Jean-Paul Gaudet

Field measurement of soil surface hydraulic properties by disc and ring infiltrometers A review and recent developments

Abstract Soil management influences physical properties and mainly the soil hydraulic functions. Their measurement becomes one of the research preferences in this branch of applied soil science. Tension disc and pressure ring infiltrometers have become very popular devices for the in situ estimates of soil surface hydraulic properties. Their use for measuring solute–water transfer parameters of soils is now well established too. A number of publications testify that both devices have been extensively used all around the world for different purposes. In this review, a short introduction is devoted to the background theory and some examples are given to show how the theory can be used to determine hydraulic conductivity and sorptivity from measured cumulative infiltration. The methods of analysis of cumulative infiltration are based either on quasi-analytical solutions of the flow equation for homogeneous soil profile or on inverse parameter estimation techniques from the numerical solution of flow equation whether the soil profile is homogeneous or not. The disc infiltrometer has also been shown as a suitable device for inferring parameters describing the water-borne transport of chemicals through near saturated soils. Associated with conservative tracers, it has been recognized as a promising tool for the determination of both hydraulic and solute transport properties as well as for other parameters such as mobile/immobile water content fraction or exchange coefficient. An emphasis is put here on some published studies performed in different soils and environmental conditions focusing on heterogeneous soil profiles (crusted soils) or structured cultivated soils (aggregated soils), either when local water transport process is studied or when field spatial variability is investigated. Some new research studies such as water–solute transfer in structured or swelling–shrinking soils and multi-interactive solute transport are emerging. A number of challenges still remain unresolved for both theory and practice for tension and pressure infiltrometers. They include questions on how to consider and characterize saturated–unsaturated preferential flow or preferential transport process (including hydrodynamic instabilities) induced by biological activity (e.g. capillary macropores, earthworm holes or root channels) by specific pedagogical conditions (e.g. cracking, crusting) and by soil management practices (i.e. conservation tillage).

Ecosystem engineering at the sediment-water interface: bioturbation and consumer-substrate interaction.

In soft-bottom sediments, consumers may influence ecosystem function more via engineering that alters abiotic resources than through trophic influences. Understanding the influence of bioturbation on physical, chemical, and biological processes of the water–sediment interface requires investigating top-down (consumer) and bottom-up (resource) forces. The objective of the present study was to determine how consumer bioturbation mode and sediment properties interact to dictate the hydrologic function of experimental filtration systems clogged by the deposition of fine sediments. Three fine-grained sediments characterized by different organic matter (OM) and pollutant content were used to assess the influence of resource type: sediment of urban origin highly loaded with OM and pollutants, river sediments rich in OM, and river sediments poor in OM content. The effects of consumer bioturbation (chironomid larvae vs. tubificid worms) on sediment reworking, changes in hydraulic head and hydraulic conductivity, and water fluxes through the water–sediment interface were measured. Invertebrate influences in reducing the clogging process depended not only on the mode of bioturbation (construction of biogenic structures, burrowing and feeding activities, etc.) but also on the interaction between the bioturbation process and the sediments of the clogging layer. We present a conceptual model that highlights the importance of sediment influences on bioturbation and argues for the integration of bottom-up influence on consumer engineering activities.

Water transfer and mobile water content measurement in a cultivated crusted soil

In crusted soils, runoff and erosion at the surface are strongly controlled by soil infiltrability. An in situ hydrodynamic characterization of a cultivated crusted soil was conducted to define the factors that reduce infiltrability. Experiments were carried out on three layers of two different profiles in the topsoil: (i) on the surface crust, which was either sedimentary or structural, (ii) within the underlying soil, and (iii) at the plow pan surface. A structural crust is the result of gradual coalescing of aggregates by raindrop compaction, whereas a sedimentary crust is formed by deposition of the particles suspended in overland flow. The purpose here was to characterize water transfer as a function of vertical heterogeneity. A tension disc infiltrometer, along with an 18O solution, was used to create a near-saturated flow. Hydrodynamic properties and mobile water fraction of the soil surface were inferred from the cumulative infiltration and the soil solute concentration at the end of the experiments. Visual observations of X-ray images obtained from thin sections were used to emphasize some of the conclusions about the hydrodynamic characterization. Results of infiltration in soil covered with either more or less developed crusts were compared. Then, comparisons were made between the infiltrability of the underlying soil, which was covered by sedimentary or structural crusts. Finally, estimated values of hydraulic conductivity and the mobile water fraction for each layer of the two profiles provided information on water transfer. Results showed that the fraction of the soil surface covered by sedimentary crusts and structural crusts was an important factor for infiltrability (the cumulative infiltration at t = 5000 s varied between 11.5 mm and 14.8 mm in soil covered by a sedimentary crust, whereas the variation was between 18 mm and 22 mm in soil covered by a structural crust). On the other hand, infiltrability did not depend on the developmental stage of the surface crusts as the differences between the cumulative infiltration in more or less developed crusts were not significantly different at P = 0.05 (according to the Student’s t test). The sedimentary crust seemed to protect the underlying soil from aggregate coalescence. Thus, collapsing was less important in the underlying plowed material covered by a sedimentary crust. As a consequence, the mobile water fraction and effective mean pore size estimations showed that in the case of strongly collapsed material, coalescing increased the pore connection: water transferred through small but well connected pores (the effective mean pore size was λm = 0.105 mm, and the mobile water fraction was f = 0.93). When there was less collapsed underlying soil, the pores participating in transfer were bigger but less connected (λm = 1.2 mm and f = 0.5). The plow pan did not show strong impermeable behavior because the macropores made by roots were not sealed by plowing.

