Papers by Jean-Christophe Comte
HAL (Le Centre pour la Communication Scientifique Directe), Dec 5, 2008
Pour résoudre ce problème de validation, les méthodes d'imagerie géophysique constituent des outi... more Pour résoudre ce problème de validation, les méthodes d'imagerie géophysique constituent des outils d'investigation d'une grande pertinence. En particulier, les méthodes électriques qui sont sensibles au contraste de résistivité existant entre l'eau douce et l'eau salé apparaissent comme des techniques privilégiées. Parmi elles, la tomographie de résistivité électrique (ERT) permet une caractérisation bi-ou tridimensionnelle des résistivités de sub-surface et donc une imagerie haute résolution de la distribution du sel dans les nappes phréatiques. Malheureusement, la modélisation
Water, Oct 27, 2021
Managed aquifer recharge operations are often conducted in near-bank areas to regulate water reso... more Managed aquifer recharge operations are often conducted in near-bank areas to regulate water resources or reduce seawater intrusion. Yet little is known about the influence of surface water level fluctuations at different temporal scales on MAR performance. A generalized conceptual model was developed based on an investigation site in Western China as a basis to simulate the response surface water level fluctuations on the water table, artificially recharged water lens (formed by the artificially recharged water), groundwater flow paths and average travel times (which is an important control on how quickly contaminants are flushed out of aquifers), and the discharge of the artificially recharged aquifer during the surface water level fluctuation. The results showed a fluctuating groundwater table in the artificially recharged near-bank aquifer under the influence of surface water level fluctuations. The peak values of the increment of the groundwater table induced by artificial recharge decreased with the increase of the period and amplitude of surface water level fluctuation, but the trough values of the increment of water table increases with that. The penetration depth of surface water into the aquifer with a fluctuating surface water level leads to a decreasing increment of the groundwater table which follows a power law. The fluctuating surface water level leads to dynamic changes of artificially recharged water lens morphology and a thinner artificially recharged water lens. A mixing zone of recharged water and ambient water could be found in the artificially recharged near-bank area, which is expected to lead to modifications in the geochemical conditions in the artificially recharged near-bank aquifer. A longer period of surface water level fluctuation leads to a longer average travel time, but the larger penetration depth of surface water and amplitude lead to a shorter average travel time. The peak discharge of the near-bank aquifer was found to decrease with the period of surface water level fluctuation, but it increases with penetration depth and amplitude. This study is important in providing insights into the performance of near-bank managed aquifer recharge with respect to surface water level fluctuation.
Hydrogeology Journal, May 10, 2015
The process of accounting for heterogeneity has made significant advances in statistical research... more The process of accounting for heterogeneity has made significant advances in statistical research, primarily in the framework of stochastic analysis and the development of multiple-point statistics (MPS). Among MPS techniques, the direct sampling (DS) method is tested to determine its ability to delineate heterogeneity from aerial magnetics data in a regional sandstone aquifer intruded by low-permeability volcanic dykes in Northern Ireland, UK. The use of two two-dimensional bivariate training images aids in creating spatial probability distributions of heterogeneities of hydrogeological interest, despite relatively 'noisy' magnetics data (i.e. including hydrogeologically irrelevant urban noise and regional geologic effects). These distributions are incorporated into a hierarchy system where previously published density function and upscaling methods are applied to derive regional distributions of equivalent hydraulic conductivity tensor K. Several K models, as determined by several stochastic realisations of MPS dyke locations, are computed within groundwater flow models and evaluated by comparing modelled heads with field observations. Results show a significant improvement in model calibration when compared to a simplistic homogeneous and isotropic aquifer model that does not account for the dyke occurrence evidenced by airborne magnetic data. The best model is obtained when normal and reverse polarity dykes are computed separately within MPS simulations and when a probability threshold of 0.7 is applied. The presented stochastic approach also provides improvement when compared to a previously published deterministic anisotropic model based on the unprocessed (i.e. noisy) airborne magnetics. This demonstrates the potential of coupling MPS to airborne geophysical data for regional groundwater modelling.
