Monte Carlo Vadose Zone Model for Soil Remedial Criteria

Many vadose zone models are available for environmental remediation, but few offer the procedures for verifying model predictions with field data and for dealing with uncertainties associated with model input parameters. This article presents a modified model combining a one-dimensional vadose-zone transport model and a simple groundwater mixing model with a function of Monte Carlo simulation (MCS). The modified model is applied to determine soil remedial concentrations for methyl tertiary butyl ether (MTBE). The modified model generates a distribution of MTBE ground-water concentrations at the point of compliance. This distribution can be used to estimate the risk of exceeding groundwater quality standard given soil remedial concentrations. In a case study, soil remedial concentration for MTBE is established to be 5?µg/kg, with a 95% and 10?µg/kg with a 50% probability that groundwater concentration will not exceed the water quality objective of 13?µg/L. Furthermore, this study uses MCS to investigate uncertainties of model input parameter hydraulic conductivity (K). One set of data (K1) is based on the results of hydraulic conductivity laboratory tests, and the other (K2) is based on the results of slug tests conducted in the field. As expected, the laboratory data show smaller K values than the field data. The comparison of the MCS results obtained from the two sets of K data indicates that the MTBE groundwater concentrations calculated based on K1 are generally 160 to 625% greater than those calculated based on K2 at the same percentiles of the MCS distribution. A higher soil remedial concentration of9jig/kg is then calculated based on the MCS results from K2 at 95%ile and 19?µg/kg at 50%ile.     

Adsorption of RDX to Soil with Low Organic Carbon: Laboratory Results,Field Observations,Remedial Implications

The purpose of this article is to review both laboratory and field observations of RDX adsorption to soils and to use those results to estimate the effects of a planned remedial action. Adsorption isotherms for RDX are generally observed to be linear and reversible. Statistical tests were performed to determine the relationship between K_d and various soil characteristics. A linear relationship between Kd and soil organic carbon was observed, as expected, but regression of K_d to organic carbon content indicated a non-zero intercept, suggesting that other sorbents may also be significant at low OC (e.g., > 0.5 %). No other soil properties were significantly related to Kd so the mechanism of adsorption at low organic carbon was not determined. These results were used to interpret observations of RDX in the vadose zone at Milan Army Ammunition Plant (MAAP), TN. MAAP exhibits widespread soil contamination by RDX. Depth to groundwater ranges from 40 to 80?ft. Unsaturated soils are fine grained near the surface, and sandy near the water table. RDX is concentrated in the upper 2?ft, where concentrations in some places exceed 1 %. Subsurface concentrations are generally less than 50?mg/kg. The distribution of RDX in soil, soil moisture and groundwater, and soil physical testing data were interpreted using simple models. The distribution of RDX is consistent with the following conceptual model: ??Water containing RDX was dis charged to the land surface (prior to 1983); ??Crystalline RDX remains in surface soil (remedial activities are ongo ing); ??Infiltrating rainwater leaches RDX from surface soils; ??This leachate carries RDX through the deeper vadose zone, resulting in significant soil contamination through out the full thickness of the vadose zone; these soils can generate leachate and adversely affect ground- water quality for many years to come. Field results were consistent with the adsorption studies. Simple models consistent with the field and laboratory observations indicate that deeper soils that are not planned to be remediated may continue to leach unacceptable concentrations to groundwater for approximately 180 years. The Army intends to evaluate whether it will be most cost-effective to address this continuing source by treating soils or groundwater.     

A Comparison of Hydrocarbon Vapor Attenuation in the Field with Predictions from Vapor Diffusion Models

This study used field data from three sites in Southern California to evaluate vapor phase transport from: (1) free product (die-sel and gasoline spill) on groundwater; (2) dissolved benzene (gasoline spill) in groundwater; and (3) hydrocarbon-impacted soil (gasoline spill) in the vadose zone. A sampling program to evaluate the vapor pathway included the following: vertical profile data, minimal purging prior to sample collection, field analysis of data, confirmation of field data using a fixed laboratory analysis, and soil physical property data. Comparison of hydrocarbon vapor concentrations measured in this field study with those calculated using vapor diffusion models suggest that an additional attenuation factor of between 500 and 35,000 is needed to account for observed concentrations. Comparison of hydrocarbon profiles with oxygen, carbon dioxide, and methane values is consistent with the interpretation that biodegradation is primarily responsible for the observed attenuation. Therefore, vapor pathway models that do not account for bioattenuation will result in a large overesti-mation of the risk at spill sites and will not be consistent with field data.     

