The influence of treated wastewater from the Lodz city agglomeration on the ice regime and water temperature of the Ner river

Until recently, because of climatic conditions in Poland, the occurrence of various river ice types was a common but periodic phenomenon. Having different intensities and durations, such phenomena used to happen annually in most rivers of Poland (Gołek 1961). They were also considered typical properties of the river regime, which were indicative of being affected by the key features of Poland's continental climate in winter. The ice phenomena in Polish rivers had different forms. Those most often noted and examined were: frazil, border ice and ice cover, while less common were frazil and freeze-up ice jams. Individual particular phenomena usually appeared in rivers in so-called ice cycles. At the beginning of the cycle, border and anchor ice usually occurred, then ice floe appeared and sometimes, at the end of the cycle, ice jams. Such cycles were sometimes completed; however, more often only part of the cycle was observed. Moreover, the sequence of individual ice phenomena varied. As a result, the number and nature of ice cycles differed between different years and from river to river.

Similarly, the durations and pause times between them were also different and depended on the dates of the beginning and the end of ice phenomena and ice cover (Prowse 2001; Pawłowski et al. 2017). Today, as a result of warming and a rise in river water temperature during winter, ice phenomena on Polish rivers are less frequent, often episodic, and complete ice cycles almost never happen.

Many years (1984–2018) of satellite research (Landsat programme) have shown that in winter (Dec, Jan, Feb) iced stretches of the rivers as a percentage of full river length was temporally and spatially variable in Poland. The percentages have changed within significantly wide limits in the period: from 0% (Baltic Coastland) to almost 100% (Carpathian Mountains, Masurian Lakeland) (Yang et al. 2020). The maps presented in Yang et al.'s study also show that in the years 2008–18, in comparison to the period 1988–94, the percentage was clearly smaller: from 5–10% in the Masurian Lakeland and Mazovia, to 10–15% in Greater Poland and the Carpathian Mountains. The authors thereof declare that the changes in Europe (including Poland) are among the largest in the world.

Magnuson et al. (2000) demonstrated that the mean date of ice cover appearance on rivers of the northern hemisphere at the beginning of the 21^st century moved forward by ca 5.7 days in comparison with the beginning of the 20^th century, while the mean date when the ice cover disappears moved backward by ca 6.3 days. Thus, the duration of this phenomena shortened by more than 12 days. It has also been shown that the changes correspond to an increase in mean winter air temperature of about 1.2 °C per 100 years. In Baltic states, the observed trends of ice phenomena, which appear considerably later, together with shorter periods of river ice cover occurrence, are commonly noted. In the ten rivers of Lithuania, Latvia, Estonia and Belarus, during the second half of the 20^th century, the duration of ice cover shortened at a pace from about three (Daugava River) to about six (Narva River) days per ten years (Klavins, Briede & Rodinov 2009). It is worth noting that similar trends calculated for 11 Norwegian rivers do not indicate a clear direction and are only sometimes statistically significant (Gebre & Alfredsen 2011).

The mechanism of ice phenomena is a complex process and is determined by many factors: meteorological, hydrological, hydraulic and hydromorphological (Prowse 2001; Takacs et al. 2013; Marszelewski & Pawłowski 2019). Although the decisive factors are always air and water temperatures (Yang, Pavelsky & Allen 2020), among the other factors that play an important role, the following should also be mentioned: atmospheric precipitation (temperature and kind), riverbed parameters (depth, width), river flow speed (magnitude and nature of friction), participation of underground water alimentation, heat exchange with the environment and a number of other local factors.

In recent years, direct and indirect human impact have gained an increasing role in shaping the ice regime (Beltaos & Prowse 2009; Takács et al. 2013). The former affects the formation of river ice through the direct impact that economic factors have on the temperature and freezing rate of water (water pollution / mineralisation); the latter indirectly influences the formation of river ice through anthropogenic changes to the morphology of river beds and river alimentation (water reservoirs, dams and weirs, channel improvement). These treatments significantly change the river longitudinal profiles and cross-sections of riverbeds, thereby affecting the speed of water inflow (Prowse 2001; Agafonova & Frolova 2007).

