When soil conditions are dry, the added water will increase soil water storage. If the moisture-holding capacity of the soil is exceeded, the excess water percolates downward through the soil to ground water. Pesticides present on vegetation or soils may be transported along with the water depending on the properties of the pesticide and the composition of the soil. Each of the transport mechanisms discussed below will be covered in greater detail in a later chapter. Surface Runoff and Adsorption Surface runoff occurs when water is applied to the soil at a faster rate than it can enter the soil.
Runoff water can carry pesticides in the water itself or by adsorption to eroding soil particles.
Runoff containing pesticides can cause direct injury to nontarget species, harm aquatic organisms in streams and ponds, and can lead to ground water contamination. The presence of vegetation or crop residue tends to slow the movement of runoff water thereby reducing the amount of pesticides which may enter surface water. Volatilization Volatilization occurs when a solid or liquid changes into a gas. When this change of state takes place the possibility of vapor drift occurs which may result in airborne chemical vapors being transported by air currents from a treated area to other locations, where rainfall can deposit them on land surfaces, lakes, streams, and vegetation. Rainwater sampled from states in the upper Midwest and Northeast resulted in the detection of low-levels of triazine and acetanilide herbicides.