In addition to the obvious impact on surface flora, fauna, and hydrology, the rock contains heavy metals and toxic compounds such as arsenic and mercury. As the rock is broken and more surface area is exposed these contaminants are leached out and collected. This concentrates these toxins, which can ofter combine to form more toxic compounds like hydrochloric acid. This collection must be disposed of, and creates a danger of release due to transport, if it isn't just pumped in to the nearest stream or river. The later of which is probably more common, although i have no data to back that up.and it leads to deforestation.