Excavation pits can be clean and I'm pretty sure they would have considered what was left in the pit.
replyWhat you're thinking of is the processing aspect of mining, which is a whole different beast.
No, not exactly. Many kinds of rocks and soil will release Acid Mine Drainage (AMD) or otherwise release sulfates and heavy metals into the environment. Of course a lot of this will already be known from the treatments they had to do when keeping the water table low when mining.
replyWhen I was a kid I remember a number of very old stone quarries that had been allowed to fill, then years later they had to put up warnings for people to stay out because the high Ph water was dissolving metals into it.
This really depends on the composition of the minerals. Butte, MT is famous for its pit that is filled with highly toxic water: https://en.wikipedia.org/wiki/Butte,_Montana#Berkeley_Pit (or https://www.youtube.com/watch?v=n-Ej2EtE744 ).
replyIt's dependent on the minerals being prospected for and the mining techniques as well as general awareness and regulations that were in place during operation. Yellowknife has a huge ecological issue due to Arsenic being actively used in Giant Mine[1] but that was introduced to ease mining (mercury is also commonly used for gold extraction and purification, though occasionally offsite). So it really depends on the mine. Mines are never good for the environment but how toxic they are will vary.
reply