The results of statistical processing of data on the concentration of oil products in river water are presented. According to the monitoring results for the period 2002-2016, the concentration of oil products in the Ufa River water and tap water in Ufa was less than 0.05 mg/dm3; in the Belaya River water it was in the range of 0.05-0.4 mg/dm3. To identify the influence of deterministic (cyclical and seasonal trending) and random factors on the concentration of oil products in the Belaya River water the method of decomposition of a time series using an additive model was used. The hypothesis of an increase in the trend in the concentration of oil products in the Belaya River water was confirmed. The probability of exceeding the concentration of oil products in river water above the established value of the maximum permissible concentration is estimated. It has been stated that the concentration of oil products in the Belaya River water sampled upstream and downstream the city is dominated
