The article presents general positions and features of a water quality assessment according to the norms of the European Union (EU), shows their difference from the national (Ukrainian) standards. It is proposed to improve the Ukrainian standards for assessing a water quality in accordance with the EU standards. On the example of the Danube and the Dniester rivers, a chronological variability of the water quality indicators is considered, the time trends of the indicators are approximated and the parameters of their distribution laws are determined. The following dependences are established: when approximating the distribution of the indicators it is better to use the lognormal law; an indicators time trend is reflected more accurately by the exponential dependence; a lognormal distribution of a trend-normalized indicator can be formally considered as one-parameter (one of the parameters - an average value of the logarithms of the normalized series - is zero). It is shown that for the previous period of time and in the future when normalizing the discharges of pollutants together with wastewater, a water quality assessment in the control points of the water bodies will meet the requirements of the EU standards on a frequency of exceeding the maximum permissible concentrations with the security which is equal to 5 or 10% depending on the purpose of the water body (5% - for fishery facilities, 10% - for drinking and recreational water use facilities).