# Change in Water Quality of the Vistula River During the Emergency Discharge of Untreated Wastewater
## 1 Introduction  
The migration of people from rural to urban and industrial areas contributed to an increase in urbanization [1, 2]. Numerous city studies show positive and negative socioeconomic processes occur here [3, 4]. In communist countries such as Poland, industrial districts of cities, such as Nova Huta in Cracow, were developed rapidly. Unfortunately, at that time, ecological aspects were often overlooked, which harmed the natural environment [5]. After joining the European Union, Poland implemented the wastewater and water framework directives [6, 7]. Poland's environmental protection required a reduction in the amount of nutrients from municipal sources discharged into water and soil. Poland is a top country contributing to nutrient pollution because the Baltic Sea provides 25% of the total nitrogen and 32% of the total phosphorus load [8]. The treaty obligations also indicated that all agglomerations with a population equivalent greater than 2000 people must be equipped with collective sewage systems and sewage treatment plants. Water pollution may be caused by planned or accidental release of various pollutants and chemicals [9]. Sewage discharge can cause multiple environmental damages due to changes in water resources and reductions in biodiversity. Untreated sewage affects water quality in water bodies and human health [10]. A total of 55% of used freshwater resources are discharged as municipal and industrial sewage. One of the sustainable development goals is to ensurethat people have access to clean drinking water and emphasize the sustainable management of sanitation [11; 12]. SDG 6.3 states, \"By 2030, improve water quality by reducing pollution, eliminating the storage and minimizing the release of hazardous chemicals and materials, halving the share of untreated wastewater and significantly increasing recycling and safe reuse globally\". Due to the inflow of pollutants, the water cannot be used for drinking and irrigation purposes. The problem of untreated sewage discharged into water bodies occurs in Antarctica, USA, Serbia [13; 14; 15]. One of the most common point water pollutants is leaky septic tanks, which are only used for periodic accumulation of sewage. Emergency discharges of pollutants from large sewage treatment plants in cities are much less common. More than 2 billion people live in areas where there is a risk of limited access to drinking water. One of the factors limiting the use of water is its pollution. Pollution can be point- or surface-based. Point pollution mainly includes sewage treatment plants, storm sewer outlets, and industrial activities. Contemporary studies on the impact of sewage and rainwater discharged from urbanized and industrial areas mostly concern qualitative aspects and are related to environmental monitoring [16; 17; 18]. Due to the high content of nutrients and organic substances, untreated municipal sewage is considered the most dangerous. The impact of various chemicals and microorganisms discharged into rivers on their hydrochemical regime and ecological properties was mainly analyzed [19; 20]. Studies on the variability of hydrochemical parameters of river waters and the impact of these changes on the ecological status of streams dominate qualitative analyses. Due to its properties, water is a major source of infections in developing countries. In the case of concentrated pollution, the concentration of nutrients increases, which causes a decrease in the amount of oxygen in the water [21; 22]. In addition, some studies focused on the functioning of aquatic organisms, especially fish, invertebrates, and algae. Ecological studies have shown the negative effects of the deterioration of water quality (reduction in biodiversity) [23]. Sewage inflow most often deteriorates water quality by increasing water temperature, concentration of nutrients, and organic pollutants, but also by reducing dissolved oxygen levels [24]. Organic pollutants from untreated industrial sewage were found in the cities of Sambalpur and Dhaka.