Controls on atrazine leaching through a soil-unsaturated fractured limestone sequence at Brévilles, France

The objective of this study was to identify the main controls on atrazine leaching through luvisols and calcisols overlying fissured limestone using the dual-permeability model MACRO. The model parameterisation was based on a combination of direct measurements (e.g. hydraulic properties, adsorption and degradation), literature data and calibration against bromide leaching experiments in field plots. A Monte Carlo sensitivity analysis was carried out for a typical application pattern, considering two different depths of unsaturated limestone (15 and 30 m). MACRO calibrations to the field experiments demonstrated the occurrence of strong macropore flow in the luvisol, while transport in the calcisol could be described by the advection-dispersion equation. MACRO simulations of tritium and atrazine leaching qualitatively matched tritium concentration profiles measured in the limestone and atrazine concentrations measured in piezometers and in aquifer discharge via a spring. The sensitivity analysis suggested that the thickness of the limestone, as well as the transport properties and processes occurring in the unsaturated rock (e.g. matrix vs. fissure flow) will have little significant long-term effect on atrazine leaching, mainly because degradation is very slow in the limestone. No mineralization of atrazine was detected in one-year incubations and a mean half-life of 10 years was assumed in the simulations. Instead, processes occurring in the soil exerted the main control on predicted atrazine leaching, especially variations in the degradation rate and the strength of sorption and macropore flow. However, fissure flow in unsaturated rock is expected to exert a much more significant control on groundwater contamination for compounds that degrade more readily in the deep vadose zone.     

Modeling depth-variant and domain-specific sorption and biodegradation in dual-permeability media

A dual-permeability model (S_1D_DUAL) was developed to simulate the transport of land-applied pesticides in macroporous media. In this model, one flow domain was represented by the bulk matrix and the other by the preferential flow domain (PFD) where water and chemicals move at faster rates. The model assumed the validity of Darcian flow and the advective-dispersive solute transport in each of the two domains with inter-domain transfer of water and solutes due to pressure and concentration gradients. It was conceptualized that sorption and biodegradation rates vary with soil depth as well as in each of the two flow domains. In addition to equilibrium sorption, kinetic sorption was simulated in the PFD. Simulations were conducted to evaluate the combined effects of preferential flow, depth- and domain-variant sorption, and degradation on leaching of two pesticides: one with strong sorption potential (trifluralin) and the other with weak sorption potential (atrazine). Simulation results for a test case showed that water flux in the PFD was three times more than in the matrix for selected storm events. When equilibrium sorption was considered, the simulated profile of trifluralin in each domain was similar; however, the atrazine profile was deeper in the PFD than in the bulk matrix under episodic storm events. With an assumption of negligible sorption in the PFD, both the atrazine and the trifluralin profiles moved twice deeper into the PFD. The simulated concentrations of the chemicals were several orders higher in the PFD than in the matrix, even at deeper depths. The volume fraction of the macropores and the sorption and biodegradation properties of the chemicals could also affect the amount of pesticides leaving the root zone. For an intense storm event, slow sorption reaction rates in the PFD produced higher breakthrough concentrations of atrazine at the bottom of the simulated soil profile, thus posing the risk for breakthrough of chemicals from the root zone.     

Uncertainty in Nonpoint Source Pollution Models and Associated Risks

The usefulness of water quality simulation models for environmental management is explored with a focus on prediction uncertainty. The specific objective is to demonstrate how the usability of a flow and transport model (here: MACRO) can be enhanced by developing and analyzing its output probability distributions based on input variability. This infiltration-based model was designed to investigate preferential flow effects on pollutant transport. A statistical sensitivity analysis is used to identify the most uncertain input parameters based on model outputs. Probability distribution functions of input variables were determined based on field-measured data obtained under alternative tillage treatments. Uncertainty of model outputs is investigated using a Latin hypercube sampling scheme (LHS) with restricted pairing for model input sampling. Probability density functions (pdfs) are constructed for water flow rate, atrazine leaching rate, total accumulated leaching, and atrazine concentration in percolation water. Results indicate that consideration of input parameter uncertainty produces a 20% higher mean flow rate along with two to three times larger atrazine leaching rate, accumulated leachate, and concentration than that obtained using mean input parameters. Uncertainty in predicted flow rate is small but that in solute transport is an order of magnitude larger than that of corresponding input parameters. Macropore flow is observed to contribute to the variability of atrazine transport results. Overall, the analysis provides a quantification of prediction uncertainty that is found to enhance a user's ability to assess risk levels associated with model predictions.     

