40 CFR 118.9: Mixtures.

For the purposes of determining the CWA hazardous substance maximum quantity onsite at the non-transportation-related onshore facility of CWA hazardous substance(s), under § 118.3(a), the following provisions apply to CWA hazardous substances mixtures:

(a) If the quantity of all of the CWA hazardous substance constituent(s) of the mixture or solution is known, the mixture meets the threshold quantity when the maximum quantity onsite, as defined in § 118.2, meets or exceeds the threshold quantity of any CWA hazardous substance in the mixture.

(b) If the quantity of one or more of the CWA hazardous substance constituent(s) of the mixture or solution is unknown, the mixture meets the threshold when the maximum quantity onsite of the mixture or solution meets or exceeds the quantity for the CWA hazardous substance established in § 118.3(a) with the lowest threshold quantity.