The inflow of municipal sewage has a particularly important role. In the winter, it is warmer than river water (the calefaction of the river). At the same time, the pollutants contained in it significantly reduce the freezing point of water (e.g., chlorides). Simultaneously, the large amount of suspended material reduces the albedo of water and raises its temperature (Nilsson & Malm Renöfält 2008; Takacs et al. 2013; Sheng et al. 2014; Pawłowski 2017). Moreover, the artificial controlling of water stage and river discharge play an important role in ice regime modification (Graf 2018).

The aim of the present paper is to assess the scale and direction of long-term ice regime change and river water temperature change in a medium-size Polish river (the Ner river). This is presented in the context of many years’ changes in its anthropogenic alimentation, through sewage produced by the Lodz (Łódź) city agglomeration.

Changes in the ice regime of the Ner river were analysed on the basis of multiyear data from the observation point situated just above its mouth (Dabie water gauge). In the pre-industrial period, the river was a medium-size watercourse draining the eastern side of the southern Greater Poland lowlands and the western part of the Lodz Heights (Burchard et al. 2002). Its water resources were average then, and the ice regime was typical for this part of Poland. At the beginning of the 20^th century, especially in the years 1951–80, the situation changed. The Ner became a receiver of a large amount of municipal and industrial sewage produced by the rapidly developing urban and industrial agglomeration of Lodz. This strongly affected the river's nature and regime, as well as other river features, e.g., the ice regime.

The Ner is a 3^rd-order right-hand tributary of the Warta river. For almost 100 years, sewage from nearly 900,000 residents living in the Lodz agglomeration was discharged into the river (Fig. 1). The river is almost 126 km long and the area of the catchment up to the Dabie water gauge is 1,725.8 km². The Ner flows into the Warta River at a distance of 448.5 km from its mouth to the Oder River at an altitude of about 89 m a.s.l. The mean river gradient is ca 0.6‰. The total catchment area is 1,834.3 km² and the average height is 149 m a.s.l. at a denivelation of 190 m. Agricultural areas, forests, meadows and pastures constitute the majority (about 80%) of the Ner up to the Dabie water gauge catchment. Meanwhile, urban and industrial areas occupy over 13% of the area. In the upper part of the catchment, the percentage of urbanised area is even over 25% (Jokiel & Bartnik 2020).

Figure 1

The mean long-term (1951–2017) Ner discharge at the Dabie water gauge is 10.0 m³·s⁻¹, and the specific runoff from the catchment is equal to 5.8 dm³·s⁻¹·km⁻². The latter is slightly higher than the Polish mean (5.5 dm³·s⁻¹·km⁻²) and almost 20% higher than the mean runoff from the other catchments of the region (Jokiel 2018). The participation of the base flow in the total runoff in the Ner catchment is also unnaturally high, and averages 67%. The reason for this phenomenon is a constant inflow of treated wastewater from the Lodz city agglomeration. The water is withdrawn both from local aquifers (shallow and deep, fractures and porous, occurring in Jurassic, Cretaceous, Paleogene, Neogene and Quaternary rocks) and from the neighbouring Pilica's catchment (Bartnik 2019). The wastewater is drained into the Ner river through the right-side tributaries network and through the large collector of the Combined Wastewater Treatment Plant of Lodz Agglomeration (‘GOŚ ŁAM’ = CWTP LA). This causes a large, unnatural increase in the inflow to the river and an apparent increase in its base flow. The inflow of wastewater is very stable over time, just like the base flow. As a consequence, the Ner river runoff regime (at Dabie water gauge) should be classified as nival and moderately well developed – type B (Wrzesiński 2016), but strongly transformed by human impact (Jokiel 2018).