Uncertainty in Nonpoint Source Pollution Models and Associated Risks

The usefulness of water quality simulation models for environmental management is explored with a focus on prediction uncertainty. The specific objective is to demonstrate how the usability of a flow and transport model (here: MACRO) can be enhanced by developing and analyzing its output probability distributions based on input variability. This infiltration-based model was designed to investigate preferential flow effects on pollutant transport. A statistical sensitivity analysis is used to identify the most uncertain input parameters based on model outputs. Probability distribution functions of input variables were determined based on field-measured data obtained under alternative tillage treatments. Uncertainty of model outputs is investigated using a Latin hypercube sampling scheme (LHS) with restricted pairing for model input sampling. Probability density functions (pdfs) are constructed for water flow rate, atrazine leaching rate, total accumulated leaching, and atrazine concentration in percolation water. Results indicate that consideration of input parameter uncertainty produces a 20% higher mean flow rate along with two to three times larger atrazine leaching rate, accumulated leachate, and concentration than that obtained using mean input parameters. Uncertainty in predicted flow rate is small but that in solute transport is an order of magnitude larger than that of corresponding input parameters. Macropore flow is observed to contribute to the variability of atrazine transport results. Overall, the analysis provides a quantification of prediction uncertainty that is found to enhance a user's ability to assess risk levels associated with model predictions.     

Atrazine distribution measured in soil and leachate following infiltration conditions

Atrazine transport through packed 10 cm soil columns representative of the 0-10 cm soil horizon was observed by measuring the atrazine recovery in the total leachate volume, and upper and lower soil layers following infiltration of 7.5 cm water using a mechanical vacuum extractor (MVE). Measured recoveries were analyzed to understand the influence of infiltration rate and delay time on atrazine transport and distribution in the column. Four time periods (0.28, 0.8, 1.8, and 5.5 h) representing very high to moderate infiltration rates (26.8, 9.4, 4.2, and 1.4 cm/h) were used. Replicate soil columns were tested immediately and following a 2-d delay after atrazine application. Results indicate atrazine recovery in leachate was independent of infiltration rate, but significantly lower for infiltration following a 2-d delay. Atrazine distribution in the 0-1 and 9-10 cm soil layers was affected by both infiltration rate and delay. These results are in contrast with previous field and laboratory studies that suggest that atrazine recovery in the leachate increases with increasing infiltration rate. It appears that the difference in atrazine recovery measured using the MVE and other leaching experiments using intact soil cores from this field site and the rain simulation equipment probably illustrates the effect of infiltrating water interacting with the atrazine present on the soil surface. This work suggests that atrazine mobilization from the soil surface is also dependent on interactions of the infiltrating water with the soil surface, in addition to the rate of infiltration through the surface soil.     

