It's likely that the Portal water source is a deep well, which is why it was easy to shutdown the entire system. If it is a well, it might be anywhere from 150 to 300 feet deep. The aquifer is recharged over a number of years from creeks, runoff water, snow and rain.
If it is a deep well, it might take a long time for the contamination to disappear.
It is not a well, it is a "developed spring", by definition within a few feet of the surface. See quote from Inyo COunty above
