Dr. Sattler's dispersion modeling maximum concentrations were developed using sampling data - collected with six SUMMA canisters possitioned a distance away from the oil & gas production site - provided by Alisa Rich. Of these seven chemicals, two (Benzene and Toulene) are part of BTEX which is a common air pollutant derved from burring fossel fuel and cigaretts. According to the EPA:
The primary HAP associated with the oil and natural gas production and natural gas transmission and storage source categories include BTEX and n-hexane. In addition, available information indicates that 2,2,4-trimethylpentane (iso-octane), formaldehyde, acetaldehyde, naphthalene, and ethylene glycol may be present in certain process and emission streams. Carbon disulfide (CS2), carbonyl sulfide (COS), and BTEX may also be present in the tail gas streams from amine treating and sulfur recovery units.The State of New York identifies 1,2,4-Trimethylbenzene as a "chemical constituent" of "chemical additives proposed to be used in New York for hydraulic fracturing operations at shale wells."
This leaves styrene and dimethyl disulfide without a recognized association with oil & gas production. Since these two chemicals were detected Alisa Rich and Dr. Sattler assume they must have come from the oil & gas production site in question and build a dispersion model as if they did. This is why you can't back in air sample contaminant levels taken some distance from the source without first understanding what the normal background level is for the chemicals in question. There is no reason why styrene or dimethyl disulfide would be coming from this site. Since styrene is a HAP, it would have been identified as chemicals of concern for oil & gas production MACT/GACT compliance under NESHAPS.
So lets say those two chemicals are background contaminants that came from some other process unrelated to oil & gas. That leaves five remaining. Both carbon disulfide and carbonyl sulfide (as well as dimethyl disulfide) were identified in the final report and in the lab reports as Tentatively Identified Compound (TICs).
This creates a bit of a problem that was not addressed in the report. Here is what the TCEQ has to say about TICs:
TICs are observed measurements in the sample for which the gas chromatograph-mass spectrometer (GC/MS) was not specifically calibrated; however, the tentative identification of a compound can be made by comparing the mass spectrum from the environmental sample to a computerized library of mass spectra. The comparison of the sample spectra and that of the library are scored for their similarity to the mass spectrum of a particular TIC and the tentative identification is made based on the most similar spectra. This is a commonly used technique; however, the absolute identity of a TIC is uncertain. Quantifying TICs is also less accurate than for target compounds because the true relative response factor is not known, since the instrument was not calibrated for the TIC. It is important to note these uncertainties when evaluating TICs.
Given the uncertainties in identification and quantification of these compounds and the method used to determine potential 1-hour maximum concentrations, it is not possible to accurately draw conclusions about the potential for adverse health effects.
I understood why, but not to the level needed to discuss it here. So I asked my professor over at SRPH that happens to know a little bit about analysis and GC/MS. "It's because of the response factor" he said. I just nodded my head making a mental note to Google that when I got back to my office. Here is what I found to be a pretty good explanation of a Response Factor:
The size of a spectral peak is proportional to the amount of the substance that reaches the detector in the GC instrument. No detector responds equally to different compounds. Results using one detector will probably differ from results obtained using another detector. Therefore, comparing analytical results to tabulated experimental data using a different detector does not provide a reliable identification of the specimen.
A “response factor” must be calculated for each substance with a particular detector. A response factor is obtained experimentally by analyzing a known quantity of the substance into the GC instrument and measuring the area of the relevant peak. The experimental conditions (temperature, pressure, carrier gas flow rate) must be identical to those used to analyze the specimen. The response factor equals the area of the spectral peak divided by the weight or volume of the substance injected. If the technician applies the proper technique, of running a standard sample before and after running the specimen, determining a response factor is not necessary."Basically," he said. "Because you did not use a standard that included these TICs, the computer makes a guess as to what chemical the peaks could represent." He then showed me how this works by letting the computer identify an unknown bunch of peaks on a spectra he had. The computer spat out a name. "The lab tech needs to look at the peaks and compare it to the peaks of the compound the computer picked out. If it's a match, great, but a lot of the time, like this example, the computer gets it wrong."
So basically what he showed me was the analytical equivalent of Damn you Auto Correct. But they quantified these chemicals I said. He just stared at me and then shook imaginary dice in his hands and let them fall. I got the message. Even if you spent the time comparing the peaks to the chemicals in the library, the lack of a standard means the quantity reported is nothing more than a guess. Which is why the TCEQ says:
Given the uncertainties in identification and quantification of these compounds and the method used to determine potential 1-hour maximum concentrations, it is not possible to accurately draw conclusions about the potential for adverse health effects.Well I called the lab and asked them if they physically compared the peaks found to the chemical in the library that the computer picked up. They said no. Damn you auto correct!
Now, to be scientific about this - which Alisa Rich should be and Dr. Sattler must be - with this information, we really need to exclude styrene, dimethyl disulfide, carbon disulfide, and carbonyl sulfide from the model.
That leaves only Benzene, Toluene, and 1,2,4-Trimethylbenzene with any reasonable plausibility for being emitted from the oil & gas production site. Lets look at this from both a comparison to the ESL and AMCV.
Note: Toluene AMCV is for health. ug/m3 were converted to ppbv using the formula 24.45 x concentration (ug/m3) ÷ molecular weight.
When compared to the AMCV - which is proper in this case, and any case where air pollution permitting is not the goal -only Benzene and 1,2,4 Trimethylbenzene exceed the Annual AMCV based on the premise that these two chemicals came solely from one source only - the oil & gas production site.
So looking at it under these conditions - straightforward, fair, scientific based - what is the conclusion for the air in the Town of Dish, Texas even if these values were backed in the model? Remember, those values reported in Table 2 are worst case possibilities.
Next Post: Air Quality in the Barnett Shale - Part 17: Dr. Sattler's Deposition - TCEQ Competency
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