Modeling Fate of RDX at Demolition Area 2 of the Massachusetts Military Reservation

The movement of explosive RDX residue from soil to groundwater at Demolition Area 2 of the Massachusetts Military Reservation was studied with mathematical models to assess its utility and limitations and to determine requisite model improvements and data needs. The Munitions Residue Characterization and Fate model, which is based on the MEPAS source term model for soil modified for solid phase residue with dissolution, the MEPAS vadose zone model, and the MEPAS aquifer model were used in this study. All three models were applied within ARAMS? to facilitate model-to-model connectivity for computing movement of RDX from soil to vadose zone, and from vadose zone to groundwater. Model parameters and a hypothetical RDX residue loading rate were adjusted to match model results to measured RDX concentrations in surface soil and in groundwater. Through an iterative process, a loading rate of 1 kg/yr for RDX residues applied for 10 years starting in 1978 was found to fit measured conditions 20 years later. Model results were sensitive to the C4-RDX dissolution flux rate and the aqueous RDX degradation rate. Results indicate that dissolution of C4 and degradation of RDX in soil and groundwater could be quite slow, and such processes warrant further study. Mechanistic models such as those presented will be useful for estimating fate of constituent residue in soil and transport to receiving waters for evaluating range residue carrying capacity and compliance issues.     

Quantifying effects of soil heterogeneity on groundwater pollution at four sites in USA

Four sites located in the north-eastern region of the United States of America have been chosen to investigate the impacts of soil heterogeneity in the transport of solutes (bromide and chloride) through the vadose zone (the zone in the soil that lies below the root zone and above the permanent saturated groundwater). A recently proposed mathematical model based on the cumulative beta distribution has been deployed to compare and contrast the regions’ heterogeneity from multiple sample percolation experiments. Significant differences in patterns of solute leaching were observed even over a small spatial scale, indicating that traditional sampling methods for solute transport, for example the gravity pan or suction lysimeters, or more recent inventions such as the multiple sample percolation systems may not be effective in estimating solute fluxes in soils when a significant degree of soil heterogeneity is present. Consequently, ignoring soil heterogeneity in solute transport studies will likely result in under- or overprediction of leached fluxes and potentially lead to serious pollution of soils and/or groundwater. The cumulative beta distribution technique is found to be a versatile and simple technique of gaining valuable information regarding soil heterogeneity effects on solute transport. It is also an excellent tool for guiding future decisions of experimental designs particularly in regard to the number of samples within one site and the number of sampling locations between sites required to obtain a representative estimate of field solute or drainage flux.     

Simulating effects of climate change on boreal ecosystem carbon pools in central Canada

Aim Possible effects of current and future climates on boreal vegetation dynamics and carbon (C) cycling were investigated using the CENTURY 4.0 soil process model and a modified version of the FORSKA2 forest patch model. Location Eleven climate station locations distributed along a transect across the boreal zone of central Canada. Methods Both models were driven by detrended long-term monthly climate data. Using a climate change signal derived from the GISS general circulation model (GCM) 2×CO₂ equilibrium climate scenario, the output from the two models was then used to compare simulated current and possible future total ecosystem C storage at the climate station locations. Results After allowing for their different underlying structures, comparison of output from both models showed good agreement with local field data under current climate conditions. CENTURY 4.0 was able to reproduce spatial variation in soil and litter C densities satisfactorily but tended to overestimate biomass productivity. FORSKA2 reproduced aboveground biomass productivity and spatially averaged biomass densities relatively well. Under the GISS 2×CO₂ scenario, both models generally predicted small increases in aboveground biomass C density for forest and tundra locations, but CENTURY 4.0 predicted greater decreases in soil and litter pools, for overall decreases in ecosystem C storage in the range 16–19%. Main conclusions With some caveats, results imply that effects of increased precipitation (as simulated by the GISS GCM) would more than compensate for any negative effects of increased temperature on forest growth. Increased temperature would also increase decomposition rates of soil and litter organic matter, however, for a net overall decrease in total ecosystem C storage.     

