What term describes the process of chemicals being washed down through the soil?

The term that best describes the process of chemicals being washed down through the soil is leaching. Leaching refers specifically to the process where soluble substances, often including nutrients and contaminants, are dissolved in water and subsequently transported downward through the soil profile, typically due to rainfall or irrigation. This movement can lead to the loss of valuable nutrients from the soil, which may affect plant growth and soil health.

In contrast, infiltration pertains to the process of water entering the soil surface, without specifically mentioning the movement of dissolved substances. Runoff refers to the flow of water, often overland, that occurs when the soil is saturated, which does not directly describe the downward movement through the soil. Percolation describes the movement of water through the soil at various rates and depths but does not specifically refer to the washing away of chemicals. Hence, leaching is the most accurate term for the process described in the question.