Informing users of waterworks systems about the quality of tap water is an obligatory trend. It should be accompanied by studies on the influence of the risk of threats on public health. Waterworks systems, being included in a critical infrastructure of the city, should be subject to special protection in this respect. In the paper, the authors' method of assessing threats to people and property from waterworks systems functioning in self-government units (SGUs), is proposed. Four categories of factors affecting the risk of threat to tap water consumers were assumed: the frequency or the probability of exposure-P, financial losses-C, damages to peoples' health-HL, the degree of the security-S. Based on this, a four-parametric risk matrix was developed. It was assumed that risk is a function of the parameters mentioned above: r = f(P, C, HL, S). For every parameter the five-parametric weight scale was assumed. An example of applying the method is presented. The proposed method should be an important element of water safety plans. It can also be adopted for other municipal systems subject to SGU.
Keywords: risk; risk matrix; self-government units; waterworks systems.