Air Quality in the Barnett Shale - Part 3: TCEQs Effects Screening Levels.

According to Wilma Subra's summary report based on the Wolf Eagle Environmental's final report, the following values for Benzene and Xylene (m & p) were given for each of the sample locations where an exceedance was noted:

• Benzene exceeded the Long-term  ESL by 8.7 times the standard.
• Xylene (m & p) exceeded the Long-term ESL by a factor of 1.1 times.
• Benzene in the air exceeded the Short-term ESL by 1.45 times and the Long-term ESL by 55 times.
• Xylenes (m & p) exceeded the Long-term ESL by 2 times.
• Benzene in the air exceeded the Long-term ESL by a factor of 7.6 times.
• Three sample locations exceeded TCEQ ESLs [for Benzene]
• Two sample locations exceeded TCEQ ESLs [for Xylene (m & p)].

The Wolf Eagle Environmental's final report makes the following statement (using bold type):

(I'll discuss the use of the term 'disaster potential' in an upcoming blog)

So what is an ESL that these two environmental professionals keep referencing as having been exceeded?  Lets look at the TCEQ definition (emphasis mine):

Effects Screening Levels [are] used to evaluate the potential for effects to occur as a result of exposure to concentrations of constituents in the air. ESLs are based on data concerning health effects, the potential for odors to be a nuisance, and effects on vegetation.

If predicted airborne levels of a constituent do not exceed the screening level, adverse health or welfare effects are not expected.  If predicted ambient levels of constituents in air exceed the screening levels, it does not necessarily indicate a problem but rather triggers a review in more depth.

ESLs [are] expressed in terms of microgram per cubic meter (µg/m3) or parts per billion by volume (ppbv) in air.  

If you recall, here is what the TCEQ has said about the air quality in this area:

“After several months of operation, state-of-the-art, 24-hour air monitors in the Barnett Shale area are showing no levels of concern for any chemicals. This reinforces our conclusion that there are no immediate health concerns from air quality in the area, and that when they are properly managed and maintained, oil and gas operations do not cause harmful excess air emissions.”

So if the test results of Wolf Eagle Environmental are correct, how can all these samples that exceeded the ESLs be dismissed by the TCEQ?  I mean, what gives?

Well - and I hate to say this about two of my fellow environmental professionals - they obviously either do not understand how to use the ESLs, or - and I hope this is not the case - they are using them to show the appearance of a risk that is necessary to further a particular agenda.

Now I am only addressing Benzene and Xylene (m & p) in this post - not to cover anything up about the rest of the chemicals found, but to make this post a bit more readable.  I will address all the chemicals that exceeded the ESLs in an upcoming post.  According to the TCEQ in their document titled ESL List June2010.xls:

“Short-term” generally indicates a one-hour averaging period. Exceptions are noted parenthetically after a constituent name [for reproductive/developmental effects]. “Long-term” indicates an annual averaging period.  Both short- and long-term ESLs are listed on the ESL List.

 The reason that there are two values has to do with effects that are 'acute' or 'chronic.'  Acute effects are what we expect to see with a one time short duration exposure.  Chronic effects are what results when one is exposed constantly over a very long period - usually calculated as 70 years of exposure at that particular concentration.  The sampling performed has a different duration when looking at Short-term ESLs or Long-term ESLs.  According to the TCEQ:

The exposure duration generally associated with short-term ESLs is 1 h, although exposure may occur on an intermittent basis.  Long-term ESLs are are used to evaluate modeled 1-year average concentrations.

What this means is that you cannot use sampling data that was collected during a single 24 hour period and use it to compare against ESLs that have been established for sampling data averaged over one year.  Unless you want to infer that a one time sampling event will represent what would be seen each day for 365 days, regardless of wind direction, humidity, temperature, rain, and snow, you cannot do this.  Well at least I would never make such a bold assertion, especially knowing that one of my peers might come across my report....

But besides all that, if you are going to make a statement that the Long-Term ESLs were exceeded, and you are going to use that bit of information to assert excess exposure to known human carcinogens, then you need to use them as they were intended.  You cannot compare Long-term ESLs with sampling results collected from a one time sampling event.  This would be similar to looking at my bank account on the first of the month (payday) and making the statement that I have that much money in my account everyday.

So based on this, for all the chemicals analyzed, there were a total of thirteen that exceeded the short-term ESL (the only ESL that can be compared against) and of these, only Benzene was found to exceed the Short-term ESL once.

OK, so if we ignore the Long-term comparison because they only took one single 24 hour hour sample, there were still thirteen times one or more chemicals exceeded the Short-term ESL.  And one of those chemicals - Benzene - a known human carcinogen - was found to have exceeded the Short-ESL.  Shouldn't that cause concern?

No, not really.  Remember what the TCEQ says about exceeding the ESL:

If predicted ambient levels of constituents in air exceed the screening levels, it does not necessarily indicate a problem but rather triggers a review in more depth.

Next post: Air Quality in the Barnett Shale - Part 4:  Exceeding TCEQs Short-term ESLs


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