Question:
I believe drainfields start ponding when the user of the onsite system adds too much water to the drainfield. With a well-maintained septic tank biomat, I believe hydraulic conductivity tends to stabilize at approximately 0.20 gpd/sf, based on a scientific analysis of the flow into a soil. If the owner has 100 square feet of trench bottom, they can drain 0.20 x 100 = 20 gpd. Most homes would need 500 square feet of trench bottom to avoid ponding. What are your comments about ponding and drainfield sizing?
Answer:
I agree ponding will take place in the bottom of the soil treatment area when the system is being used. However, this ponding will occur long before the owner “adds too much water to the drainfield.”
I do not agree ponding should be avoided. As I indicated in the March issue of Pumper, the biomat develops in the bottom of a trench as the soil filters out suspended solids and pathogens (disease causing bacteria) in the sewage tank effluent. The biomat is a necessary part of the adequate sewage treatment by a soil. Unless the biomat forms at the rock-soil interface so that partially saturated flow takes place in the soil, there will not be effective pathogen removal.
An extremely porous soil that allows the sewage tank effluent to flow through without forming a biomat (and ponding) will not adequately treat sewage tank effluent and will not remove pathogens. An onsite soil treatment system must not be installed in such a soil.
As the biomat develops at the head end of the bottom of a trench installed in a suitable soil, ponding begins and the effluent moves along the length of the trench. When the entire bottom of the trench has developed a biomat and there is ponded effluent, the liquid level will begin to rise in the trench as more effluent is added.
Effluent begins to filter into the soil sidewalls of the trench and the biomat also forms at this trench-soil interface. If the soil treatment system is properly designed, the effluent level should rise to the top of the trench rock, or other distribution medium, before any effluent flows to the second trench in the system. This is called sequential distribution.
How much effluent will be treated by a trench full of sewage tank effluent? This of course depends on the soil texture, the width of the trench and the depth of the sidewalls of the trench.
The following table of soil capacities to treat sewage tank effluent is used in Minnesota and many other states. I believe it to be quite similar to what the U.S. Environmental Protection Agency recommends. The table presents the Long Term Acceptance Rate of various soil textures as reflected by their percolation rates.
Using the above table for sizing the drainfield, a soil with a percolation rate of 0.1 to 5 minutes per inch (loamy sand) will treat 1.20 gallons per day per square foot.
If there are 24 inches of trench rock placed below the so-called distribution pipe, a 40 percent reduction in bottom area may be taken. Thus, 0.60 of a square foot will treat 1.20 gallons per day. Stated differently, the flow rate through the biomat of the loamy sand would be 2.0 gpd/sq ft. Only 10 square feet of trench bottom area in this soil would be needed to treat 20 gallons per day.
I understand your proposed value of 0.20 gpd/sq ft. was obtained from a mathematical analysis of soil texture using various assumptions as to flow conditions and the application of hydraulic flow theories. Has this value been tested in actual onsite systems?
You state 100 square feet of trench bottom would treat 20 gallons per day. This seems to be an extremely small value and there is no distinction for various soil textures. Your proposed value of 0.20 gpd/sq ft. is smaller than any of the values in the above table. It would oversize the loamy sand system by a factor of 10.
One note on the table should have further explanation. That figure is the design value of 1.67 sq ft/gpd for fine and very fine sand having a percolation rate in the range of 0.1 to 5 minutes per inch. When I was an Extension Agricultural Engineer at the University of Minnesota, we performed research on soils rated as fine and very fine sand. Onsite systems had been failing on these soils. The soil treatment systems had been sized using the value of 0.83 sq ft/gpd or an acceptance rate of 1.20 gpd/sq ft.
In our research we made accurate measurements of effluent flow into trenches installed in these soils. We found the trenches were able to treat only 0.60 gpd/sq ft. The systems installed in this soil were only half as large as they should have been. That was the reason for the failure of the onsite systems.
Our research is the reason a separate value for fine and very fine sand has been added to the table for Long Term Acceptance Rates in Minnesota.
I hope my explanation will help you better understand the values commonly used for sizing soil treatment areas of onsite sewage treatment systems in Minnesota and other states. These values have been used successfully for many years.
