Atrazine Transport Research Articles

Differential Transport of Atrazine and Glyphosate in Undisturbed Sandy Soil Column

With increasing awareness and concern for environmental quality, it is important to study the fate of pesticides in the subsurface. Laboratory studies were conducted to determine the behavior of atrazine and glyphosate within the root zone of an undisturbed sandy soil in Jianghan Plain, central China. Chloride as a tracer for water movement was applied to the soil as KCl for 26 hours before pesticide application for another 160 hours. Glyphosate, atrazine, and Cl concentrations (conc.) were determined as a function of time in breakthrough curves (BTCs). Atrazine BTC was fitted better in convection-dispersion equation equilibrium model. For glyphosate, however, a two-site non-equilibrium model was chosen. Leaching rate of atrazine from sandy soil was much higher than that of glyphosate and it took longer for glyphosate to leach through the column due to stronger sorption and degradation to its major metabolite, AMPA (aminomethylphosphonic acid, CH6NO3P), which was detected (up to 8890 ng/l) in the final leachate.

Influence of flow concentration on parameter importance and prediction uncertainty of pesticide trapping by vegetative filter strips

Flow concentration is a key hydrologic factor limiting the effectiveness of vegetated filter strips (VFS) in removing pesticides from surface runoff. Numerical models, such as VFSMOD-W, offer a mechanistic approach for evaluating VFS effectiveness under various hydrological conditions including concentrated flow. This research hypothesizes that the presence of concentrated flow drastically alters the importance of various hydrological, sedimentological, and pesticide input factors and the prediction uncertainty of pesticide reduction. Using data from a VFS experimental field study investigating chlorpyrifos and atrazine transport, a two-step global sensitivity and uncertainty analysis framework was used with VFSMOD-W based on (1) a screening method (Morris) and (2) a variance-based method (extended Fourier Analysis Sensitivity Test, FAST). The vertical, saturated hydraulic conductivity was consistently the most important input factor for predicting infiltration, explaining 49% of total output variance for uniform sheet flow, but only 8% for concentrated flow. Sedimentation was governed by both hydrologic (vertical, saturated hydraulic conductivity and initial and saturated water content) and sediment characteristics (average particle diameter). The vertical, saturated hydraulic conductivity was the most important input factor for atrazine or chlorpyrifos trapping under uniform sheet flow (explained more than 46% of the total output variance) and concentrated flow (although only explained 8% of the total variance in this case). The 95% confidence intervals for atrazine and chlorpyrifos reduction ranged between 43% and 78% for uniform sheet flow and decreased to between 1% and 16% under concentrated flow. Concentrated flow increased interactions among the system components, enhancing the relative importance of processes that were latent under shallow flow conditions. This complex behavior warrants the need for process-based modeling to be able to predict the performance of VFS under a wide range of specific hydrological conditions.

Impact of non-ideal sorption on low-concentration tailing behavior for atrazine transport in two natural porous media

The impact of non-ideal sorption on atrazine transport was investigated for two sandy porous media with 0.38% and 0.03% organic-carbon contents. In contrast to prior investigations, effluent atrazine concentrations were monitored over a range of five orders of magnitude to examine long-term elution behavior. As characterized by batch-experiments, atrazine experienced nonlinear sorption for both media. The results of the column experiments showed that atrazine exhibited extensive elution tailing (delayed approach to relative concentration of zero). This non-ideal transport was more pronounced for the medium with higher organic-carbon content. A mathematical model incorporating nonlinear, rate-limited sorption/desorption described by a continuous-distribution function was used to successfully simulate atrazine transport.

