Heterogeneous Soil Systems Research Articles

Unexpected bias of freeze-drying on the performance assessment of chemical oxidation of soils contaminated by polychlorinated biphenyls

Processes based on chemical oxidation are widely used for environmental remediation. Analysis of environmental samples such as soils often requires a freeze-drying step prior to the measurement of the concentration of contaminants. However, the effect of freezing on the quantification of the removal efficiency is poorly known. Here we studied the removal of polychlorinated biphenyls (PCBs) in contaminated soils using H2O2, persulfate, permanganate and ferrate(VI), firstly at room temperature. To assess the chemical oxidation performance, soil samples were freeze-dried and extracted with hexane and PCBs were quantified in the extract by gas chromatography–mass spectrometry. Unexpectedly, in batch samples which were freeze-dried after 5 min of ferrate(VI) oxidation, the removal of PCBs was about two times higher than in samples lyophilized after 3 h of reaction time. Similar oxidation treatments performed at − 20 °C, instead of room temperature, showed that the enhanced degradation in soil is related to freezing conditions. This phenomenon is explained by the freezing concentration effect, i.e., the increased concentration of reactants in the liquid brine. Although this freezing effect is already known in aqueous media, this is the first time it is demonstrated in a heterogeneous soil system, particularly for the remediation by chemical oxidation. The non-consideration of this unforeseen effect during the soil preparation in laboratory studies may lead to an incorrect assessment of oxidation performance.

Macroscopic Observation of Soil Nitrification Kinetics Impacted by Copper Nanoparticles: Implications for Micronutrient Nanofertilizer.

The potential agricultural use of metal nanoparticles (NPs) for slow-release micronutrient fertilizers is beginning to be investigated by both industry and regulatory agencies. However, the impact of such NPs on soil biogeochemical cycles is not clearly understood. In this study, the impact of commercially-available copper NPs on soil nitrification kinetics was investigated via batch experiments. The X-ray absorption near edge structure spectroscopy analysis showed that the NPs readily oxidized to Cu(II) and were strongly retained in soils with minimum dissolution (<1% of total mass). The Cu2+ (aq) at 1 mg/L showed a beneficial effect on the nitrification similar to the control: an approximately 9% increase in the average rate of nitrification kinetics (Vmax). However Vmax was negatively impacted by ionic Cu at 10 to 100 mg/L and CuNP at 1 to 100 mg/L. The copper toxicity of soil nitrifiers seems to be critical in the soil nitrification processes. In the CuNP treatment, the suppressed nitrification kinetics was observed at 1 to 100 mg/kg and the effect was concentration dependent at ≥10 mg/L. The reaction products as the results of surface oxidation such as the release of ionic Cu seem to play an important role in suppressing the nitrification process. Considering the potential use of copper NPs as a slow-release micronutrient fertilizer, further studies are needed in heterogeneous soil systems.

Nonstationarity of the electrical resistivity and soil moisture relationship in a heterogeneous soil system: a case study

Abstract. Understanding the role of vegetation in the interface between the atmosphere and groundwater is the most decisive key in analyzing the processes involved in water transfer. The main effect of vegetation is its root water uptake, which significantly modifies the processes involved in water transfer in the vadose zone. This paper focuses on mapping temporal and spatial changes in soil moisture using electrical resistivity tomography (ERT). The main objective is to assess how electrical resistivity (ER) is useful for mapping water distribution along a heterogeneous toposequence crossed by a hedgerow. Ten ERT were performed over the studied period for a 28 m long toposequence and compared to matric potential and groundwater level measurements. Soil volumetric water content (VWC) was predicted with two methods: (i) from ER using the Waxman and Smits model (ii) and from matric potential using an experimental retention curve fitted by a Van Genuchten model. Probability density functions (PDFs) of our set of data show that the largest change in mean ER and matric potential was observed in the topsoil layer. We then analyzed the consistency between ER and point measurements in this layer by extracting the arrays at the junction of ER grids and point measurements. PDFs of ER maps at each monitoring time (from T01 to T10) were also calculated to select the most contrasting distributions, corresponding to the wettest (T06) and driest states (T10). Results of ER were consistent with matric-potential measurements, with two different behaviors for locations inside and outside the root zone. A consistent correlation between VWC values from the Waxman and Smits model and those obtained from the retention curve was observed outside the root zone. The heterogeneous soil system inside the root zone shows a different pattern in this relationship. A shift in the relationship between ER and soil moisture for the locations outside and inside the root zone highlights the nonstationarity between wet and dry periods inside the root zone. The equivocal behavior of this relationship shows the limitation of using ER to predict soil moisture in a heterogeneous soil system. Such systems were actually related to the high hedgerow root density and also to a particular topographical context (ditch and bank) that is encountered in Brittany and throughout northwestern Europe.

