It may have been the pool chemicals but more likely it got so hot under there that it overheated the grass and top layer of soil and cooked the grass to death. Now you’ll need to rough the area up with a rake and reseed and feed with a starter lawn food. Keep the area constantly moist until the grass is well established and keep it mowed no lower than 5″ all this summer. Next spring you can start treating life the rest of your established lawn. I would dry the pool cover on the driveway next time.