Olivier Atteia

A comparison between Ca and Sr cycling in forest ecosystems

In favourable conditions, the 87Sr/86Sr isotope ratios of the Sr delivered by rain and soil mineral weathering differ. Assuming that Ca and Sr behave similarly in forest ecosystems, several authors have used the 87Sr/86Sr variation in forest compartments to calculate the contribution of rain and mineral weathering to Ca fluxes and pools. However, there are a number of experimental reports showing that Ca and Sr may behave differently in the soil and in the plant. We have tested this Ca–Sr analogy in the field by measuring the variation of Sr and Ca concentrations, fluxes and pools in spruce, beech and maple stands on granite, sandstone and limestone. Results show that (1) variations of Ca and Sr concentrations are generally correlated at each level of the ecosystems. (2) In spruce on acid soils, a preferential uptake of Ca over Sr occurs (Aubure spruce Sr/Ca = 0.8×10−3; soil exchangeable Sr/Ca between 2 and 6×10−3). On calcareous soils, a preferential uptake of Sr over Ca by spruce may occur. (3) In spruce and beech on acid and calcareous soils, a preferential translocation of Ca over Sr from roots to leaves occurs ((Sr/Ca) in leaves was between 10 and 90% of that in roots). (4) The biological cycling of Ca and Sr leads to an enrichment of the upper soil layers in Ca and Sr. Compared to Sr, Ca accumulates in the upper layer of acid soils because Ca cycling through litterfall is favoured over Sr cycling, and possibly because of the selectivity of acid organic exchangers for Ca.

Semianalytical Model Predicting Transfer of Volatile Pollutants from Groundwater to the Soil Surface

Volatilization of toxic organic contaminants from groundwater to the soil surface is often considered an important pathway in risk analysis. Most of the risk models use simplified linear solutions that may overpredict the volatile flux. Although complex numerical models have been developed, their use is restricted to experienced users and for sites where field data are known in great detail. We present here a novel semianalytical model running on a spreadsheet that simulates the volatilization flux and vertical concentration profile in a soil based on the Van Genuchten functions. These widely used functions describe precisely the gas and water saturations and movement in the capillary fringe. The analytical model shows a good accuracy over several orders of magnitude when compared to a numerical model and laboratory data. The effect of barometric pumping is also included in the semianalytical formulation, although the model predicts that barometric pumping is often negligible. A sensitivity study predicts significant fluxes in sandy vadose zones and much smaller fluxes in other soils. Fluxes are linked to the dimensionless Henrys law constant H for H < 0.2 and increase by approximately 20% when temperature increases from 5 to 25 degrees C.

Vertical heterogeneities of hydraulic aquitard parameters: preliminary results from laboratory and in situ monitoring

Abstract A borehole is developed in a shallow multi-layered aquifer and used to derive the porosity, specific storage and hydraulic conductivity of the aquitard. Local values of hydrodynamical parameters are estimated from petrophysical analysis of core samples, and the empirical relationship between porosity and permeability. Vertical diffusivity is determined from the response of the aquitard to a loading cyclic signal using pressure records at different depths. Hydraulic conductivities deduced from the petrophysical analysis ranged from 10−8 to 10−10 m s−1 and are comparable with those of facies of marine/lacustrine clay observed in samples. The permeability values calculated based on diffusivity are within the range 10−9 to 10−11 m s−1 with a quasi-systematic bias of one order of magnitude. These values are average for a larger part of the aquitard and correspond to an integrated value. The methodology retained for the aquitard characterization is discussed with emphasis on the implications for the management of a complex aquifer system. Citation Larroque, F., Cabaret, O., Atteia, O., Dupuy, A., and Franceschi, M., 2013. Vertical heterogeneities of hydraulic aquitard parameters: preliminary results from laboratory and in situ monitoring. Hydrological Sciences Journal, 58 (4), 912–929.

