Safe drinking water is a vital matter of our daily life. In Switzerland, drinking water also reflects a high emotional value. A vast number of citizens of the city of Zurich use their tap water as drinking water. This reflects the trust in the Zurich Water Supply (WVZ) also depicted in excellent ranking of WVZ in relevant polls. To keep up this trust, it requires that WVZ undertakes all efforts in order to keep the drinking water safe. In this paper a microbiological contamination event, measured in a groundwater harvesting field of WVZ, is presented, which necessitated immediate action in order to modify the safety control concept. The lack of adequate measurement tools required the temporal definition of indirect control points to prevent any repetition of the contamination event.