Would it be possible to have a mountain spring that only flows occasionally, perhaps only for a few days, following moderate rainfall?
The spring water would feed into an otherwise stagnant lake, causing it to overflow and allowing the water to flow into several streams and rivers further downhill.
There comes a time when the stagnant water becomes tainted, and this goes unnoticed until the isolated lake is filled with additional spring water, allowing it to flow into the water supply of a human settlement all at once and claim several lives.
For reasons relevant to the story’s setting the poisoning of the lake water is completed over the course of a single week, and I need a reason to justify the water becoming isolated because I imagine it would be much more difficult to do the same to a continually flowing water source.
I would like to be able to justify this without the use of a dam to control the spring water’s flow.