SWAT—A Semi-empirical Model to Predict Concentrations of Pesticides Entering Surface Waters from Agricultural Land

A semi-empirical model called SWAT has been developed to predict concentrations of agriculturally applied pesticides moving to surface waters, an aspect which is not well described by current models for pesticide fate. The model is based upon a direct hydrological link established between soil type and the amount of water moving rapidly to streams in response to rainfall. Attenuation factors describe the decrease in concentrations of pesticide between field application and loss in water moving from the site into surface waters. Evaluation of model predictions against available field data from three sites and four soil types in England shows that SWAT is capable of predicting the transient peak concentrations of a wide range of pesticides during rapid water movement to streams in response to rainfall. Predicted concentrations were too great when rainfall initiated water movement to streams very soon after pesticide application, particularly for the more mobile pesticides, and some predictions for pesticides sorbed very strongly to soil were relatively poor. Almost all of the predicted concentrations were within one order of magnitude of measured values.     

Factors influencing entry of pesticides into soil water

Leaching of pesticides and hence the risk of contamination of ground-water depends on the physicochemical properties of the pesticide, the properties of the soil and the weather. Lipophilicity is the most important physicochemical property influencing the movement of un-ionised pesticides through soil. Water solubility is usually only an important factor in leaching for a few moderately polar solids with high melting points. Organic matter content is the most important property of the soil for un-ionised pesticides whilst the mobility of weak acids depends on soil pH. Permanent anions and weak acids can be very weakly adsorbed and hence might easily reach groundwater. Applications in autumn are more likely to reach groundwater than those in spring because soil temperatures are low and rainfall exceeds evaporation in winter, enabling mobile pesticides to penetrate to subsoils where degradation rates can be very slow. Concentrations of pesticide in water entering subsoils cannot be reliably simulated to an accuracy of better than an order of magnitude because the complex patterns of water flow and the slow diffusion processes of the pesticide are insufficiently understood. The consequences of applying a mobile pesticide to soil where drainage is impeded or where the water table is near the surface need to be anticipated before it is registered for treatment of the soil.     

Effect of pesticide fate parameters and their uncertainty on the selection of 'worst-case' scenarios of pesticide leaching to groundwater

BACKGROUND: For the registration of pesticides in the European Union, model simulations for worst‐case scenarios are used to demonstrate that leaching concentrations to groundwater do not exceed a critical threshold. A worst‐case scenario is a combination of soil and climate properties for which predicted leaching concentrations are higher than a certain percentile of the spatial concentration distribution within a region. The derivation of scenarios is complicated by uncertainty about soil and pesticide fate parameters. As the ranking of climate and soil property combinations according to predicted leaching concentrations is different for different pesticides, the worst‐case scenario for one pesticide may misrepresent the worst case for another pesticide, which leads to ‘scenario uncertainty’. RESULTS: Pesticide fate parameter uncertainty led to higher concentrations in the higher percentiles of spatial concentration distributions, especially for distributions in smaller and more homogeneous regions. The effect of pesticide fate parameter uncertainty on the spatial concentration distribution was small when compared with the uncertainty of local concentration predictions and with the scenario uncertainty. CONCLUSION: Uncertainty in pesticide fate parameters and scenario uncertainty can be accounted for using higher percentiles of spatial concentration distributions and considering a range of pesticides for the scenario selection. Copyright © 2010 Society of Chemical Industry     

Herbicide leaching as affected by macropore flow and within-storm rainfall intensity variation: a RZWQM simulation

