At the end of 2023, the Washington Department of Ecology (Ecology) released several documents and tools that impact cleanup of petroleum-contaminated sites. One of these documents, the Guidance for Silica Gel Cleanup in Washington State, is brand-new (though a draft version was released for public comment in 2022). The other new releases are updated versions of previously existing documents and tools that have been used for petroleum cleanups for many years, but contain some significant changes that may substantially impact cleanup sites.
Silica Gel Cleanup Guidance
Many areas of Washington state have naturally-occurring organic materials in subsurface soil and groundwater, and the laboratory method used in this state (Ecology’s NWTPH-Dx method) quantifies semi-volatile petroleum compounds (diesel-range organics and oil-range organics) as well as non-petroleum “polar metabolites” which can originate from degradation of petroleum compounds but can also originate from naturally-occurring organic materials. An optional step for the NWTPH-Dx method is to use “silica gel cleanup” (or SGC), which introduces silica gel to remove the non-petroleum compounds from a sample, so that the results of the analysis only quantify semi-volatile petroleum compounds.
SGC is an important analytical step for sites that have diesel and heavy oil-range contamination. It can help identify how much of a detected diesel or oil concentration is due to true petroleum contamination, and how much may be due to interference from other compounds, including those naturally-occurring organic materials. Effectively quantifying the actual petroleum concentration in a sample can be critical in developing an understanding of a cleanup site, which in turn has a dramatic effect on selecting appropriate cleanup approaches and the cost associated with those actions.
The use of SGC is a controversial topic that Ecology and the consulting community have been working on for quite some time. One element that the community has struggled with is that SGC will also remove polar metabolites that originated from the breakdown of the petroleum contamination, and it is currently unclear how toxic those compounds may be. The new guidance will help the agency provide a consistent understanding of the many issues associated with silica gel cleanup on petroleum cleanup sites across the state.
Some of the key elements of this guidance include:
- The guidance indicates that the use of SGC for soil samples is generally acceptable. This is largely consistent with how the agency has viewed SGC with soil samples in recent years.
- The guidance indicates that the use of SGC for groundwater samples is also now generally acceptable – however, it also details how that data should be used. This is a significant change to how the agency has dealt with SGC of groundwater samples and provides a number of new options for remediation practitioners working on sites with diesel and oil contamination in groundwater.
- The guidance also lays out a concrete methodology for evaluating “background” samples, and methods of comparing analysis with and without silica gel cleanup to determine how much of a detected result is petroleum, and how much is polar organics.
- Most critically, the agency has derived two new cleanup levels for polar organics. Until the release of this guidance there had been no consistent understanding in how to evaluate the compounds that SGC removes from a sample. Now:
- At sites where there are detectable levels of petroleum hydrocarbons, the new groundwater cleanup level for semi-volatile petroleum hydrocarbons is 500 µg/L.
- At sites where there are no detectable concentrations of petroleum hydrocarbons, the new groundwater cleanup level for semi-volatile petroleum hydrocarbons is 700 µg/L. This is likely to be an uncommon situation, as there is generally no reason to test for NWTPH-Dx at sites without petroleum releases.
- The guidance also confirms that the results for diesel range petroleum hydrocarbons and oil range petroleum hydrocarbons should be added together to compare to the generic MTCA Method A Cleanup Levels in soil and groundwater, and not compared individually. This had been inconsistently implemented in the past, so this clarity may be helpful in discussions with the agency, though it is a more conservative interpretation of the regulations than many had previously understood.
- The agency also presented considerations for use of SGC at sites where MTCA Method B, a more site-specific approach to deriving cleanup levels, will be applied.
- Finally, Ecology notes that the “sulfuric acid” cleanup step that is an additional optional step to the NWTPH-Dx should not be used, as it may skew the data. They also note that the NWTPH-Dx method itself is being updated and is anticipated to be released in 2024.
There seems to be some concern and controversy in the comments received by the agency in response to the 2022 draft surrounding the new cleanup levels for semi-volatile petroleum hydrocarbons. Select commenters indicated they were concerned that the 500 µg/L and 700 µg/L levels were far too conservative given the available data on the toxicity of polar metabolites, which Ecology addresses to some degree in an Appendix to the guidance document.
New Excel Tools to Calculate Site Specific Petroleum Cleanup Levels
Ecology also released a set of updated documents and tools that practitioners can use to calculate site-specific cleanup levels at their petroleum sites. Earlier versions of these tools have been in wide use for many years, but had been plagued by an aging interface and some uncertainty in how to use the tools themselves.
MTCA sets out three sets of cleanup levels for contaminants:
- The most commonly used set, MTCA Method A, are generic levels intended to be used at “simple” sites, and includes levels for the most common petroleum products (gasoline, diesel, oil, and select chemicals such as benzene, that are commonly found in one or more of those products). The levels presented in MTCA Method A are typically considered conservative and include a set of assumptions in terms of mixtures and chemical fate and transport that may be more conservative than is applicable to many sites. These levels are largely unchanged by these updates.
- MTCA Method B is presented as an option for sites where practitioners may feel that other cleanup levels would be appropriate. These cleanup levels are derived using a set of equations presented in MTCA and in an updated workbook which can enable a user to enter in information specific to their site. The resulting cleanup level, particularly based on direct-contact risks, may be significantly higher than the generic Method A levels.
- MTCA Method C is used specifically for industrial sites to calculate risk primarily to workers at that site and uses the same workbook and equations as MTCA Method B, but with different default exposure assumptions.
Setting cleanup levels for petroleum mixtures is a complicated matter due to the large number of different chemicals that make up petroleum products. These individual chemicals may have wildly different toxicity levels. MTCA Method A assumes a specific, conservative composition for the most common petroleum product mixtures; while MTCA Method B and C provides a method to evaluate for the specific mixture found at a site (using different analytical methods) in order to derive a more appropriate cleanup level by identifying the specific chemical compounds in the mixture. The updated spreadsheets enable easy development of these cleanup levels by enabling a practitioner to enter in this site-specific chemical data.
The new and updated tools include:
- The updated petroleum mixture calculation workbook (MTCATPH Version 12.0), which includes detailed embedded instructions and a much more user-friendly interface than prior versions.
- A workbook enabling calculation of single-chemical hazard cleanup levels for MTCA Method B and C (MTCASGL Version 12.0) was also released.
- An updated User’s Guide for these workbooks.
- A brief guidance document detailing when and how to use Method B and C for petroleum mixtures.
Ecology has also updated some of the toxicity and chemical fate-and-transport assumptions built into these spreadsheets, some of which may significantly change site-specific petroleum cleanup levels – in some cases, some assumptions have been changed by multiple orders of magnitude (for instance, one of the chemical ranges in the spreadsheet is now essentially assumed to be 342 times more toxic than in the previous iteration).
Have a petroleum site? The new tools and guidance include many steps that may be involved in cleaning up such a site, including a review of analytical results and an understanding of site history and geologic characteristics to understand just how SGC, the different cleanup levels, and the new workbooks should be applied. Landau has been using these methods and tools to successfully clean up petroleum sites and can help you understand these steps and your options for investigation and development of a pathway to cleanup and site closure.
Further information about the new guidance can be found from Ecology here: