Rocks and Soil

• Soil comes in many layers with differing consistencies and densities. Some layers, especially those nearest the surface, are porous, or able to soak up water quickly and easily, such as topsoil or sand. Other layers, typically deeper layers, are less porous, which means that it takes water a longer time to soak in, such as clay. Water may also filter through porous rock, such as limestone. Each layer is part of the filtration process.
  
  

Function

• Water soaks through the successive layers of soil, each one finer than the next, with less space between molecules. Larger contaminated molecules in the water, such as sediment, certain pathogenic microbes and chemicals, are left behind in the soil until less and less remain. Cryptosporidium, a microbe found in fecal matter, is so large that it is filtered away.
  
  

Effects

• Some compounds in the layers react with, attract or neutralize contaminants in the water. For example, copper has anti-microbial properties, so soil full of copper will kill some of the bacteria in the water. Limestone is a neutralizer of acids. Vegetative remains, such as charcoal, soak up compounds.
  
  

Considerations

• As the water moves through the soil, it loses more and more oxygen. Aerobic bacteria, including many pathogens found in animal waste, die when they are deprived of oxygen. Also, many of the pathogens that make people sick are very delicate and cannot handle changes in temperature, pH and moisture, so the process of filtering through all of the different layers kills them.
  
  

Misconceptions

• While groundwater is filtered during its descent to the water table, it is not necessarily safe or healthful to drink. Most groundwater still needs additional purification, filtering or softening (removal of hard minerals). While many harmful contaminants are removed, others remain. Other minerals and elements, such as arsenic and lead, may enter the water as it is filtered through rock containing these elements.