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This layer is a few inches thick, starts the cleansing process, and eases water down to the next layer. If it becomes saturated, runoff begins.
The second layer
Second is the topsoil, which in our locale is generally only three to four inches thick, much having been lost previously by erosion over endless decades.
It is where living animals, bugs, plant roots, and endless trillions of microorganisms (bacteria, yeasts, molds, fungi, algae, viruses, etc.) are concentrated.
It takes hundreds of years to form an inch of topsoil.
Most developers bulldoze topsoil into huge piles for later use.
Others sell it and people are left with the mostly dead dirt of the subsoil.
In topsoil, microorganisms decompose organic material*' make available mineral , jQijtrients, eventually creating all manner of nutrients for plants and trees, sometimes to excess.
Pollutants are worked over by the chemistry of organic components and minerals, using the air and water in the soil.
Some pollutants are converted into nutrients for plant growth, or they might trigger chemical reactions that nullify the pollution.
They could even cause new pollutants to form, such as sulfur dioxide triggering soluble aluminum that gets into streams and kills fish.
After being filtered through the topsoil, water may flow somewhat clear to the third layer, but still contain some pollutants.
The third layer
The third layer is the subsoil, with more roots. Here, dissolved minerals and nutrients washed down by slowly descending water undergo further filtration and change, but there is not much organic material available.
Most of the material leached from the topsoil stops in this area, which may be about two feet thick.
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