1. IntroductionProgressive urban development is adversely affecting the quality of surface water and the performance of sewage systems. This impact can be clearly seen through the disruption of the natural, dynamic, quantitative balance between precipitation and surface runoff processes. Urban catchments are characterized by a dynamic increase in sealed surfaces as a result of the construction of new buildings, roads, sidewalks, or parking lots, which contribute primarily to rapid and fast surface runoff [1], but also to an increase in the concentrations of pollutants entering the receiving watershed. The immediate cause is runoff from roofs, roads, or parking lots, which transport more and more pollutants in an increasingly shorter time [2]. Another factor that negatively affects surface water quality is the development of automobile transportation. It indirectly affects water quality because it is the source of a large amount of various pollutants entering the air.As a result of the greenhouse effect, an increase in the number of extreme weather events, e.g., hurricanes, droughts, and heavy rainfall, is observed every year, thus determining the need to change the approach to the design of sewage systems [3]. There is a constant search for more and more effective tools for predicting possible hydrological risks and for the assessment of the functioning of the network (its overloading) and individual elements of the stormwater or combined sewer system [4,5].Another important environmental issue, mainly for surface water quality reasons, is the development of methods for estimating the impact of rapid precipitation on the stormwater recipient, taking into account both the quantity of surface runoff and its quality [6]. The contact of precipitation with airborne pollutants causes it to be already initially polluted. Subsequently, pollution occurs as a result of contact with the catchment area and the formation of surface runoff, where along its path the concentration of pollutants gradually increases, culminating in the incorporation of the sewer system into the receiver [7]. The largest loads of pollutants enter the receiver from high-intensity and short-duration precipitation. Extended over time, low-intensity precipitation does not contribute significant flush loads of pollutants. The level of stormwater pollution is also influenced by the processes of accumulation and leaching of pollutants both in the catchment area and in the drainage system [8]. The rate of pollutant build-up and wash-off is very variable and depends to a large extent on the development and location of the catchment. Additionally, the variability of these processes results from the variability and intensity of precipitation during the year, which determine the load remaining after rainfall and the variability in the occurrence and length of precipitation-free periods [9,10]. Similar relationships also apply to the quality of surface water, which deteriorates in particular during floods (pluvial and fluvial floods) as a result of increased inflow of pollutants from drainage systems and polluted surface runoff, and also as a result of erosion processes [11,12].Depending on the type of catchment development, surface runoff varies, both hydraulically (culmination time, volume of runoff) and qualitatively (concentration, pollutant load). Surface runoff is often classified in relation to the type of development [13,14,15].It cannot be overlooked that the progressive pollution of the environment (in this case, surface water) affects not only nature, but also human health. It is estimated [16] that in the last 10 years as many pollutants have entered the Earth’s environment as in the previous 70 years. This is because the rate of spread of pollutants is increasing, which means that, for example, cancer in humans will occur more frequently. This fact is also confirmed by the World Health Organization’s announcement that as many as 75% of human diseases are due to poor environmental conditions [16].It is therefore necessary to take appropriate measures to improve water and wastewater management in urban areas. This involves conducting studies or a creating a continuous monitoring system for the quantity and quality of stormwater discharged into surface waters [17]. Actions that should also be taken in the long term include the use of the potential of the blue–green infrastructure (BGI) and the management of water at the place of precipitation [18,19]. There are a number of combinations of gray and green infrastructure elements that make it possible to combine individual functions—e.g., retention and infiltration reservoirs that stop water runoff and allow it to slowly seep into the ground— with sedimentation ponds, plant passages, and other bioretention solutions, apart from retention, that participate in mechanical and biological water purification. Changes to the existing underground rainwater drainage systems require significant financial outlay. Their way of functioning requires improvement, so efforts should be made to relieve the network by building retention reservoirs, retention and infiltration reservoirs, or rainwater management at the site of precipitation. A number of studies prove that a greater degree of implementation of green solutions brings lower maintenance costs and better reduction of the amount of surface runoff and improvement in its quality [20,21,22,23]. In urbanized