The role of microbial populations in the containment of aromatic hydrocarbons in the subsurface

A survey of soil gases associated with gasoline stations on theSwan Coastal Plain of Western Australia has shown that 20% leak detectable amountsof petroleum. The fates of volatile hydrocarbons in the vadose zone at one contaminatedsite, and dissolved hydrocarbons in groundwater at another site were followed in anumber of studies which are herein reviewed. Geochemical evidence from a plume ofhydrocarbon-contaminated groundwater has shown that sulfate reduction rapidly developedas the terminal electron accepting process. Toluene degradation but not benzene degradationwas linked to sulfate reduction. The sulfate-reducing bacteria isolated from the plumerepresented a new species, Desulfosporosinus meridiei. Strains of the speciesdo not mineralise ¹⁴C-toluene in pure culture. The addition of large numbersof cells and sulfate to microcosms did stimulate toluene mineralisation but not benzenemineralisation. Attempts to follow populations of sulfate-reducing bacteria byphospholipid signatures, or Desulfosporosinus meridiei by FISH in the plume were unsuccessful, but fluorescently-labeled polyclonal antibodies were successfully used.In the vadose zone at a different site, volatile hydrocarbons were consumed in thetop 0.5 m of the soil profile. The fastest measured rate of mineralisation of ¹⁴C-benzenein soils collected from the most active zone (6.5 mg kg^-1 day^-1) could accountfor the majority of the flux of hydrocarbon vapour towards the surface. The studiesconcluded that intrinsic remediation by subsurface microbial populations in groundwateron the Swan Coastal Plain can control transport of aromatic hydrocarbon contamination,except for the transport of benzene in groundwater. In the vadose zone, intrinsicremediation by the microbial populations in the soil profile can contain the transportof aromatic hydrocarbons, provided the physical transport of gases, inparticular oxygen from the atmosphere, is not impeded by structures.     

Modeling the Soil Water Retention Curves of Soil-Gravel Mixtures with Regression Method on the Loess Plateau of China

Soil water retention parameters are critical to quantify flow and solute transport in vadose zone, while the presence of rock fragments remarkably increases their variability. Therefore a novel method for determining water retention parameters of soil-gravel mixtures is required. The procedure to generate such a model is based firstly on the determination of the quantitative relationship between the content of rock fragments and the effective saturation of soil-gravel mixtures, and then on the integration of this relationship with former analytical equations of water retention curves (WRCs). In order to find such relationships, laboratory experiments were conducted to determine WRCs of soil-gravel mixtures obtained with a clay loam soil mixed with shale clasts or pebbles in three size groups with various gravel contents. Data showed that the effective saturation of the soil-gravel mixtures with the same kind of gravels within one size group had a linear relation with gravel contents, and had a power relation with the bulk density of samples at any pressure head. Revised formulas for water retention properties of the soil-gravel mixtures are proposed to establish the water retention curved surface models of the power-linear functions and power functions. The analysis of the parameters obtained by regression and validation of the empirical models showed that they were acceptable by using either the measured data of separate gravel size group or those of all the three gravel size groups having a large size range. Furthermore, the regression parameters of the curved surfaces for the soil-gravel mixtures with a large range of gravel content could be determined from the water retention data of the soil-gravel mixtures with two representative gravel contents or bulk densities. Such revised water retention models are potentially applicable in regional or large scale field investigations of significantly heterogeneous media, where various gravel sizes and different gravel contents are present.     

Quantifying effects of soil heterogeneity on groundwater pollution at four sites in USA

Four sites located in the north-eastern region of the United States of America have been chosen to investigate the impacts of soil heterogeneity in the transport of solutes (bromide and chloride) through the vadose zone (the zone in the soil that lies below the root zone and above the permanent saturated groundwater). A recently proposed mathematical model based on the cumulative beta distribution has been deployed to compare and contrast the regions' heterogeneity from multiple sample percolation experiments. Significant differences in patterns of solute leaching were observed even over a small spatial scale, indicating that traditional sampling methods for solute transport, for example the gravity pan or suction lysimeters, or more recent inventions such as the multiple sample percolation systems may not be effective in estimating solute fluxes in soils when a significant degree of soil heterogeneity is present. Consequently, ignoring soil heterogeneity in solute transport studies will likely result in under- or overprediction of leached fluxes and potentially lead to serious pollution of soils and/or groundwater.The cumulative beta distribution technique is found to be a versatile and simple technique of gaining valuable information regarding soil heterogeneity effects on solute transport. It is also an excellent tool for guiding future decisions of experimental designs particularly in regard to the number of samples within one site and the number of sampling locations between sites required to obtain a representative estimate of field solute or drainage flux.     

