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�•►' \.MWI GEOMATRIX Mr. Don O. Culbertson Chevron Pipe Line Company 8 March 1995 Page 4 Analysis of groundwater samples by "total petroleum" methods (EPA Method 8015 and EPA Method 418.1) As we have shown, the detectable WSF of fresh PHCs is limited to the alkanes C4 - C6 and the aromatics with 14 carbons or fewer. These are discrete compounds that are best analyzed for by GC or GC/MS methods such as EPA Method 8020, 8240, 8270, 8310, etc. However, we are routinely required to include analysis of "TPH" by EPA Method 8015 (as modified) or "TRPH" by EPA Method 418.1 in our analytical suite for groundwater samples. EPA Method 8015 is a GC method, but is aggregate in quantitation approach because it is based on the amount of hydrocarbons that elute within a specified range of carbons, not discrete peaks. The importance of this is that the TPH quantitation and subsequent report from the commercial laboratory tells us nothing about which constituents (alkanes, alkenes, aromatics) are present in the sample. Furthermore, hydrocarbons that are not PHCs (such as soluble oxidized biodegradation products, e.g., alcohols, or biogenic materials, e.g., lipids) also get incorporated into the TPH quantitation if they elute within the specified range of carbons (Zemo, et. al., 1995).- Biodegradation is an important natural process at most sites where petroleum has been released to the environment and it is reasonable to expect that samples may include the byproducts of biodegradation. We know from the preceding section that the only PHC constituents that should be measurable in the dissolved-phase TPH analysis are the alkanes (C4 - C6) and the aromatics (C6 C14). If peaks are present other than these, it means one of two things: 1) the sample contains non-dissolved PHC constituents also and/or 2) the sample contains soluble non-PHCs (such as biogenic materials or biodegradation products, etc). This lack of resolution is what makes EPA Method 8015 analyses problematic for assessment of groundwater quality, assuming that we are interested in the dissolved, therefore mobile, PHC constituents. This dilemma is compounded by the fact that most commercial laboratories do not routinely interpret or provide the GC trace from water samples analyzed by EPA Method 8015. Method 418.1 (5520C,F), known as total recoverable petroleum hydrocarbons (TRPH), measures the methylene group carbon-hydrogen bonds in a sample by infrared (IR) spectroscopy. The sample is extracted by Freon 113 and the extract is treated by a silica gel cleanup to remove biogenic materials that are a source of false positives. The remaining mass is subjected to IR light which is absorbed by carbon- hydrogen bonds at a selected wavelength. A detector measures the amount of energy lost as the IR light passes through the sample. The quantitation is performed by comparing the reduced amount of light at the selected wavelength to a reference standard. Besides incomplete removal of biogenic material by the silica gel, another