Within-event variability in rainfall intensity may affect pesticide leaching rates in soil, but most laboratory studies of pesticide leaching use a rainfall simulator operating at constant rainfall intensity, or cover the soil with ponded water. This is especially true in experiments where macropores are present--macroporous soils present experimental complexities enough without the added complexity of variable rainfall intensity. One way to get around this difficulty is to use a suitable pesticide transport model, calibrate it to describe accurately a fixed-intensity experiment, and then explore the affects of within-event rainfall intensity variation on pesticide leaching through macropores. We used the Root Zone Water Quality Model (RZWQM) to investigate the effect of variable rainfall intensity on alachlor and atrazine transport through macropores. Data were used from an experiment in which atrazine and alachlor were surface-applied to 30 x 30 x 30 cm undisturbed blocks of two macroporous silt loam soils from glacial till regions. One hour later the blocks were subjected to 30-mm simulated rain with constant intensity for 0.5 h. Percolate was collected and analyzed from 64 square cells at the base of the blocks. RZWQM was calibrated to describe accurately the atrazine and alachlor leaching data, and then a median Mid-west variable-intensity storm, in which the initial intensity was high, was simulated. The variable-intensity storm more than quadrupled alachlor losses and almost doubled atrazine losses in one soil over the constant-intensity storm of the same total depth. Also rainfall intensity may affect percolate-producing macroporosity and consequently pesticide transport through macropores. For example, under variable rainfall intensity RZWQM predicted the alachlor concentration to be 2.7 microg ml(-1) with an effective macroporosity of 2.2 E(-4) cm(3) cm(-3) and 1.4 microg ml(-1) with an effective macroporosity of 4.6 E(-4) cm(3) cm(-3). Percolate-producing macroporosity and herbicide leaching under different rainfall intensity patterns, however, are not well understood. Clearly, further investigation of rainfall intensity variation on pesticide leaching through macropores is needed.     

Characteristics of deep drainage and soil water in the mobile sandy lands of Inner Mongolia,northern China

 Quantification of deep drainage and the response of soil water content to rainfall patterns are critical for an effective management strategy of soil water conservation and groundwater utilization. However, information on how rainfall characteristics influence soil water dynamics and deep drainage in mobile sandy lands are lacking. We used an underground chamber to examine the response of deep drainage and soil water content in mobile sandy lands to rainfall characteristics during the growing season of 2010, 2011 and 2012. Results showed that rainfall in this area was dominated by small events (≤5 mm), which increased soil water content in the surface soil layers (0–40 cm), but did not increase soil water content at the deeper soil layers (greater than 40 cm). Soil water content at the 0–100 cm depth increased significantly when the total amount of rain was >20 mm. Rainfall amount, intensity and the duration of dry intervals were significantly related to the soil water content at different soil layers. Deep drainage was significantly correlated with rainfall amount and intensity, but not with the duration of the dry interval. The coefficients of deep drainage in mobile sandy lands ranged from 61.30% to 67.94% during the growing seasons. Our results suggested that rainfall infiltration in these widespread mobile sandy lands had considerable potential to increase soil water storage while recharging the groundwater in this region.     

Simulation of mefenacet concentrations in paddy fields by an improved PCPF-1 model

An improved simulation model (PCPF-1) has been evaluated for the prediction of the fate of mefenacet in an experimental paddy field. This model simulates the fate and transport of pesticide in paddy water and the top 1 cm of paddy soil. Observed concentrations of mefenacet in the paddy water and the surface soil exponentially decreased from their maximum concentrations of 0.70 mg litre(-1) and 11.3 mg kg(-1), respectively. Predicted mefenacet concentrations both in the water and surface soil were in excellent agreement with those measured during the first 2 weeks after herbicide application, but concentrations in paddy water were appreciably overestimated thereafter. The model simulated mefenacet losses through runoff, percolation and degradation to be respectively 41.9%, 6.4% and 57.3% of applied, and the mass balance error was about -6%. The model simulation implied that drainage and seepage control, especially shortly after application when herbicide concentrations are high, is essential for preventing pesticide losses from paddy fields. In focusing on pesticide concentrations in this early period the PCPF-1 model can be a beneficial tool for risk assessment of pesticide losses and in the evaluation of agricultural management for reducing pesticide pollution associated with paddy rice production.     

From laboratory to field: uses and limitations of pesticide behaviour models for the soil/plant system

Modelling is an economic way of assessing pesticide behaviour under field conditions; it is cheaper and faster than field experiments. Modelling attempts to generalize knowledge of pesticide field behaviour through identification of the most important pesticide/soil properties that can be measured in the laboratory. The technology to simulate volatilization of volatile pesticides that are incorporated or injected into the soil is well developed. However, modelling of volatilization rates from plant and soil surfaces before the first significant rainfall event after application is barely possible with current knowledge. The technology to simulate pesticide persistence in the plough layer is well developed; the PERSIST model has been tested at least 178 times, usually resulting in a slightly faster decline in the field than was simulated. In general, available pesticide leaching models are reliable enough to assess the leaching of the bulk of the dose (leaching levels above 1%). The EU drinking water limit of 0.1 μg L^?1 implies leaching of less than 0.1% of a dose of 1 kg ha^?1. At such a low leaching level, the validation status of the models is still low, mainly because preferential flow processes in both structured and unstructured soils and the factors controlling the transformation rate in subsoil are not well enough understood.     