Macro-porosity and leaching of atrazine in tilled and orchard loamy soils

Atrazine is the most commonly detected herbicide in the groundwater. Leaching of atrazine largely depends on soil management practices. The aim of this study was to examine leaching of atrazine in tilled and orchard silty loam soils. The experimental objects included: conventionally tilled field (CT) with main tillage operations including pre-plow (10 cm) + harrowing, mouldboard ploughing (20 cm), and a 35 year-old apple orchard (OR) with a permanent sward. To determine leaching of atrazine soil columns of undisturbed structure were taken with steel cylinders of 21.5 cm diameter and 20 cm high from the depth of 0–20 cm. All columns were equilibrated at water content corresponding to field capacity (0.21 kg kg⁻¹). Atrazine suspended in distilled water was dripped uniformly onto the surface of each column. Then water was infiltrated and breakthrough times of leachates were recorded. Atrazine concentration in the leachates was determined by means of HPLC Waters. Macro-porosity and percolation rate were higher in OR than CT soil. Cumulative recovery % of the atrazine applied was 1.267% for OR and approximately one third more from the CT soil but the rate of leaching (per unit of time) was greater from the OR soil. The lower leaching under OR than CT can be due to a greater SOM and the presence of earthworm burrows with organic burrow linings that could adsorb atrazine and contribute to preferential flow allowing solutes to bypass parts whereas the greater rate of leaching due to a greater infiltration rate.The results indicate potential of management practices for minimizing atrazine leaching.     

Continuous time random walk model better describes the tailing of atrazine transport in soil

Contaminant transport in soils is complicated and involves some physical and chemical nonequilibrium processes. In this research, the soil column displacement experiments of Cl⁻ and atrazine under different flow velocities were carried out. The data sets of Cl⁻ transport in sandy loam fitted to the convection dispersion equation (CDE) and the two-region model (TRM) indicated that the effects of physical nonequilibrium process produced by immobile water on the breakthrough curves (BTCs) of Cl⁻ and atrazine transport through the repacking soil columns were negligible. The two-site model (TSM) and the continuous time random walk (CTRW) were also used to fit atrazine transport behavior at the flow rate of 19.86 cm h⁻¹. The CTRW convincingly captured the full evolution of atrazine BTC in the soil column, especially for the part of long tailing. However, the TSM failed to characterize the tailing of atrazine BTC in the soil column. The calculated fraction of equilibrium sorption sites, F, ranging from 0.78 to 0.80 for all flow rates suggested the contribution of nonequilibrium sorption sites to the asymmetry of atrazine BTCs. Furthermore, the data sets for the flow rates of 6.68 cm h⁻¹ and 32.81 cm h⁻¹ were predicted by the TSM and the CTRW. As to the flow rate of 6.68 cm h⁻¹, the CTRW predicted the entire BTC of atrazine transport better than the TSM did. For the flow rate of 32.81 cm h⁻¹, the CTRW characterized the late part of the tail better, while the TSM failed to predict the tailings of atrazine BTC.     

Second-order modeling of arsenite transport in soils

Rate limited processes including kinetic adsorption-desorption can greatly impact the fate and behavior of toxic arsenic compounds in heterogeneous soils. In this study, miscible displacement column experiments were carried out to investigate the extent of reactivity during transport of arsenite in soils. Arsenite breakthrough curves (BTCs) of Olivier and Windsor soils exhibited strong retardation with diffusive effluent fronts followed by slow release or tailing during leaching. Such behavior is indicative of the dominance of kinetic retention reactions for arsenite transport in the soil columns. Sharp decrease or increase in arsenite concentration in response to flow interruptions (stop-flow) further verified that non-equilibrium conditions are dominant. After some 40-60 pore volumes of continued leaching, 30-70% of the applied arsenite was retained by the soil in the columns. Furthermore, continued arsenite slow release for months was evident by the high levels of residual arsenite concentrations observed during leaching. In contrast, arsenite transport in a reference sand material exhibited no retention where complete mass recovery in the effluent solution was attained. A second-order model (SOM) which accounts for equilibrium, reversible, and irreversible retention mechanisms was utilized to describe arsenite transport results from the soil columns. Based on inverse and predictive modeling results, the SOM model successfully depicted arsenite BTCs from several soil columns. Based on inverse and predictive modeling results, a second-order model which accounts for kinetic reversible and irreversible reactions is recommended for describing arsenite transport in soils.     

