I did some more research on materials. I'm not used to working like this, so I'm kind of proud of myself for working out the details:
The big idea here was that instead of using bottlecaps, I use pennies. At the Berkeley Pit they mined for copper, so using pennies provides a convenient mental model for players -- they're digging for copper... Except pennies are actually made of zinc and coated in copper. I started thinking of a chemistry experiment I did in high school, where we filed the copper coating off the penny and dropped it in a container of hydrochloric acid. The zinc burned away.
So if I poured diluted hydrochloric acid into the game board at the end of the game, the pennies would produce a chemical effect -- like the acid lake that has formed at the Berkeley Pit. To augment the acid lake effect, I was also planning on using green chalk for the chemical tokens -- the chalk would react with the acid and produce carbon dioxide as well. In addition, I could mix baking soda into the soil itself, which would react with the acid to produce a foaming effect.
(My only fear is that the three chemical reactions would interact, with the products reacting with each other. I'll have to consult a chemist before attempting this in a live demonstration...)
The best part? There'll be this soggy cardboard box foaming, with acidic mud bubbling out onto the floor -- how do you clean it up? That's the same question facing the EPA at the real-life pit site, a toxic lake.