Microbial denitrification dominates nitrate losses from forest ecosystems

Denitrification removes fixed nitrogen (N) from the biosphere, thereby restricting the availability of this key limiting nutrient for terrestrial plant productivity. This microbially driven process has been exceedingly difficult to measure, however, given the large background of nitrogen gas (N2) in the atmosphere and vexing scaling issues associated with heterogeneous soil systems. Here, we use natural abundance of N and oxygen isotopes in nitrate (NO3 (-)) to examine dentrification rates across six forest sites in southern China and central Japan, which span temperate to tropical climates, as well as various stand ages and N deposition regimes. Our multiple stable isotope approach across soil to watershed scales shows that traditional techniques underestimate terrestrial denitrification fluxes by up to 98%, with annual losses of 5.6-30.1 kg of N per hectare via this gaseous pathway. These N export fluxes are up to sixfold higher than NO3 (-) leaching, pointing to widespread dominance of denitrification in removing NO3 (-) from forest ecosystems across a range of conditions. Further, we report that the loss of NO3 (-) to denitrification decreased in comparison to leaching pathways in sites with the highest rates of anthropogenic N deposition.

Numerical modelling of contaminant transport using FEM and meshfree method

Groundwater contamination is seeking a lot of attention due to constant degradation of water by landfills and waste lagoons. In many cases heterogeneous soil system is encountered and hence, a finite element model is developed to solve the advection-dispersion equation for layered soil system as FEM is a robust tool for modelling problems of heterogeneity and complex geometries. Recently developed Meshfree methods have advantage of eliminating the mesh and construct approximate solutions and are observed that they perform effectively as compared to conventional FEM. In the present study, both FEM and Meshfree method are used to simulate phenomenon of contaminant transport in one dimension. The results obtained are agreeing with the values in literature and hence the model is further used for predicting the transport of contaminants. Parametric study is done by changing the dispersion coefficient, average velocity, geochemical reactions, height of leachate and height of liner for obtaining suitability.

How the chemotactic characteristics of bacteria can determine their population patterns

Spatial distribution of soil microorganisms is relevant for the functioning and performance of many ecosystem processes such as nutrient cycling or biodegradation of organic matters and contaminants. Beside the multitude of abiotic environmental factors controlling the distribution of microorganisms in soil systems, many microbial species exhibit chemotactic behavior by directing their movement along concentration gradients of nutrients or of chemoattractants produced by cells of their own kind. This chemotactic ability has been shown to promote the formation of complex distribution patterns even in the absence of environmental heterogeneities. Microbial population patterns in heterogeneous soil systems might be, hence, the result of the interplay between the heterogeneous environmental conditions and the microorganisms' intrinsic pattern formation capabilities.In this modeling study, we combined an individual-based modeling approach with a reactive pore-network model to investigate the formation of bacterial patterns in homogeneous and heterogeneous porous media. We investigated the influence of different bacterial chemotactic sensitivities (toward both substrate and bacteria) on bacterial distribution patterns. The emerging population patterns were classified with the support of a geostatistical approach, and the required conditions for the formation of any specific pattern were analyzed.Results showed that the chemotactic behavior of the bacteria leads to non-trivial population patterns even in the absence of environmental heterogeneities. The presence of structural pore scale heterogeneities had also an impact on bacterial distributions. For a range of chemotactic sensitivities, microorganisms tend to migrate preferably from larger pores toward smaller pores and the resulting distribution patterns thus resembled the heterogeneity of the pore space. The results clearly indicated that in a porous medium like soil the distribution of bacteria may not only be related to the external constraints but also to the chemotactic behavior of the bacterial cells.

