How do I determine whether my release of a mixture exceeds a reportable quantity?

The mixture rule

Most hazardous substances and EHSs produced, used, or stored by facilities do not exist in pure forms, but are found in mixtures or solutions. Facilities must follow special rules when determining the RQ for releases of mixtures or solutions.

If a mixture of hazardous substances or EHSs is released and the concentration of all hazardous substances and EHSs in the mixture are known, the Clean Water Act (CWA) mixture rule may be used to calculate whether an RQ of any hazardous substance or EHS has been released.

RQs of different substances are not additive. This means that spilling a mixture containing half an RQ of one hazardous substance or EHS, and half an RQ of another hazardous substance or EHS does not add up to trigger reporting requirements.

To use the CWA mixture rule, calculate the amount of each hazardous substance or EHS that has been released. For instance, a release of 100 pounds of a mixture that is 40 percent acrylonitrile would be equivalent to a release of 40 pounds of acrylonitrile. This release would not be reportable, since the RQ for acrylonitrile is 100 pounds.

If a mixture includes more than one hazardous substance or EHS, check the RQ for each substance. The release must be reported if the RQ for any hazardous substance or EHS has been met or exceeded.

If the concentrations of the hazardous substances or EHSs are unknown, reporting is required when the total amount of the mixture released equals or exceeds the RQ for the component with the lowest RQ (40 CFR 302.6).

The CWA mixture rule can be particularly confusing when applied to RCRA hazardous wastes.

Determining RQs for RCRA wastes

RCRA has identified listed and unlisted wastestreams that are regulated as hazardous wastes and thus regulated as CERCLA hazardous substances. The RQ for each RCRA hazardous wastestream is listed in 40 CFR 302.4.

RCRA wastes may be treated as mixtures only if all hazardous components and their concentrations in the mixture are known. Otherwise, the RQ for the wastestream must be used. The CWA mixture rule also applies to unlisted characteristic wastes if the concentrations of all the constituents in the waste are known.

F- and K-listed hazardous wastes

As with all releases of CERCLA hazardous substances and EHSs, F- and K-listed RCRA hazardous wastes are reportable if the spill of the waste equals or exceeds the designated RQ.

You can apply the CWA mixture rule to RCRA F- and K-listed wastestreams if the concentrations of the hazardous constituents within the waste are known. If the concentrations are unknown, the RQ of the specific listed hazardous waste applies.

For example, if a release of F001 occurred and the concentrations of the constituents in the waste were unknown, the RQ for the wastestream (10 pounds) would apply. By contrast, if you can determine that the F001 hazardous waste contains 50 percent 1,1,1-trichloroethane and 50 percent water, the CWA mixture rule can be applied. Since 1,1,1-trichloroethane has a 1,000-pound RQ, in this example the spill is not reportable until 2,000 pounds of the mixture are released.

P- and U-Listed hazardous wastes

The definition of hazardous substance in CERCLA incorporates all RCRA hazardous wastes, including commercial chemical products listed in 40 CFR 261.33(e) and (f).

Under RCRA, in order for a material to be defined as a commercial chemical product and to receive a P or U hazardous waste code, the material has to be an unused product in which the chemical is the sole active ingredient. These hazardous wastes, in addition to all other RCRA hazardous wastes, are listed as hazardous substances under CERCLA.

Releases of these P and U hazardous wastes, listed in Table 302.4, are reportable when the designated RQ is equaled or exceeded.

Remember that the RCRA regulatory parameters for commercial chemical products do not apply to individually listed CERCLA hazardous substances. For example, a release of amitrole (U011) need not meet the unused, sole active ingredient restrictions that are applied to RCRA commercial chemical products in order to be reportable, since amitrole is specifically listed as a hazardous substance. Releases of amitrole are reportable when the release equals or exceeds 10 pounds.

RCRA characteristic wastes

Unlisted wastes exhibiting the characteristics of ignitability, corrosivity, and/or reactivity (ICR) have a RQ of 100 pounds.

If a waste known to be hazardous solely because of the characteristic of ignitability was released into the environment, the RQ would be 100 pounds. If an unlisted ICR waste is analyzed and the concentrations of all of its hazardous components are identified, the waste is no longer an unlisted waste, but one characterized by its components. Therefore, the RQ of the specific listed components of the hazardous substance can be used to determine when reporting is required.

For example, if a waste is known to be corrosive solely because of its sulfuric acid content, and no other ICR characteristics are present, the RQ of the waste is reached when 1,000 pounds of the sulfuric acid are released. If the corrosive waste is a 25 percent solution of sulfuric acid in water, in this example the RQ of the waste is not reached until 4,000 pounds of the waste solution are released.

Toxicity characteristic hazardous wastes (D004-D043) are hazardous substances under CERCLA and are listed with their applicable RQs in Table 302.4 under “Unlisted Hazardous Wastes: Characteristic of Toxicity.” Unlike other unlisted hazardous wastes (ICR), toxicity characteristic hazardous wastes have reportable quantities specific to the contaminant on which the characteristic of toxicity is based (e.g., lead or selenium).

If the composition of a wastestream can be determined, then the waste is no longer an unlisted waste, and the CWA mixture rule would apply. The test used to determine the toxicity characteristic, the Toxicity Characteristic Leaching Procedure (TCLP), measures the concentrations within a leachable extract of a waste sample, not total concentrations of all the substances in a wastestream. Accordingly, data gathered when conducting a TCLP will not automatically provide the total composition of a waste. Should the owner/operator have measures of the total concentrations of the hazardous constituents in the waste from another test or through application of knowledge, the CWA mixture rule would apply.