But you say here that the water is so irradiated you'd need a LARGE water purification system to purify semi large amounts of water. If the water is that irradiated they'd need a large purification system to be able to make enough water to trade AND purify it for their own use as well as stock piling it. You aren't going to get caravans coming all the way to your city for small amounts of water. The danger + resources would need to match the amount of potential profit. That amount of water is like the amount you'd need to grow stuff or to sustain a town for more than a week or two. Megaton doesn't have that kind of facilities. For that matter, why doesn't Megaton sustain themselves better by simply growing their own crops if they have the ability to pull water out and purify enough to make it worth caravans traveling to them?