Prediction of the Fate and Transport Processes of Atrazine in a Reservoir

The fate and transport processes of a toxic chemical such as atrazine, an herbicide, in a reservoir are significantly influenced by hydrodynamic regimes of the reservoir. The two-dimensional (2D) laterally-integrated hydrodynamics and mass transport model, CE-QUAL-W2, was enhanced by incorporating a submodel for toxic contaminants and applied to Saylorville Reservoir, Iowa. The submodel describes the physical, chemical, and biological processes and predicts unsteady vertical and longitudinal distributions of a toxic chemical. The simulation results from the enhanced 2D reservoir model were validated by measured temperatures and atrazine concentrations in the reservoir. Although a strong thermal stratification was not identified from both observed and predicted water temperatures, the spatial variation of atrazine concentrations was largely affected by seasonal flow circulation patterns in the reservoir. In particular, the results showed the effect of flow circulation on spatial distribution of atrazine during summer months as the river flow formed an underflow within the reservoir and resulted in greater concentrations near the surface of the reservoir. Atrazine concentrations in the reservoir peaked around the end of May and early June. A good agreement between predicted and observed times and magnitudes of peak concentrations was obtained. The use of time-variable decay rates of atrazine led to more accurate prediction of atrazine concentrations, while the use of a constant half-life (60 days) over the entire period resulted in a 40% overestimation of peak concentrations. The results provide a better understanding of the fate and transport of atrazine in the reservoir and information useful in the development of reservoir operation strategies with respect to timing, amount, and depth of withdrawal.

Atrazine biodegradation in the lab and in the field: enzymatic activities and gene regulation

SummaryAtrazine is an herbicide of the s‐triazine family that is used primarily as a nitrogen source by degrading microorganisms. While many catabolic pathways for xenobiotics are subjected to catabolic repression by preferential carbon sources, atrazine utilization is repressed in the presence of preferential nitrogen sources. This phenomenon appears to restrict atrazine elimination in nitrogen‐fertilized soils by indigenous organisms or in bioaugmentation approaches. The mechanisms of nitrogen control have been investigated in the model strain Pseudomonas sp. ADP. Expression of atzA, atzB ad atzC, involved in the conversion of atrazine in cyanuric acid, is constitutive. The atzDEF operon, encoding the enzymes responsible for cyanuric acid mineralization, is a target for general nitrogen control. Regulation of atzDEF involves a complex interplay between the global regulatory elements of general nitrogen control and the pathway‐specific LysR‐type regulator AtzR. In addition, indirect evidence suggests that atrazine transport may also be a target for nitrogen regulation in this strain. The knowledge about regulatory mechanisms may allow the design of rational bioremediation strategies such as biostimulation using carbon sources or the use of mutant strains impaired in the assimilation of nitrogen sources for bioaugmentation.

Transport of Atrazine in Residue-Amended Soils

In this study, our aim was to quantify the transport of atrazine in soil columns where sugarcane (Saccharum spp. hyb.) residue was incorporated. The residue is a result of sugarcane combine harvest technology where massive amount of residue is deposited on the soil surface after each harvest. Miscible displacement experiments were carried out to quantify the mobility of atrazine in soil columns where sugarcane residue was mixed a Commerce soil. The residue was either incorporated uniformly with the soil or applied as a thin mulch layer over surface soil. The incorporation of residue with the soil resulted in retarded atrazine mobility because of strong sorption and slow release of the breakthrough curves (BTCs) during leaching. Moreover, early arrival of atrazine in the column effluent was observed from BTC results where the residue was uniformly incorporated with the soil, which is possibly due to physical nonequilibrium conditions, that is, preferential flow or macropore flow. An equilibrium-type model was incapable of describing atrazine BTCs from all columns regardless of whether the residue was placed as a surface layer or incorporated in the soil. In contrast, good overall description of measured BTC results was realized when a fully kinetic model was used. Therefore, our hypothesis that kinetic rather than equilibrium-type reactions are the dominant mechanisms for the release of adsorbed atrazine from the residue is supported.

