Chemical Spill - Water System Contaminated

[Steve]

I have a spring that's flowing at least 3000 litres per hour, but have no way of testing the water, so if there was a fallout incident or perhaps a spill at a farm up in the hills it might be contaminated. I have thousands of litres stored from rain, off the barn roof, which will be uncontaminated as long as I know about the problem and don't flush them out at the wrong time.

I guess I should keep a note of which tanks have been stored for how long, so if it emerges that there was an event, say, two days ago, I'll know which water is safe and which to dump.