Percolation Tests shall be made in at least 3 test holes within 5 feet of where the proposed absorption system or shallow wastewater system is to be located. The holes shall be randomly located in soil representative of and similar in character to the rest of the area where the system will be placed. An additional test hole shall be made to a depth of 4 feet beneath the bottom of the proposed absorption system, unless groundwater or bedrock is encountered first, to determine the type and depth of absorption system. 

The horizontal dimension or diameter of the percolation test hole shall be from 6 to 12 inches. The vertical sides shall extend to the maximum depth of the proposed absorption system or to a depth of at least 30 inches, whichever is greater. 

Test holes shall be located in unfrozen soil and shall be filled at least 50 percent full with water for at least 8 hours but not more than 16 hours before making the soil percolation test. Immediately before making the test, each hole shall be refilled with water to at least 50 percent of its volume. When the water reaches the lower 25 percent of the test hole, the test shall begin. The percolation rate of a test hole shall be expressed in the number of minutes it takes the water level to drop 1 inch. The percolation rate for the area where the subsurface infiltration system is desired is the average percolation rate of all the test holes. The percolation tests shall be conducted for 2 hours unless the percolation rate is slower than 45 minutes per inch, in which case the percolation tests shall be run for at least 4 hours. 

Percolation tests required prior to approval and installation of absorption system. The owner of the land on which a subsurface absorption system is to be constructed or installed shall ensure that a percolation test is conducted in accordance with the guidelines in the “ percolation test” before installation of any such system. Absorption systems shall not be constructed in soils rated as having severe or very severe limitations for subsurface wastewater disposal by the Soil Conservation Service, U.S. Department of Agriculture, unless that limitation is not present as shown by field investigation or prior written approval is granted by the secretary. 

Percolation Rate

Minimum Absorption Trench Area

(Minutes for water to drop one inch)     (Square feet of trench bottom per bedroom)  
1 but less than 5 minutes per inch     Not permitted 
5 but less than 10 minutes per inch  125 square feet  
10 but less than 15 minutes per inch      165 square feet 
15 but less than 30 minutes per inch                   200 square feet 
30 but less than 45 minutes per inch                   250 square feet 
45 but less than 55 minutes per inch                   300 square feet  
55 but no more than 60 minutes per inch             350 square feet  
Over 60 minutes per inch                                  Not permitted  

If the percolation rate falls into the “not permitted” area (to fast or to slow) then either a mound or and evapotranspiration will be required. 
2009 Rapid City Business of the Year !
R. Chris Howie & Darin R. Howie