Call the police, water security agency, and health department. Get the word out that tap water cannot be consumed in any way and that personal treatment (boiling, filtering, etc) is not possible. Get samples for a wide range of tests. Drain the reservoir, then start filling again to replace and dilute. Arrange for a tank of treated water from Lucky Lake. Deal with about a million questions from understandably concerned residents and businesses.
Some samples may not produce results before the weekend, so.....
And so it goes.
One interesting thing was that it was very challenging to get the word out regarding the possible hazard, but the all clear took much less than an hour. Both processes were the same (posters, phone calls, direct visit), but word of mouth was far more effective for the all clear.