Movement of pesticides to surface waters from a heavy clay soil

A field experiment at Cockle Park, Northumberland on a clay loam soil (Dunkeswick series) cropped with winter wheat investigated the effects of drainage and season of application on pesticide movement. Isoproturon, mecoprop, fonofos and trifluralin were applied in two consecutive seasons at normal agricultural rates to three hydrologically isolated plots each of 0.25 ha. Two of the plots were mole-drained and the third was an undrained control. Surfacelayer flow and drainflow from each plot were monitored at 10-min intervals. Samples of flow were analysed for pesticides to evaluate transport of applied chemicals from the site. Despite widely differing properties (K_oc 20–8000 ml g^?1, t_1/2 10–60 days), all four pesticides were found in surface-layer flow and mole drainflow from the site. Maximum concentrations of pesticides in flow ranged from 0.1 to 121 μg litre^?1 (aqueous phase) and < 0.2 to 48 μg litre^?1 (particulate phase). Over two contrasting seasons, total losses of pesticides in flow followed total amounts of flow and were approximately four and five times larger, respectively, in 1990/91 than in 1989/90. The maximum loss occurred from the undrained plot and was 2.8 g isoproturon (0.45% of that applied). Total losses of autumn-applied pesticides from an undrained plot were up to four times greater than losses from a mole-drained plot. Mole drainage decreased movement of pesticides from this slowly permeable soil by reducing the amount of surfacelayer flow. Maximum concentrations of mecoprop and isoproturon in drainflow were 10–20 times larger following spring application than after application in autumn. Bypass flow down soil cracks was an important process by which pesticide was lost from the site, with transport to the drainage system via mole channels (55 cm depth) after less than 0.5 and 6.7 mm net drainage in the two winters.     

Test of the Root Zone Water Quality Model (RZWQM) for predicting runoff of atrazine, alachlor and fenamiphos species from conventional-tillage corn mesoplots

The Root Zone Water Quality Model (RZWQM) is a comprehensive, integrated physical, biological and chemical process model that simulates plant growth and movement of water, nutrients and pesticides in a representative area of an agricultural system. We tested the ability of RZWQM to predict surface runoff losses of atrazine, alachlor, fenamiphos and two fenamiphos oxidative degradates against results from a 2-year mesoplot rainfall simulation experiment. Model inputs included site-specific soil properties and weather, but default values were used for most other parameters, including pesticide properties. No attempts were made to calibrate the model except for soil crust/seal hydraulic conductivity and an adjustment of pesticide persistence in near-surface soil. Approximately 2.5 (+/- 0.9), 3.0 (+/- 0.8) and 0.3 (+/- 0.2)% of the applied alachlor, atrazine and fenamiphos were lost in surface water runoff, respectively. Runoff losses in the 'critical' events--those occurring 24 h after pesticide application--were respectively 91 (+/- 5), 86 (+/- 6) and 96 (+/- 3)% of total runoff losses for these pesticides. RZWQM adequately predicted runoff water volumes, giving a predicted/observed ratio of 1.2 (+/- 0.5) for all events. Predicted pesticide concentrations and loads from the 'critical' events were generally within a factor of 2, but atrazine losses from these events were underestimated, which was probably a formulation effect, and fenamiphos losses were overestimated due to rapid oxidation. The ratios of predicted to measured pesticide concentrations in all runoff events varied between 0.2 and 147, with an average of 7. Large over-predictions of pesticide runoff occurred in runoff events later in the season when both loads and concentrations were small. The normalized root mean square error for pesticide runoff concentration predictions varied between 42 and 122%, with an average of 84%. Pesticide runoff loads were predicted with a similar accuracy. These results indicate that the soil-water mixing model used in RZWQM is a robust predictor of pesticide entrainment and runoff.     

