**1. Introduction**

Urban development, resulting in changes in spatial management, drought, and extreme flood events caused by climate change, significantly influences hydrogeological conditions and water efficiency [1,2]. Changes in spatial management directly influence groundwater recharge, including the infiltration, lateral inflow, surface runoff, evapotranspiration, and other elements of groundwater balance [3,4]. Urban development, therefore, results in serious problems in many areas, e.g., seawater intrusion beneath coastal cities [5,6], changes in groundwater recharge and discharge [7], and groundwater pollution [8–10].

A separate problem is the water supply in urban areas in the context of changing hydrogeological conditions. During the last 50 years, the water demand in Europe has risen gradually, which is related to the increase in the human population. Around 55% of the world's human population lives in urban areas. The population living in 17 capital cities of the EU was 71.1 million in 2014, and a stable increase in population was observed in all large European cities except Athens in the last decade [11]. This has led to a general decrease in renewable groundwater resources by 24% per person in Europe [12]. An indispensable element in preventing such hazards is groundwater valorization in urban areas, with regard to both its quantity and chemical status assessment. According to the assessment rules determined by the Water Framework Directive [13], poor chemical status was observed in 9% of groundwater bodies, and, in 75% of cases, the main reason was the decrease in groundwater levels [14]. Despite the increased population and problems related to water supply in urban areas, stabilization of water abstraction has been observed in some large European cities [14]. Diversified possibilities of water supply in a particular area, local hydrogeological conditions and uneven social–economic and

industry activities result in various ranges of groundwater level positions and recharge are possible in many urban areas.

In the studies of urban areas, the recharge assessment is extremely important but difficult. The usual methods of estimating recharge are available for use in urban areas as described by Lerner [15], Spalvins [16], and Schirmer [17]. Natural infiltration is modified as a result of land use, and the total balance should include the amount of water from leaks in water supply and sewage networks, which are difficult to estimate. Water loss in numerous underground water systems in Poland and the world is large, and the actual loss varies widely [18]. As much as 60% of distributed water may be lost through leakage from the distribution system [14], and losses in the water system reach, e.g., 18% in Great Britain, 30% in France, 20–34% in Spain and the Czech Republic, 30–60% in Croatia, and as much as 75% in Albania. Based on studies in 1998, the mean loss for 195 cities in Poland was 18.6% [19]. The cumulative impact of natural and anthropogenic factors, difficult to quantify individually, is reflected in the position of the groundwater table. Determining the range of dynamics of groundwater level change and their causes in short- and long-term scales is of crucial significance for social-economic activities, politics, and planning for sustainable development [1]. The influence of changes in land use and the climate on the hydrogeological conditions in urban areas depends on the location of the study area and the hydrogeological conditions [1,20]. This requires individualized studies with regard to specific known hydrogeological conditions. Methods of analyzing the impact on hydrological and hydrogeological conditions may be subdivided into three categories: experimental studies, statistical analysis, and modelling [20].

The aim of the study was to identify cumulative natural and anthropogenic causes controlling groundwater recharge and to assess their effects reflected in the groundwater table location. The cumulative impact of natural and anthropogenic factors on groundwater in an urban area and the dynamics of groundwater levels in particular aquifers have been determined by analysis of groundwater levels based on a 27-year (1993–2019) cycle of daily measurements of groundwater levels position derived from a monitoring system located in the centre of Warsaw (Poland). The analysis was focused on three aquifers: the shallow and deeper Quaternary aquifers and the Oligocene aquifer. The reliability of this analysis was assessed on standard measurements of groundwater levels in piezometers and wells, excluding the consideration of arbitrary elements or schematization of conditions for modelling studies. Direct measurements were subject to detailed statistical analysis. Assessment of the dynamics of groundwater level changes and trends of changes in various time intervals on a long-term basis allows for determining the range and variability of groundwater recharge in an urban area. Defining the variability of groundwater level changes, particularly aquifers, change trends, and range of groundwater recharge, points to the cumulative impact of factors characteristic of urban areas. The combined influence of both geogenic and anthropogenic factors shaping groundwater levels has been analyzed.

#### **2. Background of the Study Area**

#### *2.1. Location of the Study Area*

The groundwater monitoring system is located in the center of Warsaw (Poland) in the Research Station of the Faculty of Geology at the University of Warsaw. The research station is located on the crossroads of two busy streets, Zwirki i Wigury and Banacha, in the Ochota district. The district ˙ is characterized by the prevalence of multifamily buildings, similarly to most of left-bank Warsaw. In turn, single-family houses dominate right-bank Warsaw. Larger areas of undeveloped zones (park and forest complexes) are found in the urban area peripheries (Figure 1). Warsaw is located on two sides of the Vistula River and is slightly elongated along its banks (extending ca. 30 km along the N–S direction and at ca. 28 km along the W–E direction). The urban area covers an area of 517.24 km2. The population is 1.8 million at a density of 3500 people/km2.

