Sewer/Groundwater Interactions

[MTE85DRM]

Has anyone had any experience with groundwater contamination (solvents) originating from a sanitary sewer.

I am involved in a situation where two properties have been impacted by TCE. A concrete sanitary sewer trunk runs along the property line. It is constructed below the water table elevation. We have installed over 20 monitoring wells on these two properties and the distribution of impacts occurs generally around the sanitary line.

Due to the pressure of the groundwater around the sewer I would have expected that any fluids in the pipe would remain in the pipe. However, in addition to the solvents we have noticeable concentrations of trihalomethanes (THMs) in the groundwater - highest concentration are in the vicinity of the sewer.

If solvents were released into the concrete sewer could they migrate out of the piping to impact the surrounding shallow groundwater ? If so, How ? - given the obvious pressure head.

Thanks

 

[4tuna]

Yes, I have this problem at a few sites. At one site I have two separate, small cis -1,2 DCE plumes (intermediate decholrination product of TCE) clearly related to sewer laterals. This is a not too uncommon problem; the guys at Regenesis studied 480 dry cleaner sites and found PCE in 30% of the nearby sewer corridors.

Because TCE and other solvents are heavier than water, then tend to migrate along the bottom of sewer pipes. They will seep out of any minute crack they encounter, which includes joints in intact piping. This process is independant of pressure -- sewer pipes are generally not far enough below that water table that the pressure head of surrounding groundwater is significantly different than head in the line, and even if it were I'm not sure it would prevent migration given the density difference. Once in a corridor, contaminants tend to migrate along them as the corridors are usually more permeable than surrounding sediments.

If you are in sediments, the way to clearly distinguish whether it is from the sewer corridor is to probe a few transects perpendicular to the line using a Geoprobe MIP/SC probe with an ECD detector. This will give you not only relative concentration at each point, but show you which layers the TCE is in.