On November 18, 2020 EPA released its Fall 2020 update to their “Regional Screening Levels for Chemical Contaminants at Superfund Sites.” Screening levels (SL) are risk-based concentrations derived from standardized equations combining exposure information assumptions with EPA toxicity data. The EPA website is the source of screening levels for all the EPA regions, thus the term Regional SLs or RSLs.
SLs are used for site “screening” and as initial cleanup goals, if applicable. SLs are not de facto cleanup standards and should not be applied as such. The SL’s role in site “screening” is to help identify areas, contaminants, and conditions that require further attention at a particular site. Generally, at sites where contaminant concentrations fall below SLs, no further action or study is warranted under the Superfund and RCRA Corrective Action programs, so long as the exposure assumptions at a site match those considered by the SL calculations. Chemical concentrations above SLs would not automatically designate a site as “dirty” or trigger a response action; however, exceeding a SL suggests that further evaluation of the potential risks by site contaminants is appropriate.
SLs can be obtained on the RSL website from either the generic summary tables or using the RSL calculator. The generic summary tables are presented at a target cancer risk (TR) of 1E-06, and at either target hazard quotients (THQ) of 1.0 or 0.1. The generic summary tables provide a list of contaminants, CAS number, toxicity values and chemical-specific information, MCLs, and the lesser (more protective) of the cancer and noncancer SLs for resident soil, industrial soil, resident air, industrial air, tapwater, and leaching to groundwater exposure scenarios. The summary tables are available as PDF or Excel files. The web calculator provides users considerably more flexibility to develop site-specific SLs using a combination of user-defined and default input variables.
SLs are usually updated twice a year, typically in November and May. The prior update occurred in November 2020. Changes to the RSLs tables are summarized in the “What’s New” page, including spreadsheet files comparing the newly updated RSLs and toxicity information to the previous version. Only the chemicals for which changes occurred are included in the comparison files. This is a good way to see if an RSL for a contaminant of interest changed and if it went up or down.
Of the constituents for which a change in RSL occurred during the November 2020 update, one of the more common at our sites is trans-1,2-dichloroethylene (trans-1,2-DCE). This compound can form during the breakdown of the common chlorinated solvents PCE and TCE and, until now, there were no indoor air SLs for trans-1,2-DCE. Indoor air SLs have also been added for molybdenum, consistent with other RSL changes over the last couple years that provide more information for evaluating vapor intrusion of non-organic chemicals. We do not expect the new SLs to drive remedial decisions at most sites. However, many vapor intrusion investigations identify trans-1,2-DCE alongside other chlorinated solvents. It is now even more important (and simpler) to include an evaluation of vapor intrusion risk from trans-1,2-DCE when performing investigations.
One of the more problematic issues associated the use of RSLs is what happens when RSLs change. This always seems to happen in the middle of putting together a large report or risk assessment. What do you do? Many practitioners feel that you are obliged to use the most recent screening levels in every case; others feel that screening levels identified in an approved plan or agreement become set in stone. To best manage the change, the workplan or agreement should clearly state how this will be addressed. Also, do not count on EPA or states informing you of new updates, especially if a critical screening level has gone up. I recommend signing up to automatically receive notification when new releases are available.
We will continue to keep a close eye on RSL changes when they come out and consider how they could affect the projects we manage. If you have a project that you think may be affected by these changes, please contact us so that we can discuss whether any actions are required.
Nate Wanner is a Cox-Colvin Senior Scientist with over 15 years of experience leading and completing environmental projects, in addition to six years as an educator and IT director. He holds a BS in Geology: Water Resources from Ohio University and a Masters in Geographic Information Systems from Penn State University. His areas of expertise include brownfields, underground storage tanks, due diligence and database services. Nate is an Ohio EPA VAP Certified Professional (CP), a Certified Professional Geologist (CPG) with the American Institute of Professional Geologists (AIPG) and is a registered Professional Geologist (PG) in Kentucky, and a Licensed Professional Geologist (LPG) in Indiana.