This is a really interesting discussion. From what I’ve read and seen, arsenic tends to build up more in the soil over time rather than showing immediate effects on plants, so testing both soil and water seems like a good step.
Rainwater collection sounds like a solid option if it’s practical, since it avoids adding more contaminants. For the pond water, I’d probably be a bit cautious during heavy bloom periods, especially if there’s any chance of algae or runoff from surrounding areas, but otherwise it seems like it could be usable depending on the source.
It’s one of those situations where small differences in water source and soil conditions can really matter.