areas in particular, there are many conflicting economic, social, and economic interests related to water management. Urban rivers have long been used at suitable sites to discharge sewage and stormwater, leading to severe damage to aquatic ecosystems, often to the point that they no longer provide ecosystem services to society [24,25]. The current approach is primarily to pay attention to and properly value ecosystem services and to implement the idea of sustainable development [26,27,28]. Among the many benefits of this approach are those that relate to the environmental and social impacts. The environmental impact includes the following benefits: (a) reduction of rapid floods in watercourses, (b) reduction of water pollution, and (c) improvement of soil and water conditions in the catchment area. In turn, benefits of a social nature include: (a) eliminating (at least partially) losses due to flooding, (b) strengthening the ecological awareness of the inhabitants, and (c) improving the aesthetic values of urban areas [17].Despite a number of studies cited above, it is still unclear what impact urban development without proper stormwater management has on surface water quality. Recognizing these problems, the present study investigated the quality of surface water runoff from urbanized areas and the quality of receiver waters. The aim of the study was to identify pollutant emissions from stormwater drainage systems in urbanized areas in the studied real catchment area of the Sudół River in Krakow. Additional objectives were: analysis of the impact of stormwater pollution on the quality of the Sudół River (threat to achieving the environmental goal: good water status) as well as assessment of environmental risk, i.e., the likelihood of negative effects as a result of exposure to potentially toxic environmental pollution. It is possible to assess the threat to achieving the environmental objective and the environmental risk in the studied catchment area using data from the measurement of concentrations of the following parameters: water quality in outflows from the stormwater drainage system and water quality of the Sudół River, to which stormwater is discharged. The influence of the type of development on the quality of surface runoff was evaluated by estimating and comparing the average concentrations for two different areas: (1) residential development and (2) commercial–service area. The results of pollutant concentrations determined in stormwater and the river were also compared to the limits contained in current regulations [29]. In addition, the authors compared changes in development in the study catchment in 2000 and 2018 and estimated how these changes affected the increase in surface runoff. The impact of the quality of discharged stormwater on the quality of the receiving water body—the Sudół River—was assessed. An innovative aspect of the work is the carrying out of an environmental risk assessment of pollution indicators for surface runoff from residential and service–communication areas and surface water to prevent environmental risks. 4. DiscussionFigure 11 shows the analyses carried out for the Sudół River catchment case study, which is subject to intensive changes in land use in Krakow.The analysis of CLC 2000 and 2018 shows an increase in sealed areas (Figure 7 and Figure 8 and Table 5), which causes an increase in flow (Table 6). In the section closing the catchment for the analyzed design precipitation of 19.38 mm (with p = 20% and duration of 15 min), an increase in flow of 2.62% was estimated, but in selected areas it may cause a larger increase in runoff, e.g., analyzed outlet 2 may increase by 16.22%. Similar analyses were performed by, among others, Ociepa and Suligowski for the urbanized catchment area in Kielce, Poland [11,100], Sjöman and Gill for a catchment area in Sweden [67], and Li et al. for the city of Shenyang in China [66]. However, the results are difficult to compare due to the individual nature of each location (land use and land cover, soils, climate, etc.). The main objective of this work was to investigate the qualitative aspects of stormwater. Stormwater discharged from areas of different land use have different quality parameters. Measurement campaigns were undertaken to determine the concentrations of 12 key parameters: (1) the quality of stormwater in the outflows from the drainage system from two urban areas of different sizes and different land uses, and (2) the water quality of the Sudół River, to which the stormwater is discharged. The results of 10 measurements of the quality of stormwater are presented in Table 8 and Table 9. The obtained results of the study were also referenced in a review study by De Buyck et al. in 2021 [101], which reviewed 39 publications from 1999–2019, based on which, among others, the average and maximum concentrations of pollutants in stormwater were calculated. A comparison of the obtained values of the average and maximum concentrations of the pollutants studied in the present study and the calculations made by De Buyck et al. is presented in Table 12. In order to relate the obtained results of the study to previous Polish studies, a comparison was made with the results of Strzebońska et al. [102], who conducted a study of the quality of roof runoff in Krakow, and studies of the quality of stormwater in cities by Poznań [103], Częstochowa [104], and Kielce [105].

