In the United States, contamination by petroleum hydrocarbons in soil is regulated at the state and/or federal level. A common metric for regulation is the concentration of total petroleum hydrocarbons (TPH) using EPA Method 8015. Method 8015 entails solvent extraction of petroleum hydrocarbons from the soil matrix and analysis using gas chromatography (GC) with flame ionization detection (FID). An important limitation of the TPH GC-FID analytical method is its lack of specificity for petroleum hydrocarbons leading to detection of soil compounds that are neither of petroleum origin nor composed of only hydrogen and carbon. As such, various components of soil organic matter (SOM) which are extractable by dichloromethane or n-pentane are conceivably quantified as TPH. The type of SOM compounds contributing to an apparent TPH concentration or the magnitude of the concentration in various soil types has yet to be not documented. In this work, assembled a library of 25 soils unimpacted by petroleum hydrocarbons and subjected them to solvent extraction and analysis by GC-FID according to Method 8015. We showed that all uncontaminated soils showed detectable concentrations of TPH when extracted with dichloromethane or n-pentane. The TPH C₉-C₄₀ concentration in the examined soils ranged from 150 mg kg^–1 to 3300 mg kg^–1. A high concentration of organic carbon and a low pH in the soils were positively correlated with the concentration of TPH. GC-MS identification revealed that all unimpacted soils contained hydrocarbons, including methylated- and/or ethylated-benzenes, straight-chain, branched-chain, and cyclic alkanes, but also a variety of non-hydrocarbons, including long-chain fatty aldehydes, phytosterols, long- or very long-chain primary fatty alcohols, and unknown compounds. Most of the compounds that were identified, including hydrocarbons, have a biological origin/function in plants, animals, and microorganisms. The measured concentrations of TPH originating from SOM in our soil library are higher than many action levels or clean-up levels in soils in US states. Data from this work can better inform the TPH regulatory landscape.

Rachel Mohler

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