IF, and only IF, we agree that the definitions posted by Shorts are correct, then:
-ALL "negligent discharges" are in fact also "accidental" under both the primary and secondary definitions shown there, since they are both "unforeseen/unplanned", and quite precisely "an unfortunate event resulting especially from carelessness or ignorance".
So, ALL "ND"s are in fact also correctly called "AD"s, but calling the ADs, though correct, is not quite as descriptive.
But, NOT all ADs are also NDs - SOME (a minority) are not the result of carelessness/negligence - they could be the whim of fate/act of god, etc.