Some soils are better at trapping pollutants than others. Clay soils, common in north Missouri, are made of tiny particles, so they slow the downward movement of water and sometimes impede water movement completely. Sandy soils, as found in the Missouri Bootheel and along large rivers, allow rapid water movement, and silty soils occupy the middle range. Soils made of large particles pose the greatest risk, because water seeps downward through them readily without filtering out or decomposing pollutants. The ideal soil is a mix of midsize particles to allow infiltration and tiny particles, like clay or organic matter, to slow water movement and filter pollutants.
Soil type can also affect surface water contamination.
Although runoff occurs from all soil types, clay soils are more likely to cause surface water runoff because they are less permeable. During a storm or flood, or even when watering your lawn, this runoff can wash contaminants from the land's surface into nearby surface waters. Eroding soil is also considered a water pollutant. Bare soil, especially on sloping land, can run off into streams, rivers, lakes or estuaries. The depth of soil influences risks to groundwater.