Comparison of three labeled silica nanoparticles used as tracers in transport experiments in porous media. Part II: transport experiments and modeling.

Three types of labeled silica nanoparticles were used in transport experiments in saturated sand. The goal of this study was to evaluate both the efficiency of labeling techniques (fluorescence (FITC), metal (Ag(0) core) and radioactivity ((110m)Ag(0) core)) in realistic transport conditions and the reactive transport of silica nanocolloids of variable size and concentration in porous media. Experimental results obtained under contrasted experimental conditions revealed that deposition in sand is controlled by nanoparticles size and ionic strength of the solution. A mathematical model is proposed to quantitatively describe colloid transport. Fluorescent labeling is widely used to study fate of colloids in soils but was the less sensitive one. Ag(0) labeling with ICP-MS detection was found to be very sensitive to measure deposition profiles. Radiolabeled ((110m)Ag(0)) nanoparticles permitted in situ detection. Results obtained with radiolabeled nanoparticles are wholly original and might be used for improving the modeling of deposition and release dynamics.

Comparison of three labeled silica nanoparticles used as tracers in transport experiments in porous media. Part I: syntheses and characterizations.

The synthesis and the characterization of three kinds of labeled silica nanoparticles were performed. Three different labeling strategies were investigated: fluorescent organic molecule (FITC) embedded in silica matrix, heavy metal core (Ag(0)) and radioactive core ((110m)Ag) surrounded by a silica shell. The main properties and the suitability of each kind of labeled nanoparticle in terms of size, surface properties, stability, detection limits, and cost were determined and compared regarding its use for transport studies. Fluorescent labeling was found the most convenient and the cheapest, but the best detection limits were reached with chemical (Ag(0)) and radio-labeled ((110m)Ag) nanoparticles, which also allowed nondestructive quantifications. This work showed that the choice of labeled nanoparticles as surrogates of natural colloids or manufactured nanoparticles strongly depends on the experimental conditions, especially the concentration and amount required, the composition of the effluent, and the timescale of the experiment.

Effects of water content on reactive transport of 85Sr in Chernobyl sand columns.

It is known that under unsaturated conditions, the transport of solutes can deviate from ideal advective-dispersive behaviour even for macroscopically homogeneous porous materials. Causes may include physical non-equilibrium, sorption kinetics, non-linear sorption, and the irregular distribution of sorption sites. We have performed laboratory experiments designed to identify the processes responsible for the non-ideality of radioactive Sr transport observed under unsaturated flow conditions in an Aeolian sandy deposit from the Chernobyl exclusion zone. Miscible displacement experiments were carried out at various water contents and corresponding flow rates in a laboratory model system. Results of our experiments have shown that breakthrough curves of a conservative tracer exhibit a higher degree of asymmetry when the water content decreases than at saturated water content and same Darcy velocity. It is possible that velocity variations caused by heterogeneities at the macroscopic scale are responsible for this situation. Another explanation is that molecular diffusion drives the solute mass transfer between mobile and immobile water regions, but the surface of contact between these water regions is small. At very low concentrations, representative of a radioactive Sr contamination of the pore water, sorption and physical disequilibrium dominate the radioactive Sr transport under unsaturated flow conditions. A sorption reaction is described by a cation exchange mechanism calibrated under fully saturated conditions. The sorption capacity, as well as the exchange coefficients are not affected by desaturation. The number of accessible exchange sites was calculated on the basis that the solid remained in contact with water and that the fraction of solid phase in contact with mobile water is numerically equal to the proportion of mobile water to total water content. That means that for this type of sandy soil, the nature of mineral phases is the same in advective and non-advective domains. So sorption reaction parameters can be estimated from more easily conducted saturated experiments, but hydrodynamic behaviour must be characterized by conservative tracer experiments under unsaturated flow conditions.