Description of sorbing tracers transport in fractured media using the lattice model approach

The transport of contaminants in fractured media is a complex phenomenon with a great environmental impact. It has been described with several models, most of them based on complex partial differential equations, that are difficult to apply when equilibrium and nonequilibrium dynamics are considered in complex boundaries. With the aim of overcoming this limitation, a combination of two lattice Bathnagar, Gross and Krook (BGK) models, derived from the lattice Boltzmann model, is proposed in this paper. The fractured medium is assumed to be a single fissure in a porous rock matrix. The proposed approach permits us to deal with two processes with different length scales: advection-dispersion in the fissure and diffusion within the rock matrix. In addition to the mentioned phenomena, sorption reactions are also considered. The combined model has been tested using the experimental breakthrough curves obtained by Garnier et al. (Garnier, J.M., Crampon, N., Préaux, C., Porel, G., Vreulx, M., 1985. Tra?age par 13C, 2H, I- et uranine dans la nappe de la craie sénonienne en écoulement radial convergent (Béthune, France). J. Hidrol. 78, 379-392.) giving acceptable results. A study on the influence of the lattice BGK models parameters controlling sorption and matrix diffusion on the breakthrough curves shape is included.     

Monod kinetics rather than a first-order degradation model explains atrazine fate in soil mini-columns: Implications for pesticide fate modelling

Pesticide transport models commonly assume first-order pesticide degradation kinetics for describing reactive transport in soil. This assumption was assessed in mini-column studies with associated batch degradation tests. Soil mini-columns were irrigated with atrazine in two intermittent steps of about 30 days separated by 161 days application of artificial rain water. Atrazine concentration in the effluent peaked to that of the influent concentration after initial break-through but sharply decreased while influx was sustained, suggesting a degradation lag phase. The same pattern was displayed in the second step but peak height and percentage of atrazine recovered in the effluent were lower. A Monod model with biomass decay was successfully calibrated to this data. The model was successfully evaluated against batch degradation data and mini-column experiments at lower flow rate. The study suggested that first-order degradation models may underestimate risk of pesticide leaching if the pesticide degradation potential needs amplification during degradation.     

The stratospheric ozone layer-an overview

This paper summarises the knowledge on the properties of the stratospheric ozone layer. Dynamic, chemical, and microphysical aspects are reviewed with emphasis on chemistry. The questions addressed are as follows. Do we have a quantitative understanding of the Antarctic ozone hole? What lies behind the trend of slowly decreasing ozone columns over northern mid-latitudes? To what degree was chemistry responsible for the extremely low ozone levels over northern Europe in January 1992? The discovery of the ozone hole in 1985 exposed scientific neglect of the category of fast heterogeneous reactions taking place on particulate matter in the stratosphere. But even now after the wide acceptance of some heterogeneous reactions it is difficult to fully account for the rate at which Antarctic ozone is depleted each year in August. After reviewing the known heterogeneous reactions, possible hitherto unrecognised mechanisms are briefly outlined. The paper also includes a discussion of the chemical reactions which can occur even under relatively warm conditions on the ubiquitous, stratospheric aerosol particles and which could contribute to the observed mid-latitudinal ozone depletion. Finally, the paper underlines the importance of dynamic processes, that is, horizontal transport and vertical adiabatic motion, which appear to be the main cause of the anomalously low northern hemispheric ozone values during the 1991/1992 winter.     

Particle-facilitated lead and arsenic transport in abandoned mine sites soil influenced by simulated acid rain

The role of acid rain in affecting Pb and As transport from mine tailings was investigated by pumping simulated acid rain at a infiltration rate of 10.2 cm/h through soil columns. Simulated acid rain with pH of 3.0, 4.5 and 5.6 were used as leaching solutions. Results showed that 86.9–95.9% of Pb and 90–91.8% of As eluted from the columns were adsorbed by particles in the leachates. Scanning electron microscopy (SEM) analysis showed that particles released from the columns were mainly composed of flocculated aggregates and plate or rod shaped discrete grains. Transmission electron microscopy (TEM) coupled with energy dispersive X-ray analysis (EDX) showed that these particles were predominantly silicate minerals. Results from our experiments demonstrated that when rapid infiltration conditions or a rainstorm exist, particle-facilitated transport of contaminants is likely to the dominant metal transport pathway influenced by acid rain.     

