Comparison of Shake and Column Silica Gel Cleanup Methods for Groundwater Extracts to Be Analyzed for TPHd/DRO

Transcription

1 Technical Note Comparison of Shake and Column Silica Gel Cleanup Methods for Groundwater Extracts to Be Analyzed for TPHd/ by Dawn A. Zemo, Karen A. Synowiec, Renae I. Magaw, and Rachel E. Mohler Introduction Groundwater samples from petroleum release sites are frequently analyzed for bulk extractable total petroleum hydrocarbons (TPH) quantified as TPHd/ using EPA Method 8015B/C (GC-FID) or equivalent. Prior to the quantitative analysis, the groundwater samples are extracted with organic solvents and the extraction step is commonly performed following EPA Method 3510 with methylene chloride (DCM) or DCM mixtures with other solvents. The purpose of the TPHd/ analysis is to quantify the dissolved hydrocarbons in the groundwater sample. However, neither the extraction method, nor the quantification method (Method 8015), is specific to hydrocarbons (compounds containing only carbon and hydrogen [Hart 1991], and thus are nonpolar in molecular structure), and nonhydrocarbons (compounds containing oxygen, nitrogen, sulfur or other atoms in addition to carbon and hydrogen, and thus are polar in molecular structure) can be extracted from the sample and quantified as TPHd/ when these techniques are used. Polar nonhydrocarbons (polars) can occur in groundwater at petroleum fuel release sites for several reasons including (1) natural organics unrelated to the petroleum, (2) oxygenated metabolites from biodegradation of the petroleum (e.g., organic acids, alcohols, ketones, aldehydes, and phenols), (3) laboratory or sampling equipment artifacts (e.g., phthalates), and (4) nonpetroleum-related chemicals (e.g., pesticides, chlorinated solvents, etc.) (Zemo et al. 1995, 2013; Zemo and Foote 2003). Polars can comprise a large percentage of the dissolved organics in groundwater at biodegrading petroleum fuel release sites (Zemo and Foote 2003; Lang et al. 2009; Zemo et al. 2013). The polars are not hydrocarbons, but will be included in a TPHd/ 2013 The Authors. Groundwater Monitoring & Remediation published by Wiley Periodicals, Inc. on behalf of National Ground Water Association. doi: /gwmr This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made. measurement unless a silica gel cleanup () is used to separate hydrocarbons and polars in the sample extract prior to quantification. Silica gel is used for the separation because silica gel is itself a polar material that will adsorb polars present in the sample. Hydrocarbons, which are nonpolar in molecular structure, are not adsorbed by the silica gel (as verified by the internal surrogate) and will be appropriately quantified as TPHd/. Currently, the method for extracts to be analyzed for TPHd/ is not standardized and, as shown in this study, different methods used by laboratories are not comparable in effectiveness. This means that results reported as TPH with may or may not reflect adequate cleanup, which can lead to inaccurate reporting and understanding of the levels of dissolved hydrocarbons and polars in groundwater samples. This was noted by Zemo and Foote (2003), but data from side-by-side comparisons of different methods was not provided in that article. This article is provided as a technical note contribution because the larger issue of polar compounds in groundwater at petroleum release sites has been published elsewhere (Zemo and Foote 2003; Lundegard and Sweeney 2004; Lang et al. 2009; Zemo et al. 2013). This article focuses only on the itself, and compares results from two different but commonly used methods. Background on Methods for Extracts to be Analyzed for Hydrocarbons Historically, was an integral part of EPA Method ( total recoverable petroleum hydrocarbons [TRPH]); the differentiated TRPH from total oil and grease (TOG; EPA Method 413). Currently, is also an integral part of EPA Method 1664A, which uses n-hexane for extraction and replaced Methods 413/ The step in Methods and 1664A is one in which a certain amount of silica gel is added to the sample extract in the vial and the mixture is shaken or stirred to remove the polars. The cleaned up extract is then analyzed. This method is referred to in this article as the Shake method. Laboratories have long used the Shake method, and some laboratories use this technique for sample extracts to be analyzed for TPHd/. This technique was assumed 108 Groundwater Monitoring & Remediation 33, no. 4/ Fall 2013/pages NGWA.org

