Sure, it remains concentrated in its vein, where it belongs, but not in the river bed where it is suddenly introduced in these cases. Time-gated natural seepage isn't the same as dumping literal shit-tons of the stuff into one spot where you would never naturally find it; let alone in resources that you depend on for human health.
I'm not really sure what the impact would be (in places like the Gulf, where oil seepage is normal, you've got massive niche ecosystems that actually feed off of the stuff and process it pretty well), but there is nothing natural about introducing a generally toxic resource (usually, the runoff from mining is worse than the actual rock, right?) within the very recent, geologically insignificant fart that is human existence--especially the only ~170 years or so that we have been mining this stuff--compared to the natural balance previously reached over 10s of millions of years.