al Protection Agency. 1. Six test holes over the area of the drain field site are recom- mended. This number may be modified up or down (to a minimum of three), depending upon the uniformity of the soils in the area. 2. The Manual of Septic Tank Practice specifies holes from 4 to 12 in. in diameter, and as deep as the bottom of the proposed drain field. A 6-in. diameter hole is now a widely accepted standard and is recommended by the EPA. Use a 6-in. hole dug to the depth of the proposed drain field bottom. 3. The process of digging or boring may smear the bottom and sides of the hole, SO these surfaces should be scratched to expose an undisturbed surface. Remove the loose dirt from the hole bottom. 4. Add 2 in. of coarse sand or gravel to prevent the bottom from becoming clogged with sediment. 5. Gently pour water into the hole to a level of about 12 in. over the gravel. Maintain this water level for at least four hours, and preferably overnight. An automatic filling mechanism (Figure 3.13) is useful. If the original filling and a refill both disappear in 20 minutes or less, proceed immediately with falling head measurements. 6. In 15 to 30 hours from the time water was first added to the hole, begin measurements of the fall rate of the water level. If the last amount of water added has fallen faster than about 6 in. in 30 minutes, take measurements every 10 minutes for an hour, or until the water level doesn't change by more than one-sixteenth of an inch in 10 minutes. If the water level has fallen more slowly than this, take measurements every 30 minutes for four hours, or until there is no change more than one-sixteenth of an inch over a half- hour period. Start with the water level no more than 6 in. above the SELECTION OF SITE AND SYSTEM Figure 3.13 Percolation test water supply. and refill to that level as necessary. stick The with water a level batter change board gravel, determined using a measuring reference point (Figure 3.14). easily placed across final the few test rates hole of the as a drop in the 