## Material and Methods
### Assessment of Recharge Value  
Hydrodynamic modeling studies in the Vistula valley were performed to provide a quantitative description of the components of groundwater balance within the Quaternary aquifer and create spatial distribution of groundwater depth. The modelling was carried out using the Visual ModFLOW 4.2. software [31]. The model was built under the steady state, defined as the average annual hydrodynamic state in the period of 1999-2008 and it concerned variants related to renaturalization of wetland areas [32; 33]. Lateral boundaries of this hydrogeological system were delimited along the main rivers of the region: in the north the Vistula River, in the west the Bzura River, in the south along the edge of the accumulative-erosional level, and in the east along the border of Warsaw. The lower boundary was set as the top part of poorly permeable Neogene clays.  
The relationship between the aquifer system and its environment was projected as boundary conditions: RIVER type--for Bzura, Vistula and small canals; General Head type--along south boundary with groundwater flow into Vistula valley from Warsaw-Blonie terrace side. The discretization step was set to 100 \(\times\) 100 m. The calibration process consisted of applying various values of infiltration recharge and underground evaporation with simultaneous verification of flow rates in rivers and canals [32]. Then, the hydrodynamic model was updated by applying three calculation variants of infiltration: modeling for average groundwater levels from the period of 1999-2013, for the lowest level (in 2003), and for the highest level (in 2011) [30]. The correlation statistics obtained at 49 monitoring points (located in the model area) and calculated head were as follows for all variants: standard error of the estimate 0.023-0.044 m; root mean squared 0.158-0.308 m; normalized RMS 0.953-1.849%.  
The rate of infiltration recharge was calculated from the hydrodynamic model for the period of 1999-2013 (average value) and for the lowest and highest groundwater levels (Figure 3). The greatest recharge rate values are found predominantly in dune areas. In marsh zones, which are characterized by the highest evaporimetric potential (hydrogenic soils), due to the shallow depth to groundwater, intensification of evapotranspiration-related losses causes a so-called negative recharge in the annual cycle. An analysis of seasonal changes [22] has shown that the effective recharge in the marsh areas usually occurs in springtime due to thawing. This is why pollutants can migrate with the percolating water, although, in the annual cycle, the observed losses are higher than the recharge rate. This phenomenon was the reason for covering the areas of negative annual balance by the vulnerability analysis. The recharge rate varies spatially within the range of \(-\)89-65 mm/year, and in the individual model variants, the differences in the average values are 6-12 mm/year. For the average groundwater level, the average recharge rate in this area was approximately 21 mm/year, with standard deviation of 35 mm/year. In 2003, the evaporation rate from the groundwater table in the marsh areas was higher than the recharge rate, and remained at a similar level to the average (\(-\)50-0 mm/year). In the dune areas, the recharge rate was clearly lower, by ca. 20 mm/year. The average recharge rate for this period was 15 mm/year, with an excursion up to 25 mm/year. In 2011, the recharge rate was 3 mm/year. In the dune areas, high recharge rates dominated (\(>\)60 mm/year), whereas in the marsh areas, low values prevailed (\(<\)\(-\)50 mm/year), which was related to intense evaporation from the groundwater table occurring at the shallowest depths as compared to the other variants.