Origine et évolution du soufre au sein de l'aquifère des Sables infra-molassiques du Bassin aquitain

Abstract New data on isotopic composition of sulphur and oxygen in dissolved sulphates are used to determine the origin of sulphate ions (SO 4 2− ) in the water of the Infra-Molassic Sands Eocene aquifer. Two very distinct origins appear: one is the gypsum dissolution, from the molasse, and the second is the pyrite oxidation, present at the bottom of and within the aquifer. These isotopic analyses helped, in agreement with hydrogeology, to identify geochemical basins whose properties modify the chemical water composition. They also highlight the existence of bioreduction areas. To cite this article: L. Andre et al., C. R. Geoscience 334 (2002) 749–756.

On the role of low-permeability beds in the acquisition of F and SO4 concentrations in a multi-layer aquifer, South-West France.

Fluoride (F(-)) commonly threatens groundwater quality. This is the case around the city of Bordeaux (France), where numerous wells tapping the thick and complex Eocene aquifer are contaminated by fluoride, which presents an issue for drinking water supply. The joint analysis of the spatial distribution of fluoride with other species like sulfate suggests that concentrations are mainly related to the occurrence of low-permeability layers containing evaporites or fluorite deposits. In order to identify the origin of the observed concentrations, a radial flow and transport model is implemented at the borehole scale. The hydraulic conductivity of the low-permeability layers and the vertical dispersivity of the aquifer were optimized to match the observed values of sulfate and fluoride concentrations. Interestingly, each of these parameters influences differently the simulated concentrations. This model has been successfully implemented to a neighboring well with the same parameter values, which tests the approach. The major conclusions drawn are: (i) the contamination in fluoride originates from the low-permeability layers, (ii) every low-permeability layer intercepted by the well releases fluoride (iii) Contamination not only originates from pore water of low-permeability layers, but may persist with long-term pumping due to mineral dissolution. As a consequence, fluoride contamination is likely to persist for a long time and the only solution to reduce fluoride concentration in abstracted water is to seal well screens facing low-permeability layers.

Comments on "Analytical modelling of fringe and core biodegradation in groundwater plumes." by Gutierrez-Neri et al. in J. Contam. Hydrol. 107: 1-9.

In this comment, we revisit equations concerning the analytical solutions presented by Gutierrez-Neri and co-workers for reactive transport for a pollutant undergoing core and fringe degradations. We state that a correction needs to be made in Eq. (9) of the work of Gutierrez-Neri et al. in order that the equation follows closely previous work published by J. Bear (in 1-D) and P.A. Domenico (in 3-D). Furthermore we derive alternative solutions for Eqs. (13)-(16) which separate more clearly the first-order reaction and the instantaneous reaction. It is shown that the corrected solution agrees better with the results from the numerical model than the previous solution. An improvement is also made by giving a solution which avoids negative concentrations. Furthermore, the corresponding solution for the electron acceptor reacting with the pollutant is given.

Comparison of residual NAPL source removal techniques in 3D metric scale experiments

This study compared four treatment techniques for the removal of a toluene/n-decane as NAPL (Non Aqueous Phase Liquid) phase mixture in identical 1 cubic meter tanks filled with different kind of sand. These four treatment techniques were: oxidation with persulfate, surfactant washing with Tween80®, sparging with air followed by ozone, and thermal treatment at 80°C. The sources were made with three lenses of 26×26×6.5cm, one having a hydraulic conductivity similar to the whole tank and the two others a value 10 times smaller. The four techniques were studied after conditioning the tanks with tap water during approximately 80days. The persulfate treatment tests showed average removal of the contaminants but significant flux decrease if density effects are considered. Surfactant flushing did not show a highly significant increase of the flux of toluene but allowed an increased removal rate that could lead to an almost complete removal with longer treatment time. Sparging removed a significant amount but suggests that air was passing through localized gas channels and that the removal was stagnating after removing half of the contamination. Thermal treatment reached 100% removal after the target temperature of 80°C was kept during more than 10d. The experiments emphasized the generation of a high-spatial heterogeneity in NAPL content. For all the treatments the overall removal was similar for both n-decane and toluene, suggesting that toluene was removed rapidly and n-decane more slowly in some zones, while no removal existed in other zones. The oxidation and surfactant results were also analyzed for the relation between contaminant fluxes at the outlet and mass removal. For the first time, this approach clearly allowed the differentiation of the treatments. As a conclusion, experiments showed that the most important differences between the tested treatment techniques were not the global mass removal rates but the time required to reach 99% decrease in the contaminant fluxes, which were different for each technique.

