Hydrogeological Experimental Site Research Articles

The Hydrogeological Experimental Site of Poitiers: Hydrogeological versus geophysical investigations

The University of Poitiers (France) has a Hydrogeological Experimental Site (HES) built near the Campus for the sole purpose of providing facilities to develop long-term monitoring and experiments investigating the water and mass transfer processes. The site has been investigated by conventional hydrogeological surveys including flow and temperature measurements, pumping and slug tests. The site was also subjected to geophysical investigations both in surface (3D seismic) and in wells (vertical seismic profile (PSV) and acoustic logging). The paper presents an overview of the different field experiments and shows their relative contribution to knowledge of the karstified Dogger limestones of the Poitou threshold.

Seismic and stratigraphic characterization of karstogenic horizons in a sequence of carbonate deposits: Example of the Dogger limestones of the Poitou threshold

The Deffend Hydrogeological Experimental Site (HES), near Poitiers, has been the site of numerous studies that have shown that the Bajocian and Aalenian limestones were modified by the horizontal development of multiple karstification planes. These investigations have demonstrated the occurrence of stacked circulation. The origin of the structures that generated these sub-horizontal flows has not been specifically explained. Geophysical investigations were conducted at the site, both at the surface (3D seismic) and in wells (vertical seismic profile (PSV) and acoustic logging). Comparison of the geophysical data, mainly acoustic logs, and a stratigraphic interpretation of cuttings and borehole wall imaging makes it possible to propose a coherent karstogenesis scheme that does not involve tectonic constraints. This article offers an explanation based on gaps recorded in the sequence of Dogger deposits. The karstogenic horizons are linked to sedimentary discontinuities that geologists have identified on the Poitou threshold.

Delineation of discrete conduit networks in karst aquifers via combined analysis of tracer tests and geophysical data

Abstract. Assessment of the karst network geometry based on field data is an important challenge in the accurate modeling of karst aquifers. In this study, we propose an integrated approach for the identification of effective three-dimensional (3D) discrete karst conduit networks conditioned on tracer tests and geophysical data. The procedure is threefold: (i) tracer breakthrough curves (BTCs) are processed via a regularized inversion procedure to determine the minimum number of distinct tracer flow paths between injection and monitoring points, (ii) available surface-based geophysical data and borehole-logging measurements are aggregated into a 3D proxy model of aquifer hydraulic properties, and (iii) single or multiple tracer flow paths are identified through the application of an alternative shortest path (SP) algorithm to the 3D proxy model. The capability of the proposed approach to adequately capture the geometrical structure of actual karst conduit systems mainly depends on the sensitivity of geophysical signals to karst features, whereas the relative completeness of the identified conduit network depends on the number and spatial configuration of tracer tests. The applicability of the proposed approach is illustrated through a case study at the Hydrogeological Experimental Site (HES) in Poitiers, France.

Parameter Identification by High-Resolution Inverse Numerical Model Based on LBM/CMA-ES: Application to Chalk Aquifer (North of France)

The present paper proposes the numerical solution of an inverse problem in groundwater flow (Darcy’s equation). This solution was achieved by combining a high-resolution new code HYSFLO-LBM (Hydrodynamic of Subsurface Flow by Lattice Boltzmann Method), based on LBM, to solve the direct problem, and the metaheuristic optimization algorithm CMA-ES ES (Covariance Matrix Adaptation-Evolution Strategy) to solve the optimization step. The integrated optimization algorithm which resulted from this combination, HYSFLO-LBM/CMA-ES, was applied to the hydrogeological experimental site of Beauvais (Northern France), instrumented by a set of sensors distributed over 20 hydrogeological wells. Hydrogeological parameters measured by the sensors are necessary to understand the aquifer functioning and to serve as input data for the identification of the transmissivity field by the HYSFLO-LBM/CMA-ES code. Results demonstrated an excellent concordance between the integrated optimization algorithm and hydrogeological applied methods (pumping test and magnetic resonance sounding). The spatial distribution of the transmissivity and hydraulic conductivity are related to the heterogeneous distribution of aquifer formations. The LBM and CMA-ES were chosen for their proven excellent performance and lesser cost, in terms of both money and time, unlike the geophysical survey and pumping test. The model can be used and developed as a decision support tool for integrated water resources management in the region.

