TECHNICAL UPDATE Southern California Ambient Arsenic Screening Level

Arsenic and other metals are frequently detected in soils at concentrations exceeding risk-based screening criteria due to naturally occurring conditions and regional anthropogenic contributions including wide-spread application of arsenical pesticides, regional placement of fill material or a site-specific release. In general, agencies do not require remediation or mitigation to levels below naturally-occurring or ambient background levels. Thus, untangling ambient or background conditions from site-specific releases is a key component of environmental site characterization and risk assessment activities.  

Regional Ambient Levels for Arsenic in Soil  
The California Department of Toxic Substances Control (DTSC) recently published Human Health Risk Assessment (HHRA) Note 11 regarding ambient levels for arsenic in soils in five Southern California counties: Los Angeles, Orange, Riverside, San Bernardino, and San Diego. Specifically, DTSC established a regional, ambient screening level of 12 milligrams per kilogram (mg/kg) for arsenic, which is approximately 100 times the risk-based screening level for residential soil of 0.11 mg/kg) developed by DTSC. This regional ambient level was previously established by DTSC as the upper bound of arsenic in soil from 19 Los Angeles Unified School District sites, and DTSC formally established this ambient level for Southern California with the publication of HHRA Note 11.

Even though 12 mg/kg is billed as an upper-bound on background concentrations[i], finding a single sample above that level does not necessarily mean arsenic is present above background levels background levels because naturally occurring levels will be present as a distribution of concentrations. In addition, there are some areas in Southern California, and elsewhere in the State, known to have naturally occurring arsenic levels higher than the levels serving as the basis for HHRA Note 11. In such areas, it may be necessary to develop a characterization of the local, naturally-occurring levels of arsenic in soil. 

There are a variety of published studies of regional, naturally-occurring arsenic levels in soil. Based on such studies and experience with arsenic data from local investigations, many regulators in local jurisdictions have a sense of background levels in their region. In some areas, where such information is available, it may only be necessary to collect a limited number of background samples to demonstrate that the levels of arsenic detected at a site are consistent with local understanding of natural background concentrations. Absent such information, it may be necessary to undertake a more extensive study to characterize local background levels. To aid in this process, the San Francisco Bay Area, the Regional Water Quality Control Board[1] has recommended 11 mg/kg as an upper-bound, regional ambient level of arsenic in undifferentiated urbanized flatland soils in the Bay Area based on a published study.

Site-specific Evaluations of Naturally-Occurring Metals in Soil 
If it appears that a release has occurred, a health risk assessment may be required. When a risk assessment for arsenic is performed under agency oversight, it is generally required that the risk associated with the level of total arsenic measured at the site is evaluated (i.e., without adjusting for background levels). The risk associated with the background level of arsenic is usually subtracted from the level of risk associated with the total arsenic concentration to determine if the incremental risk associated with arsenic levels above background warrants mitigation.

DTSC reported that the 95UCL of the mean arsenic concentration in the Southern California data set is 3.1 mg/kg. This level of arsenic in soil corresponds to a cancer risk of about 26 in a million and would be subtracted from the health risk associated with the total arsenic concentration at a site to support environmental management decisions.

Summary
The regional background levels recommended by DTSC and the SFBRWQCB are intended to streamline evaluations where arsenic is detected at concentrations exceeding the risk-based screening levels. Local, naturally-occurring arsenic concentrations may exceed regional background levels, and the establishment of regional ambient levels does not preclude the need to develop site-specific background values through a site-specific background evaluation. Further, if a release of arsenic has occurred at a site, it may be necessary to evaluate the incremental risk above that associated with background levels to determine if some form of risk management is needed. When remediation is needed, the remedial goal is usually to restore the site to background levels, and having a good characterization of local background levels can provide a basis for designing such a remedy.
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[i] The recommended screening level of 12 mg/kg is the 95% upper confidence limit estimate of the 99th percentile of the background data set used by DTSC to establish this regional screening level. 
[1] San Francisco Bay Regional Water Quality Control Board. 2019. User’s Guide: Derivation and Application of Environmental Screening Levels (ESLs) Interim Final.