Modeling spatial variation in microbial degradation of pesticides in soil.

Currently, no general guidance is available on suitable approaches for dealing with spatial variation in the first-order pesticide degradation rate constant k even though it is a very sensitive parameter and often highly variable at the field, catchment, and regional scales. Supported by some mechanistic reasoning, we propose a simple general modeling approach to predict k from the sorption constant, which reflects bioavailability, and easily measurable surrogate variables for microbial biomass/activity (organic carbon and clay contents). The soil depth was also explicitly included as an additional predictor variable. This approach was tested in a meta-analysis of available literature data using bootstrapped partial least-squares regression. It explained 73% of the variation in k for the 19 pesticide-study combinations (n = 212) in the database. When 4 of the 19 pesticide-study combinations were excluded (n = 169), the approach explained 80% of the variation in the degradation rate constant. We conclude that the approach shows promise as an effective way to account for the effects of bioavailability and microbial activity on microbial pesticide degradation in large-scale model applications.

Identification of key climatic factors regulating the transport of pesticides in leaching and to tile drains.

BACKGROUND: Key climatic factors influencing the transport of pesticides to drains and to depth were identified. Climatic characteristics such as the timing of rainfall in relation to pesticide application may be more critical than average annual temperature and rainfall. The fate of three pesticides was simulated in nine contrasting soil types for two seasons, five application dates and six synthetic weather data series using the MACRO model, and predicted cumulative pesticide loads were analysed using statistical methods. RESULTS: Classification trees and Pearson correlations indicated that simulated losses in excess of 75th percentile values (0.046 mg m(-2) for leaching, 0.042 mg m(-2) for drainage) generally occurred with large rainfall events following autumn application on clay soils, for both leaching and drainage scenarios. The amount and timing of winter rainfall were important factors, whatever the application period, and these interacted strongly with soil texture and pesticide mobility and persistence. Winter rainfall primarily influenced losses of less mobile and more persistent compounds, while short-term rainfall and temperature controlled leaching of the more mobile pesticides. CONCLUSIONS: Numerous climatic characteristics influenced pesticide loss, including the amount of precipitation as well as the timing of rainfall and extreme events in relation to application date. Information regarding the relative influence of the climatic characteristics evaluated here can support the development of a climatic zonation for European-scale risk assessment for pesticide fate.

Testing MACRO (version 5.1) for pesticide leaching in a Dutch clay soil.

Testing of pesticide leaching models against comprehensive field-scale measurements is necessary to increase confidence in their predictive ability when used as regulatory tools. Version 5.1 of the MACRO model was tested against measurements of water flow and the behaviour of bromide, bentazone [3-isopropyl-1H-2,1,3-benzothiadiazin-4(3H)-one-2,2-dioxide] and imidacloprid [1-(6-chloro-3-pyridylmethyl)-N-nitroimidazolidin-2-ylideneamine] in a cracked clay soil. In keeping with EU (FOCUS) procedures, the model was first calibrated against the measured moisture profiles and bromide concentrations in soil and in drain water. Uncalibrated pesticide simulations based on laboratory measurements of sorption and degradation were then compared with field data on the leaching of bentazone and imidacloprid. Calibrated parameter values indicated that a high degree of physical non-equilibrium (i.e. strong macropore flow) was necessary to describe solute transport in this soil. Comparison of measured and simulated bentazone concentration profiles revealed that the bulk of the bentazone movement in this soil was underestimated by MACRO. Nevertheless, the model simulated the dynamics of the bentazone breakthrough in drain water rather well and, in particular, accurately simulated the timing and the concentration level of the early bentazone breakthrough in drain water. The imidacloprid concentration profiles and its persistence in soil were simulated well. Moreover, the timing of the early imidacloprid breakthrough in the drain water was simulated well, although the simulated concentrations were about 2-3 times larger than measured. Deep groundwater concentrations for all substances were underestimated by MACRO, although it simulated concentrations in the shallow groundwater reasonably well. It is concluded that, in the context of ecotoxicological risk assessments for surface water, MACRO can give reasonably good simulations of pesticide concentrations in water draining from cracking clay soils, but that prior calibration against hydrologic and tracer data is desirable to reduce uncertainty and improve accuracy.

