**3. Exposure routes of pesticides in water bodies through crop production**

To cope with the growing world's population, crop production has also been increasing. Till 2015, cereal production has increased threefold, production of vegetables increased fourfold, production of tomatoes increased fivefold, and production of soybean increased eightfold as compared with 1970 [13]. This huge increase in crop production has been achieved through the expansion of crop lands, cultivation of high-yielding crop varieties, and most importantly through the use of pesticides. In India, cotton is at the top of the list consuming the highest amount of pesticides (45%) followed by rice (22%), vegetable (9%), plantation crops (7%), wheat (4%), and other crops (9%). Among the vegetables, cabbage consumes the highest amount of pesticides. On an overall basis, pesticide consumption is the highest in fruit and vegetable cropping. In developing countries such as India, about 600g/ha of pesticides are used, whereas the amount is 6000g/ha in developed nations. According to estimation, about 4.6 million tonnes of pesticides are being integrated into the environment each year through crop production of which 51.3% was consumed in Asia, 33.3% in the Americas, 11.8% in Africa, and 1.4% in Oceania in 2016.

Pesticides are usually directly applied on plant parts or plant parts are subject to pesticide pretreatment. However, only 1% of the applied pesticide reaches the target pest, and the rest amount gets incorporated into different environmental compartments exerting its harmful effects on biodiversity, and nontarget organisms. The aerial application of pesticides may pollute surrounding areas with macro-droplets or micro-droplets of pesticides. Several studies showed that pesticide spraying enhances the distribution of pesticides in areas far from the spraying site. For an instance, spraying of pesticides caused health-related issues in children living within 1000 m of a greenhouse [14].

Depending on the chemical composition of pesticides, they show different degrees of solubility, according to which they follow different pathways to reach the water bodies after their application to crop fields (**Figure 3**). The common pathway through which pesticides enter the surface water sources such as ponds, pools, ditches, lakes, streams, rivers, etc., is through irrigation or when immediate rainfall occurs after pesticide application. Small water bodies situated adjacent to agricultural fields are more prone to pesticide pollution as the pesticides applied to the crop fields directly washed away into those water sources. These water bodies receive considerably higher amounts of pesticide as compared with farther or larger water bodies [15]. In the case of groundwater systems, the common pathway for the entry of pesticides is through leaching. Also, other routes of exposure of pesticide molecules include soil erosion, direct disposal, or sedimentation, etc.

Some major pathways through which pesticides reach water sources are as follows.

## **3.1 Leaching**

Leaching of pesticides is the vertically downward movement of pesticide molecules through the minute capillaries formed by soil particles or channels formed by roots and root hairs to the groundwater table and deeper aquifers. The pesticides with

#### **Figure 3.**

*Pathways of pesticide entry into different ecosystems.*

a lower persistency value tend to degrade within less time posing a comparatively lesser threat to groundwater. There are two types of leaching observed, which may provide pathways for pesticide movement toward the groundwater [16].


Among the pesticides used, atrazine, an herbicide, has a high potential to leach into the underground water table due to its high persistency. In contrast to this, cyanazine and methyl parathion show low leaching potential due to their shorter halflife, high rate of adsorption to soil particles, and low persistency. Herbicide, 2, 4-D, is a hydrophilic pesticide and easily gets broken down by the actions of microorganisms, hence less chance of accumulation in soil [17], thereby exerting a lesser chance of water contamination. Pesticide leaching to the groundwater may be enhanced by rainfall or through irrigation only when the concerned pesticide is fairly soluble in water. The pesticide may get dissolved in water or form suspension or emulsion. Water that is moving at a higher speed as in rivers or streams as compared with ponds or ditches is more likely to carry heavy pesticides and to a farther distance. Several factors affect the rate of leaching of pesticides such as physical and chemical characteristics of pesticides, the permeability of pesticides in soil, volatilization of pesticide molecules, crop-root uptake, methods and doses of pesticide application and types of weather conditions, variation in temperature and precipitation pattern [18].

## *3.1.1 Soil organic matter*

The organic matter content of the soil is the most important soil property that affects pesticide breakdown by microorganisms. Organic matter present in the soil helps in better adsorption of pesticide molecules by providing a larger surface area. The presence of organic matter also helps the soil to hold more amount of water,

thereby increasing the chance of degradation by microorganisms. This ultimately decreases the rate of leaching of pesticides to the groundwater table.

#### *3.1.2 Soil texture*

Soil is composed mainly of sand, silt, and clay. This composition affects the movement of water through the soil. The coarse-textured soil will have more sand particles and large pores, allowing water to move rapidly carrying pesticides to the water table. Clay-textured soils will have more clay and hence will have less pore size that provides low permeability. This slows the downward movement of pesticides and increased the rate of degradation of pesticides on the soil surface.

#### *3.1.3 Soil structure*

In the soil where particles are loosely packed, pesticides tend to leach faster in the soil. Compact soil holds water back and prevents the free flowing of water through it. the soil in which openings and channels are formed, for an instance, burrows formed by earthworms or crevices due to freezing and thawing allow downward movement of water that may contain pesticides. Plant roots penetrate the soil, thus creating channels that allow water to carry pesticides downward toward the water table.

## *3.1.4 Soil water content*

The amount of water present in the soil determines the leaching of pesticides into the groundwater sources. Pesticides that are more soluble will have a greater chance of leaching to the water table when the soil is fully saturated. However, in the case of dry soil when water is added, the water molecules just fill the pores in the soil surface, decreasing the chances of carrying the pesticide residues down through the soil profile via water.
