Written by Abimbola Baejo, Staff Reporter
This report is from a webinar conducted by the Interstate Technology and Regulatory Council (ITRC) Total Petroleum Hydrocarbon Risk Evaluation Team and the US EPA Clean up Information Network on the 19 of June 2019. https://tphrisk-1.itrcweb.org/
The webinar was made to facilitate better-informed decisions made by regulators, project managers, consultants, industries and stakeholders, on evaluating the risk of TPHs at petroleum contaminated sites.
What is TPH?
In environmental media, crude oil and individual refinery products are typically characterized as TPH. They are made up of hydrocarbons along with other elements such as nitrogen, oxygen, sulphur, inorganics and metals. The refining process generates various commercial products such as kerosene, diesel, gasoline; with over 2,000 petroleum products identified. These products are made up of various number of carbon atoms which may be in straight or branched chain forms.
TPHs can be found in familiar sites such refineries, air- and seaports, offshore sheens, terminals, service stations and oil storage areas. Hydrocarbons can be broadly classified into aliphatic (e.g. alkanes and alkenes) and aromatic (e.g. benzene and naphthalene) hydrocarbons.
For TPH assessment at contaminated sites, relevant properties to consider are water-solubility, polarity, boiling point and evaporation ranges. Aliphatic hydrocarbons are non-water soluble, non-polar, have lower boiling points and are more prone to evaporation compared to the aromatic hydrocarbons. At a typical petroleum contaminated site, substances such as fuel additives (such as oxygenates), naturally occurring hydrocarbon components, metabolites from degraded substances and individual petroleum constituents (such as BTEX).
TPHs are made up of various constituents with similar or different carbon atoms. This means that there is the challenge of analytically separating TPH constituents in a risk assessment context since hydrocarbon constituents from a specific range of carbon atoms could be a challenge, especially if they are diesel, jet fuel or petroleum. With this knowledge, one can conclude that bulk TPH analysis, though a good screening method, is not a suitable method for TPH risk evaluation. A good way of summarizing this is in shown below.
The same concentration of TPHs in different areas of a site might be composed of different products; which in turn, may present different risks to the ecological environment. Therefore, we can safely say that TPH is:
- a complex mixture with an approximate quantitative value representing the amount of petroleum mixture in the sample matrix
- is defined by the analytical measure used to measure it, which varies from one laboratory to another.
- is either made up of anthropogenic products freshly released into the environment (or weathered) or natural products from ecological activities
- not totally of petroleum origin and may simply be detected by the analytical method used.
This definition then enhances the challenges faced with TPH risk assessing such as dealing with continual changes in TPH composition due to weathering brought on by site-specific conditions, trying to analyze for hundreds of individual constituents in the mixture and having limited data on the toxicological effects of the various constituents.
To overcome the challenge of drawing erroneous conclusions about a contaminated site therefore, the project manager should not focus only on TPH individual constituents when making remedial decisions, which mostly degrade before the toxic fractions do, but should collect samples for both fractions and individual constituents. A detailed Conceptual Site Model (CSM) is suggested as a good guide in assessing TPH risks as it shows where the the remediation focus should be, away from human exposure routes; and periodic revision of this CSM will assist in documenting contaminant plume changes and identifying areas with residual contamination.
Due to the complexity of TPH mixtures, analytical methods should be selected based on the data quality objective, application of the results (whether to delineate a contaminated area or to conduct a risk assessment), the regulatory requirements, the petroleum type and the media/matrix being tested. As long as the method is fit for its purpose and cost effective. TPH mixtures require separation and most laboratories use GC as a preferred method as it separates I the gas phase based on its volatility. Since it is difficult to evaluate risk for a TPH mixture, most methods suggest separation into fractions. Guidelines are usually provided on what methods suit a purpose best by governmental records but if such records are inaccessible, getting information from seasoned chemists is the best option.
Prior to TPH mixture separation, removing method interferences, such as non-petroleum hydrocarbons, is ideal for more accurate results. US EPA method 3630C describes the use of silica gel to remove polar, non-PH and naturally occurring compounds from the analysis. This gel cleanup leaves only the hydrocarbons in the sample which is the analyzed for bulk TPH. The silica gel used is a finer version of the common ones found in clothing accessories and using it in a gel column setup is most effective at removing non-hydrocarbons. Quality controls using laboratory surrogates is also advised. Cleaning up prior to bulk TPH analysis is ideal in determining the extent of hydrocarbon impact, biodegradation locations and knowing where to focus remediation activities.
Silica gel can also be used to fractionate samples into aliphatic and aromatic fractions; and the technique can be applied to all matrices. However, alternative fractionation method is suggested for volatile samples. The eluted fractions are then run on the GC instrument to obtain information on the equivalent carbon ranges. It is good to note that fractionation is more expensive compared to bulk TPH analyses as it provides a more detailed information, removes non-hydrocarbons from the analyses and raises reporting limits.
