A related use is to float part-filled plastic bottles in your dams to reduce evaporation. I recall reading about a farmer here in Australia who did that as it was the only economical way he could minimise losses to evaporation.
IF those are plastic bottles, I suspect the sun will eat them up in a very few years, rendering the tank useless.
Glass bottle walls could last for decades and decades.
Most beverage bottles are polyethylene, which will be steadily degraded by UV exposure, unless they use a lot of uv absorber/inhibitor. I doubt they do that for beverage bottles because of the expense and short anticipated lifespan.
Yup, doesn't work with newer greener plastics for nearly as long. If covered with a skim coat to keep them out of the light, then it will. Similar as Earthships.
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