The composition of water and wastewater, varying temporally and spatially, depends on factors such as environmental context, types of pollution sources, weather conditions leading to dilution or solids transportation, length of sewer network, etc. Because quantitative parameters are often not adapted for the characterization of wastewater quality variability, a non-parametric measurement is proposed, based on comparison of the UV absorption spectra of samples. The presence of isosbestic points, occurring in the set of spectra either directly or indirectly after normalization, allows quantification of the variability of a given water or effluent. A normalization step is used when dilution exists in the case of a mixture of water types (discharge or rain). Several examples show how to calculate the variability or to estimate the dilution factor from UV spectra data, even without results of physicochemical parameters.