The mean amount of water used in the Lodz city for municipal and industrial needs (V_u) in the years 1951–2017 was almost equal to 75 million m³ annually. Under the previous assumption, this was approximately the average discharge of sewage, whereas after 2002 it is the discharge of treated wastewater from Lodz to the Ner river. However, the amount of these waters considerably varied over the years (Fig. 3). At the beginning of the research period (1951–54), the amount of wastewater discharged from the Lodz agglomeration did not exceed 10·10⁶·m⁻³ per year, and in the early 1970s increased to over 100·10⁶·m⁻³ per year. The maximum was observed in 1981, when the city of Lodz discharged over 177·10⁶·m⁻³ per year of raw sewage to the river. After 1985, the volume of discharged sewage decreased for many reasons that have already been mentioned. Usually, it decreased slowly and systematically (e.g., in the period 1999–2005), but in some years the registered decline was sharp – for example in 1985–86 and 1989–92. As a result, in the last decade of the entire research period, the amount of treated wastewater discharged to the Ner river stabilised at 37–50·10⁶·m⁻³ per year. Until 1995, the discharged wastewater was only treated mechanically or not at all. However, after 1995 (opening of CWTP LA) full treatment (biological and chemical) began, and most of the sewage that went to the river was treated.

In the last 70 years, sewage water discharged to the Ner has changed, both quantitatively and qualitatively. This was due to various processes and phenomena associated with the increase in population of the entire Lodz agglomeration, its economic and industrial development, changes in local water management, and river water temperature changes due to the warming climate (Graf, & Wrzesiński 2020). All of these caused long-term fluctuations and periodic trends in the quantity of sewage discharged into the river from the Lodz agglomeration, as well as similar changes in the degree of its biochemical, chemical and thermal pollution (only since 1995 has this wastewater been biologically treated in CWTP LA). As a result, not only did the fluctuations in the Ner flow and runoff regime change, but so too the river was actually a sewage channel with “biologically dead” waters in the years 1951–95. The significant transformation of the thermal and ice regime of the Ner river, especially in its middle and lower course, was an obvious consequence of this situation.

Within the last 25 years, the purity state of the Ner river has changed, and slowly improved. Less and less sewage has been flowing into it, and thus the situation has gotten better and better. Slowly, the Ner is ceasing to be a sewer and is becoming a river again. The reason for the gradual improvement in river water quality is primarily the commissioning in 1995 of a large combined sewage treatment plant for the Lodz agglomeration (CWTP LA) and over a dozen other smaller treatment plants spread all over the river catchment.

The reduction in sewage volume and decrease in treated wastewater discharge into the Ner river was a consequence of a decrease in water demand. This was primarily caused by the collapse of the water-consuming textile industry in Lodz and its neighbouring cities (mid 1980s). Moreover, the process was a consequence of an increase in water prices for industry and the municipal economy due to the transformation to a market economy in Poland (in the 1990s). It was also the result of an end to supplying the city with large volumes of cheap but poor quality water from the Sulejowski Reservoir intakes on the Pilica river (Jokiel & Bartnik 2020). Hence, the quantity of sewage water discharged to the Ner was reduced and its quality relatively improved at the turn of the 21^st century. This caused a slight decrease in the mean water discharge at the Dabie water gauge of about 0.5 m³·s⁻¹ (Jokiel 2018).

Observations of various kinds of phenomena and ice forms in the Ner river at Dabie have been conducted since the water gauge was established here in 1921. They were conducted by observers of the Polish Institute of Meteorology and Water Management – National Research Institute (‘IMGW-PIB’ = PIMWM-NRI) service. In the pre-war years, during World War II and in the period 1945–50, the observations were incomplete (the type of phenomenon was not always noted), and the data available from these years contain numerous gaps. The series from the years 1951–2017 analysed below is complete and reliable. Its only disadvantage is that the shape of the Ner river bed has been changing so often (river control works), as has the place of observation. Nevertheless, the series of multiyear flows of the Ner river at Dabie is homogeneous (Jokiel & Juszczak 1995).

In the years 1951–17, standard hydrometric observations and measurements at the Dabie water gauge have recorded different types of ice phenomena. The combinations of these phenomena, their forms, the dates of appearance and disappearance of ice sheets, and other ice forms were documented. Ice thickness has occasionally been measured. To standardise the research presented below and simplify the diagrams and calculations, the series of appearances and disappearances of ice cover and all ice phenomena were analysed (Fig. 2). The start dates and end dates of all ice phenomena occurrences, as well as the start dates and end dates of ice cover appearances, have been analysed together with their duration in the period 1951–2017.

