## 3 Results and Discussion  
The DRASTIC index was calculated for three variants of parameter D: variant A--for the average depth to groundwater table from 1999 to 2013; variant B--for the lowest groundwater levels in a long period in 2003; and variant C--for the highest groundwater levels in a long period in 2011. The groundwater vulnerability to contamination, determined for the average level from the period 1999-2013, is characterized by a high proportion of the area with a moderately high vulnerability class (IPZ = 151-175), accounting for 41% of the study area, i.e., 231.87 km\({}^{2}\). The moderately high vulnerability occurs mainly in the dune zones (Figure 5). Despite the shallow depth to groundwater table in the marsh zones, the vulnerability is lower by 1-2 points, ranging from medium (IPZ = 126-150) in the margins of the zone, to low (IPZ = 101-125) in its central parts. The low vulnerability class is determined for the Vistula floodplain. The total area classified as having low vulnerability covers 138.80 km\({}^{2}\). The high vulnerability class (IPZ > 176) is identified in the area of 5.68 km\({}^{2}\) in some patches of the southern zone and in the suburbs of Warsaw, in the eastern part of the study area. The assessment of groundwater vulnerability to contamination for the average depth and recharge rates in the years 1999-2013 was the basis for the analysis of vulnerability changes caused by the fluctuations in the depth to groundwater table and recharge rates. The vulnerability determined for average groundwater levels from a long-term period has been defined as the base state. Vulnerability maps, constructed based on average input data values in various vulnerability assessment methods and procedures, are the basis for making spatial development and groundwater protection decisions. In the case of hydrogeological systems with unconfined groundwater table and significant changes in its dynamics, it is necessary to analyze vulnerability changes for various variants. The vulnerability determined for the lowest average levels has been defined as the vulnerability during hydrogeological drought. In the conditions of hydrogeological drought, the vulnerability is distinctly lower throughout the area. The medium vulnerability class occupies the greatest part of the study area, i.e., 216.26 km\({}^{2}\) (338%), mainly in the dune zones (Figure 5). The moderately high vulnerability covers an area of 190.75 km\({}^{2}\). Despite the shallow depth to groundwater table in the marsh zones, the vulnerability score is lower by 1-2, ranging from medium in the margins of the zone to low in its central parts. The low vulnerability class covers the Vistula River floodplain. The total area classified as having low vulnerability is 138.80 km\({}^{2}\). The high vulnerability class is determined for the total area of 2.12 km\({}^{2}\) in a few patches of the southern dune zone and in the suburbs of Warsaw, in the eastern part of the study area. The assessment results were the basis for analyzing the differences in the areas occupied by the individual vulnerability classes determined for the base state and for the conditions of hydrogeological drought. In general, the class distribution pattern has not changed significantly. The most distinct changes refer to the size of highly vulnerable areas (a decrease from 0.99% to 0.37%) and to the increase in the very low class area (from 0.21% to 0.32%). The analysis of spatial variations in the IPZ index values shows that its greatest decrease in relation to the average value was 15, and the average decrease was 3.19. Considering this change with regard to the size of the classes (IPZ = 25), the changes can be deemed insignificant. In variant C, for the highest groundwater levels in 2011, the vulnerability is close to the base level. The greatest area is covered by the moderately high vulnerability class, which is 221.75 km\({}^{2}\) of the study area, predominantly in the dune zones (Figure 5). Highly vulnerable areas occupy 197.99 km\({}^{2}\).