Label‐Free Colorimetric Detection of Trace Atrazine in Aqueous Solution by Using Molecularly Imprinted Photonic Polymers

Based on the combination of colloidal-crystal templating and a molecular imprinting technique, a sensor platform for efficient detection of atrazine in aqueous solution has been developed. The sensor is characterized by a 3D-ordered interconnected macroporous structure in which numerous nanocavities derived from atrazine imprinting are distributed in the thin wall of the formed inverse polymer opal. Owing to the special hierarchical porous structure, the molecularly imprinted polymer opals (or molecularly imprinted photonic polymer; MIPP) allow rapid and ultrasensitive detection of the target analyte. The interconnected macropores are favorable for the rapid transport of atrazine in polymer films, whereas the inherent high affinity of nanocavites distributed in thin polymer walls allows MIPP to recognize atrazine with high specificity. More importantly, the atrazine recognition events of the created nanocavities can be directly transferred (label-free) into a readable optical signal through a change in Bragg diffraction of the ordered macropores array of MIPP and thereby induce color changes that can be detected by the naked eye. With this novel sensory system, direct, ultrasensitive (as low as 10(-8) ng mL(-1)), rapid (less than 30 s) and selective detection of atrazine with a broad concentration range varying from 10(-16) M to 10(-6) M in aqueous media is achieved without the use of label techniques and expensive instruments.

Atrazine remediation in agricultural infiltrate by bioaugmented polyvinyl alcohol immobilized and free Agrobacterium radiobacter J14a

Bench-scale sand column breakthrough experiments were conducted to examine atrazine remediation in agricultural infiltrate by Agrobacterium radiobacter J14a (J14a) immobilized in phosphorylated-polyvinyl alcohol compared to free J14a cells. The effects of cell loading and infiltration rate on atrazine degradation and the loss of J14a were investigated. Four sets of experiments, i) tracers, ii) immobilized dead cells, iii) immobilized cells, and iv) free cells, were performed. The atrazine bioremediation at the cell loadings of 300, 600, and 900 mg dry cells l(-1) and the infiltration rates of 1, 3, and 6 cm d(-1) were tested for 5 column pore volumes (PV). The atrazine breakthrough results indicated that the immobilized dead cells significantly retarded atrazine transport. The atrazine removal efficiencies at the infiltration rates of 1, 3, and 6 cm d(-1) were 100%, 80-97%, and 50-70% respectively. Atrazine remediation capacity for the immobilized cells was not significantly different from the free cells. Both infiltration rate and cell loading significantly affected atrazine removal for both cell systems. The bacterial loss from the immobilized cell system was 10 to 100 times less than that from the free cell system. For long-term tests at 50 PV, the immobilized cell system provided consistent atrazine removal efficiency while the atrazine removal by the free cells declined gradually because of the cell loss.

Atrazine removal in agricultural infiltrate by bioaugmented polyvinyl alcohol immobilized and free Agrobacterium radiobacter J14a: A sand column study

Bench-scale sand column breakthrough experiments were conducted to examine atrazine removal in agricultural infiltrate by Agrobacterium radiobacter J14a (J14a) immobilized in phosphorylated-polyvinyl alcohol compared to free J14a cells. The effects of cell loading and infiltration rate on atrazine degradation and the loss of J14a were investigated. Four sets of experiments, (i) tracers, (ii) immobilized dead cells, (iii) immobilized cells, and (iv) free cells, were performed. The atrazine biodegradation at the cell loadings of 300, 600, and 900 mg dry cells L −1 and the infiltration rates of 1, 3, and 6 cm d −1 were tested for 5 column pore volumes (PV). The atrazine breakthrough results indicated that the immobilized dead cells significantly retarded atrazine transport. The atrazine removal efficiencies at the infiltration rates of 1, 3, and 6 cm d −1 were 100%, 80–97%, and 50–70%, respectively. Atrazine degradation capacity for the immobilized cells was not significantly different from the free cells. Both infiltration rate and cell loading significantly affected atrazine removal for both cell systems. The bacterial loss from the immobilized cell system was 10–100 times less than that from the free cell system. For long-term tests at 50 PV, the immobilized cell system provided consistent atrazine removal efficiency while the atrazine removal by the free cells declined gradually because of the cell loss.

