Pesticide Leaching Models in a Brazilian Agricultural Field Scenario

Introduction

Pesticides used in agriculture can pose contamination risks to water resources. Therefore, using pesticides while maintaining water resources quality is an important challenge. Groundwater contamination by pesticide residues has been a worldwide problem (Laabs et al., 2002; Bortoluzzi et al., 2006; Terry et al., 2008; Steele et al., 2009; Vryzas et al., 2009). Leaching through the soil profile has been identified as the major cause for the occurrence of pesticides in groundwater. Losses of pesticides by leaching to groundwater can be as high as 5% of the applied amount, but are typically less than 1% (Flury, 1996).

There is an urgent need to assess the risks of groundwater contamination by pesticides as part of national pesticide registration procedures. This assessment at field scale is time- and money consuming considering all combinations of pesticides, soil and climatic conditions. However, to overcome this limitation, mathematical models have been developed to simulate pesticide leaching at field scale and considering all diversity of pesticides, soils and climate. The first attempts to simulate pesticide field behavior started around 1970 (Leistra, 1971; Walker, 1974). At that time, computational resource was considered a limitation for simulations. Also models were mostly used to optimise pesticide efficiency. After the introduction of personal computers in 1985, complex models were created that considered the interplay of factors affecting pesticide field behavior and more emphasis was given to environmental risk assessment. 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. Moreover, PLM are useful for guiding experimentation and integrating, testing and improving the understanding of pesticide field behavior at process-level (Jarvis et al., 1995).

Before the use of any PLM for risk assessment, it is important to evaluate their performance by testing them against field measurements. Vanclooster et al. (2000) states that the validation status of PLM is considered low although several studies aimed to validate these models under various conditions (Scorza Júnior & Boesten, 2005; Castro et al., 2006; Bouraoui, 2007; Scorza Júnior et al., 2007; Bayless et al., 2008). PEARL (Leistra et al., 2002; Tiktak et al., 2002) and MACRO (Larsbo & Jarvis, 2003) are two PLM that have been used within European Union (EU) since 2000 to assess leaching risks as part of the procedure of pesticide registration (see http://focus.jrc.ec.europa.eu/). Although not all countries use PLM for risk assessment of water resources contamination by pesticides, its use has potential for growth by official agencies of various governments.

The main objective of this chapter is to present a successful application of two PLM (i.e. PEARL and MACRO) in a Brazilian very intensive agricultural area to simulate the leaching of a water flow tracer and two pesticides. We first present some background information about definitions, types and classifications of PLM and their use within EU for risk assessment. Thereafter, we describe the theory of the PEARL and MACRO models and their application in a Brazilian agricultural area including aspects such as input parameter estimation and calibration. Finally, some conclusions are drawn for future use of PLM in Brazil.