LNAPL source zone delineation using soil gases in a heterogeneous silty-sand aquifer

Source delineation of hydrocarbon contaminated sites is of high importance for remediation work. However, traditional methods like soil core extraction and analysis or recent Membrane Interface Probe methods are time consuming and costly. Therefore, the development of an in situ method based on soil gas analysis can be interesting. This includes the direct measurement of volatile organic compounds (VOCs) in soil gas taken from gas probes using a PID (Photo Ionization Detector) and the analysis of other soil gases related to VOC degradation distribution (CH4, O2, CO2) or related to presence of Light Non-Aqueous Phase Liquid (LNAPL) as (222)Rn. However, in widespread heterogeneous formations, delineation by gas measurements becomes more challenging. The objective of this study is twofold: (i) to analyse the potential of several in situ gas measurement techniques in comparison to soil coring for LNAPL source delineation at a heterogeneous contaminated site where the techniques might be limited by a low diffusion potential linked to the presence of fine sands and silts, and (ii) to analyse the effect of vertical sediment heterogeneities on the performance of these gas measurement methods. Thus, five types of gases were analysed: VOCs, their three related degradation products O2, CO2 and CH4 and (222)Rn. Gas measurements were compared to independent LNAPL analysis by coring. This work was conducted at an old industrial site frequently contaminated by a Diesel-Fuel mixture located in a heterogeneous fine-grained aquifer. Results show that in such heterogeneous media migration of reactive gases like VOCs occurs only across small distances and the VOC concentrations sampled with gas probes are mainly related to local conditions rather than the presence of LNAPL below the gas probe. (222)Rn is not well correlated with LNAPL because of sediment heterogeneity. Oxygen, CO2, and especially CH4, have larger lengths of diffusion and give the clearest picture for LNAPL presence at this site even when the gas probe is somewhat distant.

Improved constraints on in situ rates and on quantification of complete chloroethene degradation from stable carbon isotope mass balances in groundwater plumes

Spills of chloroethenes (CEs) at industrial and urban sites can create groundwater plumes in which tetrachloro- and trichloroethene sequentially degrade to dichloroethenes, vinyl chloride (VC) and ethene, or ethane under reducing conditions. For detoxification, degradation must go beyond VC. Assessments based on ethene and ethane, however, are difficult because these products are volatile, may stem from alternative sources, can be further transformed and are not always monitored. To alternatively quantify degradation beyond VC, stable carbon isotope mass balances have been proposed where concentration-weighted CE isotope ratios are summed up and compared to the original source isotope ratio. Reported assessments, however, have provided not satisfactorily quantified results entailing greatly differing upper and lower estimates. This work proposes an integrative approach to better constrain the extent of total chloroethene degradation in groundwater samples. It is based on fitting of measured concentration and compound-specific stable carbon isotope data to an analytical reactive transport equation simulating steady-state plumes in two dimensions using an EXCEL spreadsheet. The fitting also yields estimates of degradation rates, of source width and of dispersivities. The approach is validated using two synthetic benchmark cases where the true extent of degradation is well known, and using data from two real field cases from literature.

Field demonstration of foam injection to confine a chlorinated solvent source zone

A novel approach using foam to manage hazardous waste was successfully demonstrated under active site conditions. The purpose of the foam was to divert groundwater flow, that would normally enter the source zone area, to reduce dissolved contaminant release to the aquifer. During the demonstration, foam was pre generated and directly injected surrounding the chlorinated solvent source zone. Despite the constraints related to the industrial activities and non-optimal position of the injection points, the applicability and effectiveness of the approach have been highlighted using multiple metrics. A combination of measurements and modelling allowed definition of the foam extent surrounding each injection point, and this appears to be the critical metric to define the success of the foam injection approach. Information on the transport of chlorinated solvents in groundwater showed a decrease of contaminant flux by a factor of 4.4 downstream of the confined area. The effective permeability reduction was maintained over a period of three months. The successful containment provides evidence for consideration of the use of foam to improve traditional flushing techniques, by increasing the targeting of contaminants by remedial agents.