I've had water softeners for almost 50 years, and my parents before that, and I have no idea how much water they pass in a regeneration! I agree with Jeff, call the manufacturer, and/or put a tub out there to catch the waste water when it regenerates. And I think a drywell would work fine, as long as it's dry and big enough. If they put the softener outside you aren't in Texas are you? They had a lot of plumbing outside and it didn't turn out well for them! Prepare for the worse, the 10 year freezes are going to happen every 3 years in the future!