E NVIRONMENTAL E XPOSURE

12.3.1 E NVIRONMENTAL E XPOSURE

12.3.1.1 Point and Nonpoint Source Pesticide Pollution Environmental exposure of pesticides can be occurred by point and nonpoint

sources. A point source can be any single identifiable source of pollution from which pesticides are discharged such as the effluent pipes, careless storage, and disposal of pesticide containers, accidental spills, and overspray. Pesticide move- ment away from the targeted application site is defined as nonpoint source pollution

334 Analysis of Pesticides in Food and Environmental Samples and can occur through runoff, leaching, and drift. Nonpoint source pollution occurs

over broad geographical scales and because of its diffuse nature it typically yields relatively uniform environmental concentrations of pesticides in surface waters, sediments, and groundwater. Runoff is the surface movement of pesticide in water or bound to soil particles, while leaching is the downward movement of a pesticide through the soil by water percolation. Drift is the off-target movement by wind or air currents and can be in the form of spray droplet drift, vapor drift, or particle (dust) drift.

12.3.1.2 Environmental Parameters Affecting Exposure The environmental parameters that affect pesticide exposure could be classified as

follows:

1. Soil characteristics and field topography: Texture composition and pH are the main soil properties that affect pesticide fate and transport, whereas topographic characteristics of the fields like watershed size, slope, drainage pattern, permeability of soil layers affect greatly the potential to generate runoff water or leachates.

2. Weather and climate: Climatic factors such as the amount and timing of rainfall, duration, and intensity, as well as temperature and air movement influence the degree to which pesticides are mobilized by runoff, leaching, and drift. In addition, temperature and sunlight affect all abiotic and biotic transformation reactions of pesticides [84,85].

12.3.1.3 Pesticide Parameters Affecting Exposure The pesticide factors affecting exposure could be organized on three main sets:

1. Application factors: These include the application site (crop or soil surface) and method, the type of use (agricultural, nonagricultural applications, indoor pest management, etc.), the formulation (e.g., granules or suspended powder or liquid) and the application amount, and frequency. In addition, the application time does affect its possible routes of transport in the environment.

2. Partitioning and mobility of pesticides in the environment: The main physicochemical properties of pesticides that affect their mobility are the water solubility, vapor pressure, and soil –water partition coefficient (K oc ). K oc defines the potential for the pesticide to bind to soil particles. Off-target movement by drift also depends on the spray droplet size and the viscosity of the liquid pesticide while plant uptake from the soil is another important pathway in determining the ultimate fate of pesticide residues in the soil [84,85].

3. Persistence in the environmental compartments: Persistence is usually expressed in terms of half-life that is the time required for one-half of the pesticide to decompose to products other than the parent compound. The longer a pesticide persists within the environment, the greater the risk it

Monitoring of Pesticides in the Environment 335 poses to it. Hydrolysis, direct and indirect photolysis, and biodegradation

are the principal pesticide degradation processes and their rates depend on pesticide chemistry, as well as on environmental conditions [84].

12.3.1.4 Modeling of Environmental Exposure Monitoring data and environmental modeling are interconnected to each other.

Monitoring could provide the correct input data to models for calibration and validation or could be devoted to collect data on the timing and magnitude of loadings. Mathematical models that simulate the fate of pesticides in the environment are used for developing Environmental Estimated Concentrations (EECs) or Pre- dicted Environmental Concentrations (PECs). This means ‘‘predicting exposure’’ in space and time, drawing on available environmental fate data, physicochemical data, and the proposed agricultural practices and usage pattern associated with the pesti- cide [86]. A complete presentation of environmental models describing the exposure of pesticide in the environment is outside the scope of the present chapter. Thus, only common environmental models that are used to estimate environmental exposure concentrations for aquatic systems in the context of current risk assessing techniques will be presented.

The Generic Estimated Environmental Concentration (GENEEC) model, devel- oped by the EPA, determines generic EEC for aquatic environments under worst- case conditions (i.e., application on a highly erosive slope with heavy rainfall occurred just after the pesticide application, the treatment of the entire area — essentially 10 acres of surface area with uniform slope —with the pesticide, and the assumption that all runoff drains directly into a single pond). The model uses environmental fate parameters derived from laboratory studies under standard pro- cedures as well as soil and weather parameters. The outputs of the model are the pesticide runoff and environmental concentration estimates [87]. This model can be used as first tier approach since it is based on a single event and a high-exposure scenario. On a higher tier approach (second and third), models that can account for multiple weather conditions and=or multiple sites are used. Such models are the Pesticide Root Zone Model (PRZM), edge of field runoff=leaching the Exposure Analysis Modeling System (EXAMS), fate in surface water, and AgDrift (spray drift) [87] that used additional parameters, more descriptive of the site studied. PRZM simulates the leaching, runoff, and erosion from an agricultural field and EXAMS simulates the fate in a receiving water body. The water body simulated is a static pond, adjacent to the crop of interest. Typical conditions of the site including the soil characteristics, hydrology, crop management practices, and weather infor- mation are used. The output of this higher tier analysis is to define the EEC that can

be reasonably expected under variable site and weather conditions. The model yields an output of annual maxima distributions of peak, 96 h, 21 days, 60 days, 90 days, and yearly intervals. AgDrift includes generic data for screening level assess- ments including pesticide formulation, drop height, droplet size, nozzle type, and wind speed. The earlier approaches are used by pesticide registrants to address environmental exposure concerns and are frequently combined with geographical information systems (GIS) to produce regional maps.

336 Analysis of Pesticides in Food and Environmental Samples The fugacity approach has also proven particularly suited for describing the

behavior of pesticides in the environment. A tiered system of fugacity models has been introduced which distinguishes four levels of complexity, depending on whether the system is closed or in exchange with the surrounding environment. The four levels are Level I, close system equilibrium; Level II, equilibrium steady state; Level III, Nonequilibrium steady state; and Level IV, Nonequilibrium non- steady state. Levels I and II are used in lower tier approaches, whereas Level III is widely used in higher tiers to obtain exposure concentrations due to emission flux into a predefined standard environment. A detailed introduction into fugacity-based modeling can be found in Ref. [88].

For evaluating the impact of management practices on potential pesticide leach- ing, the Groundwater Loading Effects of Agricultural Management Systems (GLEAMS) is a widely used, field-scale model. GLEAMS assumes that a field has homogeneous land use, soils, and precipitation. It consists of four major compon- ents: hydrology, erosion, pesticide transport, and nutrients. GLEAMS estimates leaching, surface runoff, and sediment losses from the field and can be used as a tool for comparative analysis of complex pesticide chemistry, soil properties, and climate. The model output data are daily, monthly, annual pesticide mass and concentrations in runoff and sediment.

Finally, a fourth tier approach can be used based on watershed site assessments. These assessments are very complex since the landscape studied has a very high surface area, high diversity of soils and weather conditions, varied proximities of agricultural lands to receiving waters and various water bodies. Thus, GIS are commonly used to distinguish high-risk versus low-risk areas on a watershed basis. Finally, modeling and monitoring are often combined within tier 4 to provide more accurate distribution of pesticide exposure.