2 to produce effective separations, based on the assumption made by EPA that 3 g of silica gel would adsorb 100 mg of polar material in the extract. The Shake method is performed quickly, and uses little silica gel. A column based on EPA Method 3630C has been used for sample extracts to be analyzed for TPHd/ since the mid-1990s, and has been used by petroleum geochemists for decades to obtain compound class information on oil extracts. This method uses a glass column filled with 10 g of silica gel. Briefly, the sample extract and quality control surrogates are placed on the column, the column is eluted with an appropriate rinse solvent, and the cleaned up extract is re-concentrated and analyzed. This method, referred to in this article as the Column method, takes longer to perform, requires technician training, and uses more materials (glassware, silica gel, and solvents); however, it does allow for quality control measures to be taken to ensure that polars are removed and that hydrocarbons are not removed from the sample extract. It is noted that Method 3630C was not originally written for extracts to be analyzed for TPHd/, but the method does allow for its use with these and other analytes as long as appropriate quality assurance requirements are met. When used for TPHd/, the elution solvents and volumes must be adjusted so that the recovery of total hydrocarbons is acceptable. Study Methods Groundwater samples were collected during routine monitoring events from monitoring wells at four different sites, all with biodegrading petroleum sources. Polar compounds, most likely biodegradation metabolites in these cases, were known to be present in groundwater at the sampled locations based on previous monitoring events that included TPHd/ without and with. A single commercial laboratory performed all the groundwater sample extractions using DCM and Method 3510, and yielding a 1 ml extract from a 1 L field groundwater sample. Shake and Column methods were performed on duplicate extracts, and all extracts (including one with no and one each with Shake and/or Column method) were analyzed for TPHd/ using Method Both methods used 100 to 200 mesh silica gel that had been activated by heating to 130 C for at least 16 h. The Shake method used 1 g silica gel added to the extract vial, and the Column method used 10 g silica gel packed into a 300 mm glass column. The Column method used a pentane pre-wash and 25 ml of a 2:3 DCM:pentane elution solvent mixture. For three of the four sites, the Column method incorporated 500 µg of capric acid (a polar compound) as a reverse surrogate to evaluate effectiveness of the removal of polars from the extract. Recovery of the Method 8015 recovery surrogate OTP (for all samples) and the diesel spike (for the Column method laboratory control samples) were within the laboratory s acceptable control range for all samples, confirming that hydrocarbons were not removed by the. Capric acid recoveries for the Column method indicated acceptable removal of polars from the extract (recovery of 0% to 1%) for each sample, except for at Site 4 where recoveries were greater than 1%. At the time of this study, the Shake method was the routine method for extracts to be analyzed for TPHd/ at this commercial laboratory. Results and Discussion Detailed results from each of the four sites are provided on Tables 1 through 4. The study results are summarized on Table 5 and Figure 1. The TPHd/ concentrations for sample extracts without ranged from 15,000 µg/l to 150 µg/l. The percentage reduction in TPHd/ concentrations for duplicate sample extracts with was calculated for each sample pair. Results for each site demonstrate that the Column method is much more effective at separating polars from hydrocarbons than the Shake method for these DCM extracts. The average percent reduction in TPHd/ concentrations with the Shake method ranged from 27% to 62%, and averaged 46% for all four sites combined (Table 5 and Figure 1). The average percent reduction in TPHd/ concentrations with the Column method ranged from 88% to 96%, and averaged 91% for all four sites combined (Table 5 and Figure 1). In practical terms, these results show that if a Shake method has been used on a DCM extract, it is very likely that polars remained in the sample extract and were included in the reported TPHd/ concentrations. These results also show that 1 g of silica gel did not adsorb even a few hundred µg/l of polars in most cases, so the adsorption ratio/capacity EPA assumed for Methods and 1664 was not appropriate for these DCM extracts of groundwater samples. Table 1 Results for Site 1 with Shake % Reduction with Shake with Column Column < < < < Average Notes: concentrations in µg/l; range C10 to C28. Shake performed on samples collected 3Q2010, column performed on samples collected 3Q2011. <100 is not detected at a reporting limit of 100 µg/l. Reduction calculated using ½ RL for nondetect samples. NGWA.org D.A. Zemo et al./ Groundwater Monitoring & Remediation 33, no. 4:

3 Table 2 Results for Site 2 with Shake Shake with Column Column MW-28A <50 93 MW-29A <50 94 MW-30A <50 96 MW-31A <50 94 MW-32A <50 97 MW-51A <50 93 MW-65B <50 97 MW-73UA 190 <50 87 <50 87 MW-76UA <50 83 MW-76A <50 93 MW-76B <50 88 MW-77A <50 86 MW-77B <50 92 MW-78UA 170 <50 85 <50 85 MW-78A <50 93 MW-78B <50 91 Average Notes: concentrations in µg/l; range C10 to C28. s performed on samples collected 4Q2010 or 1Q2011. <50 is not detected at a reporting limit of 50 µg/l. Reduction calculated using ½ RL for nondetect samples. Table 3 Results for Site 3 with Shake Shake with Column Column MW-1-A MW-1-B 1,800 1, MW-2-A 11,000 8, MW-2-B 7,300 5, MW-3-A 11,000 7, MW-3-B 14,000 11, Average Notes: concentrations in µg/l; range C10 to C28. s performed on samples collected 2Q2011. Table 4 Results for Site 4 with Shake Shake with Column Column MW-21 15,000 6, nc 90 MW-22 9,400 5, nc 87 MW-5 1, nc 91 Average Notes: concentrations in µg/l; range C10 to C28. Shake performed on samples collected 1Q2011 except MW-22 collected 1Q2010; Column performed on samples collected 2Q2011. nc = was not complete based on either reverse surrogate (capric acid) recovery or review of the chromatograms. On the basis of the review of the capric acid recoveries or TPHd/ chromatogram patterns, Site 4 samples had incomplete even with the Column method. s can be incomplete for several reasons including (1) low polarity of the polars present, (2) the mass of the polars present exceeds the adsorptive capacity of the silica gel and causes breakthrough (this can be assessed using a polar reverse surrogate such as capric acid), and/or (3) incomplete 110 D.A. Zemo et al./ Groundwater Monitoring & Remediation 33, no. 4: NGWA.org