Unearthed
Numerical modelling of aquifers is a standard process in the sustainable management of groundwate... more Numerical modelling of aquifers is a standard process in the sustainable management of groundwater resources. To be reliable, a groundwater model requires an accurate geological framework, particularly where structure is complex. The Lagan Valley aquifer near Belfast is an example of an otherwise generally homogenous sandstone aquifer intruded by extensive swarms of low-permeability igneous dykes. The dissection of the aquifer by these dykes affects groundwater flow direction and borehole yield. We have used the Tellus aeromagnetic data to map the extent of these dykes and so improve the geometrical parameterisation of the models. Two methods are described for incorporating the effect of the dykes: a visual deterministic approach and a stochastic approach using Multiple Point Statistics. Both approaches resulted in models that significantly improved matches to observed groundwater levels and flow directions, demonstrating the value of aero-magnetic data for constraining these models.
Frontiers in Ecology and Evolution
Karst ecosystems represent up to 25% of the land surface and recent studies highlight their poten... more Karst ecosystems represent up to 25% of the land surface and recent studies highlight their potential role as a sink for atmospheric methane. Despite this, there is limited knowledge of the diversity and distribution of methane-oxidizing bacteria (MOB) or methanogens in karst caves and the sub-surface environment in general. Here, we performed a survey of 14 shotgun metagenomes from cave ecosystems covering a broad set of environmental conditions, to compare the relative abundance and phylogenetic diversity of MOB and methanogens, targeting biomarker genes for methane monooxygenase (pmoA and mmoX) and methyl-coenzyme M reductase (mcrA). Taxonomic analysis of metagenomes showed 0.02–1.28% of classified reads were related to known MOB, of which Gammaproteobacterial MOB were the most abundant making up on average 70% of the surveyed caves’ MOB community. Potential for biogenic methane production in caves was also observed, with 0.008–0.39% of reads classified to methanogens and was dom...
Unearthed, 2016
Mapping a waste disposal site using Tellus airborne geophysical data' in M.E. Young (ed.), Uneart... more Mapping a waste disposal site using Tellus airborne geophysical data' in M.E. Young (ed.), Unearthed: impacts of the Tellus surveys of the north of Ireland. Dublin. Royal Irish Academy.
NSG2020 26th European Meeting of Environmental and Engineering Geophysics, 2020
Advances in Geoethics and Groundwater Management : Theory and Practice for a Sustainable Development, 2021
Across the Island of Ireland, hard rock aquifers which are found to underlie over 65% of Ireland ... more Across the Island of Ireland, hard rock aquifers which are found to underlie over 65% of Ireland (Robins and Misstear, 2000) are commonly referred to as poorly productive bedrock aquifers (PPA), characterised by low well yields (GSI, 2006). Despite their limited role in water supply, PPAs are found to contribute significantly to the maintenance of river baseflow levels, especially in upland catchments during sustained dry periods (Comte et al., 2012). Typically, these units are considered to be devoid of intrinsic matrix porosity: instead, the flow of groundwater is confined to secondary porosity generated through interconnected fractures and fissures. Previous studies have highlighted the challenges in characterising PPA systems due to the heterogeneity of geological structures at micrometre–metre (cf. Neuman, 2005) and regional scales (km to 10s km, cf. Krasny, 2002). This is particularly the case in hard rock environments where the permeability of bedrock is largely dictated by t...
Journal of Hydrology, 2021
This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Sedimentology, 2021
Sediments deposited by glacial meltwaters (for example, ice‐contact delta deposits) form permeabl... more Sediments deposited by glacial meltwaters (for example, ice‐contact delta deposits) form permeable packages in the subsurface that can act as reservoirs for both water and hydrocarbons. They are also an important source of aggregate for the construction industry. As reservoirs they are challenging to characterize in terms of their structure, flow and storage properties due to their complex depositional history. In this study, ice‐contact deltas of Salpausselkä I and II end moraines in Southern Finland are studied using a combination of geomorphological mapping, sedimentological studies and near surface geophysical methods. Sedimentary logs from isolated outcrops were correlated to ground penetrating radar (GPR) profiles to unravel the internal structure and depositional history of these ice‐contact deltas. Subsequently, electrical resistivity tomography (ERT) and gravity data were analysed to estimate the depth to bedrock and to model porosity distribution within the sediments. Resu...
Environmental Modelling & Software, 2021
• We present a strategy for hydrogeophysical inversion and uncertainty analysis. • PEST/PEST++ an... more • We present a strategy for hydrogeophysical inversion and uncertainty analysis. • PEST/PEST++ and COMSOL Multiphysics are fully integrated. • The approach is applied to electrical resistivity monitoring in a coastal aquifer. • The coupled inversion achieves better delineation of seawater intrusion.
