Nitrates are worldwide recognized as one of the most common types of groundwater pollution in intergranular aquifers. Due to the fact that wellfields are particularly vulnerable because of its impact on groundwater flow, today more attention is focus to the protection of their impact zones. The purpose of this paper is to make a scientific contribution to understanding the transport of pollution in groundwater in the area of wellfield impact zones and based on mathematical, statistical, geostatistical and numerical methods to locate and quantify diffuse and point sources of pollution. For this purpose, a research area has been determined and it includes three wellfields in the wider area of the town of Varaždin. The methods used in this paper can also be applied to the locations of other wellfields in intergranular aquifers. However, each wellfield case needs to be approached individually in order to properly set up the research concept and obtain reliable results. The high nitrate concentration was a long-standing problem in groundwater in the study area, as a result of which one wellfield was already closed, while on the others a slight increase in the concentration of this parameter has been recorded. For this reason, the locations of these wellfields are suitable for conducting this type of scientific research. Within this paper certain parameters of groundwater quality at wellfields Varaždin, Bartolovec and Vinokovščak in the period from 1993 to 2017 are analyzed. Groundwater is generally of good quality, except in terms of nitrate concentration at the Varaždin wellfield, where the values of this parameter are above the maximum allowable concentration. In recent years, the pumping rate has been increasing at the wellfields Bartolovec and Vinokovščak and in the same period an increase in nitrate concentration was observed, therefore the relationship between nitrate concentration and pumping rate is analyzed. A good correlation of the data sets is observed in the upper aquifer, while the correlation is absent in the lower aquifer. The correlation between precipitation and nitrate concentration at the observed wellfields is analyzed and a good to strong correlation between the two data sets is observed at almost all locations, and the values of the correlation coefficient are generally higher than 0,75. Based on the developed groundwater flow models, the influential areas of the observed wellfields in working conditions are determined and the sources of nitrate pollution are located, which can generally be divided into non-point and point sources. Considering the time of groundwater retention in the horizontal flow, it is concluded that the water protection zones at all three observed wellfields are marked in accordance with the applicable ordinance. Spatial distributions of nitrate concentration at the observed wellfields obtained by the Kriging method show an increase in concentration in a certain direction, and at the locations of Varaždin and Vinokovščak wellfields the increase is in the direction of point sources of pollution. The statistical and geostatistical methods are used to quantify the sources of pollution at the Varaždin wellfield, and the results indicate that the poultry dumpsite in the influential zone of the wellfield is the main source of nitrate pollution. Based on the developed numerical groundwater flow model at the Varaždin wellfield, a simulation of nitrate transport by advection and dispersion processes is performed. The simulation results also indicate the location of the poultry dumpsite as the largest source of nitrate pollution, however, poultry farms, which are located in the area of influence of the wellfield, also contribute significantly to the nitrate concentration at the wellfield.