TPH is defined as the measurable amount of petroleum-based hydrocarbon in an environmental media, and it is dependent on analysis of the medium in which it is found TPHCWG 1997b. TPH is a gross quantification of the petroleum mixture without identification of individual constituents and can be measured using one of several analytical methods. Depending on the analytical method used, TPH measurements may not always represent petroleum hydrocarbons but rather a broader total organics measurement. The measured TPH value represents a mixture; however, the analytical method may not limit the quantification solely to hydrocarbons (i.e., compounds containing only hydrogen and carbon). The resulting concentration can include nonhydrocarbons, such as naturally occurring organic matter, chemicals, and metabolites (i.e., degradation byproducts). Understanding the physical and chemical properties of petroleum hydrocarbons as well their behavior in the environment is essential in the development of risk-based approaches.

As illustrated in the gas chromatograms presented in Figure 2-1, the distributions of hydrocarbons are quite different for different products. A more in-depth discussion on petroleum chemistry is provided in TPH Fundamentals.

Figure 2‑1. Example analytical chromatograms for petroleum mixtures and petroleum products.
(Source: Texas Commission on Environmental Quality,2010.)

There are refined analytical methods for TPH that provide an improved approach for identifying and characterizing the extent of petroleum releases. However, the indicator compounds (e.g., BTEXN) can be missed due to high detection limits in highly contaminated media. The absence of BTEXN is often misinterpreted as the absence of contamination and lack of any risk.

Some regulatory programs link the bulk TPH concentrations to the presence of light nonaqueous phase liquids (LNAPL). LNAPL is present when TPH concentrations in soil are sufficient for LNAPL to occur in the pore space or when dissolved-phase concentrations exceed solubility limits considering the petroleum hydrocarbon mixture. Discussion of risks associated with LNAPL is not included in this document. Details regarding risk-based decision making at LNAPL sites are available in ITRC LNAPL-3 2018.

The thousands of compounds associated with TPH- and petroleum-related degradation compounds span a wide range of chemical, physical, and toxicological properties. Because of this complexity, a variety of laboratory analytical methods to quantify TPH concentrations in different environmental media have been used historically and/or are available today (see Conceptual Site Models), and no single method encompasses all components and breakdown products of TPH. Bulk analysis involves reporting petroleum hydrocarbon concentrations in aggregate, as a single, total number (e.g., TPH, gasoline range TPH, total volatile hydrocarbons, etc.). These data may also be reported by groups with similar volatility (i.e., equivalent carbon number). Individual compounds known to be included in TPH may be measured and used to represent TPH as indicator compounds. Fractionation methods separate petroleum compounds into two broad classes, aliphatics and aromatics, based on distinct chemical behavior. Note, however, that not all petroleum compounds or metabolites may be quantified in a particular TPH laboratory analysis (see Definition of TPH Fact Sheet and TPH Analytical Methods Fact Sheet).

Different methods can produce different results for the same sample Zemo 2016. In addition, petroleum-related degradation compounds (which are not strictly considered to be hydrocarbons, because these compounds contain oxygen) will often be reported in combination with parent petroleum compounds in TPH test methods that do not include removal of polar compounds, a step referred to as silica gel cleanup. This step may or may not be acceptable, depending on the data quality objectives of the investigation and state regulatory requirements. Some regulatory agencies assume for initial screening purposes that petroleum-related degradation products have a similar toxicity as the parent compounds and require data for samples both with and without degradation compounds removed as part of a risk assessment CASWB-SFBR 2016a, 2016b; HIDOH 2017; WADOE 2016a, 2016b (see Human Health Risk Assessment or Ecological Risk Assessment).

Because TPH data are dependent on the analytical method used for a particular sample, use of historical TPH measurements for risk characterization may require special consideration and interpretation (see Case Studies). Additionally, toxicity studies for TPH mixtures have not gone through the same level of peer review as the indicator compounds that are considered at petroleum release sites, so this will increase the uncertainty of the human health and ecological risk characterization estimates.