Field leaching of pesticides at five test sites in Hawaii: modeling flow and transport

BACKGROUND: Physically based tier‐II models may serve as possible alternatives to expensive field and laboratory leaching experiments required for pesticide approval and registration. The objective of this study was to predict pesticide fate and transport at five different sites in Hawaii using data from an earlier field leaching experiment and a one‐dimensional tier‐II model. As the predicted concentration profiles of pesticides did not provide close agreement with data, inverse modeling was used to obtain adequate reactive transport parameters. The estimated transport parameters of pesticides were also utilized in a tier‐I model, which is currently used by the state authorities to evaluate the relative leaching potential. RESULTS: Water flow in soil profiles was simulated by the tier‐II model with acceptable accuracy at all experimental sites. The observed concentration profiles and center of mass depths predicted by the tier‐II simulations based on optimized transport parameters provided better agreements than did the non‐optimized parameters. With optimized parameters, the tier‐I model also delivered results consistent with observed pesticide center of mass depths. CONCLUSION: Tier‐II numerical modeling helped to identify relevant transport processes in field leaching of pesticides. The process‐based modeling of water flow and pesticide transport, coupled with the inverse procedure, can contribute significantly to the evaluation of chemical leaching in Hawaii soils. Copyright © 2011 Society of Chemical Industry     

三种拟除虫菊酯类农药在砂土和壤土中的淋溶行为

采用土柱淋溶法和气相色谱法研究了3种拟除虫菊酯类农药三氟氯氰菊酯、联苯菊酯和高效氯氰菊酯在热带地区主要土壤类型砂土和壤土中的淋溶特性。结果表明:3种拟除虫菊酯类农药在砂土和壤土中主要残留于第1段土壤 (0～5 cm) 中,且驻留量随土壤深度增大而减少。三氟氯氰菊酯、高效氯氰菊酯和联苯菊酯在砂土中的R_i值分别为52.86%、94.73%和83.19%,在壤土中的R_i值分别为54.70%、77.28%和55.33%,均大于50%。根据农药在土壤中的淋溶性等级划分标准,3种药剂均属于难淋溶农药,不易对地下水造成污染。本研究结果可为热带地区土壤和地下水中农药污染修复提供参考。     

Quantifying interactions between compound properties and macropore flow effects on pesticide leaching

The objective of this study was to investigate the interactions between compound properties and macropore flow effects on pesticide leaching. To this end, the dual‐porosity MACRO model was used to simulate leaching of 60 hypothetical compounds with widely differing sorption and degradation characteristics using a pre‐calibrated scenario from Lanna, south‐west Sweden, representing a structured clay soil. The model predicts that, in the worst case, macropore flow increases leaching by more than four orders of magnitude for moderately to strongly sorbed compounds with relatively short half‐lives. However, it was also notable that leaching of some very mobile compounds is actually reduced by macropore flow. For pesticides leaching between 0.0001 and 10% of the applied dose (without macropore flow), the impact of pesticide properties on leaching is markedly reduced. This suggests that reductions in applied dose become a relatively more attractive and effective means of decreasing leaching from structured soils. © 2000 Society of Chemical Industry     

Pesticide soil sorption parameters: theory, measurement, uses, limitations and reliability

The soil sorption coefficient Kd and the soil organic carbon sorption coefficient KOC of pesticides are basic parameters used by environmental scientists and regulatory agencies worldwide in describing the environmental fate and behavior of pesticides. They are a measure of the strength of sorption of pesticides to soils and other geosorbent surfaces at the water/solid interface, and are thus directly related to both environmental mobility and persistence. KOC is regarded as a 'universal' parameter related to the hydrophobicity of the pesticide molecule, which applies to a given pesticide in all soils. This assumption is known to be inexact, but it is used in this way in modeling and estimating risk for pesticide leaching and runoff. In this report we examine the theory, uses, measurement or estimation, limitations and reliability of these parameters and provide some 'rules of thumb' for the use of these parameters in describing the behavior and fate of pesticides in the environment, especially in analysis by modeling.     