Concomitant behavior of arsenic and selenium from the karst infillings materials of the fractured carbonate Dogger Aquifer (Hydrogeological Experimental Site, Poitiers, France)

Petrographic and mineralogical analyses combined with sequential extractions and leaching experiments as a function of pH were performed on black clayey sediments fulfilling karsts in the Hydrogeological Experimental Site (HES) of Poitiers (France) to investigate the behavior of arsenic and selenium in a fractured limestone aquifer.Sequential extractions showed that arsenic is mainly associated with pyrite (about 35%) and secondary iron oxyhydroxides (around 13%), along with a substantial exchangeable fraction (about 13%). The soluble fraction and the fraction associated to organic matter are ∼2% and ∼5%, respectively. The distribution of selenium is mainly pyritic (around 39%) or associated with organic matter (about 18%). Its association to secondary iron oxyhydroxides minerals is low (around 2%), whereas its soluble fraction is around 5%.SEM analyses revealed the presence of arsenic “hot spots” into euhedral pyrite crystals surrounded by a halo of iron oxyhydroxides resulting from their alteration, and both are enriched with arsenic. Selenium has a similar pyritic origin but after alteration, it is predominantly associated with organic matter.Despite different distributions, the leaching experiment as a function of pH showed that the mobilization of arsenic and selenium overlapped below pH 2 and above pH 8. The main differences were observed between pH 2 and 8 with a plateau at 5% of released selenium, whereas the amount of mobilized arsenic continuously decreased. The pH-dependence of both elements is attributed to the partial dissolution of pyrite in acidic conditions combined with desorption processes at higher pH values.

Contribution of Full Wave Acoustic Logging to the Detection and Prediction of Karstic Bodies

A 3D seismic survey was done on a near surface karstic reservoir located at the hydrogeological experimental site (HES) of the University of Poitiers (France). The processing of the 3D data led to obtaining a 3D velocity block in depth. The velocity block was converted in pseudo porosity. The resulting 3D seismic pseudo-porosity block reveals three high-porosity, presumably-water-productive layers, at depths of 30–40, 85–87 and 110–115 m. This paper shows how full wave acoustic logging (FWAL) can be used to validate the results obtained from the 3D seismic survey if the karstic body has a lateral extension over several seismic. If karstic bodies have a small extension, FWAL in open hole can be fruitfully used to: detect highly permeable bodies, thanks to measurements of acoustic energy and attenuation; detect the presence of karstic bodies characterized by a very strong attenuation of the different wave trains and a loss of continuity of acoustic sections; confirm the results obtained by vertical seismic profile (VSP) data. The field example also shows that acoustic attenuation of the total wavefield as well as conversion of downward-going P-wave in Stoneley waves observed on VSP data are strongly correlated with the presence of flow.

Investigating karst aquifers: Using pseudo 3-D electrical resistivity tomography to identify major karst features

This work presents the electrical resistivity imaging of the Hydrogeological Experimental Site (HES) karst aquifer of Poitiers (France) from pseudo three-dimensional (3-D) Electrical Resistivity Tomography (ERT), i.e., a three-dimensional model obtained from the joint inversion of resistivity data collected along parallel profiles. The inverse resistivity model provided a fairly reliable definition of the main hydrogeological units. The survey also allowed the recognition of possible shallow dissolution-enlarged fractures that do not show any ground expression at present but have been revealed at depth by vertical flowmeter data and high-resolution video footage from wells. These features seem to connect with middle-depth sub-vertical features down to a depth of 25 m, which are likely to correlate with soil-mantled, incipient sinkhole formation features that do not show any ground expression at present, as well as karst conduits. At greater depth, the resistivity model allowed the recognition of the first two main karst intervals (35–40 m and 85–87 m deep) associated with water productive layers, as revealed by well logs. The opportunity to cross-validate geophysical results with extensive ground truth provided by the HES allowed an assessment of the error the pseudo 3-D ERT model makes in estimating the depth of the karst intervals. Synthetic dataset modelling provided the detectability of the pseudo 3-D ERT model in identifying karst intervals. Synthetic dataset modelling also allowed an estimation of the response of the experimental and inversion setup to a variable degree of heterogeneity of the aquifer, using well-log data as geological a priori information.

