New pond construction, possible problem with mortar?

[brokensword]

if they truly used mortar, it wouldn't hold anyhow as it is not waterproof. Cement/concrete (which is made with cement) is, though. All said, cementing/mortaring stones like that seems pointless and surely won't hold. Exactly how arethe floating stones situated that they needed to be mortared? I'd assume there's a good base upon which they are sitting. Weight should hold them, imo, and should be stable without any bond to worry about. Sounds like there's a bit of engineering if you've got any cantilever effects going on.
That said, you might consider hydraulic cement; it's built for underwater situations.

The mortar will/might affect the pH as mentioned, but it depends on how much was used, as Lisa noted. I doubt a bead of morter would do much in a pond that size but if you're concerned, by all means drain and clean. If they used additives other than pure mortar, then of course the wildlife and flora might be affected adversely.

 

[Bluerooster]

I used portland/sand mix to secure pea gravel on a sloping side to prevent the stones from being washed into the pond, as well as to secure some stones in the stream bed, and water fall. I kept an eye on the PH and kh which took a spike, but then leveled out at an acceptable level. But my pond is only about 3000 gallons. For the displacement you are talking about, I may well just go ahead and add the other 29,000 gallons to it, and check it. It will surely dilute the mortar. Also, if the water you have is acidic, they could cancel each other out.
On to the sediment, it will settle out with time. But that's just my two pennies. It may behoove you to check the alkalinity before getting too far with decision making. But keep any future mortar out of the water.

 

[Doctor mcboy]

i like the pond. without waterflow of some kind the water in your pond start will start to smell bad. i have a natural pond with no liner under large oak trees and the water looks inky black like the bayous i used to play in during my childhood in Shreveport la. i love that look but not all do. without some sort of filtration yours will probably look like this also. at least use a aerator to put some oxygen in the water to keep the smell down.