system and the water table (10 ft for a seepage pit). Soil percolation rates are determined using the percolation test in conjunction with an evaluation of soil texture and structure. Soils too coarse to allow the formation of a continuous bacterial slime layer will allow wastewater to pass with inadequate treatment, SELECTION OF SITE AND SYSTEM Figure 3.15 Determining soil conditions. while soils too fine to pass water at a reasonable rate will promote the surfacing of wastewater. Ground surface slope can be measured using a staff and carpenter's level (Figure 3.16). One person holding a staff paces off 100 ft, then the other sights along the level to the point on the staff at the same level as his eye. The slope is equal to the vertical distance between the sighted spot on the staff and the mark on the staff correspond- ing to the "instrument level" above the ground. Flooding potential is often considered a limiting factor in the instal- lation of onsite liquid waste treatment and disposal systems. How- ever, no alternative to the standard system will completely over- come this limitation. It is best simply not to build in areas subject to frequent flooding. Flooding potential can be determined by asking neighbors about past floods. Another source of information is the United States Geological Survey. The Army Corps of Engineers may also be able to provide some flooding data, as may state or local governmental officials (Figure 3.16). 34 ONSITE WASTEWATER DISPOSAL % Slope Figure 3.16 Measuring ground slope. SYSTEM SELECTION Whenever it is reasonable to do so, the standard septic tank/drain field should be installed. It is the most reliable and least expensive method of onsite liquid waste disposal. Where site limitations pre- vent the use of a standard system, the least complex (and therefore least expensive) alternative that will overcome those site limitations should be selected. There are systems available to handle any kind of site limitation. These range from minor variations of the standard septic tank/dr