The Importance of Natural Water Filtration

• Pollutants such as toxins, viruses, manufacturing oils and bacteria enter the water system every day, and it is thanks to the soil in forests, wetlands and along rivers that many of these potentially harmful substances don't make it into the drinkable water supply. Pathogens that are otherwise difficult to remove are naturally caught and disabled before they can cause any harm. Wetlands are capable of filtering out dangerous metals, trapping excessive amounts of runoff sediment and eliminating excessive amounts of nitrogen that can contribute to the proliferation of invasive water plants. Woodlands located along streams and rivers, known as riparian forests, filter out sediments and eliminate excess pollutants that leaches from nearby land, while microorganisms in the soil also play an important role in breaking down harmful substances.
  
  

How Soil Filters Water

• All soils are blends of sand, silt and clay, with sand particles being the largest and clay particles the smallest. The ratio of one type of component to the others determines how efficiently water moves through the soil in a particular area. Some soil component combinations allow water to pass through its pores too quickly, while in other cases, the soil is too dense and holds on to the water. Porosity is the term used to describe the shape of soil particles and how they are arranged. The spaces between the particles are where water enters and exits, with sand having the largest pore spaces and clay having tiny but many spaces between its particles. How these open spaces are connected play a crucial role in water movement, and the soil's ability to do this is measured in terms of permeability. Soil particles filter water by capturing solid bits of matter moving through its pores and by hanging on to some long enough to transform them into harmless substances.
  
  

How Water Moves Through the Soil

• Water is always moving, whether through the soil, through plants or in bodies such as streams and rivers. It enters the soil through rain or snow, through irrigation, by moving over the soil's surface from other areas or from groundwater seepage. As the main storage system for the water that plants need, it sinks into the soil through a process known as infiltration. Once past the soil's surface, water moves downward and sideways, moving quickly through the larger pores in sandy soil and more slowly between the smaller pores in clay soils. As it moves, it is essentially scrubbed by the particles of soil, which also latch on to some of the helpful nutrients that plants need for growth.
  
  

Impediments to Natural Water Purification

• Certain conditions can interfere with natural water filtration in the soil. Paved areas, such as roads and parking lots, can channel contaminated water into rivers and lakes. Changing the course of waterways upsets the soil-to-water balance necessary for effective filtration, and clear-cutting along riverbanks removes the water's ability to cleanse itself. The natural filtration system is also thrown off course when invasive species of plants and animals take over an aquatic area, and nitrogen pollution threatens to acidify the soil to harmful levels.