**Figure 1.** Location of the study area.

The city is located on two sides of the Vistula River, the largest river in Poland. Two types of terraces can be distinguished along the river: a modern floodplain covering a larger surface area in the southern part of the city and older terraces with the largest surface area in the northern part of right-bank Warsaw. Moraine plateaus extend from the river to the west as the Warsaw Plain and to the east as the Wołomin Plain. The research station is located on the Warsaw Plain (Figure 1).

Groundwater recharge and drainage are variable within the urban area. In the centre of Warsaw, infiltration and its drainage are restricted by spatial management and a sewage system (stormwater drainage) typical for urban infrastructure. Precipitation from surfaces covered by non-permeable materials (roads, pavements, and roofs) is distributed by the sewage system, and thus effective infiltration is completely reduced. Groundwater recharge takes place through lateral inflow from the neighboring areas and to a large degree through water loss from the water–sewage system of the urban area [21,22]. Infiltration has a significant contribution to groundwater recharge and resource formation in other parts of the urban area.

#### *2.2. Geology and Hydrogeological Conditions*

The research station is located within a denuded post-glacial plateau located at an elevation of 108–115 m a.s.l. and incised by a glacial valley. The oldest series drilled in the Warsaw region are Upper Cretaceous strata, developed as grey marls interbedded with limestones and covered by Oligocene strata developed as sands, silts, and clays with glauconite and phosphorites. Miocene deposits include sands, silts, and clays with interbeds of lignite. The Pliocene comprises variably colored compact clays, the so-called variegated clays, and clay and sandy silts. The Pleistocene includes glacial, fluvioglacial, and ice-dammed deposits of the Podlasian, Sanian, Odranian, and Vistulian glaciations, and the Cromerian, Mazovian, and Eemian interglacials. In the vicinity of the research station, these deposits are developed as glacial tills located on silts, sands, and ice-dammed clays (Figure 2). The geological succession of the subsurface zone comprises (from the top) anthropogenic embankment deposits, sandy silts, clayey sands, glacial tills, and fine-grained and silty sands (Figure 2). At a depth of about 7.6 m b.g.l. within fine-grained and silty sands is the first Quaternary aquifer (first aquifer) with a water table monitored by piezometers. The deeper Quaternary aquifer (second aquifer) with a confined water table is located within fine-grained and poorly sorted sands at a depth interval from 26 to 46 m b.g.l. The water table drilled during the well construction at the depth of 18 m b.g.l. became stabilized at 9.95 m b.g.l. (Figure 2).

**Figure 2.** Geological profile and location of the groundwater tables in aquifers.

Within the Warsaw agglomeration, waters of the Quaternary aquifer remain in hydraulic contact and are combined in one aquifer in the northern part of the urban area. Generally, groundwater in Quaternary deposits with hydraulic conductivity in the range of 25–30 m/day, whose thickness increases northwards and flows from the east with a velocity of 30–100 m/year [23].

Below the Quaternary aquifers is a 100-m thick series of Pliocene clays and Miocene silty sands, silts, and clays with lignite interbeds, with variable thicknesses. The deepest Oligocene aquifer (third aquifer) recognized in Warsaw in the monitoring system is found in fine- and medium-grained sands with glauconite at a depth interval from 225 to 263 m b.g.l. The groundwater table drilled at a depth of 221 m b.g.l. became stabilized at the depth of 35.75 m b.g.l. during the drilling.

#### *2.3. Water Supply in Warsaw*

The Warsaw agglomeration is supplied about 96% of its water from surface water sources, and the remaining 4% is from groundwater sources. Surface water comes from the Vistula River and the Zegrze Reservoir (located 7 km north of the Warsaw border). The water system, with a total length of 4215.7 km, covers almost the entire area of the Warsaw urban agglomeration. The population of Warsaw uses about 340,000 m3 of water per day [24], and the daily use of water is about 135 L/person [25]. Groundwater is exploited mainly by industrial plants. About 500 intakes from the Quaternary aquifers and about 100 intakes from the Oligocene aquifer, exploiting almost 25,000 m3/day, are located in the Warsaw agglomeration. Due to its quantity and quality, groundwater represents the strategic groundwater resources of the urban area. In the entire Mazovian voivodeship, of which Warsaw is the largest urban area, the exploitable groundwater resources from Quaternary deposits are in the range of about 205,000 m3/h, and from the Palaeogene-Neogene deposits, they are in the range of 17,500 m3/h [26].
