They settle out all of the heavier radioactive elements. They then dilute the remaining heavy water with additional water to drive the level of tritium to an acceptable level. It is then dumped into the ocean and rapidly mixes with the surrounding seawater. If you were to look at a map of ocean currents you'd see generally where it would go from there, but it doesn't really matter because tritium isn't really a significant concern. If they were dumping significant quantities of cobalt 60 you should care more, but they aren't, so you shouldn't.