Modelling the leaching of imazapyr in a railway embankment.

The use of herbicides on railway tracks is known to present a risk to groundwater, but little is known of the mechanisms influencing leaching through the coarse material used to construct railway embankments. Therefore, in the present study, four different models based on the convection-dispersion equation (CDE) were compared with previously reported field data on the leaching of imazapyr. In particular, the significance of non-equilibrium processes was investigated by comparing different CDE formulations accounting for preferential finger flow, particle-facilitated transport and kinetic sorption. The traditional CDE assuming 'local equilibrium' based on 24 h batch sorption data gave poor results (model efficiency - 1.1). It strongly underestimated leaching of imazapyr in the first 4 months following application, thus confirming the importance of non-equilibrium transport processes. Accounting for short-term sorption kinetics made little difference, giving similar results to the 'local equilibrium' CDE simulation. A simulation accounting for particle-facilitated transport could accurately match this accelerated transport, and also gave the best overall fit to the data (model efficiency 0.76). However, not even this model could match the long-term retention of imazapyr residues observed close to the soil surface more than 1 year after application, and it also underestimated the time of breakthrough to groundwater. This strongly suggests that a long-term retention/sorption process not included in any of the models tested (i.e. sorption hysteresis or bound residues) acted to retard leaching. The formation of 'protected' residues was also indicated by a much slower degradation of imazapyr more than 1 year after application. Industry.

Stochastic modeling of diffuse pesticide losses from a small agricultural catchment.

The objective of this study was to identify the main sources of variation in pesticide losses at field and catchment scales using the dual permeability model MACRO. Stochastic simulations of the leaching of the herbicide MCPA (4-chloro-2-methylphenoxyacetic acid) were compared with seven years of measured concentrations in a stream draining a small agricultural catchment and one year of measured concentrations at the outlet of a field located within the catchment. MACRO was parameterized from measured probability distributions accounting for spatial variability of soil properties and local pedotransfer functions derived from information gathered in field- and catchment-scale soil surveys. At the field scale, a single deterministic simulation using the means of the input distributions was also performed. The deterministic run failed to reproduce the summer outflows when most leaching occurred, and greatly underestimated pesticide leaching. In contrast, the stochastic simulations successfully predicted the hydrologic response of the field and catchment and there was a good resemblance between the simulations and measured MCPA concentrations at the field outlet. At the catchment scale, the stochastic approach underestimated the concentrations of MCPA in the stream, probably mostly due to point sources, but perhaps also because the distributions used for the input variables did not accurately reflect conditions in the catchment. Sensitivity analyses showed that the most important factors affecting MACRO modeled diffuse MCPA losses from this catchment were soil properties controlling macropore flow, precipitation following application, and organic carbon content.

Modelling pesticide sorption in the surface and subsurface soils of an agricultural catchment.

BACKGROUND: Sorption models that improve upon the koc concept are urgently needed for reliable spatial modelling of pesticide leaching. Sorption of glyphosate, bentazone and isoproturon was measured in surface and subsurface soils to test an 'extended' partitioning model that also accounts for inorganic sorbents and pH. Best-subset regression and Akaike information criteria were used to justify the inclusion of predictors and identify suitable models. RESULTS: The extended partitioning model improved upon the koc concept for all three compounds: inorganic sorbents dominated sorption in subsurface soils, and their effects were only masked by organic matter in surface soils with organic carbon contents larger than ca 2%. Interactions between organic and inorganic sorbents affected glyphosate sorption, but apparently not that of bentazone or isoproturon. CONCLUSION: Information on clay, iron and aluminium oxides and soil pH, in addition to organic carbon, is needed for accurate prediction of pesticide leaching. The variables foc , fclay and pH are generally available, whereas measurements of oxides of Al and Fe are rarely reported. The authors therefore emphasise the need to measure and report contents of oxides of Al and Fe in soil survey databases, because small variations in their concentrations may contribute significantly to large variations in sorption, especially of ionisable pesticides.