4 Figure 1. Summary of silica gel cleanup () method comparison study results. Table 5 Summary of Study Results Average % Reduction in with Shake Average % Reduction in with Column Site Site Site Site Average for study activation of the silica gel. Silica gel or portions of the cleanup apparatus can also be contaminated and therefore yield unreliable data. The data user must check with the laboratory for these potential problems. The study results also show that about 90% to 100% of the dissolved organics in groundwater at these wells reported as TPHd/ (without ) were polar compounds, and were not dissolved hydrocarbons. This finding is consistent with results from other studies (Zemo and Foote 2003; Lundegard and Sweeney 2004; Lang et al. 2009; Zemo et al. 2013). Conclusions This study provides side-by-side comparison of Shake and Column methods for 1-L groundwater samples collected from four sites and demonstrates that a Column method is much more effective than a Shake method in separating polars from hydrocarbons in DCM extracts to be analyzed for TPHd/. A Column method is recommended for DCM extracts in the 2012 California State Water Resources Control Board LUFT Manual (2012). Consultants and regulators should understand which method is being used on samples at a given site so that data interpretation is not confounded by poor separation of polars and hydrocarbons. Acknowledgments The authors thank Drew Noel and Mike Bauer, both with Chevron Environmental Management Company, for their support in obtaining and analyzing samples. Financial support for this work was provided by Chevron Energy Technology Company via the Remediation Technology Development Initiative. The authors also thank anonymous reviewers for their constructive comments on this manuscript. References Calif o rnia State Water Resources Control Board Leaking Underground Fuel Tank Guidance Manual. Sacramento, California. Hart, H Organic Chemistry: A Short Course, 8th ed., Boston: Houghton Mifflin. Lang, D.A., T.P. Bastow, B.G.K. van Aarssen, B. Warton, G.B. Davis, and C.D. Johnson Polar compounds from the dissolution of weathered diesel. Ground Water Monitoring and Remediation 29, Lundeg ard, P.D. and R.E. Sweeney Total petroleum hydrocarbons in groundwater Evaluation of nondissolved and nonhydrocarbon fractions. Environmental Forensics 5, Zemo, D.A., J.E. Bruya, and T.E. Graf The application of petroleum hydrocarbon fingerprint characterization in site investigation and remediation. Ground Water Monitoring and Remediation. 15, Zemo, D.A. and G.R. Foote The technical case for eliminating the use of the TPH analysis in assessing and regulating dissolved petroleum hydrocarbons in ground water. Ground Water Monitoring and Remediation Zemo, D.A., K.T. O Reilly, R.E. Mohler, A.K. Tiwary, R.I. Magaw, and K. A. Synowiec Nature and estimated human NGWA.org D.A. Zemo et al./ Groundwater Monitoring & Remediation 33, no. 4:

5 toxicity of polar metabolite mixtures in groundwater quantified as TPHd/ at biodegrading fuel release sites. Ground Water Monitoring and Remediation. DOI: /gwmr Biographical Sketches Dawn A. Zemo, corresponding author, is Principal Hydrogeologist at Zemo & Associates, Inc. She has a M.S. in Geology from Vanderbilt University. She is a Professional Geologist (PG) and a Certified Engineering Geologist (CEG) in California. She can be reached at Zemo & Associates, Inc., 986 Wander Way, Incline Village, NV 89451; ; dazemo@ zemoassociates.com Karen A. Synowiec is a Sr. Staff Hydrogeologist at Chevron Energy Technology Company (CETC). She has a M.S. in Geology from University of Wisconsin-Milwaukee. She can be reached at CETC, 6001 Bollinger Canyon Road, San Ramon, CA 94583; ; kasy@chevron.com Renae I. Magaw is a Sr. Staff Toxicologist at CETC. She has a Masters in Public Health (M.P.H.) from University of California -Berkeley. She can be reached at CETC, 6001 Bollinger Canyon Road, San Ramon, CA 94583; ; rmagaw@chevron.com Rachel E. Mohler is a Lead Research Scientist at CETC. She has a PhD. in Analytical Chemistry from University of Washington. She can be reached at CETC; 100 Chevron Way, Richmond, CA 94802; ; rmohler@chevron.com ENABLE DISCOVERY WILEY ONLINE LIBRARY Access this journal and thousands of other essential resources. Featuring a clean and easy-to-use interface, this online service delivers intuitive navigation, enhanced discoverability, expanded functionalities, and a range of personalization and alerting options. Sign up for content alerts and RSS feeds, access full-text, learn more about the journal, find related content, export citations, and click through to references. wileyonlinelibrary.com 112 D.A. Zemo et al./ Groundwater Monitoring & Remediation 33, no. 4: NGWA.org