The successful experience of IGCP636 project “Unifying international research forces to unlock and strengthen geothermal exploitation of the Americas and Europe”

Geothermal fluids are extracted through deep wells drilled in a geological reservoir, which can be represented by highly heterogeneous volcanic complex, sedimentary basins or deep basement rocks. The main objective of this project was to propose suitable methodologies and techniques for the characterization and modeling of fractured geothermal reservoirs, to ensure their sustainable exploitation and, therefore, ensure acceptation of this kind of energy by local communities. This goal was achieved with experiences from the volcanic complex of the Nevado del Ruiz (Colombia), the St. Lawrence Lowlands sedimentary basin (Quebec, Canada), the Carboniferous limestone reservoir in the area of Mons (Belgium), the Soultz-Sous-Forets power plant and the Ploemeur fractured rock hydrogeological experimental site (France), and various areas of interest for geothermal exploration in Chile. The work conducted provided new insights into the construction of conceptual and numerical models for geothermal reservoirs and allowed the creation of a solid research network between Europe and Americas, fostering participation in geosciences of young researchers, students, and women.

Distribution of arsenic in an argillaceous sample using sequential chemical extractions: application to the Hydrogeological Experimental Site of Poitiers (HESP)

Arsenic mobility and transport into groundwater is highly dependent on its chemical distribution in sediments. In this study, a combination of sequential extraction and mineralogical investigation by X-ray diffraction on a sample collected from the hydrogeological experiment site of Poitiers (France) was tested to evaluate and predict the mobility of arsenic in different conditions. Results showed that the highest fraction of As (~35 %) was released by 25 % nitric acid however this fraction is poorly mobile under natural chemical conditions due to its high stability. Exchangeable As (~14 %) and As bound to iron and aluminium oxides (~12 %) exhibited a significant fraction that could be of high concern in case of change in the ionic composition and redox conditions, respectively. Soluble fraction comprised only 1 % of As which was in accordance with the composition of groundwater in the site.

Leaching behavior of selenium from the karst infillings of the Hydrogeological Experimental Site of Poitiers

Abstract The selenium enrichment in groundwater may be caused by the contact with sedimentary formations. In many French regions, geogenic sedimentary sources of Se were identified as responsible of the Se concentrations exceeding the European limit in many wells designated to produce drinking water. This study focuses on the black argillaceous infillings of the Hydrogeological Experimental Site of Poitiers (HESP) and aims to better understand the Se release mechanisms at the interface between these infillings and aqueous media using batch experiments with variable pH and contact time. The Se behavior as function of pH was similar in the ultrapure water and in the HESP water and can be discriminated in three main domains. For very acidic domain the speciation of the released Se, that can be associated to the dissolution of some mineral phases, was majorly unidentified. For 2 in-situ data confirm the diversity and the complexity of the release mechanisms of Se that is not associated with a specific carrier phase but disseminated within the sedimentary matrix.

