About the mangels, my plan was to plant them in various areas and once they were grown, move the pigs to those areas. In theory (one of the links I put shows this), they would eat the tops and then root up the bottoms. When the mangels were gone, I would move them to a new area. Unfortunately, I didn't get to test this plan, as my hubby decided not to retire then (2012) and we've spent the last six years moving around the country LOL. We're back on our homestead now and starting from scratch.
We have a well but I haven't had the water tested and I don't have a well spigot near my greenhouse, so I use city water, which is treated with chlorine. It's highly unlikely you have chlorine in your well unless you shock it, and if you do, you can just fill a container and let it sit for 24 hours and the chlorine will evaporate. This is my first experience with duckweed, so I don't know whether PH really affects it. From what I've read, it seems to grow anywhere in still water.