The goal of an early warning water monitoring system (EWWMS) for quality control in a water distribution network is to identify low probability/high impact contamination events in source water or distribution systems. It should be able to detect not only intentional contamination, but also contaminants introduced accidentally or natural occurrences. Firstly, to ensure the full protection of drinking water, a technology-based EWWMS should be one component of the program. While laboratory technology exists to measure a wide range of contaminants, it's not the same for on-line and real time technology. A number of research projects are investigating rapid and on-line monitoring technologies, including biosensors and biochips, fiber optics and microelectronics. In most cases the performances of these systems has not been fully characterized. In this paper, a submergible UV/VIS spectrometer has been extensively tested in laboratory experiments, with the aim to check its limits of sensitivity, its ability to identify and quantify specific contaminants and its rate of false positive/negative.