The consideration of TPH and other petroleum-related compounds (e.g., metabolites) that are not addressed individually in a risk assessment is important. Evaluation of the composition of petroleum products, including gasoline, crude oil, jet fuels, kerosene, diesel, home heating oils, and lubricating oils, identified up to 250 constituents with available physical properties TPHCWG 1998b. Hydrocarbons can degrade in the subsurface where microbes and sufficient moisture content are present. Degradation of parent compounds adds to the complexity of an ever-changing mixture of compounds. Obtaining analytical data for each of these individual constituents and/or degradation components is unrealistic and cost prohibitive. In addition, the development of theoretical data requires a series of broad assumptions. Furthermore, the toxicity data and fate and transport modeling parameters required for the assessment of human and ecological health risk are currently not available for most of these individual compounds.

The most common approaches used to address human health and ecological risks posed by these compounds include

• Evaluation of whole products
• Reliance on a short list of targeted indicator compounds
• Use of carbon range fractions

(See Regulatory Framework, Human Health Risk, and Ecological Risk Assessment for more information).

Additionally, ecological risks may include consideration of other compositional aspects of the hydrocarbon mixture (e.g., low molecular weight versus high molecular weight fractions). Some combination of these approaches may be used for TPH risk characterization. Ecological risks may also be evaluated using site-specific toxicity testing approaches using the above-listed analyte definitions or by using operationally defined fractions, such as water-soluble fractions (WSFs) or water-accommodated fractions (WAFs).

Comparison of whole product TPH concentration data (e.g., TPH as gasoline or TPH as diesel) to risk-based screening levels based on assumed carbon range compositions of fuels is used by some states to directly assess risk to human health and the environment ITRC 2015; HIDOH 2017. Use of whole product data may be appropriate for petroleum releases from known sources (for example, tank failures onto secondary containment, fuel releases from pipelines and trucks, etc.). Alternatively, the toxicities of well-studied indicator compounds such as BTEXN represent the toxicity of the petroleum mixture as a whole and have been used to develop screening levels for TPH.

Under the second approach, data for individually targeted indicator compounds are used to evaluate the risk posed by complex mixtures, even though they might make up a small fraction of the total compounds present. If calculation of risk or comparison to screening levels specific to these chemicals suggested minimal risks, then no further action was required. This led in some cases to a premature and potentially erroneous conclusion that the risk posed by the “TPH” component of the contamination and associated degradation products will be minimal if the risk posed by individual indicator compounds falls within acceptable levels. Additional details on these items are presented in the discussions on toxicity assessment, ecological risk assessment, and risk calculators.

The fraction approach involves resolving petroleum constituents into fractions based on chemical properties and then assigning representative toxicity criteria to each fraction. This allows screening levels to be developed or risk directly calculated for individual carbon ranges in the same manner as is done for individual compounds. Several regulatory agencies (see States Survey) have published guidance that employs fractions along with indicator compounds for assessment of potential contaminated sites MADEP 2002b; WADOE 2016a; CASWB-SFBR 2016a; HIDOH 2017; TCEQ 2017a, 2017b.

The Conceptual Site Models and Investigative Strategies section compares the different methods currently approved for use by the state regulatory agencies. The TPH Criteria Working Group (TPHCWG) guidance advocated the use of benzene and carcinogenic polycyclic aromatic hydrocarbons (PAHs) as indicator compounds to evaluate human health cancer risk at such sites and assessment of 13 TPH fractions for evaluation of noncancer hazards TPHCWG 1997a. Evaluation of ecological risk would not involve consideration of cancer end points.

Quantification of TPH in contaminated media is based on the analytical method that is used. Selection of analytical methods for TPH characterization is important because different analytical methods may yield entirely different results for a given sample. Common analytical techniques include gas chromatography (GC), infrared spectrometry, gravimetry, immunoassay, and liquid chromatography. Similarities and differences between these methods are discussed in Conceptual Site Models and Investigative Strategies. Most common analytical methods include the following steps, and each influences the identification of TPH constituents and compositional data reporting:

1) Collection and preservation for applicable media (soil, water, vapor, etc.)

