One thing that really puzzles me is that if the plant us "shut...

One thing that really puzzles me is that if the plant us "shut down" why is there a necessity to measure the bores?

yes although the plant was shutdown, the plant still ran for a couple of months before its shutdowns. Hence if there were presence of contamination, it is still possible to detect it in those bores. But the fact that we found nothing, means that the simple explanation, is that there is nothing to be found.

You can also see it in another way: oh yeah but by the time it was shutdown and conducting the sampling test now, all the contaminants will have disappeared and run off to another location. Yes this is possible, but benzene/toluene don't break down so easily, in fact they are bio accumulative chemical, meaning they get absorbed by animal and stay in the system for a very very long time. Hence if its is present in the ecosystem, it will be detected.

But the fact that we found 1ppm with is significant lower than drinking std, simply suggest that is a natural level occurrence.