In the sixties people began to raise awareness on the need of water protection, triggered by numerous contamination of the water environment. Consequently, extra care has been taken to protect public supplied water sources, so that further contamination is avoided. The regional well field located in Eastern Slavonia, is very important for its supplies of high quality groundwater. The Water Management Strategy (Official Gazette 91/08) marks this area as strategically important for drinking water supply for the Republic of Croatia. So far the quality of the groundwater was not affected, due to the hydrogeological characteristics of the covering layers that provide good protection for the aquifer system. However, it is very important to protect this natural resource from increasing anthropogenic impact, particularly through the protection of groundwater in the regional well field “Eastern Slavonia”. The study encompasses the wider area of the regional well field “Eastern Slavonia” – Sikirevci. The well field is situated in the Brod-Posavina County, to the east of Slavonski Brod and west of Slavonski Šamac which is between the settlements Sikirevci and the Sava River. The wider area of Eastern Slavonia included in the study is located between Županja in the east, Strizivojna in the North and Donji Andrijevci in the west. Due to the spatial characteristics of captured aquifer, the study area was expanded to include the Federation of Bosnia and Herzegovina, to Kostrč in the east, Donji Svilaj in the west and Odžak in the south. The wider area of regional well field "Eastern Slavonia" is built of Quaternary deposits with high, medium and small transmissivity. The area with high transmissivity is situated mainly by the Sava River and on the border of the Federation of Bosnia and Herzegovina, where the aquifer is composed of gravel and sand with high transmissivity and high hydraulic conductivity. The regional well field Eastern Slavonia - Sikirevci is situated in the area where the Quaternary deposits with medium transmissivity are captured. The aquifer is composed of gravel and sand layers with thickness that reaches values over 90 meters. The results of the study show that it is evident that the groundwater in the area is good quality and can be used for water supply. However, the values of certain indicators, such as arsenic, manganese and iron show a positive trend, i.e. its concentration is increasing in time. Available data from analyses of the groundwater quality from the pumping wells in the wider area of the regional well field indicate that the concentrations of arsenic, ammonium, manganese, iron, chemical oxygen demand and nitrates sometimes exceed the maximum allowable concentration for drinking water. That can be the result of reductive conditions in the aquifer system. Multivariate statistical cluster analysis yielded the overview of the groups of elements based on their geochemical affinity and/or origin. Based on the mean values of the compared parameters, groundwater quality is clearly higher in the area of Sikirevci than in other studied areas (Bosanski Šamac and Odžak), which is most likely due to contamination sources located in the part of the study area located in the Federation of Bosnia and Herzegovina. Anthropogenic contamination derived probably from domestic wastewater due to lack of public utility infrastructure and wastewater treatment facilities, as well as unsupervised application of agrotechnical chemicals. Therefore, there is a need of assessing the impact of potential contamination on the aquifer system that could endanger groundwater quality, despite the favourable geological and hydrogeological characteristics of covering layers and their protective role in study area. The present paper displays the intrinsic vulnerability of the wider area in the regional well field "Eastern Slavonia" by applying SINTACS, AVI, GOD, COP, Germany and Ireland methods was evaluated. Also, the specific vulnerability to nitrates and lindane was estimated using the COP method. Areas with very low and high vulnerability by applying AVI, GOD and COP methods occupy most of the study area. Results SINTACS, GOD and AVI methods indicate that the areas around the Sava and Bosnia Rivers and the areas around Slavonski and Bosanski Šamac, Zorica and Kruševice the most vulnerable. These areas are in the hydrological sense singled out as areas with a minimum thickness of covering deposits, which means the minimum depth to groundwater. Areas with the highest degree of vulnerability were recorded using GOD and the German method, whilst the average level of groundwater is observed. Using COP and AVI methods, there were only low to very low vulnerability and COP methods negligible in central vulnerability. The results of the application of the COP method for determining the specific vulnerability show that the area of low specific vulnerability to nitrates and lindane is on the central part of the study area. This is in the area where the groundwater level at the time of high water are nearest the surface, i.e., the thickness of the unsaturated zone is the smallest. The wider area of regional well field "Eastern Slavonia" is partially in the category of very low vulnerability, and partially in the category of low vulnerability to nitrate and lindane contamination. The vulnerability maps from the results of the analysis can be one of the bases when defining the approach for appropriate management of this strategically important groundwater reserve. Intrinsic and specific vulnerability maps should be used when defining the protection zones of groundwater in alluvial aquifers in other similar areas in lowland part of the Croatia. They can be used as well as irreplaceable background for decision making in management and land use.