Figure 2

Data on the volume of wastewater discharged from the Lodz agglomeration to the sewage system and then to the Ner river are very fragmentary and imprecise, and therefore cannot be analysed for the long-term period. Information on the amount of water used in Lodz in 1951–2017 is available (V_u; Fig. 3). Despite the fact that it can be found in various sources, it is difficult to verify. The data have multiple gaps and are therefore very inaccurate. Nevertheless, they can provide some approximation to the amount of wastewater discharged into the Ner River during this period. It needs to be highlighted that the Ner river has been collecting the vast majority of sewage flowing from the entire Lodz agglomeration from the moment the city was created (Jokiel & Bartnik 2020). The existing gaps in data on the volume of annual water consumption of the Lodz agglomeration (occurring in 1974–80 and 1993–98) have been supplemented using regression equations for the following relationship between the variables: the amount of water consumed and the amount of water supplied to homes; and the amount of water consumed and the volume of sewage discharged from the Lodz agglomeration. The determination coefficients of these equations are, respectively: 0.99 and 0.94, and the formulas are statistically significant at 1% (Mann–Kendall test). The changes in the amount of water that was consumed yearly by the Lodz agglomeration in the years 1951–2017 are presented in Fig. 3.

Figure 3

At the first stage of the investigation, the specified terms of appearance and disappearance of ice phenomena and ice cover were determined for each year of the multiyear period. On their basis, the duration of ice phenomena and ice cover on the Ner river was calculated for specific years. Further calculations of the long-term changes in the occurrence of ice phenomena on the river were made using several basic statistical methods and procedures (mean, standard deviation, coefficient of variation, etc.). The existence of trends in the series was examined by the non-parametric Mann–Kendall statistical test (Mann 1945; Kendall 1975) mentioned above. Nowadays, this method is often used to detect the existence of trends in various hydrological time series. Examples of works in which this test was also used include: Steele et al. 1974; Hirsch et al. 1982; Hirsch & Slack 1984; Crawford et al. 1983; van Belle & Hughes 1984; Cailas et al. 1986; Hipel et al. 1988; Taylor & Loftis 1989; Demaree & Nicolis 1990; Zetterqvist 1991; Yu et al. 1993; Chiew & McMahon 1993; Burn & Hag Elnur 2002; Lettenmaier et al. 1994; Gan 1998; Lins & Slack 1999; Douglas et al. 2000; Hamilton et al. 2001; Yue & Hashino 2003; Yue & Wang 2004. In the next stage, the directions and strength of linear trends occurring in the series of dates of appearance and disappearance of phenomena and ice cover were determined (using least squares method) and the coefficients of determination (R²) of their equations were calculated (Wright 1921; Sen & Srivastava 1990).

The double mass method was used to assess the impact of water consumption and relationships between the number of days with phenomena and ice cover on the Ner river and water consumption in the Lodz city agglomeration. This is a recognised method of detecting heterogeneity in data series. The double-mass curve is based on the concept that a graph of the cumulative data of one variable versus the cumulative data of another is a straight line, as long as the relation between the variables is a fixed ratio (Searcy & Hardison 1960; Linsley et al. 1975). When an inconsistency is noted, further investigation into its cause is warranted. Major breaks in slope indicate that the data is nonhomogeneous.

In the years 1951–2017, the ice phenomena on the Ner river changed significantly and were very diverse. Over the years, ice cover and other ice phenomena have become briefer. They started occurring later and disappeared earlier. The duration and frequency of these phenomena also decreased. However, these changes have not been a continuous process for many years, but they were very diverse in pace, scale and direction. The reasons for the changes in the Ner river ice regime were not only changes in river water temperature caused by global warming (the constant direction and pace of change), but also additional fluctuations in this temperature determined by changes in the quantity and quality of wastewater discharged into the river from the Lodz city agglomeration. The observed fluctuations in the duration and the frequency of ice phenomena allowed four periods to be distinguished in the examined period that are characterised by differing paces and directions of changes in the river ice regime (compare Fig. 2 & Fig. 7):

1951–65: the phase of slow disappearance of ice phenomena in the river caused by a systematic but small increase in water temperature.

The observed changes and fluctuations in the frequency and duration of ice phenomena in the Ner river are a good example illustrating the overlapping of changes caused by a local anthropogenic factor (here, sewage discharge) and the relatively time-stable changes caused by a regional factor (increase in water temperature) caused by global warming.