2) Extraction techniques (methylene chloride, etc.)

3) Adjustment for compound interferences

4) Adjustment for concentration dilutions

5) Quantification of analytes

The same concentration of TPH in different areas of a site might include dissimilar fractions, which, in turn, may represent different risks to human or ecological health and the environment. For example, samples from two different sites may have similar TPH concentrations; however, the composition and weathering of the petroleum hydrocarbons may be significantly different (Figure 2-3).

Figure 2‑3. TPH composition changes.

The risk at a specific site will change with time due to weathering/degradation. Some states recommend the collection of carbon range data as part of the site investigation and assessment of risk MADEP 2002a; ORDEQ 2011; WADOE 2016a; TCEQ 2017a, 2017b. In practice, some combination of both approaches is often used, with site-specific carbon range data being used to develop more site-specific screening levels and bulk TPH data being used to identify areas of potential concern and develop remediation plans (see States Survey). For example, bulk analyses may be used for scoping mass removal/reduction cleanups, but fractionated data are more appropriate for risk evaluations. However, bulk TPH data may be acceptable for cases of fresh spills of known origins. Examples of both approaches are presented in the Case Studies.

• Analytical characterization—fractions based on distinguishing aromatic and aliphatic components as well as molecular weight
• Fate and transport parameters—representative chemical and physical properties were selected for different hydrocarbon fractions based on surrogate chemicals for each fraction
• Toxicity characterization—representative toxicity factors were selected for different hydrocarbon fractions based on surrogate chemicals for each fraction. For risk characterization based on surrogate chemicals for each fraction, a hazard quotient (HQ) was calculated for each TPH fraction and a TPH Hazard Index was calculated as the sum of the individual HQs.

Details of the fraction approach are provided in Conceptual Site Models and Investigative Strategies, Human Health Risk Assessment, and Risk Calculators.

A survey has been conducted to identify the range of regulatory approaches used across the United States to evaluate TPH risk at petroleum release sites. This survey identified the range of regulatory approaches used across the United States. The findings of the States Survey are summarized in Appendix C.

The decision framework identifies critical activities that are conducted in a stepwise fashion and others that are considered throughout multiple steps in the TPH risk characterization process. Activities that are considered throughout the process include:

• Regulatory Framework
• TPH Fundamentals (including chemical and physical properties)
• Updating the conceptual site model, in an iterative process, as data are collected or risk characterization assessments are completed
• Stakeholder Concerns

The stepwise process consists of the following actions:

1. If emergency response activities are warranted for a site, then these should be implemented prior to data collection for assessment of chronic risks/hazards and corrective action planning.
2. A preliminary conceptual site model (CSM) should be developed based on an initial survey of the site. This CSM will be used to develop the site investigation strategy and should be updated throughout the remediation, risk evaluation, and closure life cycle stages.
3. The site investigation should be conducted to assess the vertical and horizontal distribution of petroleum-related contaminants at the site and collect data necessary for risk characterization. However, in some cases, data may have already been collected. Multiple phases of environmental investigation may be necessary to collect data of known quality needed for risk characterization and corrective action planning.
4. The risk characterization step consists of an assessment of human health risks, ecological risks, and other factors considered in risk-based decision making (i.e., regulatory requirements, aesthetic criteria, or safety assessment). Various tools (i.e., calculators) are available that can quantify site-specific TPH risk calculations that may be used in the risk characterization process.
5. The risk characterization results are used to assess risk management and corrective action needs (e.g., remedial actions, engineering/institutional controls) or if site closure is appropriate.

The corrective action planning that may entail remediation and/or engineering/institutional controls should be implemented, as appropriate. Note that these aspects are beyond the scope of this guidance; the reader is directed to other ITRC or state guidance for reference.