Leaching is the technical term for the process when liquid passes through granular solids, such as when rain water passes through soil or sand. The process exposes various mineral and chemicals to the passing water on the way to its destination – a stream or an underground reservoir. Many such chemicals, both elemental (zinc, carbon, etc.) and compounds (sodium chloride, potassium hydrochloride, etc.) are acted upon by their immersion in water – they break up into simpler compounds or combine into more complex compounds, or simply are carried along with the water to its final destination. Leaching depletes the soil of important substances, important for vegetable growth, and consequently important for the nutrition in crops for human and animal consumption. Rain water is generally beneficial, but artificial sprinkling can sometimes hurt the general ecology of the soil.
But perhaps more importantly, the damage or changes to the water from leaching can be quite toxic; for example, artificial insecticides, herbicides, fertilizers, etc. – the chemical invasion and “enhancements” of the farming industry, especially large factory farms, can damage reservoirs of human sources of water, through the leaching of those chemicals into the water supply.