We live in a rural area and have a wheat field right behind the house. Mid January we had several shrubs loose their leaves and some appear to have died (it's a little early to be sure). We suspect the wheat field was sprayed but have not been able to verify that. We did talk to a farmer who said the most likely thing that would have been sprayed that time of year was 2,4-D.
The cover was off the pool by that time since all the trees had lost their leaves so there is a possibility that whatever it was got into the pool water.
My research so far indicates that there should not be a health risk to swimming in that water but I wanted to see if anyone had more knowledge of this.