人工模拟降雨对吡虫啉在不结球白菜中残留量的影响

为探索露地蔬菜中残留农药降解较快的主要原因,以吡虫啉为供试农药,以不结球白菜(Brassica rapa L.Chinensis Group.,俗称小白菜)为代表作物,采用人工模拟降雨研究了不同降雨强度、降雨间隔、降雨次数及农药剂型对吡虫啉在蔬菜中残留量的影响。结果表明:吡虫啉在小白菜中的残留量随着降雨强度的增大而降低,随着降雨间隔时间的延长而升高,随着降雨次数的增多而降低,且连续降雨对吡虫啉的淋失作用大于间断降雨;降雨对小白菜上吡虫啉水分散粒剂的淋洗率高于对乳油的淋洗率。     

Herbicide movement in soils: principles, pathways and processes

European legislation concerning ground- and surface-water quality and the protection of non-target organisms in surface-water from pesticide contamination has initiated more stringent data requirements from regulatory authorities concerning the movement of all pesticides in soils. Other interested parties, such as water companies, environment agencies and consumer-driven organizations, have sought to influence the use of herbicides and their impact on the environment. The resulting studies and associated research have led to a better understanding of the fate and behaviour of herbicides in the soil environment. The amount of herbicide that moves away from the area of application will depend on the physico-chemical properties of the chemical and the agroclimatic characteristics of the target site. Under average conditions, the amount of herbicide lost by movement from a soil profile is typically <0.1% to 1% of the applied mass but, under certain localized circumstances, can reach up to 5% or greater. Leaching, drain-flow and surface run-off are the main pathways responsible for herbicide movement within soils. The soil/herbicide processes determining the losses are also variable in both time and space. It is therefore necessary to understand the spatial characteristics of soils, their hydrology and the associated herbicide use patterns.     

The pesticide module of the Root Zone Water Quality Model (RZWQM): testing and sensitivity analysis of selected algorithms for pesticide fate and surface runoff

The Root Zone Water Quality Model (RZWQM) is a one-dimensional, numerical model for simulating water movement and chemical transport under a variety of management and weather scenarios at the field scale. The pesticide module of RZWQM includes detailed algorithms that describe the complex interactions between pesticides and the environment. We have simulated a range of situations with RZWQM, including foliar interception and washoff of a multiply applied insecticide (chlorpyrifos) to growing corn, and herbicides (alachlor, atrazine, flumetsulam) with pH-dependent soil sorption, to examine whether the model appears to generate reasonable results. The model was also tested using chlorpyrifos and flumetsulam for the sensitivity of its predictions of chemical fate and water and pesticide runoff to various input parameters. The model appears to generate reasonable representations of the fate and partitioning of surface- and foliar-applied chemicals, and the sorption of weakly acidic or basic pesticides, processes that are becoming increasingly important for describing adequately the environmental behavior of newer pesticides. However, the kinetic sorption algorithms for charged pesticides appear to be faulty. Of the 29 parameters and variables analyzed, chlorpyrifos half-life, the Freundlich adsorption exponent, the fraction of kinetic sorption sites, air temperature, soil bulk density, soil-water content at 33 kPa suction head and rainfall were most sensitive for predictions of chlorpyrifos residues in soil. The latter three inputs and the saturated hydraulic conductivity of the soil and surface crusts were most sensitive for predictions of surface water runoff and water-phase loss of chlorpyrifos. In addition, predictions of flumetsulam (a weak acid) runoff and dynamics in soil were sensitive to the Freundlich equilibrium adsorption constant, soil pH and its dissociation coefficient.     

Is tile drainage water representative of root zone leaching of pesticides?

Given the methods presently available, determination of flux-averaged concentrations of pesticides in structured soils is always a compromise. Most of the available methods entail major uncertainties and limitations. Tile drainage monitoring has several advantages, but the extent to which it is representative of overall leaching has been questioned because it comprises a mixture of water of different origins. This literature review evaluates whether drainage water pesticide concentrations are representative of root zone leaching of pesticides. As there are no reports quantifying the extent to which the flux-averaged concentration of pesticides in drainage water differs from that found between the drains, evidence-based conclusions cannot be drawn. Nevertheless, the existing literature does suggest that the concentration in drainage water does not always correspond to the concentration at drain depth between the drains; depending on the conditions pertaining, the concentrations may be higher or lower. As to whether the flux-averaged concentration of pesticides in drainage water is representative of the interdrain concentration at drain depth it is concluded that (1) the representativeness of drainage water concentrations can be questioned on very well-drained soils and on poorly drained soils with little capacity for lateral transport beneath the plough layer, (2) the conditions provided by relatively porous soils and moderate climatic conditions are conducive to the drainage water concentration being representative and (3) drainage water will be more representative in the case of weakly sorbed pesticides than for strongly sorbed pesticides. Used critically, it is thus believed that drainage water concentrations can serve to characterize the flux-averaged concentration of pesticides at drain depth. However, the use of drainage water for determining average concentrations necessitates thorough investigation and interpretation of precipitation, percolation, drain outflow and concentration dynamics.     