On the use of multiple-point statistics to improve groundwater flow modeling in karst aquifers: A case study from the Hydrogeological Experimental Site of Poitiers, France

Limestone aquifers often exhibit complex groundwater flow behaviors resulting from depositional heterogeneities and post-lithification fracturing and karstification. In this study, multiple-point statistics (MPS) was applied to reproduce karst features and to improve groundwater flow modeling. For this purpose, MPS realizations were used in a numerical flow model to simulate the responses to pumping test experiments observed at the Hydrogeological Experimental Site of Poitiers, France. The main flow behaviors evident in the field data were simulated, particularly (i) the early-time inflection of the drawdown signal at certain observation wells and (ii) the convex behavior of the drawdown curves at intermediate times. In addition, it was shown that the spatial structure of the karst features at various scales is critical with regard to the propagation of the depletion wave induced by pumping. Indeed, (i) the spatial shape of the cone of depression is significantly affected by the karst proportion in the vicinity of the pumping well, and (ii) early-time inflection of the drawdown signal occurs only at observation wells crossing locally well-developed karst features.

Investigation of the nature and origin of the geological matrices rich in selenium within the Hydrogeological Experimental Site of Poitiers, France

The selenium (Se) content and the associated release mechanisms in both surface and groundwater have become a major concern worldwide over the past 30 years. Within the Hydrogeological Experimental Site of Poitiers (HESP), a large range of aqueous Se concentrations (from non-detectable to more than 30 ppb) is observed in a limited area (about 10ha), where water flows are highly characterized. This site thus consists of an interesting spot to better understand the release mechanisms of selenium into groundwater. The present study consists of an identification and a characterization of the lithological sources of Se within the HESP. Total rock analyses applied to core samples from different depths and wells demonstrated that selenium is concentrated in argillaceous sediments enriched with organic matter, pyrite and uranium that fulfill a part of the karst cavities developed within the Bajocian host rocks. Mineralogical and petrographic investigations highlighted the heterogeneity of these filling materials and showed the presence of successive deposits under different climatic conditions. Extensive characterizations dated the selected Se-rich samples from the Upper Cretaceous and converged to demonstrate the external and continental origin of the studied filling materials and their transformation after deposition under reduced conditions. Only indirect correlations allow considering an agreement between the Se history and the very mature organic matter identified in the argillaceous samples. This association will be further favored to determine the mechanisms releasing selenium into groundwater.

Characterization of reciprocity gaps from interference tests in fractured media through a dual porosity model

AbstractWe analyze drawdown reciprocity gaps emerging in interference tests performed in a confined fissured karstic formation. Modeling the system as a dual porosity continuum allows characterizing the dynamics of the relative contribution of the connected fractures and the rock matrix to the total flow rate extracted at the pumping wells. Observed lack of reciprocity of drawdowns can then be linked to the occurrence of processes that are not accounted for in the classical flow models based on a single‐continuum representation of the system through flow equations grounded on Darcy's law only. We show that interpreting the system as a dual porosity continuum can cause drawdown reciprocity gaps to emerge as a consequence of local effects associated with an identifiable contribution of the matrix to the total fluid extracted at the well location during pumping. These theoretical results are then employed to identify the contribution to the flow being supplied to the pumping well by the low conductivity matrix constituting the host rock formation, in contrast to that provided by the fractures. An application to data from two interference tests performed at the Hydrogeological Experimental Site (HES) in Poitiers, France, illustrates the approach. We show that, whenever the matrix is assumed to provide a contribution to the total flow rate extracted, nonreciprocity is expected, the latter being linked to the occurrence of a differential drawdown between fracture and matrix at the pumping well. This difference decreases with time in the example presented, displaying a power law late time behavior, with nonreciprocity effects persisting up to remarkably long times.