Leaching and persistence of ametryn and atrazine in red–yellow latosol

This study evaluated the mobility and persistence of atrazine and ametryn in red–yellow latosols using polyvinyl chloride columns with a diameter of 100 mm and a height of 15 cm. The assays simulated 60-mm rainfall events at 10-day intervals for 70 days. The persistence and leaching were evaluated for these two herbicides. The analytes obtained from the samples were quantified by gas chromatography using flame ionization detection. Compared with ametryn, atrazine showed a greater potential to reach depths below 15 cm over 30 days of simulated rain. Ametryn, however, showed greater persistence in soil at 70 days after application. The persistence of atrazine and ametryn in soil under sunlight was 10 and 144 days respectively. Atrazine was more susceptible to sunlight than ametryn because sunlight favored atrazine degradation in hydroxyatrazine. The results indicate that in red–yellow latosol, atrazine has a high leaching potential in short term, but that ametryn is more persistent and has a high leaching potential in long term.     

A reaction-based paradigm to model reactive chemical transport in groundwater with general kinetic and equilibrium reactions

This paper presents a reaction-based water quality transport model in subsurface flow systems. Transport of chemical species with a variety of chemical and physical processes is mathematically described by M partial differential equations (PDEs). Decomposition via Gauss-Jordan column reduction of the reaction network transforms M species reactive transport equations into two sets of equations: a set of thermodynamic equilibrium equations representing N(E) equilibrium reactions and a set of reactive transport equations of M-N(E) kinetic-variables involving no equilibrium reactions (a kinetic-variable is a linear combination of species). The elimination of equilibrium reactions from reactive transport equations allows robust and efficient numerical integration. The model solves the PDEs of kinetic-variables rather than individual chemical species, which reduces the number of reactive transport equations and simplifies the reaction terms in the equations. A variety of numerical methods are investigated for solving the coupled transport and reaction equations. Simulation comparisons with exact solutions were performed to verify numerical accuracy and assess the effectiveness of various numerical strategies to deal with different application circumstances. Two validation examples involving simulations of uranium transport in soil columns are presented to evaluate the ability of the model to simulate reactive transport with complex reaction networks involving both kinetic and equilibrium reactions.     

Description of sorbing tracers transport in fractured media using the lattice model approach

The transport of contaminants in fractured media is a complex phenomenon with a great environmental impact. It has been described with several models, most of them based on complex partial differential equations, that are difficult to apply when equilibrium and nonequilibrium dynamics are considered in complex boundaries. With the aim of overcoming this limitation, a combination of two lattice Bathnagar, Gross and Krook (BGK) models, derived from the lattice Boltzmann model, is proposed in this paper. The fractured medium is assumed to be a single fissure in a porous rock matrix. The proposed approach permits us to deal with two processes with different length scales: advection–dispersion in the fissure and diffusion within the rock matrix. In addition to the mentioned phenomena, sorption reactions are also considered. The combined model has been tested using the experimental breakthrough curves obtained by Garnier et al. (Garnier, J.M., Crampon, N., Préaux, C., Porel, G., Vreulx, M., 1985. Traçage par ¹³C, ²H, I⁻ et uranine dans la nappe de la craie sénonienne en écoulement radial convergent (Béthune, France). J. Hidrol. 78, 379–392.) giving acceptable results. A study on the influence of the lattice BGK models parameters controlling sorption and matrix diffusion on the breakthrough curves shape is included.     

