How can a water body's sediment influence pesticide fate?

• A. Sediments have no effect on exposure.
• B. Sediments can adsorb pesticides, acting as a reservoir that releases the chemical over time and affecting exposure.
• C. Sediments always degrade pesticides immediately.
• D. Sediments only affect the color of water.

Answer: (B)

The main idea is that sediments can trap pesticides and release them later, shaping how much and how long aquatic organisms are exposed. Pesticides that are hydrophobic tend to attach to organic matter in sediment particles, a process called adsorption. This pulls them out of the water column initially, lowering immediate exposure in the water. But the bound pesticide isn’t permanently gone; it sits in the sediment and can desorb or be released back into the water over time, especially if sediments are disturbed by events like storms, dredging, or changes in water chemistry. Because exposure for aquatic life depends on what’s available in the water, this sediment-bound reservoir can lead to prolonged or delayed exposure even after the pesticide application has ended. Sediment properties, such as organic carbon content and particle size, influence how much pesticide is adsorbed and how readily it can be remobilized.

Degradation of pesticides does occur in sediments, but it is not guaranteed to happen immediately; rates vary with microbial activity, oxygen levels, and other conditions, so the idea that sediments always degrade pesticides right away isn’t accurate.

The other options miss the key point that sediments actively influence exposure through binding and potential remobilization, rather than having no effect, causing immediate degradation, or only affecting the water’s color.