Leaching is a process where water carries soluble substances or small particles through soil or rock. It is not to be confused with the use of leeches as medicine. The word “leaching” is believed to have been derived from either late Middle English “leche” or Old English “leccan” meaning to moisten and to allow leaking. Leaching is actually two important actions occurring simultaneously: (1) chemical interactions with surfaces and (2) physical movement of water. As the water passes through the rock and soil, it interacts with the surfaces of the materials. Compounds on the surface of minerals can become dissolved. In addition, the physical movement of water can dislodge and move particles.

Leaching can transport chemical compounds like dissolved substances or larger materials such as decomposing plant materials, fine rock fragments, and microbes throughout the Critical Zone. In agricultural ecosystems, leaching is an important balance between preventing salt accumulation and removing nutrients from soil). Leaching occurs when the soil pores become filled with water and water moves downward in the soil. The rate of leaching increases with the amount of rainfall, high temperatures, and the removal of protective vegetation.

Leaching is a natural environment concern when it contributes to groundwater contamination. As water from rain, flooding, or other sources seeps into the ground, it can dissolve chemicals and carry them into the underground water supply. Of particular concern are hazardous waste dumps and landfills, and, in agriculture, excess fertilizer, improperly stored animal manure, and biocides (e.g. pesticides, fungicides, insecticides, and herbicides)).

In summary, leaching is a process where water carries soluble substances or small particles through soil or rock. It can transport chemical compounds and nutrients throughout the Critical Zone and is an important balance in agricultural ecosystems. However, it can also contribute to groundwater contamination and is a natural environment concern.