Agricultural activity of spreading fertilizers may, in case they are washed away to an aquifer, result in a groundwater contamination. There was a research conducted in Croatia on the lower part of the Danube-Sava canal which is occupied by extensive agricultural production to measure the groundwater pollution by nitrate nitrogen, ammonia nitrogen and phosphorus. Amounts of these nutrients were measured six times a year over the period of fifteen years (2004. - 2018.) by installed hydrogeological piezometers. The obtained data, given for relatively long period of time, were analysed by methods of time series to determine trends. It was established that although concentrations of nitrate and ammonia nitrogen occasionally exceeded the value of maximum allowable concentration, long term trends of all pollutants are still slightly decreasing.