Unusual Septic, Bentonite Plugs

This question is for experienced septic inspectors. I have an upcoming inspection this Saturday 2/6. I just received the municipal paperwork today and the system design is unusual. Paperwork was 40 pages long, a bunch of emails from the contractor and County in regards to the new design. I’ve included just the drawing here. I have not been to the site yet, but I’m looking for specific info regarding the bentonite plugs noted in this system.

Background info: Home was built in 2013, original septic system failed in 2019 due to excessive ground water intrusion into the drain field. The system is located in a low lying topographical area, where rain water was running down nearby hills into the drainage system, flooding the system. The new system installed in 2019 has a curtain drain installed at the base of the hill, to divert rain water away from the drainage system. The septic system consists of a 2000 gal concrete tank, a new 1,000 lift pump (new drain fields are higher than old ones), a D box, and 2 drain fields.

In between the lift pump and the D box are 3 Bentonite plugs, which the paperwork says were designed to prevent effluent from reentering the lift pump from the drainage system. I’ve never encountered Bentonite plugs before, these are very uncommon here. Is there anything I should specifically look for with these? My understanding of bentonite is that it is a fast setting clay that is used to plug damns and I’m not sure how it is being used in this system, or what defects I should be looking for with this. Are these essentially acting as baffles in the drain trench? I’m assuming I should not see bentonite inside the drain lines, and I may not be able to see it at all during the inspection. Any thoughts or advice would be helpful.

1 Like

This is all I could find readily, Ryan:



Traditional Approach

Historically, anti-seep collars, also known as trench dams and trench breakers, have been constructed from a range of materials, including rubber, concrete, corrugated metal, foam, et al. With these approaches, installation is often time consuming and can be technically problematic. Another alternative uses 50lb paper bags of traditional bentonite chips or sand bags (stacked in the trench) to achieve a dam or collar around a pipe. However, for a number of key reasons, these approaches actually can be far more expensive, less safe, and much less effective than an AquaBlok approach.

1 Like

Good information Larry!

1 Like

Thanks Larry. That’s pretty similar to what I was finding. My understanding is that these are used to prevent seepage or slippage of the entire gravel/sand trench when the trench has been exposed to excessive moisture. Essentially the entire gravel trench has slid in one direction and they are using these to prevent that or mitigate it.
Probably not anything I will be able to see on the inspection because they are below grade. The stuff I have been reading about them doesn’t sound like they are too effective either. It seems to me the bentonite plug diameter would have to be larger than the original trench to be effective and have holding power.