Geochemical Processes Governing the Fate and Transport of Chromium(III) and Chromium(VI) in Soils

Chromium has served as an exceptional and necessary elemental component of many industrial processes and consumer products. Its prevalence in the global environment as both a dissolved and wind‐borne constituent has prompted concern during the last several decades due to the large migration potential and biological toxicity of various Cr chemical species. The objective of this study was to develop an improved understanding and predictive capability of the rates and mechanisms of competing geochemical redox and sorption reactions that govern the fate and transport of Cr(III) and C(VI) in heterogeneous subsurface environments. Batch and miscible displacement experiments, coupled with solid‐phase spectroscopy methods, were utilized to quantify the interaction of Cr with subsurface materials acquired from three geographically distinct locations within the continental United States that represented soils from different Department of Energy facilities known to have issues regarding Cr contamination. Soil chemical and mineralogical properties were found to be important factors controlling the mechanisms of Cr–solid phase interactions, with many of the reactive processes being time dependent. Both sorption and redox reactions impacted Cr(III)– and Cr(VI)–solid phase interactions and were modeled as nonlinear, nonequilibrium or equilibrium, reversible or nonreversible reactive processes. The research investigations within this study highlight the environmental significance of Cr speciation and solid‐phase reactivity in heterogeneous subsurface soil systems with contrasting geochemical and mineralogical properties.

Numerical study of localization in soil systems

A numerical study of the mechanical behavior of heterogeneous soil systems, consisting of a bulk of sand with embedded stiff gravel inclusions or soft clay inclusions, is performed. A solution scheme using parallel computing is employed when analyzing two different categories of problems. First, a homogenization problem is studied, where use of a single representative volume element subjected to plane strain compression offers the possibility to investigate the coupling between the response at a local scale and at a global scale. Second, a plane strain footing problem with different heterogeneous soil systems is analyzed using a traditional finite element formulation. The material model utilized for the soil is a large deformation formulation of non-associated elasto-plasticity with an isotropic hardening law, able to represent dilation. It was found that the shape of the gravel or clay inclusions in the systems had no significant effect on the global responses, whereas the strain localizations in the two different soil systems, sand–gravel and sand–clay, were found to have different character. The effect of the initial density on the response was clearly observed in the localization patterns.

The water quality of streams draining a plantation forest on gley soils: the Nant Tanllwyth, Plynlimon mid-Wales

Abstract. The water quality of the Nant Tanllwyth stream in the Plynlimon region of mid-Wales is related to the key hydrobiogeological controls and the effects of conifer harvesting based on an analysis of rain, cloud, stream and groundwater measurements. The results show the normal patterns of stream water quality response to hydrology. Thus, there is a high damping of atmospheric inputs due to storage in a highly heterogeneous soil and groundwater system. Correspondingly, there is a highly dynamic response for components such as calcium, bicarbonate and aluminium. This response links to the relative inputs of acidic and aluminium-bearing soil waters under high flow conditions and base enriched bicarbonate bearing waters from the groundwater areas under baseflow conditions. The introduction of a deep borehole near the main stem of the river opened up a groundwater flow route to the stream and other parts of the catchment. There were two aspects to this. Firstly, it caused a change to the stream water quality, particularly under baseflow conditions, by increasing the concentrations of calcium and magnesium and by reducing the acidity. The monitoring shows that this change has persisted for over eight years and that there is no sign of reversion to pre-borehole times. Secondly, it caused a change in the groundwater level and chemistry at a borehole on the other side of the river. This feature shows that the fracture system is of hydrogeochemical and hydrogeological complexity. The effects of conifer harvesting are remarkable. At the local scale, felling leads to the expected short term increase in nitrate, ammonium and phosphate from the disturbance of the soil and the reduction in uptake into the vegetation. Correspondingly, there is a reduction in sodium and chloride linked to reduced scavenging of atmospheric inputs from cloud water by the vegetation and also due to increased dilution potential due to reductions in transpiration by the trees. However, within the main stream, virtually no change is observed in stream water quality with felling, except for a decrease rather than an expected increase in nitrate concentration. It seems that the increase in phosphate, for a system that is essentially phosphate limiting, has stimulated biological activity in the stream leading to increased uptake of nitrate and ammonium. However, there is little change in the sodium and chloride in the stream and there are important issues of the representative nature of studying small scale drainage areas. Boreholes, introduced to assess the impacts of the felling programme, show unexpected variations in groundwater chemistry. These variations are associated with the complexity of both flow routing and the chemical reactivity within the groundwater and lower soil zones, rather than changes that can be linked specifically to felling. The implications of the study are discussed in relation to both process understanding and forestry management practices. Keywords: water quality, acidification, conifer, harvesting, pH, nitrate, ammonium, phosphate, nutrients, alkalinity, Plynlimon, Wales, Tanllwyth, streams, groundwater

