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.

Exposure scenarios in combination with simulation models have been used in the procedure of Environmental Risk Assessment (ERA) of pesticides to estimate exposure concentrations in environmental compartments.The appropriate definition of exposure scenarios is primordial to guarantee ERA purpose to evaluate if a pesticide use in an intended area can be considered safe. This work had the aim to present and test a geo-and statistically based approach to define worst-case groundwater exposure scenarios for Brazil. To do so, soil-climate data of Mato Grosso do Sul state consisting of 42 locations (3 soil classes and 14 meteorological stations) were used to generate populations of Predicted Environmental Concentrations (PEC) at 3 and 5-m depth for a simulation period of 24 years. A total of 3,780 graphs were generated of PEC populations for 15 "dummy" pesticides. After setting equal risks in both dimensions (spatial and temporal) equal to 20%, the overall risks exceedance of tested worst-case groundwater exposure scenarios were about 10% for both depths. Thus, this approach can be considered suitable to guarantee a pre-defined overall risk exceedance once defined by the regulatory authorities in Brazil.

This study evaluates the effects of different levels of sugarcane straw on the soil surface on the transport by runoff water of four herbicides: ametryn, diuron, hexazinone, and sulfentrazone. Two runoff experiments were conducted in a set of nine experimental plots. In the first experiment, sugarcane was cultivated and no straw was applied to the soil. In the second experiment, sugarcane was cultivated with straw levels of 0, 9, or 18 Mg ha-1 on the soil surface. The runoff water from natural rainfall was collected in reservoirs at the lower end of each plot on different occasions following the application of the herbicides to the plots. The total volume of runoff water collected from the plots was dependent on the coverage of the soil by straw, in the sequence: 0 > 9 > 18 Mg ha-1. For the herbicides hexazinone and sulfentrazone, which are molecules of high solubility in water, increasing the amount of straw on the soil produced an increase in the fraction of the applied dose appearing in the collected runoff water. However, for diuron and ametryn, which have low solubility in water, the presence of straw did not influence the amount found in the collected runoff water.

The use of Pesticide Leaching Models (PLM) for risk assessment may be an efficient and attractive way of assessing solutions to some agricultural and environmental problems. Many countries of the European Union and the USA have been using PLM for risk assessment already for a few decades. This chapter has the aim to present a successful application of two PLM (i.e. MACRO and PEARL) in a Brazilian very intensive agricultural area to simulate moisture profiles and the leaching of a water flow tracer (i.e. bromide) and the pesticides cyproconazole and thiamethoxam. Also attempts to summarize the available knowledge about the processes governing pesticide behavior in soil, types and classifications of PLM, the use of PLM for risk assessment at European Union, a theoretical description of PEARL and MACRO models and their testing in a Brazilian agricultural field scenario.