ified, it is neces- sary to find out what kinds of soils exist in that area, what flow rates of liquids they can accept, and whether there are limiting features such as high ground water or shallow bedrock. If a system is to serve its users adequately, it must be designed not only to fit the site, but also to suit its intended purpose. So the first item on the site evaluation procedure is to determine if there are or will be any unusual circumstances, such as the presence of six chil- 16 ONSITE WASTEWATER DISPOSAL leach field Street Figure 3.1 Typical plot plan. dren in a two-bedroom house, a home-based laundry service, or a plastics manufacturing operation located in the basement. The stan- dard practice in estimating design flow for residential dwellings is to assume 75 gallons per day per individual. If the number of individ- uals cannot be determined, it is usual to assume two persons per bedroom (Figure 3.2). For example, the design flow for a two- bedroom house would be 300 gallons per day. Soils The depth and type of soils in the disposal area limit the amount and rate of wastewater that can be applied. Quite often the type of soil differs from place to place within the disposal area, and these changes must be considered when planning the onsite system. Changes also occur with depth and these too must be known and taken into consideration. A 3- or 4-inch diameter hand auger is very useful for taking soil samples from various depths and at various locations on the site. A posthole digger can be used, but the hole depth is limited and it's much harder to operate, especially when SELECTION OF SITE AND SYSTEM Figure 3.2 Determining number of people in household. working in soils which contain cobbles and large rocks. The holes should be made 2 to 4 ft deeper than the expected depth of the disposal field excavations in order to find out if there will be enough suitable soil between the trench bottom and bedrock, clay or other relatively impermeable material, to accept the water percolating from the drain field.