Can the "de minimis" concept used in determining the threshold planning quantities in mixtures be applied in the determination of the reportable quantity for emergency release notification?

No.  The "de minimis" quantity was set in place for threshold planning quantities simply to make the calculation of the total amount of extremely hazardous substances at a facility more straightforward for planning purposes.  The de minimis concept does not apply to Section 304 release reporting, however, because the extremely hazardous substance is already in the environment potentially doing harm.  Facilities should follow the "mixture rule" for reporting releases under Section 304.  This rule has some relevance in reporting small quantities of hazardous substances.