NATURAL PURIFICATION CAPABILITY

The natural purification capability of the soil is, however, limited. Under certain conditions, such as heavy rainfall, chemical spills, and sandy soil, contaminants may leach through the soil. In such cases the subsurface geological material and the distance a contaminant must travel to groundwater are important factors in determining whether a contaminant actually reaches the groundwater.

Soil organic matter is important in holding contaminants. Soils high in organic matter provide an excellent environment for the chemical and biological breakdown of contaminants before they reach groundwater.

The soilorganic matter content influences the amount of chemicals the soil can bind and hold. Practices such as plowing under cover crops and applying livestock manure increase the soil's organic matter content, and help reduce the movement of chemicals.

Soil permeability is the ease with which gases, liquids, or plant roots can penetrate or pass through the soil. Permeability varies in different soil horizons. The more easily water moves through the soil, the more likely contaminants will reach groundwater.

Soil pH is the negative logarithm of the hydrogen-ion activity of a soil, ranging from zero (highly acid) to 14 (highly alkaline) with a neutral value of 7.. Soils with pH values less than 6.6 allow more chemical movement than soils with higher pH values.

This is a measure of the amount of open space between soil and rock particles. All other things being equal, chemicals are more likely to leach through more porous soils. Water holding capacity of the soil is a major vulnerability factor. Downward movement of water is greater in coarser soils (sandy soils) which have a low water holding capacity.

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