Attachment 6: The Total Petroleum Hydrocarbon Criteria Working Group (TPHCWG) Method for Determining Risk-Based TPH Soil Clean-up Levels

The Total Petroleum Hydrocarbon Criteria Working Group, TPHCWG, is a national consortium of representatives from government, industry, consulting, and academia to address the large disparity among TPH clean-up requirements being used by states at sites contaminated with hydrocarbon materials such as fuels, lubricating oils, and crude oils. The TPHCWG has formulated an approach for developing risk-based soil TPH clean-up levels protective of human health at petroleum hydrocarbon-contaminated sites. This attachment outlines the TPHCWG method and the potential benefits of using this method within the RTDF phytoremediation field demonstration program.


One of the problems associated with using traditional compound-specific risk assessments to determine clean-up levels is that petroleum hydrocarbons are composed of thousands of individual compounds. In order to include information which is representative of these compounds, the TPHCWG approach evaluates the risk to human health by examining subfractions of the hydrocarbon contained in soils contaminated with TPH. These hydrocarbon fractions were selected to have similar fate and transport characteristics (Figures 2 and 3).

Figure 2

Figure 3

Currently, the TPHCWG recognizes thirteen hydrocarbon fractions (seven aromatic and six aliphatic fractions) ranging from five carbons to thirty-five carbons. A recent PERF initiative (97-08) has examined the incorporation of carbon numbers from 36 to 60 into the TPHCWG methodology. These results will be made available in the near future. Toxicological characteristics were assigned to each fraction based on a review of available data for surrogate compounds or for mixtures representative of the fractions (Table 1).

Table 1. Fraction-Specific Reference Doses (RfDs)
Selected by the TPHCWG

Carbon Range Aromatic RfD
Aliphatic RfD
C6-C8 (Aliphatics)
C7-C8 (Aromatics)
0.20 - Oral
0.10 - Inhalation
5.0 - Oral
5.0 - Inhalation
0.04 - Oral
0.05 - Inhalation
0.1 - Oral
0.3 - Inhalation
0.03 - Oral 2.0 - Oral
>C34 20 - Oral

The TPHCWG approach is applied within the framework of the American Society for Testing and Materials' (ASTM) Risk-Based Corrective Action (RBCA). The RBCA framework integrates established risk and exposure assessment practices with remedial action selection. A tiered approach allows the decision maker to weigh the cost of additional site-specific evaluations against the benefits of less conservative clean-up criteria at each tier. Key elements of the RBCA framework are source characteristics, exposure pathways, and receptors. The first tier consists of a screening level evaluation, and results in the calculation of Risk-Based Screening Levels (RBSLs). RBSLs are defined by ASTM as risk-based, site-specific corrective action levels for chemicals of concern.

The TPHCWG approach to developing TPH RBSLs is a two-step process. The first step is to assess potential effects on human health from exposure to carcinogenic compounds in the soil (i.e., generally benzene or carcinogenic polycyclic aromatic hydrocarbons (PAHs)), since the presence of potential carcinogens in TPH typically drive clean-up efforts. If potential carcinogenic compounds are present above regulatory or risk-based levels, then these are used to develop clean-up levels. However, if potential carcinogenic compounds are not present, or are present below the regulatory or risk-based levels, then the second step is to combine fate and transport information with noncarcinogenic reference doses developed by the TPHCWG to establish RBSLs for individual hydrocarbon fractions. The individual fraction RBSLs are then combined with site-specific mixture composition data to calculate mixture-specific "whole" TPH RBSLs. In this way, the TPHCWG approach evaluates a chemical mixture comprised of different carbon fractions instead of evaluating a single chemical, and the resulting whole TPH RBSL is the risk-based clean-up level.

The State of Michigan has recently adopted this methodology to develop a TPH screening level of 10,000 ppm in soils. The State of Massachusetts has adopted a similar methodology for developing TPH risk-based screening levels.

Potential Benefits to the RTDF Phytoremediation -TPH Subgroup

The RTDF Phytoremediation - TPH Subgroup seeks to gather information from various field demonstrations which will serve as a database to determine the effectiveness of phytoremediation for meeting clean-up levels. The Craney Island study indicated that a decrease in TPH concentrations on the order of 50% over a two-year period may be achieved using phytoremediation. Assuming optimal conditions, this means that an initial soil TPH concentration of 2,500 mg/kg may be reduced to 1,250 mg/kg. If the regulatory TPH clean-up level is below this concentration, then phytoremediation will not have achieved its goal. In addition, the effectiveness of phytoremediation to meet clean-up goals will be even more tenuous at higher initial TPH concentrations. The potential benefits to the RTDF of using the TPHCWG method for determining risk-based TPH soil clean-up levels in combination with phytoremediation are:

Once a TPHCWG risk-based clean-up level is determined for a site, conventional TPH analyses can be used to determine the extent of contamination relative to the clean-up level at the same site. In addition, since the TPHCWG approach uses carbon fraction data that is specific to petroleum hydrocarbons, the TPH risk-based clean-up level will be more realistic than that derived from a small subset of compounds (i.e., BTEX and PAHs).

Preliminary Conceptual Site Model for the RTDF Field Demonstrations

The RBCA standard default depth of the surface soil is 100 cm (approximately 3 ft). Therefore, the use of the TPHCWG method would only be applicable to the surface soil, unless a deep rooted local species was used in the field demonstration. Under an industrial use scenario, the appropriate receptor for contamination in the surface soil is an industrial worker or trespasser. Under a residential use scenario, the appropriate receptor is an adult or child resident. The appropriate exposure pathways for these receptors are direct contact (i.e., ingestion, dermal absorption, or inhalation of volatiles or particulates). Exposure pathways associated with the subsurface soil (volatilization to enclosed spaces and leaching to groundwater) will only be affected if plant species are selected to impact this part of the soil.

Sampling and Analysis

In order to realize the benefits of the TPHCWG approach to developing risk-based cleanup levels, the soil sampled at the beginning of the demonstration (t=0) and at the end of the demonstration (t=3) should be analyzed using the TPH fractionation method. This method, called the Direct Analysis method, can be conducted on the same soils sampled under the RTDF Protocol. In addition, BTEX analysis should be conducted on all of the samples.

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