A finite element solution for the fractional advection–dispersion equation

The fractional advection–dispersion equation (FADE) known as its non-local dispersion, has been proven to be a promising tool to simulate anomalous solute transport in groundwater. We present an unconditionally stable finite element (FEM) approach to solve the one-dimensional FADE based on the Caputo definition of the fractional derivative with considering its singularity at the boundaries. The stability and accuracy of the FEM solution is verified against the analytical solution, and the sensitivity of the FEM solution to the fractional order α and the skewness parameter β is analyzed. We find that the proposed numerical approach converge to the numerical solution of the advection–dispersion equation (ADE) as the fractional order α equals 2. The problem caused by using the first- or third-kind boundary with an integral-order derivative at the inlet is remedied by using the third-kind boundary with a fractional-order derivative there. The problems for concentration estimation at boundaries caused by the singularity of the fractional derivative can be solved by using the concept of transition probability conservation. The FEM solution of this study has smaller numerical dispersion than that of the FD solution by Meerschaert and Tadjeran (J Comput Appl Math 2004). For a given α, the spatial distribution of concentration exhibits a symmetric non-Fickian behavior when β = 0. The spatial distribution of concentration shows a Fickian behavior on the left-hand side of the spatial domain and a notable non-Fickian behavior on the right-hand side of the spatial domain when β = 1, whereas when β = −1 the spatial distribution of concentration is the opposite of that of β = 1. Finally, the numerical approach is applied to simulate the atrazine transport in a saturated soil column and the results indicat that the FEM solution of the FADE could better simulate the atrazine transport process than that of the ADE, especially at the tail of the breakthrough curves.

Stationary and non-stationary autoregressive processes with external inputs for predicting trends in water quality

An autoregressive approach for the prediction of water quality trends in systems subject to varying meteorological conditions and short observation periods is discussed. Under these conditions, the dynamics of the system can be reliably forecast, provided their internal processes are understood and characterized independently of the external inputs. A methodology based on stationary and non-stationary autoregressive processes with external inputs (ARX) is proposed to assess and predict trends in hydrosystems which are at risk of contamination by organic and inorganic pollutants, such as pesticides or nutrients. The procedures are exemplified for the transport of atrazine and its main metabolite deethylatrazine in a small agricultural catchment in France. The approach is expected to be of particular value to assess current and future trends in water quality as part of the European Water Framework Directive and Groundwater Directives.

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 19.86cmh−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 6.68cmh−1 and 32.81cmh−1 were predicted by the TSM and the CTRW. As to the flow rate of 6.68cmh−1, the CTRW predicted the entire BTC of atrazine transport better than the TSM did. For the flow rate of 32.81cmh−1, the CTRW characterized the late part of the tail better, while the TSM failed to predict the tailings of atrazine BTC.

Spatial variability of bromide and atrazine transport parameters for a Udipsamment

Variability of soil properties can have major influence on the results of solute transport models when applied to field conducted experiments. The objective of this study was to determine spatial variability of transport and degradation parameters for a bromide tracer and atrazine herbicide in a 0.1 ha field of alluvial soils under no-till management. The soil investigated was a Sarpy (mixed, mesic Typic Udipsamment) with surface texture varying from sand to loam. The field was classified into three areas (Area I, sand; Area II, sandy loam; and Area III, loam) according to surface texture. Atrazine and bromide were applied at rates of 1.8 kg ha − 1 and 115 kg ha − 1 , respectively. Soil cores were extracted at selected dates, segmented into 75 mm increments, and analyzed for each chemical separately. Soil physical and chemical properties were determined as a function of soil depth from additional soil cores removed when the transport study was completed. The value of the average pore water velocity, v, estimated using bromide concentrations was 6.0 mm day − 1 which was about 29% lower than the value computed from meteorological information, 8.5 mm day − 1 . The average bromide dispersion coefficient ( D), 508 mm 2 day − 1 , was 80 times higher than that for atrazine, 6.13 mm 2 day − 1 . The bromide v and D parameters were found to be spatially autocorrelated with ranges between 20 and 23 m and 12 and 24 m, respectively. Atrazine retardation and degradation parameters were found to have spatial structure for only one sampling date each with a range between 18 and 24 m. Soil textural parameters were found to have spatial structure for the first three depths (0–45 cm) of the five depths monitored (0–75 cm). This study found spatial autocorrelation within about 20 m for bromide transport parameters and a few atrazine transport parameters for an alluvial site.

