Jumping in to an older thread here. I'm also NW of Chicago. We're going to be out of town for a week over Memorial Day, so I'm planning on waiting until then to open. If I were to get a sample of water and test chlorine, could I add more bleach if required, or would lack of circulation whiten liner?
I don't recommend testing after the water has been sitting all winter. Leave it covered and deal with it after you return. It won't get much worse covered, over a week, and the forecast till then still has us in Seattle type weather. I think you'll be fine.
I have always waited to Memorial Day weekend, never had a "Swamp" as others caution you about. As you can tell from this past week and the forecast for the next week, it is not exactly swimming weather. Why open it and pay for electricity to run a pump for 2 weeks while the temps are in the 60s?