Effect of NAPL entrapment conditions on air sparging remediation efficiency

The effect of soil heterogeneity and the entrapment condition of NAPL source on the mass removal efficiency of air sparging coupled with soil vapour extraction (AS/SVE) was investigated using an intermediate scale two-dimensional laboratory soil tank. Four different NAPL entrapments were created by varying the height of the water table in heterogeneous soil models. Different mass removal efficiencies were achieved for different NAPL entrapment conditions, which were governed by soil heterogeneity and water table height before and during AS/SVE operation. Remobilization and redistribution of toluene and water improved the mass removal. Overall results suggested that it was difficult to achieve the complete remediation of NAPL source due to complex entrapment in heterogeneous soil system. In order to assess the potential contamination in the post-remediation stage, gas and dissolved concentrations of toluene were measured after the AS/SVE operation. The results showed that gas concentration close to remaining NAPL source zone increased rapidly and reached to steady state values, which were much smaller than the vapour pressure, whereas the aqueous phase concentrations increased continuously toward the solubility limit.

Detailed characterization of solute transport in a heterogeneous field soil.

There is a necessity for improved physical understanding of solute transport processes in heterogeneous soil systems. In situ nondestructive techniques like time domain reflectometry (TDR) and fiber optic miniprobes (FOMPs) permit the collection of unique measurements of solute transport processes in soils for the purposes of model development and validation. This study examined the application of TDR and FOMPs to measure solute transport at various points laterally and at two depths in a heterogeneous clay-loam soil. A miscible displacement experiment was performed at a constant irrigation flux to examine the applicability of these probes to field soils. In their first application to a field soil, the FOMPs were successfully calibrated and performed well in measuring solute breakthrough curves. Two flow regimes were identified in the soil profile, the first where lateral spreading of the solute occurred in the surface horizon, followed by convergence into preferential flow pathways in the second transport zone. The measured transport response was heterogeneous with at least two identifiable vertical flow phases. It was demonstrated using transfer function modeling and data from a corresponding laboratory study that the FOMPs were measuring the slower phase, while the TDR probes captured a composite of the fast and slow phases. The combination of these two techniques may be a means to separate solute transport phases in heterogeneous media and relate laboratory column results to field studies.

Modeling flow and transport in a two-dimensional dual-permeability system with spatially variable hydraulic properties

Most field soils exhibit soil spatial variability as well as soil structure. The challenge is to account adequately for both types of spatial heterogeneity in simulation models. A numerical finite element code was used to compare single- and dual-permeability approaches for modeling variably saturated flow and transport in two-dimensional heterogeneous soil systems. The code was based on the Richards' equation for water flow and the advection-dispersion equation for solute transport. Spatial variability in the soil hydraulic properties was accounted for by randomly generating a hydraulic conductivity field using a one-dimensional first-order Markov process. Soil structural effects were modeled with a two-domain concept in which a first-order kinetic expression is used to describe the transfer of water and solute between the two domains. Numerical experiments were carried out for the case of furrow irrigation, including the breakthrough of a conservative solute to the groundwater table. We compared five different scenarios: a single domain having uniform hydraulic properties (SU), a single domain with a randomly distributed hydraulic conductivity (SR), a dual-permeability system with uniform hydraulic properties (DU), a dual-permeability system with a randomly distributed fracture hydraulic conductivity (DRF), and a dual-permeability system having a randomly distributed matrix hydraulic conductivity (DRM). All scenarios started with pressure heads in equilibrium with a constant groundwater table 150 cm below the soil surface and zero initial solute concentrations. The simulated two-dimensional (2D) vertical concentration profiles showed preferential pathways resulting from both the spatial variability (SR) and soil structure (DRF) scenarios. As expected, drainage of water from the bottom of the profile occurred significantly earlier for dual-than for single-permeability scenarios. The combination of having spatial variability in the hydraulic properties and invoking the dual-permeability approach yielded the quickest and largest leaching of solute. The 2D dual-permeability approach should considerably improve the simulation of water and solute movement in naturally heterogeneous field soils.