SENSITIVITY OF ATRAZINE PARTITION COEFFICIENT ON PESTICIDE ROOT ZONE MODEL PREDICTION OF SOIL'S ATRAZINE RESIDUE DISTRIBUTION WITHIN THE CROP ROOT ZONE

Previous applications of the Pesticide Root Zone Model (PRZM) comparing predicted distributions of pesticides in the soil profile to the measured values suggest that PRZM prediction capability may be improved by selecting proper representation of the distribution coefficient values that directly influence atrazine-soil interactions. The objective of this study was to observe the variation in the predicted distribution of atrazine in the soil profile as the coefficient of distribution (Kd) was varied in one or more soil horizons using the PRZM-2 model. The goal was to identify if more attention should be devoted to understanding the atrazine-soil sorption issues better, or whether discrepancies in field data and model output are explained by the difficulties in assessing the preferential transport of atrazine. The PRZM-2 predicted atrazine distribution was compared with measured values for no-till (NT) and conventional-till (CT) corn plots. Results indicate that reducing Kd by 70% in NT plots reduced the predicted atrazine recovery in Horizon 1 (0-10 cm) by 31%, 34%, and 87% for simulations performed 2, 4, and 8 weeks after application (WAA). Reducing Kd in only the upper soil horizon improved predictions for the upper horizon but overpredicted atrazine concentrations in the second horizon. Reducing Kd throughout the profile reduced predicted atrazine concentrations in the upper horizon and increased predicted concentrations in lower horizons. Improvement in the correlation between predicted and measured atrazine concentrations was greatest for NT plots; however, discrepancies between predicted and measured data still exist and may be due to improper characterization of the atrazine degradation or dissipation parameters. These results suggest that although preferential flow appears to influence atrazine distribution, it may not be the only process influencing our ability to model atrazine fate and transport. Future research on atrazine degradation and dissipation under field conditions seems to be necessary to simulate atrazine fate and transport under preferential flow conditions.

Effects of soil type and tillage practice on atrazine transport through intact soil cores

Agricultural systems of Argentina have increased herbicides inputs, mostly associated with adoption of no tillage (NT). Several studies have revealed presence of pesticides in groundwater. Therefore, research on the behaviour of herbicides in soils is driven by the need to manage and prevent possible contamination of groundwater. Soil organic carbon (OC) is the main soil component responsible of sorption, and consequently the main tool to reduce the leaching. However, in dynamic systems transport of organic chemicals depends on soil structural and hydraulic properties. Sorption controls the physical and biological availability of chemicals. Physical, heterogeneous flow domain, and chemical, kinetic reactions and molecular diffusion into aggregates, which are nonequilibrium processes that affect solute transport. The main objective of this paper was to evaluate the effects of soil texture and tillage system on atrazine transport through intact soil columns. The study focused on the identification of processes; and determination of parameters that control atrazine transport in the upper layer of soils. Balcarce (BAL, silty clay loam, fine, thermic, illitic, Typic Argiudoll)), Tres Arroyos (TAR, clay loam, fine, thermic, illitic, Typic Argiudoll) and Coronel Dorrego (DOR, loam, fine, thermic, mixed illitic–montmorillonitic, Typic Argiudoll) soils from the southeast of Buenos Aires Province (Argentina) were selected. The soils represent a wide range of OC content (BAL 35.5, TAR 28.8 and DOR 17.3 g kg − 1 ). At each site NT and conventional tillage (CT) systems were sampled. Four replicates of intact soils cores (15 × 8 cm) were removed from each combination of soil × tillage (BAL-NT, BAL-CT, TAR-NT, TAR-CT, DOR-NT, DOR-CT). Displacement studies were done using atrazine as the reactive solute and bromide as the nonreactive solute. Equilibrium and nonequilibrium transport models (CXTFIT 2.1) were employed to describe the breakthrough curves (BTCs). The software tool SMART was used to simulate atrazine transport under steady-state flow conditions. Atrazine BTCs were skewed to the right; exhibiting an asymmetric shape and tailing that implied nonequilibrium conditions during transport. Since physical nonequilibrium was assumed to be nearly negligible, the observed nonequilibrium was interpreted as a sorption-related process. The two-site nonequilibrium model showed an acceptable fit with the observed data (72 < R 2 < 86). Recovery percentages of atrazine in effluents were: BAL-CT 54.51%; BAL-NT 45.10%; TAR-CT 44.28%; TAR-NT 29.70%; DOR-CT 18.60%; DOR-NT 48.95%. The intraparticle diffusion model provided by SMART showed the best fit. In conclusion, intrinsic soil properties were more relevant for atrazine transport than those associated with tillage practices. However, no tillage produced early detection of atrazine in effluents, and favoured atrazine leaching in coarser soils with the lowest OC contents. However, the maximum loss of atrazine in the percolate took place in the soils with the highest OC level; with no effects of tillage practices. These soils had fine texture, and were well structured and aggregated. Intraparticle and intraorganic matter diffusion appear to be responsible for nonequilibrium sorption. Delayed sorption in aggregated soils leads to high concentration of atrazine available for leaching.

