Water entering the soil may have a variety of constituents that require removal prior to reuse by society or to minimize negative environmental impact. Those constituents might include sediment, pesticides, plant nutrients, salts, bacteria, viruses, heavy metals, organic chemicals (including petroleum products), pharmaceuticals and personal care products. Considering the sources of the water that is entering the soil, it is easy to see how one or more of these constituents could be present. Soil can provide much of the filtration that is needed, either as part of the ubiquitous ecosystem services it provides or by design.

The ability of soil to provide this filtration is through one or more physical, chemical or biological processes that remove or degrade various constituents in water as it passes through on its way to groundwater. Physical filtration is directly analogous to passing water through a screen: The soil acts as a sieve and holds back particles that are too large to pass through. Unlike a simple screen, however, this characteristic is enhanced by the tortuous path that water takes through the soil, which provides multiple opportunities to capture constituents. Sediment and even bacteria can be removed by this mechanism. 

The surfaces of soil particles often are chemically reactive and provide multiple means by which contaminants in water can be adsorbed by soil particles and effectively be removed from the water by chemical processes. Many soil clay particles have a negative charge and will attract any constituents in the water that have a positive charge (e.g., some heavy metals, salts, organic chemicals and pesticides). Another mechanism is the formation of covalent bonds (sharing of electrons) that helps soil retain many organic chemicals, pesticides and some inorganic constituents. 

Soil is teeming with life - particularly a wide variety of microorganisms that have the ability to degrade or transform both organic and inorganic substances in the soil environment. This is particularly useful when water contains organic materials that have a high biological oxygen demand and would impart negative effects on water if they did not undergo further decomposition first. This would apply to effluent from wastewater treatment plants, food processing waste and water from livestock facilities. In addition, soil microorganisms can often specifically degrade organic chemicals that would be considered contaminants if present in drinking water.  

Filtration Limitations

Of course, like a filter designed and built by engineers, the ability of soil to purify water has limitations that must be recognized and taken into account. It is no coincidence that nitrates, a form of nitrogen that acts as a plant nutrient, and trichloroethylene, an organic solvent, are two contaminants that often are reported to be in groundwater if one considers the characteristics of those substances. Both are soluble in water and neither can be chemically adsorbed by soil or degraded or transformed by soil microorganisms. As a result, both essentially move with soil water and frequently end up in groundwater. 

Sandy soil allows water to move quickly through and has limited ability to chemically adsorb constituents in water, thus limiting opportunities for filtration. Groundwater quality issues are more common in areas with sandy soil and where the groundwater is close to the soil surface. Any soil has a finite capacity to purify water, and excessive applications of water or other materials requiring purification to the soil can result in limited treatment.

Soils are great water purifiers, and both society and the environment benefit from them, whether from the ecosystem services they provide that protect freshwater resources, or from their deliberate use as a filter, which allows us to effectively remove a variety of byproducts with relative ease.

Removing Pollutants From Wastewater 

Three natural processes remove pollutants from wastewater as the wastewater moves through unsaturated soil:

• Filtering is a physical treatment process. As water moves through the small soil pores, wastewater particles are removed, thus eliminating cloudiness. After passing through about 1 foot of soil, the wastewater is clear.
• Naturally occurring microbes attach and grow on the surface of soil particles and consume the wastewater organic matter and nutrients as their food. Organic matter creates bad odors as it decomposes. Ammonia in wastewater is extremely toxic to fish. After filtering through about 2 feet of soil, ammonia is transformed to nitrate and the organic matter is consumed so the wastewater has no odor.
• Adsorption of pollutants occurs due to the chemical nature of soils. Phosphorus is removed from wastewater as it attaches onto soil particles or forms insoluble compounds in the soil. Bacteria and viruses that cause disease are removed in the same way. They are attracted to and adhere onto soil particles in unsaturated soils. Bacteria are removed after filtering through 1.5 feet to 2 feet of soil. However, viruses are smaller and more difficult to remove, thus taking 3 feet to 4 feet of unsaturated soil depth for removal.