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

Introduction Groundwater samples from petroleum release sites are frequently analyzed for bulk extractable total petroleum hydrocarbons (TPH) quantified as TPHd/DRO 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/DRO 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/DRO 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/DRO measurement unless a silica gel cleanup (SGC) 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/DRO. Currently, the SGC method for extracts to be analyzed for TPHd/DRO 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 SGC” 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 SGC 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 SGC itself, and compares results from two different but commonly used SGC methods.