Mineralisation of atrazine, metolachlor and their respective metabolites in vegetated filter strip and cultivated soil

In vegetated filter strips (VFS) the presence of perennial vegetation, rhizodeposition of labile organic substrates and the accumulation of an organic residue thatch layer may enhance microbial numbers and activity, thereby increasing the potential for mineralisation of herbicides and herbicide metabolites retained during run-off events. The objective of this laboratory experiment was to compare the mineralisation of atrazine and metolachlor with that of their respective metabolites in VFS and cultivated soil. With the exception of total bacteria, propagule density of the microbial groups, endogenous soil enzymes and microbial diversity were higher in the VFS soil. This correlated with increased mineralisation of metolachlor and its metabolites in the VFS soil and indicates potential for VFS to curtail the subsequent transport of these compounds. In contrast, the mineralisation of atrazine and the majority of its metabolites was substantially reduced in VFS soil relative to cultivated soil. Consequently, the potential for subsequent transport of atrazine and many of its metabolites may be greater in VFS soil than in cultivated soil if reduced mineralisation is not offset by increased sorption in the VFS.

The Attenuation of Atrazine and its Major Degradation Products in a Restored Riparian Buffer

The fate and transport of atrazine and three of its major degradation products were studied in a restored forested riparian buffer system in southern Georgia to examine the effectiveness of the system in attenuating the chemicals. Atrazine was applied to a 10 100 m strip immediately upslope from the buffer system, and atrazine, deethylatrazine, deisopropylatrazine, and hydroxyatrazine were monitored in groundwater, soil, and runoff water for 11 months following application. Atrazine removal from groundwater was very effective, with the calculated 3810 mg entering the upslope position being reduced to 174 mg exiting the streamside landscape position. Each of the landscape positions (0, 8, 13, 18, 28, and 38 m from the application strip) in the buffer system contributed significantly to the atrazine concentration reduction in the initial three months following application, when the majority of atrazine was measured. Deethylatrazine is the primary result of biotic degradation and was the predominant degradation product in groundwater, with approximately 10% (379 mg) as much estimated to have entered the upper interface of the buffer system as did the parent atrazine. The riparian buffer system was successful in significantly reducing concentrations of deethylatrazine in groundwater in both the initial three months following application and in the final eight months of the study. Adsorption of the analytes to subsurface soils was minimal in this system. Deethylatrazine was also the predominant degradation product in the runoff water, but hydroxyatrazine was predominant in conjunction with runoff sediments. Removal efficiency of analyte mass loads in surface runoff water ranged from 92% to 100% as the chemicals were transported through the buffer system. While the mass load of atrazine associated with sediment was only 8% of that measured for the water phase, the mass loads of deethylatrazine and deisopropylatrazine (approximately 40%) and particularly hydroxyatrazine (75%) in sediments were proportionally much higher. About 98.6% of measured runoff water and sediment phase transport of atrazine occurred within the first few days following application as a result of an extended rainfall event. In contrast, roughly 45% of the deethylatrazine and deisopropylatrazine and only 16% of the hydroxyatrazine were measured in the first rain event sample. The results of this work indicate that riparian buffer systems can be successfully used to intercept atrazine and the studied degradation products in order to minimize contamination of natural water systems. Riparian buffers can provide year round attenuation, particularly for deethylatrazine. The importance of timing atrazine application to correspond with periods when there is no threat of rainfall is also emphasized.

