On March 10th, CTV reported that elevated levels of trichloroethene (TCE) were found in groundwater along Brydges Street in London, Ontario. It is not currently known if there is any immediate health risk to the local community. As we understand, indoor air will be tested to assess the potential for vapour intrusion in homes (read more about vapour intrusion).
The current London issue aside, TCE has been amongst the most common chemical contaminants in soil and groundwater we have assessed/addressed over the past 25-plus years. In fact, we are currently working on several projects where TCE is the major chemical of concern.
What is TCE?
With TCE again in the news, I sat down with our senior hydrogeologist, Dr. Michael Sklash, P. Eng., for a short primer on TCE. Over the past 25 years, Mike has assessed dozens of sites in Canada and the United States with TCE contamination.
(Questions in bold; Dr. Sklash’s responses in italics)
Why do we see so many issues with TCE in the environment?
TCE was widely used for industrial and military degreasing during the 1950s through the 1970s. However, TCE was eventually listed as a probable carcinogen, and it was gradually replaced by other degreasing chemicals. Unfortunately, since both the behaviour of TCE in the environment and its health effects were not well understood while it was in widespread use, releases to the environment were common and not necessarily accidental (remember, they didn’t know what we know today). Now, TCE is a “legacy” chemical from my parents’ generation that we run into frequently.
TCE in the Environment
Will TCE tend to float or sink in groundwater?
TCE is one of a group of chemicals called DNAPLs (dense nonaqueous phase liquids, pronounced “D napples”). There are two interesting characteristics of DNAPLs with respect to their behaviour in groundwater. First, since DNAPLs are denser than water, when a DNAPL is released in large enough quantities, it migrates through the unsaturated zone, penetrates the water table, and then sinks through the groundwater until it is absorbed in the formation or encounters a low-permeability layer. Second, since DNAPLs do not dissolve in groundwater very quickly, they tend to persist separately from water.
That said, a common misconception we encounter regarding TCE in groundwater is a misunderstanding of when TCE is a DNAPL. We frequently hear from experienced professionals that TCE sinks in groundwater because it is a DNAPL; they want you to look at the bottom of the well for TCE or at the bottom of the aquifer (permeable unit such as sand, gravel, or permeable rock) regardless of the TCE concentration in groundwater. The reality is that when TCE is at low concentrations in groundwater, TCE behaves like any other dissolved chemical in groundwater, moving where the groundwater moves.
Will TCE move at the same rate as groundwater?
No. TCE may move many times slower than groundwater because it tends to “stick” to soil and organic matter in the soil. The technical term for this behaviour is “retardation.” The velocity of TCE in groundwater can be approximated if the groundwater velocity and organic carbon content of the soil are known. However, we frequently find that even experienced professionals use an inappropriate test method to determine the organic carbon content of the soil.
I’ve heard people say that a little TCE can take a long time to remediate. Why?
Because TCE is a probable carcinogen, allowable concentrations in drinking water are very low (5 parts per billion or less). Although TCE moves much more slowly than groundwater, releases that occurred 40 to 50 years ago have created huge groundwater contamination plumes in some geologic environments. As little as a few drums of TCE released 40 or 50 years ago can result in a contaminant plume in the groundwater that is kilometres long. That’s why it is important to clearly understand not only “your release,” but also how other historic releases in the area may impact your efforts.
So TCE contamination is expensive to remediate?
The cost to remediate TCE in groundwater depends on many factors including the size and depth of the contaminant plume, the geologic conditions, and your time frame to complete the cleanup, your cleanup concentration goal, and so on. Cleanup will not be inexpensive, so the cleanup plan must be done right the first time.
If someone expects they may have had a release of TCE, what should they do?
In some cases (like a failed, above-ground, TCE-storage tank while you own the property), you know there was a release, where and when it occurred, and how much was released. The path forward is relatively simple: engage reliable, experienced, environmental professionals to guide you through the regulatory requirements, determine the nature and extent of the problem, and develop options for dealing with the release.
Unfortunately, you, or a previous owner or operator of the property, may have had a release of TCE that no one witnessed or no one documented. Tanks and degreasers may have leaked, filters and residues may have been tossed out the back door, or used TCE may have washed down an old, forgotten, dry well. The path forward in this case is much the same: engage reliable, experienced, environmental professionals to guide you through the regulatory requirements, determine the nature and extent of the problem, and develop options for dealing with the release. The key difference here is: “determin[ing] the nature and extent of the problem” is much more difficult.
Any final comments?
Don’t start to think about cleanup until you thoroughly understand the nature and extent of the problem. “Silver bullet” solutions won’t work; they won’t work in the long run or economically if you haven’t put your arms around the problem first.
Contact the Environmental Expert
If you have any questions about TCE in groundwater, contact Mike (msklash@dragun.com) at 519-979-7300, ext 120.