I think if you keep the wood outside the pond liner there will not be so much worry. You could build the treated wood frame, install the liner and then use a product called Trex which is a mix of recycled plastic and sawdust to cap the top of the frame so that the water does not come in contact with the treated timbers.
I used Trex to provide overhangs for an outdoor turtle pond and it is virtually rot proof. It's very expensive so I recommend using it where the wood or runoff is likely to contact the water and use the treated timber elsewhere where structural strength is important. Trex is great for decking and similar applications but not strong enough to carry heavy loads.
In regard to Moon's recommendation I seriously considered cypress for the wood structure of my greenhouse. A year or so before I got fully committed to the project I talked to a guy who owned a really old commercial greenhouse that had been running since the early 20th Century. Alot of the wood was cypress nearly a century old and still going strong in the humid environment of a greenhouse. Had I been
willing to wait I could have salvaged it since the owner was getting ready to tear down and replace everything with the choice of modern commercial greenhousing- metal frame and twinwall polycarbonate.
Most material was purchased new- though I did use wood salvaged from a friend's old deck for stakes and concrete forms at the beginning of the project and plastic barrels and jugs as thermal mass containers and recycle flower pots and a variety of other plastic containers for raising plants!
I've always loved the idea of building something with recycled material - but when the time came to make my decision I needed to move fast and treated lumber and a lean-to kit from Turner was the quickest and most sound way to go. New cypress or redwood- if you can even get it locally costs an outrageous fortune and I had a feeling that alot of the salvaged wood from that commercial greenhouse might be
flawed or not long or thick enough for major load bearing applications. Treated pine which resists insects was reasonable and availible in the many dimensions that the professional builder I hired needed to get the job done.
I've been careful to clean up and properly dispose of the leftover peices that were too small to be of use as well as the sawdust. When it came time to establish my planting beds- especially for the vegetables I put down leftover pond liner to deter the translocation of any residue and made my own soil mix from bags of commercial topsoil mixed with sand, coir peat, vermiculite, Profile (Aquatic Plant Soil)
and some composted sawdust (untreated from a local sawmill) and wood humus.
One more thing- according to many sources on gardening- neutral or basic soils are less prone to leaching of arsenic and other compounds from treated wood than acidic soils. To really be safe most gardeners line their beds with plastic so the treated wood does not contact the soil. That would also help prolong the life of untreated wood as well.
If you are really worried about it you could go with concrete blocks instead. The way I did my wall on the isle side of the greenhouse pond was one block high capped with a flat rectangular paver block. To protect the liner I used that black felt stuff sold for that purpose to cover the inside of the block wall and brought it under the first cap and then back over and down the inside. Then I sandwiched the
remaining flap of the liner between this and a second cap.
I did this only on one side of the pond but you can do it the whole way around and make a very formal looking one. Outdoors in a cold climate or if the wall is going to be more than a few blocks high- I'd recommend a footer. You could go with a low wall and excavate inside that to get more depth and save on construction material. That is how I did the greenhouse pond.
Or maybe just forget about blocks or wood and just dig out a hole and build a naturalistic pond. It's really the easiest way to go and you can even use the dirt as fill to build up the surrounding grade for a rock garden or waterfall!
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