Modeling Transport of Protons and Calcium Ions in an Alginate Gel Bead System: The Effects of Physical Nonequilibrium and Nonlinear Competitive Sorption

Solute transport in heterogeneous soil systems is determined by a combination of physical and chemical processes. Heterogeneity can cause physical nonequilibrium, which results in asymmetrical breakthrough curves with early initial breakthrough and tailing. In the case of reactive transport, the shape of the breakthrough curve is also effected by the sorption isotherm and by multicomponent effects. The combined effects of physical nonequilibrium and nonlinear multicomponent chemistry are the subject of this study. We show how an experimental system composed of alginate gel beads, together with two-region transport modeling, can be used to study transport phenomena in reactive heterogeneous systems. Proton transport through the gel column with ion exchange of protons and calcium ions was measured and simulated. The results show the combined effects of nonlinear sorption and of competition, which add to the effects of physical nonequilibrium. The results emphasize the importance of a thorough understanding of both physical and chemical processes for a reliable prediction of reactive solute transport.

Numerical modeling of observed effective flow behavior in unsaturated heterogeneous sands

The concept of effective parameters has been introduced in recent years to represent the spatial variability of natural soils in numerical simulation models. In the present study, effective hydraulic properties of unsaturated flow were investigated for the case of a two‐dimensional heterogeneous laboratory tank. Hydraulic parameter estimates obtained from simple statistical averages, inverse procedures, and a stochastic theory were compared to effective retention and hydraulic conductivity characteristics measured for the whole tank at steady state. The applicability of the effective parameter estimates was investigated by comparing transient flow events monitored in the laboratory tank with simulated results based on those estimates. Capillary suction measurements were simulated reasonably well using several straightforward arithmetic and geometric statistical averaging approaches, whereas most averaging approaches simulated too slow a response in the outflow rate. An alternative approach involving a combination of arithmetic and geometric averaging of the measured values more closely simulated the observed relatively fast changes in outflow rate. Generally, the simulations based on the measurements of effective properties performed quite well, indicating that the fundamental concept of effective parameters may be valid for this type of heterogeneous soil system.

Performance of a non‐nuclear resonant frequency capacitance probe. III. error analysis

All techniques used to measure soil water content have errors associated with them. These errors are further propagated when soil volumetric water contents are numerically integrated over a number of depth intervals to estimate the soil water storage for the entire profile. Previous studies had field‐tested the performance of a commercial capacitance probe (Troxler Sentry 200‐AP). The probe was calibrated and the measured data on crop water use were compared with that reported previously in literature using other instruments. However, the errors associated with the field estimates of soil water storage and crop water use measured with the Sentry 200‐AP capacitance probe were not evaluated. The objective of this study was to analyze these associated errors. Equations were derived to evaluate the errors incurred in calculating the volumetric water content using the fitted field calibration curve for the Sentry 200‐AP capacitance probe and in the estimates of the crop water use. This study suggests that the error in making in situ measurements of soil water content in heterogeneous soil systems using the Sentry 200‐AP capacitance probe depends on the degree of saturation at the point of measurement in the soil profile. The Sentry 200‐AP capacitance probe produces errors of >5% in measuring soil volumetric water contents of >35% in heterogeneous soil systems.

