You described the UTG person as a passive player. He is checkraising the turn with two people already committed. I believe he will have 2 pair or bettor here 100% of the time. Unless you think the UTG is loose enough to play T6,62,T2,T4,64,24, I would fold the turn. If you think the UTG player can very well have many of these holdings, then you should call the turn hoping for an Ace or the board to pair. If neither of these events happens on the river, I would fold. I dont care how big the pot is on the river, there is no reason to call for one more bet against this type opponent unless you improve. Well actually there is one reason, and that is if you think theres a chance that your read on the player is wrong. You said that this opponent is "loose passive as far as I can tell" this statement alone probably means you should call the river in this big pot.