Quantifying effects of soil heterogeneity on groundwater pollution at four sites in USA

Four sites located in the north-eastern region of the United States of America have been chosen to investigate the impacts of soil heterogeneity in the transport of solutes (bromide and chloride) through the vadose zone (the zone in the soil that lies below the root zone and above the permanent saturated groundwater). A recently proposed mathematical model based on the cumulative beta distribution has been deployed to compare and contrast the regions' heterogeneity from multiple sample percolation experiments. Significant differences in patterns of solute leaching were observed even over a small spatial scale, indicating that traditional sampling methods for solute transport, for example the gravity pan or suction lysimeters, or more recent inventions such as the multiple sample percolation systems may not be effective in estimating solute fluxes in soils when a significant degree of soil heterogeneity is present. Consequently, ignoring soil heterogeneity in solute transport studies will likely result in     

Investigation of Biogeochemical Activities in the Soil and Unsaturated Zone of Weathered Granite

This study, based on field and laboratory work, investigates the biogeochemical activity below the organic top soil horizons, particularly the potential for nitrate removal processes in the deep vadose zone (1–2.5 m depth) of a weathered granite. An experimental site located in the Kerbernez agricultural catchment (Brittany) has been equipped with ceramic cups from 0.5 to 2.5 m depth since November 2001. This arrangement allowed collection of water samples from the soil profile and the upper part of the unsaturated weathered granite. Samples were analysed twice a month for chloride, nitrate and sulphate concentrations over a period of 2.5 years. Laboratory measurements were carried out on three soil horizons and four weathered granite facies sampled in October 2003 for hydrolasic activity, potential nitrification, potential denitrification and batch experiments to study nutrient dynamics. Anion analyses in the field show that the nitrate and chloride concentrations were linearly correlated at each depth. The nitrate/chloride ratio decreased with depth in the upper part of the weathered granite from 4.93 to 2.82. This suggests that nitrate was removed during its vertical transport in the unsaturated zone. The laboratory experiments show that the bacterial activity decreased with depth. However, a significant potential for biogeochemical reactions exists below the organic soil layers. The denitrification rates obtained in the laboratory were significant, up to 800 ng  N h⁻¹ g⁻¹ after about 100 h of incubation for the most reactive weathered granite facies. These rates agree with effective rates usually measured in riparian zones, but they were 50 times higher than those observed in the field. This difference suggests that the denitrification processes occurring in the field were spatially limited to localised anaerobic microsites, where the bacterial activities are controlled by the availability of N and C substrate. Finally, the laboratory measurements lead us to assume that heterotrophic denitrification was clearly the predominant process occurring in the field because of the good correlation between nitrate concentration variation and carbon content (r = −0.94). Moreover, the slight increase in sulphate concentrations observed in the field and in the laboratory was insufficient to explain the complete removal of nitrate.     

PHYTOREMEDIATION OF BTEX HYDROCARBONS: POTENTIAL IMPACTS OF DIURNAL GROUNDWATER FLUCTUATION ON MICROBIAL DEGRADATION

Volatile hydrocarbons have multiple potential fates in phytoremediation. This research investigated the relationship between biodegradation and plant uptake of BTEX compounds in laboratory and field settings. At a phytoremediation site, preliminary studies revealed minimal uptake into trees and enhanced degradation potential in the rhizosphere and in the bulk soil. Increased oxygen transport to the vadose zone caused by diurnal rise and fall of the water table was hypothesized to enhance degradation in the bulk soil. A detailed greenhouse study was then conducted to investigate potential bioremediation impacts using field-site soil and DN34 hybrid poplar trees.

In rhizosphere soils, the contaminated-planted reactor had significantly higher BTEX degrader populations versus the uncontaminated-planted reactor, as was anticipated. The bulk soil in the planted-contaminated reactor had increased degrader populations than the unplanted-contaminated soil or planted-uncontaminated soil, and planting increased degradation throughout the soil profile, not just in the limited volume of rhizosphere soils. Oxygen diffusive and advective transport into reactors was modeled and calculated. Oxygen input in planted reactors was at least 3 to 5 times higher than in unplanted reactors, and increasing oxygen input lead to increased degrader populations in a linear manner. These results combined with the knowledge that high-transpiration trees draw the contaminated groundwater to the capillary fringe and the rhizosphere indicate that phytoremediation can aid microbial degradation via multiple mechanisms: increasing degrader populations, increasing oxygen input via groundwater diurnal fluctuations, and transporting contaminants to the biologically-enriched soil profile.     