The calculated mean and maximum concentrations from all measurements (outlet 1 and outlet 2) show higher values for all tested biogenic compounds; additionally, the determined mean concentration for COD is higher than in the work of De Buyck et al.

In the study [102], 31 pollutant indicators were determined, including N–NO3, Cu, and Zn indicators in common with the present work. Demonstrated concentrations in roof runoff, which should be of better quality than the runoff studied in our work covering runoff from rooftops, roads, and parking areas, were lower for N–NO3 and Cu indicators, while Zn concentrations were higher. The determined concentrations in outlets 1 and 2 were also compared to other studies on stormwater quality in Polish cities, presented in Table 12:The study in Poznań [103] includes 8 parameters in common with the present work. The results of stormwater quality in Krakow were worse in terms of average concentrations for indicators TSS, TKN, N–NO2, TP, and Cu; however, significant differences are found for N–NO2: more than 18 times higher mean and max concentration; TP: more than 2.7 times higher mean concentration and 4.8 times maximum concentration; and Cu: more than 2 times higher mean concentration, but the recorded maximum concentration is lower by half.In the study in Częstochowa [104], three parameters common to this study were taken into account: TSS, COD, and Cu. The results of the quality of stormwater in Krakow were worse in terms of mean Cu concentration (10 times higher), COD with similar mean concentration, and TSS with a two-times lower concentration.In the study in Kielce [105], four parameters common to this study were taken into account: TSS, Zn, Cu, and Hg. The results of stormwater quality in Krakow were better in terms of average concentrations for TSS (just below the lower limit of the range of mean concentrations), Cu (about 2 times lower than the lower limit of the range of mean concentrations), Hg (over 1000 times the lower limit of the range of mean concentrations), and Zn (at the upper limit of the range of average concentrations).The analyses carried out and the concentrations obtained prove that land use has an impact on the quality of stormwater and, as a result, on the quality of surface water. Concentrations from two drainage outlets were examined: a small residential area (outlet 1) and a residential and commercial area with a developed transportation network with heavy traffic (outlet 2). The comparison made in Table 10 and Figure 9 shows that the more intensive development (outlet 2), which includes, e.g., commercial areas and high-traffic roads, results in average concentrations higher than in low-density residential areas (outlet 1). In particular, this applies to such pollutants as TSS, COD, HOI, Cu, and PAHs; their average concentrations were more than two times higher in outlet 2 than in outlet 1. Similar conclusions were obtained, e.g., in a study for different types of land use in Singapore [106]: concentration in stormwater from residential area is lower than from areas such as business districts, industry, and residential roads in term of parameters TSS, Zn, and Cu. A similar study was also performed by Wang et al. [13] showing that the average concentrations of TSS, COD, Zn, and Cu in runoff of rainwater in Chongqing (China) from urban traffic roads are much higher than from residential roads, commercial areas, and roof runoff. Paule et al. [15] studied the relationship between land use change and stormwater runoff quality in Yongin, South Korea. A correlation has been shown between the increase in concentrations of TSS, COD, TN, and TP and the increase in commercial, parking lot, residential, and road areas.Threats to aquatic ecosystems were investigated through environmental risk assessment for stormwater discharged through outlets 1 and 2 and the Sudół River. The magnitude of the RQ for COD, TP, and N–NO3 was calculated, taking into account the limits for waters in which freshwater fish can live. Due to the high concentrations of N–NO3, it is reasonable to believe that this compound could cause negative effects among fish. In the river, the RQ is more than three times the values for which such an impact should be expected. Despite the fact that phosphorus and nitrogen are essential nutrients, their excess in the waters leads to eutrophication. Algal blooms limit the development of shallow-water vegetation and produce poisonous substances that are a threat to animal organisms and human health and life [107]. A significant amount of suspended matter in the water is not toxic in itself, but the threat is posed by various substances sorbing on it that are dangerous to the aquatic ecosystem [108]. In the studies conducted, a positive correlation between RQ for suspended solids and heavy metals is noticeable. Cu compounds can cause significant risks to the aquatic environment. They are considered harmful to aquatic ecosystems, and crustaceans are considered the most sensitive organisms [95]. Fish, on the other hand, exhibit a wide range of toxicity values, but their ability to reproduce and grow can be impaired when chronically exposed to Cu [109]. Zn shows toxicity to aquatic organisms, especially plankton [110]. According to Gebar et al. [91], a negative effect occurs in half of the arthropod population studied at an RQ of 7.3 calculated according to the PNEC adopted by the authors. In the calculations carried out for the realistic scenario, this value was exceeded at least twice, which clearly suggests that a negative effect of exposure of living organisms to this element is very likely to occur. Another highly toxic metal is Hg, and its presence in surface waters poses a threat to living organisms. Its compounds can accumulate in mollusks, fish, and successively further up the food chain to humans [111]. Hg concentrations at ng/L levels cause toxicity in Daphnia [112], so of the three freshwater locations studied, these organisms are most vulnerable in river waters. In contrast, a study by Zhang et al. shows that fish have a higher tolerance to Hg than do phytoplankton and invertebrates [113]. In addition to heavy metals, PAHs are well-known contaminants due to their strong carcinogenic and mutagenic properties [97]. These compounds, despite their low water solubility and hydrophobicity, have been found in surface waters. The results obtained for the realistic scenario correlate with the literature data. The RQ for four select PAHs in Yellow River waters in China is 114], while in Brazil it is up to 4 [97].The conducted research proves that urbanization and the accompanying changes in land use have led to changes in hydrology and increased pollution of surface waters, and this may pose a threat to aquatic ecosystems in the Sudół River catchment. For this reason, it is important to introduce stormwater management rules to stop such negative trends and reduce threats. There are many studies that show the beneficial effect of the use of stormwater control measures (SCMs) on reducing pollution and surface runoff. Pennino et al. [115] indicate that the use of stormwater green infrastructure brings a significant reduction in flash hydrology and pollution concentration. SCMs reduce the concentration of phosphorus [116,117], and they can limit, delay, or stabilize the supply of nitrogen [115,118,119,120]; in the case of suspension, no influence on their reduction is shown [115,116], but of course it depends on the type of SCMs, their location, and the scale of the solutions used [115,118,119]. A study by Walsh et al. [116] showed that extensive use of dispersed SCMs can reverse the negative effects of urbanization and improve stream water quality. Therefore, it seems advisable to introduce administrative recommendations (or even an obligation) to apply stormwater control measures for all new investments, as well as to strengthen their implementation through economic instruments, such as rainwater charges and investment co-financing. Economic incentives can also induce owners of already built-up real estate to change their stormwater management. As we have shown in our previous work [121,122,123], the existing economic instruments in Poland need to be changed in order to effectively encourage property owners to invest in sustainable rainwater management. 5. Conclusions