Hydrogeology Journal, 2021
Understanding the role of geological heterogeneity on the performance of managed aquifer recharge... more Understanding the role of geological heterogeneity on the performance of managed aquifer recharge (MAR) in terms of effective groundwater storage is crucial to design MAR systems. Natural aquifers are affected by a variety of geologic strata and structures at different scales, which are responsible for wide ranging hydraulic properties. This study combines physical experiments and numerical modeling to investigate the effect of geologic structures commonly encountered in sedimentary environments, on MAR-induced groundwater flow patterns using injection wells. Models were conceptualized and parametrized based on the hydrogeological conditions of Tailan River basin in arid NW China, which hosts a typical, structurally complex, alluvial-fan aquifer system affected by sediment layering, clay lenses and anticline barriers, and is extensively studied for the strategic potential of MAR in addressing water shortages in the region. Results showed that, compared to a homogeneous scenario, high-permeability aquifer layers shortened groundwater ages, decreased the thickness of the artificially recharged water lenses (ARWLs), and shifted the stagnation points downstream. Clay lenses increased groundwater residence times but had little effect on spatial flow patterns due to their elongation parallel-to-flow direction. Overall groundwater ages, as well as the thickness of ARWLs created through injection on the upstream side of an anticline, increased, and this to a larger extent than through injection on the downstream side, which did not increase significantly compared to the homogeneous scenario. Results provide insights for MAR optimization in naturally heterogeneous aquifer systems, along with a benchmark tool for application to a wide range of typical geological conditions.
Journal of Hydrology, 2021
Quantifying groundwater storage in weathered/fractured basement rock aquifers can be challenging ... more Quantifying groundwater storage in weathered/fractured basement rock aquifers can be challenging owing to both their high degree of heterogeneity and their overall low storage capacity. Therefore, in these aquifers, the use of direct borehole hydraulic data is usually insufficient. Here we assessed the popular method of electrical resistivity tomography (ERT), combined with borehole data and including associated uncertainties, to resolve the spatial variability of groundwater storage properties at high resolution within a fractured mica schist aquifer in Ireland. Porosity distributions across both the saturated and unsaturated zones were calculated from two-dimensional (2D) ERT resistivities using two standard petrophysical models, Archie and Waxman & Smits (WS), the latter accounting for the influence of clay minerals on resistivity data. Our results demonstrated the importance of the hydrogeological conceptual constraints provided by ERT when parametrizing the 2D petrophysical models from borehole point data. They also confirmed the importance of accounting for clay minerals (the products of bedrock weathering processes) in the WS model, whereas predictions from Archie's model produced unrealistically high porosity values of over an order of magnitude higher than the WS model. The WS model predicted porosities decreasing exponentially with depth, with values ranging from a few % in the shallowest, most-weathered part of the bedrock (upper 5 m on average) and deep fractured zones (to about 20 m deep), to less than 1% in the underlying fissured aquifer, and possibly down another order of magnitude in the deep massive bedrock. WS-derived porosities were in agreement with independent vertical water content profiles derived from magnetic resonance sounding (MRS), as well as point storativity values estimated from borehole hydraulic testing at the study site, with particularly good matches in the upper weathered/fractured bedrock and deeply weathered/fractured zones associated with regional faults. Detailed comparison suggested that WS provides an upper-bound estimate of groundwater storage in this environment. In the deep massive, un-weathered, and poorly fractured bedrock, however, discrepancies between groundwater storage estimate obtained from the three methods (ERT, MRS, and hydraulic) prevented reliable storage quantification, owing to the methods' inherent technical limitations in such low porosity rocks. Our results demonstrated the suitability of resistivity tomography to quantify groundwater storage heterogeneity in weathered/fractured basement rock aquifers at high resolution and with reasonable overall uncertainty given the relative high uncertainties in petrophysical parameters at the kilometric scale. The results are promising for better characterization of groundwater storage variations in these hydrogeological systems, which are crucial to predict their response to climate variability and human exploitation.
Hydrogeology Journal, 2020
The Nairobi volcano-sedimentary regional aquifer system (NAS) of Kenya hosts >6 M people, incl... more The Nairobi volcano-sedimentary regional aquifer system (NAS) of Kenya hosts >6 M people, including 4.7 M people in the city of Nairobi. This work combines analysis of multi-decadal in-situ water-level data with numerical groundwater modelling to provide an assessment of the past and likely future evolution of Nairobi’s groundwater resources. Since the mid-1970s, groundwater abstraction has increased 10-fold at a rate similar to urban population growth, groundwater levels have declined at a median rate of 6 m/decade underneath Nairobi since 1950, whilst built-up areas have increased by 70% since 2000. Despite the absence of significant trends in climatic data since the 1970s, more recently, drought conditions have resulted in increased applications for borehole licences. Based on a new conceptual understanding of the NAS (including insights from geophysics and stable isotopes), numerical simulations provide further quantitative estimates of the accelerating negative impact of abs...
