The Influence of Anthropogenic Pollution on the Physicochemical Conditions of the Waters of the Lower Section of the Sąpólna River

Abstract

River pollution resulting from unregulated and improper water sewage management is a global issue of concern. The discharge of inadequately treated sewage into rivers and the sudden release of excessive quantities during heavy rainfall can result in significant fish mortality. This phenomenon has been observed repeatedly in the case of the Sąpólna River, NW Poland. Consequently, a decision was made to monitor the water quality at two key locations: the drainage channel that feeds into the river and downstream of the channel. Seventeen water quality indicators were measured, including temperature, pH, conductivity, total suspended solids (TSS), dissolved oxygen (DO), biochemical oxygen demand (BOD₅), chemical oxygen demand (COD_Cr), total organic carbon (TOC), alkalinity, total hardness, total reactive phosphorus (TRP), total phosphorus (TP), nitrite-nitrogen (NO₂⁻-N), nitrate-nitrogen (NO₃⁻-N), total ammonia nitrogen (NH₄-N), total organic nitrogen (TON), and total nitrogen (TN). It was determined that, at the location farthest from the drainage channel, water quality still falls short of meeting the specified standards. The primary factors leading to the degradation of water quality at this point were TSS, TRP, NO₂⁻-N, and TN. It was concluded that the primary localized source of water pollution in the studied section of the Sąpólna River is the discharge from sewage treatment plants in Nowogard. Consequently, actions should be taken to address sewage quality and reduce discharge quantities.

1. Introduction

Anthropogenic water pollution and its ecological repercussions persist as an ongoing and unresolved issue in both small and large water bodies across Poland and globally [1,2,3,4,5]. Despite the dynamic nature of rivers, characterized by the linear flow of matter and energy [6,7], pollutants introduced into their waters do not consistently undergo reduction through self-purification processes. The efficacy of these processes is contingent upon various factors, including river size, flow rate, temperature, and meteorological conditions [7].

The progressive accumulation of various forms of carbon, nitrogen, and phosphorus along a river’s course is a commonplace phenomenon, even when these elements originate from natural sources [8]. This accumulation is a consequence of the steadily increasing flow of the watercourse and the rising concentrations of suspended particles [8]. However, localized surges in the levels of these compounds, introduced into rivers as a result of human economic activities, disrupt the river’s continuity and, in turn, jeopardize the proper functioning of these ecosystems [6]. Although river ecosystems strive to attain a state of equilibrium, detrimental alterations in water quality often hinder natural processes both within the river channel and its surrounding valley.

The dynamic fluctuations in physicochemical conditions within flowing waters result in a considerable diversity among rivers, leading to distinct characteristics in various segments of the same watercourse. Consequently, the European Union has established an extensive system for classifying rivers, with its regulations influencing Polish legislation. Presently, in Poland, a five-tier classification of water quality is enforced, as delineated in the Minister of Infrastructure’s Regulation dated 25 June 2021 [9]. This regulation encompasses the classification of ecological status, ecological potential, chemical status, and the methodology for assessing the ecological status of homogeneous sections of surface waters, as well as environmental quality standards for priority substances. These guidelines were formulated based on the recommendations outlined in Directive 2000/60/EC of the European Parliament and the Council, which sets the framework for Community action in the realm of water policy, applicable across all EU member states [10]. Another pivotal document aiding in the assessment of water quality conducive to fish habitat is Directive 2006/44/EC of the European Parliament and the Council, dated 6 September 2006, focusing on the quality of fresh waters requiring protection or improvement to support fish life (Official Journal of the European Union, 25 September 2006) [11]. Waters with heightened physicochemical parameter values, often stemming from anthropogenic pollution, exert a profound influence on the reproduction, embryonic development, growth, and survival of aquatic organisms, including fish [3,12,13,14].

Particularly disadvantaged are small rivers, especially in urbanized catchments, where even the inflow of treated wastewater can have a greater negative impact on biocoenoses than in the case of large rivers. This is because of the critical relationship between the amount of wastewater (including treated wastewater) discharged into the river and the natural flow of such a river [15,16].

An additional negative factor in recent years is precipitation in the form of extreme torrential rains resulting from the ongoing climate change, which occur irregularly and after long periods of drought. However, the environmental impacts of extreme hydroclimatic conditions are not well understood. The response of river water quality under climate change is driven by hydrological changes, increases in water and soil temperatures, and interactions between hydroclimatic, land use, and human factors [17,18]. As a result of heavy rainfall, the capacity of wastewater treatment plants receiving rainwater may be exceeded, and excessive amounts of untreated wastewater, additionally contaminated with activated sludge from treatment plant bioreactors, may be discharged into rivers. The river threatened by the above-mentioned factors is the Sąpólna River. It is a tributary of the Ukleja River, which feeds the waters of the Rega River, recognized as a valuable habitat for salmonids and should, therefore, be subject to special protection in order to ensure the possibility of reviving populations of these fish.

Given the above, this paper aims to assess the water quality of the Sąpólna River within the section where effluents from sewage treatment plants are discharged in accordance with current regulatory standards. The results obtained show the need to establish the permanent monitoring of watercourses at risk of pollution and the necessary investments leading to the expansion of the existing wastewater treatment plant.

2. Materials and Methods

2.1. Study Area

The Sąpólna River, with a length of 60.08 km, is situated in northwestern Poland and is a tributary of the Ukleja River, ultimately flowing into the Rega River. Its spring is in the vicinity of the village of Bagna and has two tributaries, Bukowina and Dobra. Its total catchment area is 87.7 km² [19]. The main channel of the Sąpólna River is fed by small watercourses that carry water only during the rainy season or when the snow melts. During the summer season, these watercourses dry up and become partially overgrown.

The Sąpólna River hosts a diverse range of fish species, including brown trout (Salmo trutta fario), gudgeon (Gobio gobio L.), roach (Rutilus rutilus L.), three-spined stickleback (Gasterosteus aculeatus L.), ninespine stickleback (Pungitius pungitius L.), burbot (Lota lota L.), and tench (Tinca tinca L.) [20]. Additionally, it is home to protected species, such as the European bullhead (Cottus gobio L.), stone loach (Barbatula barbatula L.), and brook lamprey (Lampetra planeri Bloch) [20].

The Sąpólna River’s catchment area is part of the Rega River basin, where the Polish Angling Association in Szczecin carries out fish stocking efforts aimed at increasing the populations of salmonid fish. Annually, the river is stocked with summer-spawning brown trout fry in quantities ranging from 3000 to 6000 individuals (information from a PZW Szczecin angler, unpublished data). In the years 2019–2020, five artificial spawning grounds were established in the river to restore natural spawning habitats for salmonid fish [21].

This watercourse is subject to significant anthropogenic pressure, primarily from agricultural pollution and the discharge of treated sewage from the mechanical–biological sewage treatment plant in Nowogard. As a consequence, fish die-off in the Sąpólna River was observed at the end of August 2021, primarily due to the discharge of a large volume of sewage during heavy rainfall, resulting in reduced wastewater treatment efficiency.

The sewage treatment plant in Nowogard was put into operation in 1975 and modernized in 1993–1996. The maximum capacity of the plant is 4125 m³·d⁻¹ (i.e., 173 m³·h⁻¹), while the average load of the plant is 2479.452 m³·d⁻¹, of which 2438.356 m³·d⁻¹ is sewage from the sewerage system and 41.096 m³·d⁻¹ is transported sewage. The treatment plant receives domestic, municipal, and industrial wastewater through a combined sewer system, which also receives rainwater. The design capacity of the population equivalent is 17484 EP [22]. According to the Provincial Inspectorate for Environmental Protection in Szczecin, the technical and operational condition of the plant is good, but due to the combined sewer system in Nowogard, the amount of incoming and treated wastewater fluctuates in a wide range, which periodically causes difficulties in maintenance and interruptions in the operation of the plant [23].

The ecological status of the Sąpólna River is also affected by quad rallies, which have a destructive effect on its bed, destroying trout nests on the bottom. The Sąpólna River, however, is not a source of drinking water in Nowogard (it takes water from two drilled wells).

The hydrochemical research presented in this study was conducted monthly from October 2021 to September 2022. Based on meteoblue.com data [24], the average air temperature in 2022 was 7.7 °C, with an average minimum temperature of −3.0 °C and an average maximum temperature of +25 °C. Total precipitation during the period was 625 mm (February had the highest precipitation—an average of about 30 mm per week). In March 2022, there was no precipitation, while in the first week of April, there was heavy precipitation of more than 20 mm [24]. During the winter season, the formation of ice cover on the river and in the drainage channel was not observed. Water samples were collected from six research sites, with sites S1, S2, and S3 located on the drainage channel that serves as a tributary to the Sąpólna River, while sites S4, S5, and S6 were situated directly on the river (Figure 1).

Site S1—the beginning of the drainage channel, 30 m upstream from the sewage plant outfall (53°67′99″ N, 15°12′69″ E). The channel has a width ranging from 0.6 to 0.8 m and a depth of 0.15 to 0.30 m, with a firm, rocky bottom and steep grassy banks. It is supplied with water infiltrating through an earthen embankment, where an asphalt road is located. During dry periods, the channel’s water flow in this section diminishes, leading to the absence of sample collection in August and September 2022. The immediate surroundings consist of uncultivated meadows.