An assessment of the impact of land use changes and stormwater management in selected developed areas (a small residential area and a larger residential–commercial area with an intensive traffic network) on the surface water quality of the Sudół River was conducted, with the following findings:

The changes in land use from 2000 to 2018 were estimated at the scale of the entire catchment and their impact on the change in sealing and changes in hydrology, showing that progressive urbanization has resulted in the conversion of land used for agriculture into residential land (an increase of more than 96%) and industrial and commercial land (an increase of 113%), resulting in an increase in the degree of sealing (the CN curve at the scale of the entire catchment changed from 77.72 to 78.28), which is reflected in an increase in surface runoff and flows in the river (hydrological modeling for precipitation with p = 20% shows a 2.6% increase in flow in the estuary section of the catchment).

Changes in development lead to changes in hydrology: a clear impact was found from the analyses in 1 of the 2 areas studied: the residential–commercial area, where 31.76 ha of land changed its use in the period 2000–2018 (which accounts for 40% of the area), resulting in changes in the CN curve from a value of 85.47 to 88.05 and a 16% increase in outflow from the stormwater drainage system for p = 20% rainfall.

We conducted a study of the quality of stormwater discharges from the analyzed 2 areas to show significant pollution, in particular, in terms of such pollutants as TSS (average concentration in outlet 1: 45 mg/L, in outlet 2: 164 mg/L), petroleum hydrocarbons (HOI in O1: 0.36 mg/L; in O2: 0.9 mg/L), PAHs (in O1: 0.1689 µg/L; in O2: 0.6438 µg/L), and heavy metals (Cu in O1: 0.03 mg/L; in O2: 0.0685 mg/L, Zn in O1: 0.4 mg/L; in O2: 0.547 mg/L, and Hg in O1: 0.0003 mg/L; in O2: 0.0004 mg/L). Concentrations of these pollutants in particular from outlet 2 from a residential–commercial area with a heavy traffic transportation network were 2 times (TSS, Cu, and Hg), 3 times (Zn), and even 4 times (HOI and PAHs) higher than in the waters of the Sudół River.

Estimated pollutant loads contributed by stormwater may account for a significant share of the loads observed in the river in the 130 m cross-section downstream of outlet 2. Calculations conducted for precipitation p = 20% and average concentrations show that outlet 2, draining from a highly urbanized, sealed catchment, may account for more than 40% of the load of petroleum hydrocarbons and PAHs, as well as 21–37% of the load of heavy metals analyzed.

Environmental risk assessment of surface runoff and waters of the Sudół catchment shows the highest risks for N–NO3, with the highest risk found in the river waters. High risks are also shown for heavy metals, the highest for Cu concentrations in stormwater discharged by outlet 2; for this outlet, a significant level of risk is found for Zn. For waters from outlet 1, a significant level of risk is found for Zn and Cu.

The existing approach to stormwater management in the form of its discharge directly into the waters of the Sudół River and drainage ditches without treatment may be responsible for the exceedances of permissible concentrations in the river in terms of the indicators TN, TP, Zn, Cu, Hg, petroleum hydrocarbons (HOI), and polycyclic aromatic hydrocarbons (PAHs), as the recorded concentrations of these indicators in particular in outlet 2 exceed the concentration limits allowed for Class II surface water.

Such problems are likely to occur in 1/3 of the Sudół catchment area—this percentage is currently made up of residential neighborhoods and industrial and commercial areas. If the catchment area is subjected to further continuous development, this may contribute to the persistence of poor physical and chemical status or even its deterioration, and thus contribute to the threat of not achieving good water status in the Sudół catchment area.

Development is inevitable, but it is necessary to strive for stormwater management that will limit surface runoff and reduce its pollution. The use of green infrastructure can reduce stormwater pollution [124,125], as studies show that it is possible to apply solutions that can reduce both biogenic pollution [20,120,126] and substances such as heavy metals [127,128] and PAHs [35,129].