Determination of the distribution and speciation of selenium in an argillaceous sample using chemical extractions and post-extractions analyses: application to the hydrogeological experimental site of Poitiers

To better understand selenium's dynamics in environmental systems, the present study aims to investigate selenium speciation and distribution in black argillaceous sediments, partially fulfilling karstic cavities into the Hydrogeological Experimental Site of Poitiers. These sediments are suspected to be responsible for selenium concentrations exceeding the European Framework Directive's drinking water limit value (10μgL(-1)) in some specific wells. A combination of a sequential extractions scheme and single parallel extractions was thus applied on a representative argillaceous sample. Impacts of the extractions on mineral dissolution and organic matter mobilization were followed by quantifying major cations and total organic carbon (TOC) in the aqueous extracts. The nature of the released organic matter was characterized using thermochemolysis coupled with gas chromatography-mass spectrometry (GC-MS). About 10% of selenium from the black argillaceous studied matrix could be defined as 'easily mobilizable' when the majority (around 70%) revealed associated with the aliphatic and alkaline-soluble organic matter's fraction (about 20%). In these fractions, selenium speciation was moreover dominated by oxidized species including a mixture of Se(VI) (20-30%) and Se(IV) (70-80%) in the 'easily mobilizable' fraction, while only Se(IV) was detected in alkaline-soluble organic matter fraction.

Hydrogeological Experimental Site of Poitiers (France)

In hydrogeology, understanding the behavior of aquifers is an important issue. May it be for forecasting the availability of water on a long term or for estimating the impact of pollutants when injected at difierent regions, modeling of an aquifer is very useful for hydrogeologists. This paper gives an overlook of the difierent types of tests on a particularsite, namely the Hydrogeological Experimental Site (HES) of Poitiers (France). The aim is to show that the HES is a huge instrumented test bed allowing difierent types of experiments. Thus, system identification users can expect to gather large series of input-output datasets so as to experiment their own tools. Note that a large database exists concerning already-run tests, but what is interesting is that new experiments can be programmed so as to account for customized datasets. As a counterpart, the use of HES data should promote proper modeling of the underlying aquifer, thus contributing to the hydrological research field.

The Nature of Selenium Species in the Hydrogeological Experimental Site of Poitiers

High concentrations in selenium - above the quality limit of 10 ppb - were detected in some wells of the Hydrogeological Experimental Site (HES) of Poitiers. Speciation measurements show that Se migrates mainly as selenate in the Dogger's aquifer water. The characterizations of drill core samples tend to attribute a geogenic origin to the selenium species observed in the Dogger's aquifer and to identify specific argillaceous levels fulfilling some karst cavities as selenium host matrix. Chemical extractions applied to these argillaceous samples show that most of Se is mobilized in parallel with the base-soluble organic matter part as Se(IV) species and other reduced forms.

Inversion of a dual-continuum approach to flow in a karstified limestone: Insight into aquifer heterogeneity revealed by well-test interferences

Summary Two large sets of interference testing data from a karstic aquifer (Hydrogeological Experimental Site, HES – Poitiers – France) are inverted using a dual-continuum approach to groundwater flow. The parameterization technique used to infer spatially distributed hydrodynamic parameters is free of any prior estimation regarding the spatial structure of the parameter fields; it merely seeks the minimal number of local values to obtain the best fit between flow simulations and data. A previous work inverting without any prior geostatistical assumption the same set of data based on a single flowing continuum produced nicely structured (spatially correlated) parameter fields. The dual-continuum approach renders fields as patchworks of subareas of quite uniform values. The statistical distribution of the hydrodynamic parameters is generally skewed toward high values, especially when interference data bear traces of rapid channeled flow in karstic conduits. It is now well established that continuous approaches to flow are well suited to simulate diffusive flow in fractured – karstified rocks, although single continuums may result in artificially high contrasts of hydraulic diffusion values between flowing and non-flowing areas. In the case of a dual continuum, the model has in its physics the capability to clearly separate conduction properties attached to fractures and channels from capacitive properties of the host matrix. We show that high contrasts of hydraulic diffusion disappear, resulting in a mildly heterogeneous medium in which high values of hydraulic diffusion are only located in areas where channeled flow occurs.