Advances in Water Resources, 2020
-Inverted resistivity data are commonly used to delineate saltwater intrusion (SWI). -Errors asso... more -Inverted resistivity data are commonly used to delineate saltwater intrusion (SWI). -Errors associated to inversion and petrophysical transformation are assessed. -A coupled modelling approach was applied to four synthetic heterogeneous aquifers. -Inversion and salinity recovery errors are equivalent when neglecting heterogeneity. -Spatial quantification of the errors is key for geophysics-aided SWI management.
In Finland, two large "moraine" ridges (Salpausselka I and Salpausselka II), extending to over 60... more In Finland, two large "moraine" ridges (Salpausselka I and Salpausselka II), extending to over 600 km in length, delineate two major stillstand/readvance positions of the Fennoscandian ice sheet during the last deglaciation (Glückert, 1986). They are inferred to be chronologically related to the cold stage known as the Younger Dryas which occurred at the end of the last glaciation. During this time the Baltic ice lobe and the Finnish Lake District ice lobe, constituting a part of the southern margin of the Fennoscandian ice sheet, were grounded in a large proglacial lake, the Baltic ice lake, a predecessor to the modern-day Baltic Sea. The "moraine" ridge is mostly composed of glaciofluvial sands, gravels and boulders rather than diamicton and deposited on crystalline, impermeable bedrock and constitute freshwater aquifer in southern and eastern Finland. The average thickness of ice-contact deltas sediments is estimated at between 20 and 60 meters and is highly variable. Outcrop studies are combined with GPR and ERTprofiles to provide insight into the aquifer architecture at different scales and depths of investigation. This study aims to improve our understanding of such deposits in the subsurface, especially about their internal structure, sedimentary facies distribution and potential barriers and/or baffles to fluid flow and poro-perm characteristics.
Journal of Soils and Sediments, 2019
Purpose The aim of this work was to identify and analyze the records of flood-drought cycles as p... more Purpose The aim of this work was to identify and analyze the records of flood-drought cycles as preserved in the sediments of the Notwane reservoir, southern Botswana, in order to better understand how extreme events affect water and sediment quality. This work represents the first attempt to study the reservoir sediments in arid to semi-arid environments and suggests that they could be used as proxies for the characterization of the effects of flood-drought cycles. Materials and methods For the first time in an arid context like Botswana, sediments from artificial reservoirs were explored through correlating sediment records with the presence and quantity of pollutants in the reservoir’s wider arid and semi-arid catchment after the latest extreme flood event of 2017. Sediments from the Notwane reservoir were collected with a push corer to a maximum depth of 80 cm. Sediments were then analyzed for grain size distribution, organic matter content, and concentrations of heavy metals (F...
Hydrological Sciences Journal, 2018
Understanding the spatio-temporal variability in groundwater recharge is a prerequisite to sustai... more Understanding the spatio-temporal variability in groundwater recharge is a prerequisite to sustainable management of aquifers. Spatial analysis of groundwater stable isotopes uncovered predominant controls on groundwater recharge in the Nairobi aquifer system (NAS) and the South Coast aquifer (SC), two exemplar East African aquifers relied upon by 7 million people. We analysed 368 samples for stable isotopes and basic physico-chemical parameters. The NAS groundwater isotopes are controlled by precipitation orographic effects and enriched recharge from impounded lakes/wetlands; the SC isotopes are correlated with water-table depth influencing evapotranspiration. Global Network of Isotopes in Precipitation (GNIP) data revealed groundwater recharge during months of heavy rains in the NAS, whilst the SC experiences spatiotemporally diffuse recharge. Inferred "isoscapes" show: in the NAS, (1) direct, rapid recharge favoured by faults, well-drained soils and ample rainfall in uplands, (2) delayed recharge from impounded lakes and wetlands in mid-lands, and (3) focused, event-based recharge in floodplains; and in the SC, diffuse recharge complicated by significant water-table evapotranspiration processes.
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Papers by Jean-Christophe Comte