Pirimiphos-methyl and benalaxyl losses in surface runoff from plots cultivated with potatoes

Abstract: Losses of pirimiphos-methyl and benalaxyl in runoff water from clay soil plots cultivated with potatoes and of differing soil surface slopes were determined over approximately 120 days (1 October 1999-28 January 2000). The plot slopes were 0, 1, 2.5 and 5%, and soil erosion increased with the slope from 610 to 1760kgha(-1). The runoff of surface water was between 3.1 and 16.6% of the rainfall. Surface runoff was highest for the fifth and seventh runoff events due to rainfall, 51 days and 72 days after the first pesticide application. The maximum concentrations of the two pesticides in runoff occurred in the plots with the greatest slope (5%) during the fifth runoff event, November 21, 1999 reaching 8.4 and 12.3 microg litre(-1) for pirimiphos-methyl and 17.8 and 20.2 microg litre(-1) for benalaxyl in tilled and untilled plots respectively. The cumulative losses of pirimiphos-methyl in surface runoff from tilled and untilled plots with a slope 5% were estimated at only 0.37 and 0.59% of the initial applied active ingredient, respectively, while for plots with a slope 0% the percentages were 0.013 and 0.018%. For benalaxyl the corresponding values from tilled and untilled plots were 1.69 and 1.76% (slope 5%), and 0.062 and 0.085 (slope 0%). Degradation of the pesticides in the topsoil was monitored from October 1999 and May 2000. Cultivation of potatoes decreased the half-life of the two pesticides compared to the untilled fields, for pirimiphos-methyl from 16.7 to 9.2 days and for benalaxyl from 26.7 to 12.6 days. The slope of soil surface and the different sorption capacities for the compounds are the main parameters which influenced the transportation of studied pesticides, pirimiphos-methyl and benalaxyl residues via surface water in soil-water systems.     

Leaching of pesticides and a bromide tracer through lysimeters from five contrasting soils

In each of two seasons, undisturbed lysimeters 0.8 m in diameter and 1.05 m in length taken from five soil types were cropped with winter wheat. They received autumn applications of the pesticides isoproturon and linuron as well as a bromide tracer and spring applications of dimethoate and MCPA. Leachate was collected at regular intervals and concentrations of the various solutes determined. Rainfall from December to March was 290 and 191 mm in the first and second seasons, respectively. Both springs were exceptionally dry with less than 50% of the mean April‐to‐June rainfall of 138 mm. Total flow from the lysimeters ranged from 335 to 477 mm (and from 0.78 to 3.95 pore volumes) over the two seasons. Leaching to drainage of bromide highlighted soils where preferential flow was influential with total losses ranging from 24% of applied for a strongly structured, alluvial clay loam to 79% for an unstructured sand. Leaching to drainage of isoproturon (K_oc ≈ 100 ml g⁻¹) was observed from all but a peat soil with losses greater (0.31–1.01% of applied) from the clay loam and a deep medium loam, where patterns of leaching clearly indicated preferential flow mechanisms, than from the sand and a light loam over gravel (0.04–0.18% of applied) where a broad breakthrough curve indicated that matrix flow was more important. Linuron (K_oc ≈ 500 ml g⁻¹) was detected in occasional samples of leachate from the clay loam, the light loam over gravel and the medium loam during the first season only (maximum loss 0.12% of applied). The sandy soil, often considered most vulnerable to leaching, gave the smallest total losses of pesticide of the four mineral soils, whilst significant preferential flow in the deep, medium loam was believed to result from a compacted topsoil. Neither of the spring‐applied pesticides was detected in the leachate, as flow following application was very small and relatively slow. © 2000 Society of Chemical Industry