Passive temperature tomography experiments to characterize transmissivity and connectivity of preferential flow paths in fractured media

International audienceThe detection of preferential flow paths and the characterization of their hydraulic properties are major challenges in fractured rock hydrology. In this study, we propose to use temperature as a passive tracer to characterize fracture connectivity and hydraulic properties. In particular, we propose a new temperature tomography field method in which borehole temperature profiles are measured under different pumping conditions by changing successively the pumping and observation boreholes. To interpret these temperature- depth profiles, we propose a three step inversion-based framework. We consider first an inverse model that allows for automatic permeable fracture detection from borehole temperature profiles under pumping conditions. Then we apply a borehole-scale flow and temperature model to produce flowmeter profiles by inversion of temperature profiles. This second step uses inversion to characterize the relationship between temperature variations with depth and borehole flow velocities (Klepikova et al., 2011). The third inverse step, which exploits cross-borehole flowmeter tests, is aimed at inferring inter-borehole fracture connectivity and transmissivities. This multi-step inverse framework provides a means of including temperature profiles to image fracture hydraulic properties and connectivity. We test the proposed approach with field data obtained from the Ploemeur (N.W. France) fractured rock aquifer, where the full temperature tomography experiment was carried out between three 100 m depth boreholes 10 m apart. We identified several transmissive fractures and their connectivity which correspond to known fractures and corroborate well with independent information, including available borehole flowmeter tests and geophysical data. Hence, although indirect, temperature tomography appears to be a promising approach for characterizing connectivity patterns and transmissivities of the main flow paths in fractured rock

Thermal-Plume fibre Optic Tracking (T-POT) test for flow velocity measurement in groundwater boreholes

International audienceWe develop an approach for measuring in-well fluid velocities using point electrical heating combined with spatially and temporally continuous temperature monitoring using Distributed Temperature Sensing (DTS). The method uses a point heater to warm a discrete volume of water. The rate of advection of this plume, once the heating is stopped, equates to the average flow velocity in the well. We conducted Thermal-Plume fibre Optic Tracking (T-POT) tests in a borehole in a fractured rock aquifer with the heater at the same depth and multiple pumping rates. Tracking of the thermal plume peak allowed the spatially varying velocity to be estimated up to 50 m downstream from the heating point, depending on the pumping rate. The T-POT technique can be used to estimate the velocity throughout long intervals provided that thermal dilution due to inflows, dispersion, or cooling by conduction do not render the thermal pulse unresolvable with DTS. A complete flow log may be obtained by deploying the heater at multiple depths, or with multiple point heaters

Plant growth drives soil nitrogen cycling and N-related microbial activity through changing root traits

Relationships between plants and nitrogen-related microbes may vary with plant growth. We investigated these dynamic relationships over three months by analyzing plant functional traits (PFT), arbuscular mycorrhizal fungal (AMF) colonization, potential N mineralization (PNM), potential nitrification (PNA) and denitrification activities (PDA) in Dactylis glomerata cultures. D. glomerata recruited AMF during early growth, and thereafter maintained a constant root colonization intensity. This may have permitted high enough plant nutrient acquisition over the three months as to offset reduced soil inorganic N. PFT changed with plant age and declining soil fertility, resulting in higher allocation to root biomass and higher root C:N ratio. Additional to root AMF presence, PR' changes may have favored denitrification over mineralization through changes in soil properties, particularly increasing the quality of the labile carbon soil fraction. Other PFT changes, such as N uptake, modified the plants' ability to compete with bacterial groups involved in N cycling. (C) 2020 Elsevier Ltd and British Mycological Society. All rights reserved.Peer reviewe

Inferring field-scale properties of a fractured aquifer from ground surface deformation during a well test

International audienceFractured aquifers which bear valuable water resources are often difficult to characterize with classical hydrogeological tools due to their intrinsic heterogeneities. Here, we implement ground surface deformation tools (tiltmetry and optical leveling) to monitor groundwater pressure changes induced by a classical hydraulic test at the Ploemeur observatory. By jointly analyzing complementary time constraining data (tilt) and spatially constraining data (vertical displacement), our results strongly suggest that the use of these surface deformation observations allows for estimating storativity and structural properties (dip, root depth, lateral extension) of a large hydraulically active fracture, in good agreement with previous studies. Hence, we demonstrate that ground surface deformation is a useful addition to traditional hydrogeological techniques and opens possibilities for characterizing important large-scale properties of fractured aquifers with short-term well tests as a controlled forcing

Heat as a tracer for understanding transport processes in fractured media: Theory and field assessment from multiscale thermal push-pull tracer tests

International audienceThe characterization and modeling of heat transfer in fractured media is particularly challenging as the existence of fractures at multiple scales induces highly localized flow patterns. From a theoretical and numerical analysis of heat transfer in simple conceptual models of fractured media, we show that flow channeling has a significant effect on the scaling of heat recovery in both space and time. The late time tailing of heat recovery under channeled flow is shown to diverge from the TðtÞ / t 21:5 behavior expected for the classical parallel plate model and follow the scaling TðtÞ / 1=tðlog tÞ 2 for a simple channel modeled as a tube. This scaling, which differs significantly from known scalings in mobile-immobile systems, is of purely geometrical origin: late time heat transfer from the matrix to a channel corresponds dimensionally to a radial diffusion process, while heat transfer from the matrix to a plate may be considered as a one-dimensional process. This phenomenon is also manifested on the spatial scaling of heat recovery as flow channeling affects the decay of the thermal breakthrough peak amplitude and the increase of the peak time with scale. These findings are supported by the results of a field experimental campaign performed on the fractured rock site of Ploemeur. The scaling of heat recovery in time and space, measured from thermal breakthrough curves measured through a series of push-pull tests at different scales, shows a clear signature of flow channeling. The whole data set can thus be successfully represented by a multichannel model parametrized by the mean channel density and aperture. These findings, which bring new insights on the effect of flow channeling on heat transfer in fractured rocks, show how heat recovery in geothermal tests may be controlled by fracture geometry. In addition, this highlights the interest of thermal push-pull tests as a complement to solute tracers tests to infer fracture aperture and geometry