Predictive modelling of hydraulic head responses to dipole flow experiments in a fractured/karstified limestone aquifer: Insights from a comparison of five modelling approaches to real-field experiments

Summary Five modelling approaches were tested against a well-studied limestone aquifer constituting the Hydrogeological Experimental Site (HES) of Poitiers, France. The modelling exercise consisted of predicting the drawdown responses of a series of observation wells for two dipole (pumping–injection) flow experiments involving two distinct well pairs at the HES. The differences between model predictions appear to be mainly related to the hydraulic datasets used for model parameterisation and, to a much lesser extent, to the conceptual modelling approach (equivalent porous medium vs. discrete fracture networks), the model dimensionality (two-dimensional vs. three-dimensional), and/or the parameterisation approach (forward vs. inverse). Despite the abundance and diversity of calibration/parameterisation data, all of the models failed to predict the drawdowns with a reasonable degree of accuracy. Only the order of magnitude of the drawdowns was correctly predicted by three of the five models, whereas all models failed to predict the drawdown behaviour at both intermediate and late times. The primary source of error is attributed to a lack of information in the single-well pumping test data, which did not capture the multi-permeability structure of the aquifer as revealed a posteriori by the responses to dipole flow experiments. This highlights the need to develop novel (or improved) approaches to characterise the hydraulic properties of limestone aquifers.

Theoretical analysis and field evidence of reciprocity gaps during interference pumping tests

We address the question of the reciprocity of drawdowns observed during a sequence of interference pumping tests performed within a porous medium, in the context of different conceptual models of subsurface flow. We provide a generalization of the work by Bruggeman (1972) [5], extending his results obtained in 1972 for Darcian flows in an unbounded, heterogeneous porous medium. We then analyze reciprocity within a dual-continuum conceptualization, where the medium is viewed as being composed of two overlapping continua, representing the porous matrix and embedded fractures, respectively. We show theoretically, and demonstrate numerically, that only drawdowns associated with the fracture continuum display reciprocity under transient flow conditions. Conversely, non-flowing matrix and fracture continua display reciprocity behavior under steady-state conditions. We then provide field evidence of the insurgence of reciprocity gaps by analyzing interference test data from the karstic limestone aquifer of the Hydrogeological Experimental Site (HES) in Poitiers, France. On the basis of our theoretical results and experimental observations, we discuss different interpretations of the observed reciprocity gaps. These include (a) non-linear dependencies of local hydraulic parameters, (b) occurrence of internal boundaries within the domain, (c) inertial effects that develop through open conduits within the rock matrix, (d) modifications of the aquifer properties between subsequent pumping tests, and (e) significant contribution of the matrix pressure in monitored wells when the behavior of the aquifer is conceptualized by a dual-continuum approach.

Inversion of a set of well-test interferences in a fractured limestone aquifer by using an automatic downscaling parameterization technique

This work inverts interference hydraulic tests from a karstic aquifer modeled as a single continuum with heterogeneous hydraulic properties (hydraulic conductivity K and specific storage capacity Ss). An automatic technique of parameterization is proposed to solve the inverse problem. Sought parameters are located at the nodes of a parameter grid made of triangular elements and independently superimposed onto the calculation grid of the flow problem. The parameter grid can be refined within the process of inversion so that the solution is improved locally because increasing the number of parameters better depicts spatial heterogeneity. Applied to hydraulic tests carried out at the Hydrogeological Experimental Site (HES) of Poitiers–France, this Adaptative Multi-scale Triangulation (AMT) method of parameterization reveals that hydraulic conductivity K and hydraulic diffusion K/Ss are structured in space as correlated stochastic continuums. This structure is not imposed a priori but is found as the mere consequence of inverting drawdown data. The sought stochastic continuums are discussed and compared to previous results obtained by processing the same data with various techniques. Even though interference data show evidences of local karstic features, the hydraulic bulk behavior of the aquifer is that of a diffusive continuous medium. This reinforces the idea that at mid to large scales, flow in fractured rocks can also be handled with relevance by means of a continuous heterogeneous approach. This result would extend to fractured aquifers with local draining in open conduits as is the case in this study.