I wonder if purifying the water with bleach would be a possibility for you. Prepper types do this for drinking water and it takes very little beach per gallon. My thought is if it can be made drinkable for humans...the trees would be fine, but the algae might have a hard time with it. Not to mention, city types have been watering their lawns with chlorinated water forever.
When I had aquariums, it was common knowledge to let city chlorinated water sit for a day or two, open top, and the chlorine would leave the water and could be put in the tank with the fish.
Something to look into at any rate.
