*3.1.6 Type of bedrock*

Bedrock is the bottommost layer present beneath soil or rock fragments. The types of bedrock determine the leaching of water that may carry pesticides. For example, in the case of limestone bedrocks, the downward water channels are comparatively larger, thereby allowing water to leach quickly. Limestone is highly soluble in water and hence dissolves in water creating underground passages that let water move out of the area rapidly, carrying pesticides to farther distances.

## *3.1.7 Slope*

The topography of an area affects the rate of movement of water flow across the earth's surface. The areas with steep slopes allow fast surface runoff but reduce the

chances for water to leach to the groundwater table. In contrast in valleys and flat areas, a runoff will be slow, but leaching to underground will be comparatively faster.

#### **3.2 Runoff**

Surface runoff is the movement of water molecules on the earth's surface in case of the availability of excess water on the soil surface that accumulates from different sources. It occurs when the amount of surface water reaches such a quantity that the soil fails to infiltrate or absorb that. It happens when there is irrigation, rainfall, or when the snow melts that add more water to soil surface that eventually flows down toward ponds, pools, ditches, canals, streams, rivers, or lakes. During runoff, the pesticide molecules present in the crop field soils tend to be carried away that get stored in the lentic water systems. Pesticides stored in the standing water systems get a longer period for leaching into the groundwater sources. Several factors such as environmental conditions, pesticide composition, soil characteristics, etc., affect the transfer of pesticides through runoff water.

#### *3.2.1 Soil moisture content*

The water content of the soil in an area will determine the amount of runoff that will occur from the site. Soil that is already saturated with water faces more risk of surface runoff. In the case of dry soils, the addition of water will lead to filling of the pores of soil decreasing the chances of runoff.

#### *3.2.2 Soil texture*

Soils that contain clay are more compact and hence more prone to runoff losses, whereas loose sandy soils possess less chance of surface runoff.

#### *3.2.3 Weather or irrigation*

Climatic conditions such as the temperature of the atmosphere, precipitation, etc., determine largely the rate of surface runoff. Pesticides applied in the crop fields when subjected to immediate rainfall lead to washing off of the applied pesticide molecules. The wasted pesticides along with surface runoff may reach the nearby water bodies. Also, pesticides that are applied where the soil is already saturated with previous rainfall or irrigation may be subjected to runoff if light rainfall or additional irrigation follows. At times when the temperature is very low, i.e., in the case of frozen soils, applied pesticides face the problem of runoff. Therefore, it is usually recommended not to apply pesticides in frozen soils and the pesticide application should not be followed by heavy rainfall or irrigation.

#### *3.2.4 Slope*

The slope is an important deciding factor for the runoff of pesticides with water. The type of landscapes where the ground has a slope will facilitate the runoff of pesticides.

#### *3.2.5 Pesticide characteristics*

The physical and chemical properties of a pesticide are the deciding factors for the surface runoff of those molecules. Pesticides that are hydrophilic or more soluble in water will get the opportunity for easy runoff. The hydrophobic pesticides get adhered to the soil particles and hence get less chance for surface runoff.

#### *3.2.6 Pesticide persistence*

Some pesticide molecules are easily degraded by the action of microorganisms and hence will not be available for surface runoff.
