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It wasn't as if Percy had forgotten.. it's just, he's been busy alright?
From the moment he was introduced to the world of monsters and gods Percy was thrown from one unbelievable task to another, with barely a moment to breathe in between.
So yes, sure, maybe it had gotten pushed down his list of priorities, but that wasn't his fault!
That being said, the war against the Titans is long ended, and the battles in the ancient lands nothing but recent memories, so it's as he's traveling through the city towards the Empire State Building (read: Olympus’ not so hidden entrance) that Percy is reminded of the promise he'd made so many years ago. His promise, to himself at the very least, that he could and would use his abilities to help nature.
More specifically his promise to help purify the waters surrounding the city he called home.
Knowing that Zeus (read: God King of Dramatic Rants and Bellows) would likely throw a tantrum at his missing the reward ceremony, but simultaneously just not caring, Percy swiveled in his path and shifted his stride towards the river.