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The soil and its layers
Once on the ground, polluted rainwater encounters nature’s second line of defense, the soil.
Here, such rainwater is joined by pollutants making their way into or along the ground as part of runoff, spills, or human application.
These might be from specific sites, such as benzene from a gasoline spill. Or, from a large area, such’as heavy metals in sludge applied to farmland.
Much comes from highways, parking lots, and sidewalks.
This runoff includes oil and other liquid hydrocarbons leaking from vehicles; heavy metals from engines, brakes, transmissions, or from abrasion of tires and brake linings.
De-icing salts put down by homeowners, stores, or highway departments under*' snowy and icy conditions become part of runoff.
Overall, there are about 90 pollutants of concern that soil might be called upon to clean up. How does it handle this situation?
Under ideal conditions, soil consists of four layers before reaching bedrock.
Water percolates through these layers, being cleansed as it goes, especially by the roots of vegetation.
Some, soils do a better job than others; and the more time it takes, the better.
The first layer consists of leaves, twigs, animal remains, and other material that is organic.
94.8%