Determination of Uranium source term in a Polluted Site: a multitechnique study

This study aims at understanding a potential migration of uranium in a polluted calcareous peat-land. Aqueous uranium monitoring indicates a seasonal variation of uranium content. During winter, aqueous uranium was found to be mostly under the +VI oxidation state, which is potentially mobile, while it was reduced into less soluble U(IV) species in summer. These observations were correlated with changes of the redox potential due to the activity of sulphate-reducing bacteria. A companion study (Phrommavanh et al., 2008 in this congress) focuses on the determination of the uranium aqueous speciation. Concurrently, one has to know the origin of the source term which is a part of the understanding of the uranium mobility. Several techniques were used in order to discriminate the chemical form of uranium: gamma spectrometry and aqua regia extractions on soil samples, porous PTFE quartz cells in order to sample the aqueous fraction, SEM observations with automatic U particles detection and XANES to assess the U redox state. The association of these different techniques gave consistent results with an increasing U content at a depth of 0.8 m. Such a localisation is explained by a redox front where anoxic and reducing conditions prevail.

Speciation of Uranium in a Polluted Site: a TRLFS study

The uranium speciation in a polluted soil was investigated to assess the possible impact on geo- and biosphere. During winter, uranium was found to be mostly under the +VI oxidation state, which is potentially mobile, while it was reduced into less soluble U(IV) species in summer. These observations were correlated with changes of the redox potential due to the activity of sulphate-reducing bacteria. The potential migration of uranium is also highly dependent on its aqueous speciation, particularly concerning U(VI). In this work, the speciation of U(VI) was investigated in real samples by using time-resolved laser-induced fluorescence spectroscopy (TRLFS) and speciation calculations. We show that U(VI) speciation can be determined in carbonate-rich solutions and at ambient temperature by using TRLFS on chemically-treated samples, to remove U(VI)-fluorescence-quenching molecules such as organics.

Water infiltration in an aquifer recharge basin affected by temperature and air entrapment

Abstract Artificial basins are used to recharge groundwater and protect water pumping fields. In these basins, infiltration rates are monitored to detect any decrease in water infiltration in relation with clogging. However, miss-estimations of infiltration rate may result from neglecting the effects of water temperature change and air-entrapment. This study aims to investigate the effect of temperature and air entrapment on water infiltration at the basin scale by conducting successive infiltration cycles in an experimental basin of 11869 m2 in a pumping field at Crepieux-Charmy (Lyon, France). A first experiment, conducted in summer 2011, showed a strong increase in infiltration rate; which was linked to a potential increase in ground water temperature or a potential dissolution of air entrapped at the beginning of the infiltration. A second experiment was conducted in summer, to inject cold water instead of warm water, and also revealed an increase in infiltration rate. This increase was linked to air dissolution in the soil. A final experiment was conducted in spring with no temperature contrast and no entrapped air (soil initially water-saturated), revealing a constant infiltration rate. Modeling and analysis of experiments revealed that air entrapment and cold water temperature in the soil could substantially reduce infiltration rate over the first infiltration cycles, with respective effects of similar magnitude. Clearly, both water temperature change and air entrapment must be considered for an accurate assessment of the infiltration rate in basins.

Estimation of saturated hydraulic conductivity from ring infiltrometer test taking into account the surface moisture stain extension

Abstract A large single-ring infiltrometer test was performed in order to characterize the saturated hydraulic conductivity below an infiltration basin in the well field of Lyon (France). Two kinds of data are recorded during the experiment: the volume of water infiltrated over time and the extension of the moisture stain around the ring. Then numerical analysis was performed to determine the saturated hydraulic conductivity of the soil by calibration. Considering an isotropic hydraulic conductivity, the saturated hydraulic conductivity of the alluvial deposits is estimated at 3.8 10−6 m s−1. However, with this assumption, we are not able to represent accurately the extension of the moisture stain around the ring. When anisotropy of hydraulic conductivity is introduced, experimental data and simulation results are in good agreement, both for the volume of water infiltrated over time and the extension of the moisture stain. The vertical saturated hydraulic conductivity in the anisotropic configuration is 4.75 times smaller than in the isotropic configuration (8.0 10−7 m s−1), and the horizontal saturated hydraulic conductivity is 125 times higher than the vertical saturated hydraulic conductivity (1.0 10−4 m s−1).