Hi Guys,
I used to work as an environmental consultant...all the negative effects of industry on the environment got me out of that! I did learn that (as majormono said) limits for releases to the environment of substances such as copper are VERY low. 500
micrograms per liter in his case, similar in the US. I think you might approach that level after a few boards.
What that means is that if you dump this in your yard, and there's a well nearby, you could easily contaminate it. Or, if someone goes to sell the house, and a guy like me in my former life does some soil or water tests (and it happens more than you might think!), we'd find it. Then we would dig up something like 50 cubic yards of your yard to dispose of the wasted soil. If it's in the groundwater, we'd have to pump and treat that, too. Makes your property worth about nothing afterwards

Very, very expensive, too!
Please keep in mind the acute problems of using etchant (burns, eye & lung damage, etc), but also the chronic, long-term things...I know all you guys take the spent solution somewhere to be properly disposed of.

Mark Hammer's method is great, I highly endorse that level of responsibility

Like with anything else in the modern world, a little thought goes a long way. There's no reason to stop etching, just do some thinking ahead of time.
Rock on!