Is there a concentration cutoff below which it is not necessary to report a release, even though a Reportable Quantity (RQ) might have been exceeded over a 24-hour period?

No. There are no concentration cutoffs for the RQs (i.e., a lower-bound concentration below which reporting would not be required). Unless it is one of the releases permitted under other federal programs or qualifies for one of the reporting exemptions for hazardous substance releases, the release of an RQ or more of a hazardous substance must be reported, regardless of the concentration of the substance released. Notification of releases of hazardous substances that equal or exceed an RQ, even those with relatively low concentrations, is mandated by CERCLA and EPA believes that such reports are essential to allow government personnel to decide whether a response action is necessary to protect public health or welfare or the environment.