Generally, sand and plants. Nature does an excellent job of filtering water on its own. Its why springs are so pure. The water is filtered as it soaked into the earth and then it sprouts up from the spring.
New home plans built in well communities require perk testing which is an analysis of the soil layers to see if a septic and well can be placed there. Larger developments in those communities, such as big box stores and condo developments, have requirements for unpaved surface area and drain basins so the land can treat the rain water. Many are also adding tree requirements. If a property can not meet the community standards, its either filled with the deficient soils or more likely remains marginally developed until city water and sewage arrives.
Odorless, colorless, tasteless contaminants are extremely rare so its not a large worry. Country water varies greatly as it sometimes tastes like bottled water and sometimes tastes metallic as metals are found in the ground, but never has the chemical taste found in artifically treated water. Areas such as Kentucky have high lime concentrations which can be beneficial for things such as making bourban. However, communities share water tables, often over several counties, so finding contamination in one home leads the county drain commission to test other homes.
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