An evaluation of abiotic carbon sinks in deserts

Recent field studies have reported anomalous CO₂ uptake using eddy‐covariance techniques in arid and semiarid ecosystems. The rates of CO₂ uptake are incompatible with changes in situ of organic carbon pools. Here, I examine several potential mechanisms of abiotic CO₂ uptake in arid and semiarid soils: atmospheric pressure pumping, carbonate dissolution, and percolation of soil water through the vadose zone. Each mechanism is deemed inadequate to explain the observations of the eddy‐covariance systems, which must now be questioned for their accuracy in desert ecosystems.     

Effect of Soil Fabric on Transport of a LNAPL through Unsaturated Fine-Grained Soils: A Centrifugal Model Study

Centrifugal model tests were performed to study the impact of the fabric of a fine-grained soil on transport of a light non-aqueous phase liquid (LNAPL). An image processing technique was developed to extract contaminant transport and fate data from the centrifugal model. Two unconsolidated sites with different moisture contents and a saturated site consolidated due to self-weight were simulated using the centrifuge. The LNAPL migrated in the vertical direction as a narrow plume and formed a free product pool above the saturated zone in unsaturated and unconsolidated soils. However, the LNAPL migrated in the horizontal direction before moving in the vertical direction as a broad plume in the consolidated site. The test results showed that the final width of the plume in the unsaturated zone of the consolidated site was nearly two times as large as that for the unconsolidated sites. In addition, the rate of leak from the underground storage tanks (USTs) on consolidated soils was substantially higher when compared with those on the unconsolidated state. The comparison of LNAPL saturation profiles at the centerline of the centrifugal models during leakage showed that, depending on the soil fabric at a given time and depth, the LNAPL phase would be different; i.e., mobile or immobile (residual) in the same soil type. The test results provided additional insight into contribution of soil fabric on transport and fate of contaminants. The soil fabric controls the geological and hydro-geological properties of fine-grained soils and hence the contamination plume.     

Organic carbon availability limiting microbial denitrification in the deep vadose zone

Microbes in the deep vadose zone play an essential role in the mitigation of nitrate leaching; however, limited information is available on the mechanisms of microbial denitrification due to sampling difficulties. We experimentally studied the factors that affect denitrification in soils collected down to 10.5 meters deep along the soil profile. After an anoxic pre‐incubation, denitrification rates moderately increased and the N₂O/(N₂O + N₂) ratios declined while the microbial abundance and diversity did not change significantly in most of the layers. Denitrification rate was significantly enhanced and the abundance of the denitrification genes was simultaneously elevated by the increased availability of organic carbon in all studied layers, to a greater extent in the subsurface layers than in the surface layers, suggesting the severe scarcity of carbon in the deep vadose zone. The genera Pseudomonas and Bacillus, which are made up of a number of species that have been previously identified as denitrifiers in soil, were the major taxa that respond to carbon addition. Overall, our results suggested that the limited denitrification in the deep vadose zone is not because of the lack of denitrifiers, but due to the low abundance of denitrifiers which is caused by low carbon availability.     

Short-term variation in benthic phosphorus transfer due to discontinuous aeration/oxygenation operation

The short-term dynamics of soluble reactive phosphorus (SRP) transport across the sediment surface in a brackish lake due to discontinuous aeration and oxygenation operations were investigated using laboratory and field experimental and analytical procedures. According to a laboratory incubation experiment using intact sediment cores, SRP release from the sediment was clearly suppressed by aeration, and substantial negative SRP transfer was observed during oxygenation treatment, while a positive value was observed for N₂ bubbled cores. A remarkable but impermanent increase in SRP release rate was observed within 1 or 2 days of discontinuing the aeration and oxygenation, respectively, and the release rate rapidly deceased to a quasi-steady value under N₂ bubbling conditions. An analytical model could quantitatively reproduce these laboratory experimental results for anoxic and aerated conditions, showing that this impermanent increase was attributable to the rapid release of accumulated SRP in the oxic surface layer of the sediment. Field experiments using an in situ oxygenator showed the same tendency as the laboratory experiments, but with much larger values of the benthic SRP transfer rate. Overall, the short-term dynamics of benthic SRP transport caused by discontinuous aeration and oxygenation are considered to be an important process for the phosphorus cycle in the field.     

VIRTUS, a model of virus transport in unsaturated soils.

