reatment. However, over a period of several months, the bacterial slime layer that accumulates at the bottom of the disposal trench slows the percolation of waste- water SO that it seeps through over a longer period of time, allowing unsaturated conditions below the drain field and more efficient cleansing. This slower percolation rate requires a larger drain field than would be indicated by the percolation test. The specific loading rates allowable for various sands are discussed in the chapter on sand filters, and it is sufficient to mention here that a loading rate of 1.2 gallons of effluent per square foot of trench bottom surface per day should not be exceeded with subsurface absorption systems. Limiting Characteristics Most of the land area in the United States is not optimally suited for installation of typical septic tank/soil absorption systems. Yet development is rapid in many of these unsuitable areas. Central sewage treatment facilities are not economically feasible in many 32 ONSITE WASTEWATER DISPOSAL cases, SO homeowners are obligated to install onsite treatment and disposal systems which will not cause pollution problems or present dangers to public health. Alternatives to the standard system inevi- tably are more expensive, and often more prone to failure if improp- erly designed, sited, constructed, or maintained. A thorough site evaluation is crucial in marginally suitable and unsuitable areas to ensure that the best system is installed at the least cost. Experience indicates that alternatives to the standard septic tank/soil absorption system should be considered if any one of the following conditions is present: 1. Bedrock, creviced bedrock, or impermeable soil exists within 4 to 6 ft of the soil surface; 2. Ground water is present within 4 to 7 ft of the soil surface; 3. The soil percolation rate is slower than about 60 to 90 minutes per inch; 4. The soil percolation rate is faster than about 1 to 6 minutes per inch; 5. The ground surface slopes at greater than about 5 to 8 ft of drop per 10