Looks like the 400,000 gallons were contained before they went anywhere, and were just being re-used by the plant. The shutdown is because they detected the smaller leak on Wednesday that got out of whatever they were using to contain it. Your drinking water should be fine (at least with respect to the tritium). Even hundreds of gallons, diluted by the ground and the Mississippi, is going to be negligible by the time it gets to our drinking water.