Water filtration is a natural part of the water cycle. When water in lakes, rivers, and the seas is heated by the sun, some of it becomes water vapor and rises into the atmosphere as clouds. The air cools the water vapor, which falls back to Earth as rain. The rain is filtered through the soil and becomes part of the water table. As the water seeps through layers of rocks and soil, they remove impurities from the water. Sand
: Whether a soil type will filter water well depends on how much organic matter, sand, silt and clay it contains. Sand is composed of coarse, large particles and is very porous, so it works as an effective filter. According to West Virginia University, sand filters are one of the oldest methods of municipal water filtration. The filters need to be large to be effective because water seeps through very slowly. Water that has organic contaminants is difficult to filter with sand because it creates clogs and limits the water flowing through the fine sand of the filter. Clay
: Clay has a fine particle size that absorbs and holds water well. Clay absorbs contaminants from the water as it passes slowly through. Clay is often mixed with organic matter, which traps and holds chemical contaminants. The fine particles of the clay, according to the Virginia Cooperative Extension Service, slow down the water while the organic matter in the soil promotes the chemical and biological breakdown of contaminants, keeping them out of the groundwater. Aggregates
: Groundwater is not found in an underground lake or pool, but in an aggregate, or mixture, of sand, stones, and silty clay. The water from rainfall, snow melt, or overflowing streams seeps through the different layers of the soil; contaminants are filtered out and held in the soil. The upper layers of soil include a healthy mix of organic matter. According to North Carolina State University, a healthy soil mix is vital to preserve the quality of existing and future groundwater.