Air Quality in the Barnett Shale - Part 28: Why Plot 2 used Carbonyl Disulfide

I was a bit curious as to why the FWLN Report sent to the FWISD included one model for carbon disulfide (Plot 1) and another model for carbonyl sulfide (Plot 2).  Everything up to this point had focused on the health concerns for carbon disulfide.

So I looked closer at the FWLN findings:

...and:

And then I looked at the statements printed on a green background in the report:

Okay...so it is all about the carbon disulfide.  So why produce Plot 2 for carbonyl sulfide?

Well duh!  Carbon disulfide was not detected by TO-15 in 46 out of 48 samples (see post) and ASTM Method D5504-08 detected carbon disulfide but had to qualify it as an artifact (see post).

But sample QLA5DC-1 did not have that inconvient qualifier:

So as I'm reading this and looking at the data, my mind is forming the question of; if there is no carbon disulfide detected in the BSEEC samples collected from different sites over different days, and carbon disulfide is the contaminant of concern, why spring carbonyl sulfide into the mix and continue to direct attention to carbon disulfide?

Doh!  Then it hit me.  FWLN is looking at carbonyl sulfide as a "byproduct" of carbon disulfide.  In other words, if you find carbonyl sulfide there must have been - or is - carbon disulfide!  Very astute FWLN!  Only you probably didn't figure on me checking into this, did you?

Yes, indeed, carbonyl sulfide is a "byproduct" of carbon disulfide.  It is produced when carbon disulfide is purposely manufactured.  Lets see what the EPA has to say on this:

You know what else the EPA has to say about carbonyl sulfide?

In the United States, the reported concentration of carbonyl sulfide in the air in rural areas was 0.27-0.80 microgram/m3.  Levels were 1.17 micrograms/m3 in Philadelphia, PA; 1.21 micrograms/m3 in Wallops Island, VA; and 1.37 micrograms/m3 in Lawton, OK.  Reported concentrations of carbonyl sulfide over salt marshes, which are major natural sources, range from about 60 to 180 micrograms/m3, and 14 to 19 micrograms/m3 over the ocean (HSDB 1994).

Well if I am going to be objective here, I need to follow this lead....

Okay...so finally we have something that indicates that we may have an issue with a contaminate in the Barnett Shale area above normally expected background.  And even if you subtract the maximum amount attributed to the artifact, in sample QLA5DC-1 you would still have 105 ug/m3.  That's more than comes off a salt marsh based on the EPA's research.

So lets work with this.  But first we need to make a few points clear.  First the presence of carbonyl sulfide in the ambient air samples collected has nothing to do with carbon disulfide.  And second, the non-detection of carbon disulfide in the BSEEC report using method TO-15 indicates that carbon disulfide is not present.

So the question I can reasonably ask now is carbonyl sulfide a concern? 

Now we are back to the one sample - low n's - issue.  However, carbonyl sulfide was detected above the artifact concentration of 25 ug/m3 in 6 of the 18 samples indicating a possible concentration above the levels found in an urban environment.  So lets assume that 130 ug/m3 (53 ppbv) is a valid number for carbonyl sulfide in the ambient air collected by BSEEC.

What does the BSEEC Report have to say about this:

Looks like the BSEEC folks got their units mixed up. This means that I need to check to see if the AMCV they report is in ppbv or ug/m3.  And guess what I find when I check?  There is no AMCV for carbonyl sulfide.  So where did "1800 ppbv" come from?

Well if I can find the acute ReV that would be the AMCV.  Search....nothing.  What about the RfC?  Checking the IRIS website....nothing, in fact they state:

The health effects data for carbonyl sulfide were reviewed by the U.S. EPA RfD/RfC Work Group and determined to be inadequate for the derivation of an inhalation RfC. The verification status for this chemical is currently not verifiable. No EPA documentation presently exists for this chemical.

Okay...so where did the BSEEC folks get a short-term AMCV of 1800 ppbv?  Well as of right now I have no idea.  I did find a short term ESL value that the TCEQ has derived in a 2008 memo:

So even at 130 ug/m3 (53 ppbv) the amount detected is well below the health-based ESL concentration of 540 ppbv which is 70% of the AMCV.....540 / 0.3...that equals 1800...hey...that's the AMCV the BSEEC folks reported!  