As a result of the recently proposed mandatory groundwater disinfection requirements to inactivate viruses in potable water supplies, there has been increasing interest in virus fate and transport in the subsurface. Several models have been developed to predict the fate of viruses in groundwater, but few include transport in the unsaturated zone and all require a constant virus inactivation rate. These are serious limitations in the models, as it has been well documented that considerable virus removal occurs in the unsaturated zone and that the inactivation rate of viruses is dependent on environmental conditions. The purpose of this research was to develop a predictive model of virus fate and transport in unsaturated soils that allows the virus inactivation rate to vary on the basis of changes in soil temperature. The model was developed on the basis of the law of mass conservation of a contaminant in porous media and couples the flows of water, viruses, and heat through the soil. Model predictions were compared with measured data of virus transport in laboratory column studies and, with the exception of one point, were within the 95% confidence limits of the measured concentrations. The model should be a useful tool for anyone wishing to estimate the number of viruses entering groundwater after traveling through the soil from a contamination source. In addition, model simulations were performed to identify parameters that have a large effect on the results. This information can be used to help design experiments so that important variables are measured accurately.     

Estimation of effective cleanup radius for soil‐vapor extraction systems

Soil‐vapor extraction (SVE) is a standard and effective in situ treatment for the removal of volatile contaminants from vadose‐zone soil. The duration of SVE operation required to reach site closure is quite variable, however, ranging up to several years or more. An understanding of the contaminant recovery rate as a function of distance from each vapor‐extraction well allows SVE systems to be designed so that cleanup goals can be achieved within a specified time frame.

A simple one‐dimensional model has been developed that provides a rough estimate of the effective cleanup radius (defined as “the maximum distance from a vapor extraction point through which sufficient air is drawn to remove the required fraction of contamination in the desired time") for SVE systems. Because the model uses analytical rather than numerical methods, it has advantages over more sophisticated, multidimensional models, including simplicity, speed, versatility, and robustness.

The contaminant removal rate at a given distance from the vapor‐extraction point is assumed to be a function of the local rate of soil‐gas flow, the contaminant soil concentration, and the contaminant volatility. Soil‐gas flow rate as a function of distance from the vapor‐extraction point is estimated from pilot test data by assuming that the infiltration of atmospheric air through the soil surface is related to the vacuum in the soil. Although widely applicable, the model should be used with some caution when the vadose zone is highly stratified or when venting contaminated soil greater than 30 ft below grade. Since 1992, Groundwater Technology, Inc. has been using this model routinely as a design tool for SVE systems.     

VIRTUS, a model of virus transport in unsaturated soils.

As a result of the recently proposed mandatory groundwater disinfection requirements to inactivate viruses in potable water supplies, there has been increasing interest in virus fate and transport in the subsurface. Several models have been developed to predict the fate of viruses in groundwater, but few include transport in the unsaturated zone and all require a constant virus inactivation rate. These are serious limitations in the models, as it has been well documented that considerable virus removal occurs in the unsaturated zone and that the inactivation rate of viruses is dependent on environmental conditions. The purpose of this research was to develop a predictive model of virus fate and transport in unsaturated soils that allows the virus inactivation rate to vary on the basis of changes in soil temperature. The model was developed on the basis of the law of mass conservation of a contaminant in porous media and couples the flows of water, viruses, and heat through the soil. Model predictions were compared with measured data of virus transport in laboratory column studies and, with the exception of one point, were within the 95% confidence limits of the measured concentrations. The model should be a useful tool for anyone wishing to estimate the number of viruses entering groundwater after traveling through the soil from a contamination source. In addition, model simulations were performed to identify parameters that have a large effect on the results. This information can be used to help design experiments so that important variables are measured accurately.     

The distribution of ammonium ion from a localized source into the soil

The distribution of ammoniun ions from a concentrated source in the soil is calculated with a computed model based on chromatographic theory. The analytical solution for the chromatographic equation with convective transport in a pistonlike flow, for a non linear adsorption isotherm, is compared with computer simulation model results and actual experimental data from soil solumns.Adsorption isotherms for the exchange of Ca⁺⁺ by NH₄ ⁺ ions in the range of 0.01 to 6 eq NH₄/I were obtained. The isotherm are described by a modification of Freundlich's isotherm.Ammonium displacement is a frontal movement. The resulting concentration profile is a uniform zone of influence within which ammonium concentration is close to 2/3 of the soil C.E.C. A relatively narrow transition zone exists between the zone of influence and the bulk soil.Good correlation was obtained between experimental and computed ammonium concentration distribution along the soil profile after leaching.The results presented in this work indicate that a quantitative estimation of the extent to which ammonium ions are transported from application site, and the processes following this transport, can be obtained.Present address of the first author: Universidad Catolica de Chile, Dept. of Soils, Chile.