puu's back flush garden

my grandmother’s water filter has a backflush cycle every night. it dumps an enormous amount of water onto the ground outside the pump house, I’m guessing it’s at least thirty gallons. so I’m going to put in a small pond to make use of all that water. this is in the planning stage at the moment, but I’ll install it and start planting soon. one concern is that everything that the filter removes will be flushed into this garden, so I don’t know how great an idea it will be to eat this stuff. the source of the water is the North Fork of the Lewis River, which is of unknown (to me) purity. if it turns out there are nasties in the water, I’ll just run it through a reed bed first. more research is clearly in order…

planned plants:
water lotus
wapato
water chestnut
watercress
canna
chufa
groundnut
azola for nitrogen
cattail and bull rush as filtering reeds?
any suggestions?

I’ll probably drop some critters in there, too:
minnows
snails
sharks
frogs