development on shallow aquifers has raised concerns, especially re... more Potential impacts of shale gas development on shallow aquifers has raised concerns, especially regarding groundwater contamination. The intermediate zone separating shallow aquifers from shale gas reservoirs plays a critical role in aquifer vulnerability to fluid upflow, but the assessment of such vulnerability is challenging due to the general paucity of data in this intermediate zone. The ultimate goal of the project reported here was to develop a holistic multi-geoscience methodology to assess potential impacts of unconventional hydrocarbon development on freshwater aquifers related to upward migration through natural pathways. The study area is located in the St. Lawrence Lowlands (southern Quebec, Canada), where limited oil and gas exploration and no shale gas production have taken place. A large set of data was collected over a ~500 km 2 area near a horizontal shale gas exploration well completed and fracked into the Utica Shale at a depth of 2 km. To investigate the intermediate zone integrity, this project integrated research results from multiple sources in order to obtain a better understanding of the system hydrodynamics, including geology, hydrogeology, deep and shallow geophysics, soil, rock and groundwater geochemistry, and geomechanics. The combined interpretation of the multidisciplinary dataset demonstrates that there is no evidence of, and a very limited potential for, upward fluid migration from the Utica Shale reservoir to the shallow aquifer. Microbial and thermogenic methane in groundwater of this region appear to come from the shallow, organic-rich, fractured sedimentary rocks making up the regional aquifer. Nonetheless, diluted brines present in a few shallow wells close to and downstream of a normal fault revealed that some upward groundwater migration occurs, but only over a few hundred meters from the surface based on the isotopic signature of methane. The 3 methodology developed should help support regulations related to shale gas development aiming to protect groundwater.
The copyright of individual parts of the supplement might differ from the CC-BY 3.0 licence. Tabl... more The copyright of individual parts of the supplement might differ from the CC-BY 3.0 licence. Table S1. Classification of nitrate concentrations for each watershed and percentage change compared to 2001 (%/2001) in average nitrate concentration for the entire Prince Edward Island for scenarios 1 to 9. B is for Background, L, M and H stand for Low, Medium and High nitrate concentration class, respectively. Locations of each watershed with associated watershed number are indicated in Figure 1.
Summary The main objective of this study was to test the application limits of a groundwater rech... more Summary The main objective of this study was to test the application limits of a groundwater recharge assessment technique based on the inversion of a vertical one-dimensional numerical model of advective–conductive heat transport, using temperature time series at three different depths (1, 3, 5 m) in the unsaturated zone. For this purpose, several synthetic hourly datasets of subsurface temperatures, representing various weather, ground cover, and soil texture conditions, thus covering a wide range of groundwater recharge values, were produced with the vertical one-dimensional coupled heat and moisture transport simulator SHAW (Simultaneous Heat and Water model). Estimates of the vertical flux of water in the soil were then retrieved from these realistic temperature profiles using a simple one-dimensional numerical simulator of advective and conductive heat transport in the unsaturated zone that was developed as part of this study. The water flux was assumed constant on a weekly, monthly, semiannual, and annual basis. From these vertical water flux estimates, annual (potential) groundwater recharge rates were then computed and results were compared to those calculated previously with SHAW to assess the accuracy of the method. Results showed that, under ideal conditions, it would be possible to estimate annual recharge rates that are above 200 mm/y, with an acceptable error of less than 20%. These “ideal” conditions include the resolution of the water flux on a weekly basis, error-free temperature measurements below the soil freezing zone, and model parameter values (thermal conductivity and heat capacity of the soil) known a priori with no uncertainty. However, this work demonstrates that the accuracy of the method is highly sensitive to the uncertainty of the input model parameters of the numerical model used to carry out the inversion and to measurement errors of temperature time series. For the conditions represented in this study, these findings suggest that, despite the best modeling and field instrumentation practices, heat-based techniques for the assessment of diffuse groundwater recharge rates are likely not well suited for real field conditions, but could still represent a viable approach for applications carried out in engineered materials and under controlled conditions.
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