Correspondence/Rebuttal pubs.acs.org/est
Response to Comment on “Identifying Well Contamination through the use of 3‑D Fluorescence Spectroscopy to Classify Coalbed Methane Produced Water”
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for CBM produced water the authors chose to classify background matrix fluorescence by creating a database of representative samples for a number of basins. A database offers representation of the range of various organic chemical mixtures based on location, fracturing water source, formation water quality, coal depositional environment, formation depth, and type of hydrocarbon produced. Given the complexity, this approach lends itself to the FRI classification method, which was created and refined by relying on scientific databases of representative samples.3 The peaks mentioned were selected based on their repeated observed occurrence in basin samples. The peak locations were described using the classification criteria described by Chen et al. (2003) to offer relative locations compared to previous studies, but are intended only for use as relative qualitative comparison criteria not to be definitive.3 (3) Peak comparison: This study did not use peak ratio comparison methods. The authors normalized the images based on DOC concentration and compared samples based on peak location. A red shift in region of 450−480 nm 3 and the innerfilter effect 4 pertaining to low and short wavelength variations and molecular weight were not quantified as the peak compositions between anthropogenic and naturally occurring organic signatures were apparent using the simpler relative peak location comparison. (4) Acetate fluorescence: Acetate is suggested as a potential indication of biogenic methanogenesis based on acetoclastic methanogenesis and controlled laboratory studies.5−7 Acetate is known to fluoresce and is used in polar solvent comparisons in the form of ethyl acetate.8 Sodium acetate was added to Milli-Q water to create the standard for this study.2 Although contamination potential exists at trace concentrations for most laboratory procedures, the suggestion of tryptophan contamination as the reason for fluorescence is not supported by images of tryptophan that display two isolated peaks around 350 nm emission.1 The acetate spectra, generated as comparison spectra, displayed maximum peak fluorescence at lower emission wavelengths. The mention of “incorrect region classification” is misapplied to this publication given the numerous advantages of a qualitative comparison technique for oil and gas operations. The adoption and implementation of this type of analysis in the industry relies on a concise method that is both easy to perform and interpret. This qualitative analysis is not suggested as standalone identification for discrete compounds, but offers a highly effective method for identifying anthropogenic contamination.
he regional classification method developed to classify 3-D fluoresce spectra, referred to by Li et al. 2012 as the fluorescence regional integration (FRI) method, was criticized in response to the Environmental Science and Technology publication “Identifying Well Contamination through the use of 3-D Fluorescence Spectroscopy to Classify Coalbed Methane Produced Water”. Recent work by Li et al. (2012) offers an alternate approach to the FRI method focused on identifying multiexcitation-peak fluorophores in excitation− emission (EEM) spectra using a coupled instrument analysis technique of high performance liquid chromatography (HPLC) with fluorescence detector.1 This more complex analysis allows the method to distinguish between the multiexcitation-peak fluorophores semiquantitatively. The method is likely to be of interest to studies looking to specifically characterize peak components for semiquantitative multilayer peak comparisons. While the more complex analysis may provide more refined characterization, the ES&T publication demonstrates that wellestablished FRI method represents a quick and effective relative indicator for anthropogenic contamination in water sources associated with oil and gas operations. The following numeric responses address the critical flaws of the FRI interpretation mentioned in the comment response as they pertain to the application of this analysis technique to samples associated with water from the oil and gas industry: (1) Ease of use: Water quality analyses and information presented are limited in the oil and gas industry, which is dominated by the production of hydrocarbon for which water is merely a byproduct. The use of organic chemicals in well production and hydraulic fracturing operations has potential environmental risks if not managed appropriately. Methods for the identification of these compounds are generally limited to analyses for discrete compounds. The opportunity exists to use early detection techniques, such as qualitative analyses, to indicate anthropogenic contamination. The use of the traditional FRI method for this type of qualitative analysis has been widely recognized as a sufficient means of producing relative spectra for comparison purposes. The authors further simplify this technique using the overall fluorescence intensity (OFI) recognizing that anthropogenic contamination causes image flooding and can be identified immediately without considering regional integration methods.2 Use of the OFI may also extend to two-dimensional hand-held fluorescence meters for use in field applications for instantaneous recognition. The ease of use of the OFI and/or FRI is a preferred method to the complex analytical instrumentation and data analysis techniques proposed as alternative by Li et al. (2012). (2) Natural signatures: The depositional environment associated with coal bed methane (CBM) production offers a complex naturally occurring organic matrix that fluoresces with unique peak locations depending on the pathway of methanogenesis.2 Given the lack of discrete organic matter information © 2012 American Chemical Society
Katharine G. Dahm†,‡ Colette M. Van Straaten† Junko Munakata-Marr†
Published: December 28, 2012 1772
dx.doi.org/10.1021/es3052735 | Environ. Sci. Technol. 2013, 47, 1772−1773
Environmental Science & Technology
Correspondence/Rebuttal
Jörg E. Drewes*,† †
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Department of Civil and Environmental Engineering, Colorado School of Mines, 1500 Illinois Street, Golden, Colorado 80401-1887, United States ‡ U.S. Bureau of Reclamation, Denver, Colorado 80225-0007, United States
AUTHOR INFORMATION
Corresponding Author
*Phone: +1 (303) 273 − 3401; e-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Li, W.-T.; Xu, Z.-X.; Li, A.-M.; Zhou, Q.; Wang, J.-N. HPLC/ HPSEC-FLD with multi-excitation/emission scan for EEM interpretation and DOM analysis. Water Res. 2012, DOI: 10.1016/ j.watres.2012.11.040. (2) Dahm, K. G.; Van Straaten, C. M.; Munakata-Marr, J.; Drewes, J. E. Identifying well contamination through the use of 3-D fluorescence spectroscopy to classify coalbed methane produced water. Environ. Sci. Technol. 2012, DOI: 10.1021/es303866k. (3) Chen, J.; LeBoeuf, E. J.; Dai, S.; Gu, B. Fluorescence spectroscopic studies of natural organic matter fractions. Chemosphere 2003, 50 (5), 639−647. (4) Li, W.-T.; Xu, Z.-X.; Li, A.-M. Comment on “Identifying well contamination through the use of 3-d fluorescence spectroscopy to classify coalbed methane produced water”. Environ. Sci. Technol. 2012, DOI: 10.1021/es3032735. (5) Gallagher, L. In Biogenic methane from coal: The oxidation factor; Secondary Biogenic Coal Bed Natural Gas International Conference, Laramie, Wyoming, 2012. (6) Harris, S. H.; Smith, R. L.; Barker, C. E. Microbial and chemical factors influencing methane production in laboratory incubations of low-rank subsurface coals. Int. J. Coal Geol. 2008, 76 (1−2), 46−51. (7) Orem, W. H.; Voytek, M. A.; Jones, E. J.; Lerch, H. E.; Bates, A. L.; Corum, M. D.; Warwick, P. D.; Clark, A. C. Organic intermediates in the anaerobic biodegradation of coal to methane under laboratory conditions. Org. Geochem. 2010, 41 (9), 997−1000. (8) Lakowicz, J. R., Principles of Fluorescence Spectroscopy; Springer: London, Limited, 2009.
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