We just bought a piece of property that has a 3-acre pond on it. The pond is a basalt depression that is surrounded by cattails and filled with weeds. I launched a rowboat into it the other day and the oars kept coming up with a ton of weeds. As I was rowing, I noticed that the pond was constantly bubbling. Little tiny air bubbles were coming up through the weeds. There is no foam. the surface is clear. What is making the bubbles? Is it oxygen, methane, or something else?
There is not really enough information to make a determination, but a best guess would be methane from anarobic decomp. Which is pretty common in the bottom cline zone of swampy waters. Alternatively, it could be radon gas, particularly considering you already know it's a basalt depression, and basalt is often contaminated with thorium. I would recommend that you get one of those radon detector kits, and set it up under a cardboard box (protected from rain) somewhere near your shoreline. If it's radon, it's not just where your pond is.
If you do find radon, DO NOT build a basement in your new home. Only a vented crawlspace.
If you can stir the bottom up enough to release a good many bubbles in one place, you might be able to light them with a flame! That will prove that methane is present for sure, and it's really quite spectacular!
Alder Burns (adiantum)
Location: The dry side of Spokane, USDA zone 6ish, 2300' elevation.