They pump in freshwater to the cooling circuit of the reactor, like from the tap. The water that comes back out of the discharge has gained temperature but has no tritium. They probably just return this water to the sewer system for now.

Just the water from the leaks has be retained at present.

If they mixed the water from the tanks, they'd have to retain 100% of the flow of the water.

Now if they built a big cooling pond and stuff they could reuse the water 'forever'. But the last thing you want is a big pond of slightly radioactive water than all kinds of wildlife can come live in.