ATIONS AND PROCESSES Figure 2.3 Drain field under construction. pass into the drain fields, the spaces between the soil particles can become packed SO tightly with sewage particles that water can't pass through quickly enough. Then the sewage effluent moves up to the ground surface. Another mechanism for cleansing sewage effluent is by biological action. The organic material carried into the drain field, including bacteria and viruses, is subject to being eaten by organisms which normally live in the soil, such as insects, worms, and soil bacteria. Some materials in the effluent, especially viruses, can become attached to soil particles by their electrical charge. Others may become stuck in a slime layer that can form around the soil particles. If they are held long enough they lose their potency through grow- ing old. Old age for a virus appears to be some age greater than three months. Some viruses can remain infective longer than three months in certain soils. Other potentially harmful organisms may not be able to survive the chemical environment of the soil. Many disease-causing bacteria require specific conditions, such as a cer- tain amount of acidity and a specific temperature range (the condi- tions found inside the body of a human, for example) in order to survive and reproduce. These bacteria cannot live long in the soil. Others survive quite well and remain infective for months. ONSITE WASTEWATER DISPOSAL Saturated soil, that is, soil with water filling the spaces between the soil particles, is not a very good cleanser of sewage effluent. In unsaturated soil, only a film of water exists around the soil particles. It is in this film that most cleansing actions occur (after the effluent has passed through the biological mat). This means that a ground water aquifer is not a very good cleanser of the effluent. Therefore, there should be a certain minimum amount of unsaturated soil between the drain field and the ground water table which acts to reduce the chance of polluting the ground water. Exactly how m