Transport of Escherichia coli through variably saturated sand columns and modeling approaches

A sand column leaching system with well-controlled suction and flow rate was built to investigate the effects on bacterial transport of air-water interface effects (AWI) correlated to water content, particle size, and column length. Adsorption of Escherichia coli strain D to silica sands was measured in batch tests. The average % adsorption for coarse and fine sands was 45.9+/-7.8% and 96.9+/-3.2%, respectively. However, results from static batch adsorption experiments have limited applicability to dynamic bacterial transport in columns. The early breakthrough of E. coli relative to bromide was clear for all columns, namely c. 0.15 to 0.3 pore volume earlier. Column length had no significant effects on the E. coli peak concentration or on total recovery in leachate, indicating retention in the top layer of sands. Tailing of breakthrough curves was more prominent for all fine sand columns than their coarse sand counterparts. Bacterial recovery in leachate from coarse and saturated sand columns was significantly higher than from fine and unsaturated columns. Observed data were fitted by the convection-dispersion model, amended for one-site and two-site adsorption to particles, and for air-water interface (AWI) adsorption. Among all models, the two-site+AWI model achieved consistently high model efficiency for all experiments. Thus it is evident from experimental and modeling results that AWI adsorption plays an important role in E. coli transport in sand columns.     

An analytical solution for vertical transport of volatile chemicals in the vadose zone

An analytical solution is presented for one-dimensional vertical transport of volatile chemicals through the vadose zone to groundwater. The solution accounts for the important transport mechanisms of the steady advection of water and gas, diffusion and dispersion in water and gas, as well as adsorption, and first-order degradation. By assuming a linear, equilibrium partitioning between water, gas and the adsorbed chemical phases, the dependent variable in the mathematical model becomes the total resident concentration. The general solution was derived for cases having a constant initial total concentration over a discrete depth interval and a zero initial total concentration elsewhere. A zero concentration gradient is assumed at the groundwater table. Examples are given to demonstrate the application of the new solution for calculating the case of a non-uniform initial source concentration, and estimating the transport of chemicals to the groundwater and the atmosphere. The solution was also used to verify a numerical code called VLEACH. We discovered an error in VLEACH, and found that the new solution agreed very well with the numerical results by corrected VLEACH. A simplified solution to predict the migration of volatile organic chemical due to the gas density effect has shown that a high source concentration may lead to significant downward advective gas-phase transport in a soil with a high air-permeability.     

Erratum

The influence of soil macro-porosity and manure on atrazine (6-chloro-N-ethyl-N′-(1-methylethyl)-1,3,5-triazine-2,4-diamine) transport was investigated under laboratory conditions using disturbed and undisturbed soil columns. The macro-porosity in the soil column was obtained with CT scanning technique. Liquid manure was applied at the surface of soil column, 19 cm long and 8 cm in diameter, at a rate of 60 m³/ha. Experimental results revealed that atrazine moves faster through the soils in the presence of manure compared to soil without application of manure. The average time for elusion and the relative peak concentration in the disturbed soil column without manure was 14.5 h and 3.1%, respectively compared to 11.0 h and 6.9% in the presence of manure, respectively. Similar behavior was observed in the case of disturbed soil columns. Soil macro-porosity has shown large impact on atrazine transport, especially in the presence of manure.     

Macroporosity and manure influence on atrazine transport through soil

The influence of soil macro-porosity and manure on atrazine (6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine) transport was investigated under laboratory conditions using disturbed and undisturbed soil columns. The macro-porosity in the soil column was obtained with CT scanning technique. Liquid manure was applied at the surface of soil column, 19 cm long and 8 cm in diameter, at a rate of 60 m3/ha. Experimental results revealed that atrazine moves faster through the soils in the presence of manure compared to soil without application of manure. The average time for elusion and the relative peak concentration in the disturbed soil column without manure was 14.5 h and 3.1%, respectively compared to 11.0 h and 6.9% in the presence of manure, respectively. Similar behavior was observed in the case of disturbed soil columns. Soil macro-porosity has shown large impact on atrazine transport, especially in the presence of manure.