Chromatograms provide information such as sample components, presence of non-hydrocarbons, presence of solvents, presence of non-dissolved hydrocarbons, poor integration and weathering. They can also be used to compare samples with interferents as shown below:
Methods used to analyze TPH in contaminated samples can yield different results when compared with one another, as well as the presence of non-petroleum hydrocarbons being quantified as TPHs. To overcome this, use field methods such as observed plume delineation during excavation, PID analysis of bag headspaces and oil-in-soil analysis for semi-volatiles, as well as the CSM to get valuable information, before using laboratory methods and chromatograms to confirm conclusions made from the field observations.
ENVIRONMENTAL FATE OF TPH
Determining the environmental fate of TPH is critical to understand how the vapor composition and dissolved plumes differ from the source zone due to partitioning and transformation processes. TPHs partition to vapor as well as water. When partitioning to vapor, the smaller hydrocarbons are more volatile and therefore dominate the vapor composition. A more complex process is involved when TPH is partitioning to water because the smaller hydrocarbons are more soluble, based on their molecular structure. Aliphatic hydrocarbons are less soluble compared to the aromatics which are likely to dominate the soil water fractions. TPH weathering on the other hand, contributes exceedingly to TPH mass reduction in the environment may be due to aerobic or anaerobic biodegradation processes in the soil or photooxidation processes; to generate petroleum metabolites which may be further degraded. Petroleum metabolites produced have oxygen atoms in their molecules, making them polar in nature and partition preferentially in water. These metabolites are measured primarily via TPH analysis without silica gel cleanup, and are identified using chromatogram patterns, understanding the solubility of the parent compound and using CSMs maps. most TPH components found in groundwater are metabolites and their toxicity characteristics are usually different from their parent compounds.
The use of TPH fraction approach with fractionation methods is considered best for assessing TPH risks because it provides accurate hydrocarbon quantitation along with the toxicity values as well as the chemical or physical parameters involved. To determine the fractionation composition in a TPH, the fuel composition and the weathering conditions are determined.
For example, Non-Aqueous Phase Liquid (NAPL) undergoing weathering process overtime will first have the mobile hydrocarbons partition out while at the same time, further NAPL depletion will occur with the generation of metabolites by continual biodegradation. There is the migration of vapor plumes to thin zones around the NAPL as well as heavily impacted media due to aerobic degradation in the unsaturated zone. Contaminated ground water could be made up of mostly small aromatic hydrocarbon fractions, some small aliphatic hydrocarbon fractions as well as medium aromatic hydrocarbon fractions.
Along a groundwater flow path, a differential fate affects the TPH composition which in turn affects the exposure.
TPH composition changes along the path of flow could be due to:
- – differential transport and sorption of individual hydrocarbons,
- – different susceptibilities of hydrocarbons to biodegradation and
- – different redox zones along the path of flow.
On the other hand, bulk TPH composition show highest hydrocarbon concentrations near the surface and diminish downwards along the gradient while the metabolites generated via biodegradation, increase in concentrations downgradient of the source area and highest parts of the dissolved hydrocarbon plume. Over time, metabolite concentrations may increase near source, shifting the apex of the triangle to the right.
ASSESSING HUMAN AND ECOLOGICAL RISK FROM TPH
TPH risk assessment is done in three tiers where the first tier is a screening-level assessment; and the site-specific assessment comprises the second and third tiers.
Screening-level assessment involves preliminary CSM development (source characterization and initial exposure pathway assessment) and initial data review (regulatory requirement evaluation, existing TPH data review).
Site-specific assessment involves more detailed assessment which includes the identification of data gaps from data obtained from screening-level assessment and collecting additional field data such as bulk TPH data and chromatograms, indicator compounds and fractions, and CSM updates.
An environmental risk assessment may not be necessary if viable habitats are absent at the TPH contaminated site, if no contamination is found below the root zones and below the burrowing zones of ecological receptors; and there is no potential release of the contaminant to nearby viable ecological habitats. However, risk assessment is necessary if it is a regulatory requirement, if the screening level values are available and if the available levels are appropriate for the site conditions or the type of release.
Site-specific assessment, therefore, is required when screening levels are lacking or exceeded; and at complex sites with multiple media, sensitive habitats and receptors. Such an assessment should focus on direct exposure, contaminant bioaccumulation and toxicity assessment which evaluates the ecological risk, physical and chemical toxicity effects and the metabolites produced.
The stakeholders involved are affected property owners or communities with regard to the risks that are specific to petroleum contamination as measured by TPH. Communicating with them requires sensitivity and a timely approach in order to help them understand facts and clear their confusions and concerns about TPH risk assessment. This could be done through factsheets, posters, outreach meetings, websites and internet links on TPH information. There should be public notification prior to sampling as well as the provision of post sampling TPH data results with appropriate explanations. Technical information and public health issues should be translated and communicated in a format that is easily understood by the general public.
Similar sensitivity should be shown to other TPH assessment impacts to public property, including property value, access, and private property rights. A major concern is the fear of property devaluation as a result of possible residual TPH and a Monitored Natural Attenuation (MNA) remedy. The fears can be effectively addressed by explaining why the selected remedy is protective and effective (especially MNA), describing how all activities are done with agency oversight (that is local organizations and government agencies); and individual property owners concerns should also be addressed.
Overall, a successful TPH risk evaluation project requires an appropriate technical approach, careful review of analytical methods chosen, a complete CSM with regular updates during remediation as well as stakeholders’ engagement.