Drainage stone is supposed to drain. After all, typical 3/4″ to 1.5″ gravel used in septic trenches has a lot of void space. In my early days working at an engineering firm with a materials testing lab, I tested some typical, but fairly dirty, septic gravel and it had approximately 40% open space. So water should flow easily through that, right?
As part of our Septic SitterTM field validation program, we have been monitoring trench ponding levels in several leachfields located in PEI, Canada. We have observed some interesting results.
Mr. & Mrs. Buns (the name is made up but the system and data is real) have a 15 year old raised-bed, C3 “Contour” drainfield. Contour drainfields were developed and popularized in Nova Scotia, and this system consists of a single gravel trench, 140 ft long by 4 feet wide and about 8″ deep.
The trench is constructed along a contour and the bottom of the trench is level throughout. The trench is trickle-fed at the centre with a Tee which should, in theory, cause the flow to split evenly in each direction.
Liquid Levels Throughout Trench
About a year ago we retrofitted inspection ports and installed Septic SitterTM sensors at three locations in the trench. As the data from the sensors show, the effluent distribution is not very even at all.
During times of high flow, rainfall events or snowmelt, the middle of the trench (sensor #2) ponds to the top or even higher (see the 27″ high spike in the blue line on the graph). Otherwise, ponding levels fluctuate between 8″ and 12″.
At the location of sensor #3 (green line on graph) the trench is typically ponded about 10″ deep with very little variation. But near the end of the trench, at location #4 (black line on graph), the trench is always empty. We have only observed effluent ponding in this location once in the past year and that was only for a day after a major rainstorm.
What Is Going On..?
So why is this happening? The septic tank has had an effluent filter on it since the system was first installed, so I don’t believe that a lot of solids could have escaped from the tank and clogged the pipe or drainfield. However a video inspection of the perforated distribution pipe is recommended to ensure that a blocked pipe is not part of the problem.
Dirty Stone Theory…
The stone used in this system was quite “dirty” and contained a lot of fines (sand, silt, clay). If you saw it however, I doubt you would think it was dirty enough to prevent water from flowing freely through it.
However, perhaps over the years, biofilm developed in between the gravel particles, so that it started to bridge the void spaces between the stones. Could biofilm have developed to the point where it has started to interfere with the free movement of effluent throughout the trench? That is my current theory, however I would be interested to hear what you think.
Please comment below, and let us know how you would investigate or remediate this system so that the infiltration capacity of the entire leachfield can be restored.
Septic Sitter Launch This Week at WWETT 2017!
Dynamic Monitors is launching the Septic Sitter system this week at the WWETT show. Come by Booth 4312 and see a live feed of the data from this site. You can tell me in person what you think is going on with this system.