Not to diss' the environmental authorities, but I seriously doubt that dumping a bucket of paint water in my backyard will bring the law down on me. There are no wells in the city and the particles will get filtered by the soil and grass before it reaches the stormwater system. In fact, it's so dry here that the stormwater system won't see any water that is backyard because the soil will drink it up.

Thanks for the info on letting it evaporate. I actually did buy a teflon paint tray. You just let the paint dry in it and peel it off and put it in the trash. It'll take a long time for a bucket of water to evaporate though, but perhaps I'll give it a try if I don't need the bucket for anything else.