On the Value of Local Measurements for Prediction of Pesticide Transport at the Field Scale

Pesticide transport through the soil profile at the field scale is notoriously difficult to predict because of a lack of appropriate field‐scale models and model parameters that take adequate account of heterogeneity in local flow rates and parameters. It is generally impossible to measure all the parameters required to describe that heterogeneity. A way of approximating pesticide transport at the field scale, given uncertainty in the representation of both local characteristics and field‐scale distributions, is presented. The methodology allows prior estimates of field‐scale distribution parameters to be conditioned on uncertain column‐scale measurements. It is shown how this conditioning can drastically constrain steady‐state flow predictions of atrazine (2‐chloro‐4‐ethylamino‐6‐isopropylamino‐1,3,5‐triazine) transport to groundwater for a site in the Rhone Valley, Switzerland. Some of the simplifying assumptions of the current analysis, in particular, to allow for transient simulations, can be relaxed in the future as more computer power becomes available. The methodology, however, should remain valid.

Comparison of Atrazine Losses in Three Small Headwater Catchments

Understanding the processes causing herbicide transport to surface waters is crucial to determine mitigation options to reduce these losses. To this end, we investigated the atrazine (2-chloro-4-ethylamino-6-isopropylamino-1,3,5-triazine) transport in three agricultural catchments (1.1-2.1 km2) in the watershed of Lake "Greifensee" (Switzerland). In 1999, atrazine application data were recorded for all three catchments. Time proportional samples were taken at a high temporal resolution at the catchment outlets. Extremely wet conditions caused large relative losses from the catchments, ranging between 0.6 and 3.5% of the amount applied. Most of the atrazine load was due to event-driven diffuse losses from the fields. Farmyard runoff contributed less but caused the highest concentrations (up to 31 microg L(-1)) in the brooks. The maximum concentrations due to diffuse losses varied between 1.2 and 8.2 microg L(-1) among the catchments. Despite different absolute concentration levels, the concentration time-series were very similar. It seems that the travel-times within the catchments were mainly controlled by the rainfall pattern with little influence of the catchment properties. These properties, however, caused the relative losses to vary by a factor of 6 between the catchments. This variability could be partly explained by differences in the connectivity of the fields to the brooks and by their hydrological soil properties. A comparison of the losses from the three catchments with those from the entire watershed of Lake Greifensee demonstrated that they were representative for the larger area. Hence, the study results provide a good data set to evaluate distributed models predicting herbicide losses.

Sulcotrione versus atrazine transport and degradation in soil columns

A soil column experiment under outdoor conditions was performed to monitor the fate of 14C-ring-labelled sulcotrione, 2-(2-chloro-4-mesylbenzoyl)cyclohexane-1,3-dione and atrazine, 6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine, in water leachates and in the ploughed horizon of a sandy loam soil. Two months after treatment, the cumulative amounts of herbicide residues leached from the soil were 14.5% and 7% of the applied radioactivity for sulcotrione and atrazine, respectively. Maximum leachate concentrations for each herbicide were observed during the first month following application: 120 and 95 microg litre(-1) for sulcotrione and atrazine respectively. After 2 weeks, 78% of the sulcotrione and atrazine was extractable from the soil, whereas after two months only 10 and 4%, respectively, could be extracted. The maximum sulcotrione content in the first 10 cm of soil was identical with that of atrazine. For both molecules, the content of non-extractable residues was low, being around 15%. Sulcotrione seems to be more mobile than atrazine but the consequences for water contamination are similar since lower doses are used.