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Abstract

In many states, the total petroleum hydrocarbons (TPH) analysis based on gas chromatography-flame ionization detection (GC-FID) is still being used to assess and regulate ground water quality at petroleum release sites. The soluble fraction of fresh crude oil or fresh products that could potentially be dissolved into the ground water is limited to relatively few petroleum hydrocarbon constituents (primarily the C6 to C14 aromatics). Research by numerous investigators has shown that the reported TPH concentrations of ground water samples frequently do not represent dissolved petroleum hydrocarbons but rather represent nondissolved petroleum or polar nonhydrocarbon compounds. Nondissolved petroleum is frequently entrained within a sample when sampling ground water within affected soil, and polar nonhydrocarbons are present in ground water as a result of petroleum biodegradation or other factors. These constituents are being measured because the TPH analysis does not include steps to remove nondissolved petroleum or a silica gel cleanup to remove polars.

Many states have regulatory action levels for TPH in ground water that are in the 0.1 to 2 mg/L range. The technical basis for these action levels appears to be toxicity and/or organoleptic (taste and odor) properties of the dissolved petroleum hydrocarbons associated with fresh oil or fresh products. The presence of compounds measured as TPH that are not dissolved petroleum hydrocarbons has proven problematic for regulatory decision-making because the comparison of concentration data to the regulatory criteria may not be correct. Because the TPH analysis does not reliably distinguish between dissolved petroleum hydrocarbons and other compounds, it should not be used to assess or regulate dissolved petroleum hydrocarbons, in ground water. We recommend that constituent-specific analyses be used to assess and regulate ground water quality at petroleum hydrocarbon release sites. If TPH must be used due to regulatory requirements, samples should be cleaned up with silica gel to remove polars (and turbidity should be removed, if present) prior to analysis so that comparison to regulatory action levels that are based on properties of dissolved petroleum hydrocarbon constituents is technically appropriate.