Figure 35. Typical septic tank detail (Source: Reference 92). 



Handles 

 For Lifting 

 Cover Slab 



m. 



Pluo 

 "^Inspection Pipe 



Reinforcing Rods 



Scientific studies have shown that the nitrate form of nitrogen is 

 commonly found in high concentrations (up to 40 ppm) at distances of 

 100 feet from septic systems. Other studies have shown unacceptable amounts 

 of nitrate at 150 feet [105][106]. Consequently, a setback distance at 

 least 150 feet inland of the annual flood line is required to minimize 

 nitrate pollution. After extensive review, L. B. Leopold concluded that 

 "for soil cleansing to be effective, contaminated water must move through 

 unsaturated soil at least 100 feet" and that "...it might be advisable 

 to have no source of pollution such as a seepage field closer than 300 

 feet to a channel or watercourse" [107]. Local regulations have typically 

 required the absorption fields of septic tanks to be set back a minimum 

 of 50 feet from the edge of the water--stream, lake, open ditch, or other 

 water body into which unfiltered effluent could escape. 



Even with an appropriate setback there may be problems with refusal 

 of saturated or impermeable soils to accept septic tank effluent. A 

 major consideration of efficiency is the rate at which effluent moves 

 into and through the soil. The amount and type of gravel, sand, silt, and 

 clay influence this rate, and movement is faster through sandy and gravely 

 soils than through clayey ones. Soil permeability for septic tanks 



141 



