A bit of dirt in the bottom of an outdoor pond is almost unavoidable from my experience. The dirt will just settle down to the bottom & stay there until the turtle stirs it up again.

Just watch out for small pebbles that can be eaten & hurt your turtle!
My pond is about 110 gal (130 - amount displaced by rocks) and there is always a bit of dirt, etc. at the bottom. I do a big water change each week or two (1/2 - 3/4 change) & kind of stir up the bottom to get more of the gunk out, but otherwise I just leave it alone.
I have a small underwater filter/pump that clogs too easily anyway (need to upgrade!) but as I clean it several times a week I don't have much of a problem.
You could try floating plants too, since they don't need any dirt at all. I've had the best luck w/ water lettuce & water hyacinth & my turtles love them both (especially the hyacinth roots) But watch out - they WILL take over your pond. You'll need to thin them frequently. My turtles love water lillies too...so much in fact I finally took them out. They never got a chance to grow.
Glad to see someone else posting about a pond - seems most members live in colder areas where the turtles need to come indoors for the winter. You are somewhere warm, right?