Cadmium: New Developments in EPA Risk Screening Methods May Affect Assessment and Remediation of Environmental Cleanup Sites
Cadmium is a contaminant of concern frequently encountered at environmental cleanup sites. It is identified as a hazardous substance by the U.S. Environmental Protection Agency (U.S. EPA) under the Comprehensive Environmental Response, Compensation and Liability Act (CERCLA), and is one of the “RCRA-8” metals under the Resource Conservation and Recovery Act (RCRA).
Recent changes in the assessment of cadmium toxicity were published by federal agencies such as the U.S. EPA and the Agency for Toxic Substances and Disease Registry (ATSDR). These changes indicate that allowable levels of cadmium in the environment (i.e., calculated concentrations in soil, sediment and water assumed to not pose an unacceptable human health risk) may be lower than previously reported. The changes may result in more extensive assessments and more complex or expensive remedial actions at contaminated sites, particularly those sites with cadmium-impacted soil.
Cadmium Occurrence and Use
Cadmium is a naturally occurring element, located between zinc and mercury as a Group 12 metal in the periodic table, with an atomic number of 48. Cadmium occurs naturally in soil, typically in combined forms such as cadmium carbonate, cadmium chloride, cadmium oxide, cadmium sulfate and cadmium sulfide. It is taken up into plant tissue and is commonly found in green leafy vegetables; it is known to sequester in the leaves of tobacco plants and has shown to be bioaccumulative in invertebrates and vertebrates.
In addition to its natural occurrence, however, cadmium is also released to the environment anthropogenically. Such releases are typically a result of the mining, processing and smelting of copper and zinc ores, as well as phosphate fertilizer use, electroplating operations and disposal of cadmium-containing waste. Cadmium is an important component of nickel-cadmium batteries and is used in several specialized applications such as pigments, stabilizers for polymers and steel coatings. Consequently, cadmium is frequently encountered in environmental assessments and cleanups.
Recent Revisions in Cadmium Toxicity Value and Implications
To facilitate the assessment and remediation and reuse of contaminated sites, the U.S. EPA publishes tables of “Regional Screening Levels” (RSLs), updated on a semi-annual basis, which provide target concentrations of environmental contaminants for soil and indoor air (based on two alternative land uses, residential or industrial), and tap water (irrespective of associated land use). In the fall of 2021, U.S. EPA revised the toxicity criteria for cadmium listed in the RSL tables, specifically reducing the oral Reference Dose (RfDo) values. Consequently, the RSL concentrations for soil and tap water were revised to levels as much as an order of magnitude lower, as provided in the summary table below.
For the evaluation of oral exposures to cadmium, the RSL tables had previously relied upon RfDo values from U.S. EPA’s Integrated Risk Information System (IRIS), a continuously updated database of toxicity criteria which have undergone extensive internal (i.e., intra-agency) and external peer review. IRIS provides two values for the RfDo: one for exposures from food of 0.001 mg/kg-d and another for exposures from water of 0.0005 mg/kg-d. These two alternative RfDo values differ by a relatively modest factor of two, accounting for the greater bioavailability of cadmium dissolved in water versus cadmium adhered onto solid particles (such as food or soil). The RSLs prior to Spring 2021 relied upon the two IRIS values for the RfDo.
However, the current RSL table (along with all versions of the RSL tables issued since Fall 2021) relies upon the ATSDR Chronic Oral Minimal Risk Level (MRL) of 0.1 µg/kg-d, which is equivalent to 0.0001 mg/kg-d. (Please note that the chronic oral MRL value is the same, irrespective of whether the exposure is from food or water.) The use of ATSDR toxicity value results in screening levels as much as one order of magnitude lower: a residential soil RSL of 7.1 mg/kg, one-tenth of the earlier value of 71 mg/kg, and a tap water RSL of 1.8 µg/L, one-fifth of the previous value of 9.2 µg/L.
Traditionally, the Oak Ridge National Laboratory (which administers the RSL tables under contract with U.S. EPA) has given precedence to the toxicity criteria found in IRIS (Tier 1 hierarchy) over the values reported by other sources (e.g., PPRTVs (Tier 2 hierarchy), CalEPA (Tier 3 hierarchy), ATSDR (Tier 3 hierarchy))5. Toxicity criteria from Tier 2 and Tier 3 sources were traditionally used for exposure routes (i.e., oral, inhalation), disease endpoints (i.e., non-cancer, cancer) or exposure periods (i.e., acute, subchronic/intermediate, chronic) for which IRIS did not provide a recommended value. In the case of cadmium, the RSL tables are apparently using an ATSDR value in lieu of an IRIS value. This is certainly a rare case, and it may be (for the moment) the only such case. This is particularly interesting since U.S. EPA reports the cadmium RfDo values as “high confidence”.
When the changes in the RfDo values for cadmium first appeared in the Fall 2021 RSL tables, it may have been an indication that a pending change in IRIS was forthcoming. However, in the intervening two years, cadmium has not been listed as one as the revised assessments in development on IRIS (IRIS Assessments | IRIS | US EPA), and is not mentioned in the current edition of the IRIS program outlook (A Message from the IRIS Program June 2023 (epa.gov). Therefore, the RfDo values for exposures to food and water remain unchanged in the IRIS database (Cadmium CASRN 7440-43-9 | IRIS | US EPA, ORD). It is expected the revised RSL values based upon the oral MRL from ATSDR will remain in effect for the foreseeable future.
The changes in RSLs for cadmium have impacted site assessments in several ways: lower RSL values means that cadmium is likely to be more frequently identified as a chemical of concern; potential ingestion exposures to cadmium are more likely to drive hazard index values above acceptable levels in human health risk assessments; and remedial actions will more frequently be required to address cadmium concentrations in soil and groundwater. More importantly, some states (e.g., Delaware, North Carolina) have automatically adopted the toxicity values EPA used for cadmium in the RSL tables to develop their own soil cleanup concentrations (e.g., Preliminary Soil Remediation Goals).
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