Accumulation and degradation of dead-end metabolites during treatment of soil contaminated with polycyclic aromatic hydrocarbons with five strains of white-rot fungi

The white-rot fungi Trametes versicolor PRL 572, Trametes versicolor MUCL 28407, Pleurotus ostreatus MUCL 29527, Pleurotus sajor-caju MUCL 29757 and Phanerochaete chrysosporium DSM 1556 were investigated for their ability to degrade the polycyclic aromatic hydrocarbons (PAH) anthracene, benz[a]anthracene and dibenz[a,h]anthracene in soil. The fungi were grown on wheat straw and mixed with artificially contaminated soil. The results of this study show that, in a heterogeneous soil environment, the fungi have different abilities to degrade PAH, with Trametes showing little or no accumulation of dead-end metabolites and Phanerochaete and Pleurotus showing almost complete conversion of anthracene to 9,10-anthracenedione. In contrast to earlier studies, Phanerochaete showed the ability to degrade the accumulated 9,10-anthracenedione while Pleurotus did not. This proves that, in a heterogeneous soil system, the PAH degradation pattern for white-rot fungi can be quite different from that in a controlled liquid system.

Advection‐Dispersion Analysis of Solute Transport in Undisturbed Soil Monoliths

AbstractThe results of an extended analysis of the chloride breakthrough curves (BTCs) obtained from transport experiments on undisturbed soil monoliths previously reported by Sassner et al. (1994) are discussed. Parameter values for different forms of the advection‐dispersion equation (ADE) were obtained by curve‐fitting using the nonlinear least‐squares optimization code CXTFIT developed by Parker and van Genuchten (1984). Good fits to the experimental BTCs from the individual soil monoliths were obtained for three different forms of the ADE, allowing for anion exclusion, immobile water, and both of these processes, respectively. Although the more complex forms of the ADE possess more flexibility to provide a slightly more refined fit to the individual breakthrough curves, larger uncertainty is associated with the fitted parameter values. Due to the limited amount of information manifested in the BTCs it is not possible on the basis of this information to distinguish which of the models is physically more meaningful. Furthermore, when more parameters are included in the optimization procedure, the resulting values are more uncertain. The same ADE models were also fitted with equally good results to a hypothetical large‐scale BTC derived by flux‐averaging the responses from the individual monoliths. However, regardless of the form of the ADE used, the resulting parameter values for the large‐scale transport (i.e., not only the dispersivity value that is expected to increase) were inconsistent with the corresponding parameter values obtained for the individual BTCs. This supports previous indications that ADE models may not be accurate for predicting large‐scale transport in heterogeneous soil systems corresponding to e.g. the scale of an agricultural field or a grid element in a numerical catchment model because model parameters are not determinable from independent measurements on a scale corresponding to a practical manageable core size or to standard intrumentation.

Probabilistic One-Dimensional Consolidation

The probabilistic analysis of one-dimensional consolidation requires as input the multivariate probability density function relating hydraulic conductivity, compressibility, and porosity. A Monte Carlo approach can be used to obtain solutions to hypothetical consolidation problems in which the soil properties are generated stochastically. Results show that the standard deviations associated with the input soil properties in heterogeneous soils can lead to large uncertainties in predicted hydraulic head values and consolidation rates. Uncertainties are increased by increasing soil heterogeneity and decreased by the presence of spatial trends. It appears that there is no simply way to define an equivalent homogeneous soil with single-valued average soil properties in heterogeneous soil systems. The practice, when utilized with the classical analytical solutions, may yield results significantly different from the most probable result for the actual stochastically-heterogeneous system.

A Functional Iteration Technique for Solving the Richards Equation Applied to Two‐Dimensional Infiltration Problems

In the solution of nonlinear parabolic partial differential equations, such as the Richards equation, classical implicit schemes often oscillate and fail to converge. A fully implicit scheme has been developed along with a functional iteration method for solving the system of nonlinear difference equations. Newton's iteration technique is mathematically the most preferable of all functional iteration methods because of its quadratic convergence. The Richards equation, Newton‐linearized with respect to relative permeability and saturation as functions of capillary pressure, is particularly aided by this new approach for problems in which saturations vary rapidly with time (infiltration fronts, cone of depression near a well bore, and so forth). Although the computing time is almost twice as long for a time step with Newton's iteration scheme, the smaller time truncation than that of classical implicit schemes and the stability in cases in which classical schemes are unstable permit the use of much larger time steps. To demonstrate the method, heterogeneous (layered) soil systems are used to simulate sharp infiltration fronts caused by ponding at the soil surface.