Okay, that gives us a value we can defend as health-based, assuming TCEQs POD and UFs are correct...they seem well supported.  And if the carbonyl sulfide ESL is listed for odor at 55 ppbm, it makes sense that the health-based threshold would be higher:

The short-term ESL is the lowest value of acute odor-, vegetation- and health-based ESLs. (Factsheet)

And at the highest carbonyl sulfide concentration detected - 53 ppbv - that amount is 10 times lower than the health-based ESL of 540 ppbv and more than 33 times lower than the ambient air concentration - the true health-based threshold - of 1800 ppbv.

Y'all double check my math (remember math makes my head hurt) to make sure I'm seeing this right....

Lab reports 53 ppbv...TCEQ states health-based ESL is 540 ppbv...at 540 ppbv the AMCV would be 1800 ppbv...okay...so the carbonyl sulfide in the Barnett Shale samples is well below health based levels.

So what does Dr. Sattler and the FWLN have to say about the carbonyl sulfide in their Report?

Huh?  How could the oil & gas source that produced sample QLA5DC-1 produce an emission rate that produces a model that that predicts concentrations one mile from the source that are "6 times greater than the health benchmark for carbonyl sulfide?"  At 1800 ppbv that would mean one mile out you would have concentrations at 10,800 ppbv?  Even if she incorrectly used the health-based ESL of 540 ppbv, that would show concentrations at 3,240 ppbv one mile from the source.

How is that physically even possible?

Maybe I'm missing something here, how can directly downwind from the source  - 50 -75 feet away - we find an actual concentration 130 ppbv but are able to get a concentration significantly higher further from the source?  How can the calculated emission rate from this actual value extend downwind one mile from the source at a concentration 6 times greater than what produced it at 50-75 feet away?  The same emission rate is being used, correct?  Doesn't Gaussian dispersion show the highest downwind concentrations nearest the source?

Wait a minute...Dr. Sattler is calculating an air concentration in and around the source (red area on her plot) of 655 ug/m3.  That's six times higher than the actual concentration detected within the red colored area around the source.  Now I'm really confused.  I mean the emission rate is staying constant, correct?  And the downwind concentration used to "back in" to obtain the emission rate is 130 ug/m3.  So with that "backed-in" emission rate, you would calculate 130 ug/m3 for the red area.  Yet in Plot 2, that area has a concentration of  655 ug/m3:

Okay...so lets assume Dr. Sattler's maximum 1 hour concentration for carbonyl sulfide is possible.  Would that maximum level pose a health-based concern?

• The highest concentration Dr. Sattler reports in Figure 1 is 655 ug/m3 or 267 ppbv.
• The maximum concentration of 267 ppbv is less than the ESL of  540 ppbv.
• 267 ppbv is less than one half the amount of the health based ESL
• 267 ppbv is less than six times the health based ambient air concentration of the AMCV.

So why would Dr. Sattler and the FWLN insist that the concentrations one mile from the source could be as high as "6 times the health benchmark for carbonyl sulfide?"  Well lets look at Dr. Sattler's Report on Plot 2 to see what's up about this:

The 1-hour Texas Commission on Environmental Quality (TCEQ) Effects Screening Level (ESL) for carbonyl sulfide is 135 µg/m3; one of the sample concentrations exceeded this level.

.... ah...that's why!  Dr. Sattler is using an odor ESL threshold for the "health benchmark" that is being exceeded:

...and if you will recall, "The short-term ESL is the lowest value of acute odor-, vegetation- and health-based ESLs."

So even if Dr. Sattler's model is correct, the carbonyl sulfide in Plot 2 does not pose and adverse health risk.

And if that is true...and the fact that 46 our of 48 BSEEC TO-15 samples showed non-detect for carbon disulfide, and the two samples that were positive were less than 0.3 ppbv... then there is no health based concern for carbon disulfide or for carbonyl sulfide and the requirement to have a one-mile setback based on the health concerns for these two contaminants is not warranted or substantiated by any of the data provided in the FWLN or BSEEC or ERG reports.

And just so we are clear on this.  The need to provide - or not provide - a setback for O&G operations is not being argued here.  The basis for a one mile setback recommendation due to health-based concerns regarding carbon disulfide and/or carbonyl sulfide is.

And that's all I can really say about this....

Next Post: Air Quality in the Barnett Shale - Part 29: Where should we go from here?

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