take a good look at the back of the card,
if im not wrong, canada custom is responsible for collecting the card, if you return from the us.
you may return the card back to canada custom at the airport,
i doubt the us embassy in ottawa would look after the card
When you were entering the US, they would have told you to do this. And this is how it works (as there is no US authority to check you when you leave the US.)