water under the bridge vs past

water under the bridge and past both are nouns.

Word	Noun	Adjective	Verb	Adverb
water under the bridge	Yes	No	No	No
past	Yes	Yes	No	Yes

As nouns, past is a hypernym of water under the bridge; that is, past is a word with a broader meaning than water under the bridge:

Other hypernyms of water under the bridge include past times, yesteryear.

water under the bridge (noun)	past (noun)
past events to be put aside	a verb tense that expresses actions or states in the past
	the time that has elapsed
	a earlier period in someone's life (especially one that they have reason to keep secret)

water under the bridge (adjective)	past (adjective)
	earlier than the present time; no longer current
	of a person who has held and relinquished a position or office

water under the bridge (adverb)	past (adverb)
	so as to pass a given point