why would water keep needed to be added after every 30 miles.wheres it going?

If you don't see it puddling under your car then it's going out the tailpipe or out the overflow tube to the ground while you're driving. Having to add it every 30 miles is a major loss, so you may have a bad head gasket. Some leaks could happen after the engine warms up too, and again most likely occur while you're driving.