About the monitoring of groundwater fluxes variations through active-DTS measurements

National audienceActive-Distributed Temperature Sensing (DTS) measurements, which consists in heating a Fiber Optic (FO) cable and in monitoring the temperature elevation, has proven to be very efficient to quantify the spatial distribution of groundwater fluxes in saturated porous media at high resolution with low uncertainties. Here, we investigate the feasibility of active-Distributed Temperature Sensing (DTS) measurements to monitor and quantify groundwater fluxes variations over time, in response to the need of characterizing temporal dynamic of groundwater and the lack of available methods. To test the method, we rely on both numerical simulations and sandbox experiments to assess the sensitivity of temperature elevation to variable flow conditions and our ability to interpret associated temperature variations. Results confirm that the temperature elevation and evolution over time is sensitive to flow conditions and that associated temperature variations can be used to characterize groundwater fluxes variations. First, experimental and numerical results show that when a flow change is followed by a long-enough steady-state flow period the temperature stabilizes independently of previous fluxes conditions. In such case, the stabilization temperature can easily be interpreted to estimate groundwater fluxes using the analytical model commonly used under steady flow conditions to interpret active-DTS measurements. Furthermore, we demonstrate here that, under certain flow conditions depending on the nature of flow variations, the approach offers the possibility of continuously monitoring fluxes variations. For instantaneous flow changes, it is even possible to go further by reproducing temperature signal variations over time by applying the superposition principle to the analytical model. These preliminary tests are particularly promising and open new perspectives for monitoring and/or quantifying the temporal dynamic of groundwater fluxes at different temporal scales including diurnal and short-term periodic fluxes variations

About the monitoring of groundwater fluxes variations through active-DTS measurements

National audienceActive-Distributed Temperature Sensing (DTS) measurements, which consists in heating a Fiber Optic (FO) cable and in monitoring the temperature elevation, has proven to be very efficient to quantify the spatial distribution of groundwater fluxes in saturated porous media at high resolution with low uncertainties. Here, we investigate the feasibility of active-Distributed Temperature Sensing (DTS) measurements to monitor and quantify groundwater fluxes variations over time, in response to the need of characterizing temporal dynamic of groundwater and the lack of available methods. To test the method, we rely on both numerical simulations and sandbox experiments to assess the sensitivity of temperature elevation to variable flow conditions and our ability to interpret associated temperature variations. Results confirm that the temperature elevation and evolution over time is sensitive to flow conditions and that associated temperature variations can be used to characterize groundwater fluxes variations. First, experimental and numerical results show that when a flow change is followed by a long-enough steady-state flow period the temperature stabilizes independently of previous fluxes conditions. In such case, the stabilization temperature can easily be interpreted to estimate groundwater fluxes using the analytical model commonly used under steady flow conditions to interpret active-DTS measurements. Furthermore, we demonstrate here that, under certain flow conditions depending on the nature of flow variations, the approach offers the possibility of continuously monitoring fluxes variations. For instantaneous flow changes, it is even possible to go further by reproducing temperature signal variations over time by applying the superposition principle to the analytical model. These preliminary tests are particularly promising and open new perspectives for monitoring and/or quantifying the temporal dynamic of groundwater fluxes at different temporal scales including diurnal and short-term periodic fluxes variations

Comportement à long terme d’un aquifèrehétérogène dans des conditions transitoires

National audienceAvec l’augmentation de la consommation en eau, la connaissancedes mécanismes de recharge dans les aquifères hétérogènesdevient d’une importance majeure. En particulier il est nécessairede comprendre comment la pression anthropique et les variationsclimatiques déstabilisent les systèmes naturels afin de faire face àune possible évolution dans le temps. La réponse équivalente d’unsystème aquifère est généralement interprétée comme complexe,non-linéaire et hystérétique. Un autre point de vue considérerait laréponse actuelle comme la superposition de nombreux événementstransitoires en milieu complexe et donc dépendante de l’état passéde l’aquifère et des conditions limites.Ici, nous présentons une méthode pour modéliser et caractériserl’impact à long terme des déstabilisations anthropiques et climatiques.En effet l’état actuel d’un aquifère peut se reconstruirecomme l’impact cumulé des anciennes conditions limites, qui sontnaturellement considérées comme non-stationnaires. Nous présentonsplusieurs modèles semi-analytiques et numériques basés surdifférents types de conditions aux limites.Ces modèles sont employés pour interpréter la réponse hydrauliqued’un système aquifère plurikilométrique en domaine fracturé, ense basant sur 25 ans de données piézométriques. Le terrain expérimentalfait partie du réseau SOERE (H+) et se situe à Ploemeur(Morbihan). Deux systèmes en contexte géologique similairesont comparés, l’un étant pompé à un débit de 1,1 Mm3/andepuis 1991, l’autre demeurant encore à l’état naturel. Nosmodèles pourraient prédire le comportement des systèmes hydrogéologiquesen réponse à la variabilité climatique et estimer laperturbation du bilan hydrologique et des flux naturels engendréspar les pompages. L’accent est mis sur l’importance d’intégrer lecaractère transitoire des processus dans la modélisation des systèmespour une gestion précise des ressources d’eau souterraine.Nous montrons également le rôle prépondérant des conditions limitessur l’hétérogénéité des aquifères