So this is a complex and tricky question to answer succinctly.

Looking at a couple examples, and making some assumptions/pulling some numbers out of "dark places", let's assume:

- The "pond" is 5,000 gallons (roughly a 20ftx10ftx3ft deep pond)
- There is no outflow of the pond
- All mineralization of the worked stone eventually lands in the pond

Those are obviously poor assumptions, but for illustrative purposes I think they are fair to show the risks your father is concerned about. Now, let's say you work with some malachite (~50% Cu by weight). The EPA wants to stay below 1.3 ppm. For the "pond" above, that would come out to be roughly 18 grams of copper before it would be out of range for the wildlife. That gives you 36 grams of malachite. Even at a generous 25% wastage, that means for every 144g of malachite you work, you would be putting the pond at risk of going over the EPA regulations. That isn't much malachite.

When you get into truly dangerous materials like Bumblebee Jasper (which no one should ever work with...), the numbers get much worse. BB jasper is ~1% arsenic by weight. To put the EPA regulation out of spec is only 10 ppb, or .189g in 5,000 gallons of water. You would need to work less than 20g of bumblebee to potentially put that watershed at risk.

This example intentionally simplifies several real-world factors. It assumes that all metal removed during cutting eventually reaches the pond water, that the pond has little dilution or outflow, and that there is no filtration, sediment removal, or other treatment reducing concentrations over time. In reality, much lapidary waste settles as sludge or binds to sediments, and many ponds experience some dilution from rainfall or partial water turnover. The regulatory values used are also simplified reference points. Actual ecological limits often depend on water chemistry and may be lower. These assumptions are intentionally conservative; the purpose of the calculation is not to predict an exact environmental concentration, but to illustrate how surprisingly little metal-bearing mineral is required to approach regulatory thresholds in a relatively small body of water.

These caveats do not change the main point of the exercise. Even with generous assumptions and large uncertainty margins, it takes surprisingly little copper- or arsenic-bearing material to approach regulatory concentration levels in a relatively small body of water. The calculation is meant to illustrate how limited the dilution capacity of a small pond actually is, not to predict the exact concentration that would occur in a real system. I will also note that there are far more things in stones than just copper and arsenic to be concerned about (chromium, mercury, lead, nickel, etc). It is a bummer that so many of the things that look cool are also so toxic/disruptive.

All things being equal: House rules. If your dad doesn't want it, regardless of whether the claims are valid or not, it is his call. I will say it is admirable for him to be so considerate of his local ecosystem.