Site S2—30 m downstream from the sewage plant’s effluent discharge point into the drainage channel, representing the mixing zone of sewage effluents and channel water (53°68′00″ N; 15°12′70″ E). Morphometric characteristics and the environment at this site are similar to those at site S1 (Figure 2).

Site S3—40 m upstream from the confluence of the drainage channel with the Sąpólna River (53°67′84″ N; 15°12′70″ E). The channel has a width of approximately 2 m and a depth ranging from 0.6 to 1.0 m. Its bottom is muddy, with banks covered by vascular vegetation and grassy slopes. The channel is supplied with water infiltrating from nearby meadows and pastures and partially from the adjacent forest on the right bank.

Site S4—a site on the Sąpólna River, located 50 m upstream from the confluence of water from the drainage channel (53°67′80″ N; 15°17′21″ E). The site is situated above a defunct fish weir, with a river width of approximately 5 m and a depth ranging from 0.5 to 0.7 m at the sampling point. The regulated banks are covered with vegetation and feature distinct slopes. The immediate surroundings consist of a forest, and the riverbed is sandy.

Site S5—a site on the Sąpólna River, located 100 m downstream from the confluence of the drainage channel carrying water and sewage from the treatment plant. The river width ranges from 3.5 to 4.0 m, with steep and regulated banks featuring plant growth and reed beds in narrow strips. The river has a well-defined current, with depths ranging from 0.6 to 1.0 m. The riverbed is sandy, transitioning to muddy near the banks. The immediate surroundings consist of meadows and pastures (Figure 3).

Site S6—a location on the Sąpólna River, located 10 km downstream from site S5 (53°73′38″ N; 15°24′46″ E). The riverbed has a width of approximately 8 m, with depths ranging from 0.6 to 1.0 m, and a sandy gravel bottom. Upstream from this point, there are artificial spawning grounds for salmonid fish, while the immediate surroundings consist of a forest. Beyond the tree line, the river takes on a natural mountainous character (Figure 4). Water flow is continuously measured at this site. Flow values during the study period ranged from 0.230 m·s⁻¹ during the summer months (low water) to 0.461 m·s⁻¹ during high water.

2.2. Measurement and Analysis of Water Quality

The quality parameters of water were determined using standard methods recommended by APHA [25] (

Table 1. Laboratory methods used to determine the values of analyzed parameters in Sąpólna river water [25].

Parameter	Method	Units
Temperature	Standard Method 2550	°C
Conductivity	Standard Method 2510	μS cm−1
pH	Standard Method 4500-H+
Total suspended solids (TSS)	Standard Method 2540D	mg·dm−3
Alkalinity	Standard Method 2320	mg CaCO3·dm−3
Total hardness	Standard Method 2340	mg CO3·dm−3
Dissolved oxygen (DO)	Standard Method 4500-O B	mg O2·dm−3
Biochemical Oxygen Demand (BOD5)	Standard Method 5210 B	mg O2·dm−3
Chemical Oxygen Demand (CODCr)	Standard Method 5220 B	mg O2·dm−3
Nitrite-nitrogen (NO2−-N)	Standard Method 4500-NO2−	mg·dm−3(as NO2-N)
Nitrate-nitrogen (NO3−-N)	Standard Method 4500-NO3−	mg·dm−3 (as NO3-N)
Total ammonia nitrogen (NH4-N)	Standard Method 4500-NH3	mg·dm−3 (as NH3-N)
Total nitrogen (TN)	Standard Method 4500-N	mg·dm−3 (as N)
Total reactive phosphorus (TRP)	Standard Method 4500-P	mg·dm−3 (as P)
Total phosphorus (TP)	Standard Method 4500-P	mg·dm−3 (as P)

).

Colorimetric methods utilized a UV-VIS double beam spectrophotometer U-2900 (Hitachi High-Technologies Corporation, Tokyo, Japan). Total nitrogen (TN) and total organic carbon (TOC) were determined using a VarioTOC SELECT analyzer (ELEMENTAR Analysensysteme GmbH, Langenselbold, Germany). Total organic nitrogen was calculated as TON = TN − TIN, where TIN (total inorganic nitrogen) is the sum of ammonium nitrogen, nitrite-nitrogen (III), and nitrate-nitrogen (V).

The assessment of the values of analyzed water quality indicators was conducted by applying the requirements outlined in Directive 2006/44/EC [11] and in the Regulation of the Minister of Infrastructure [9] (

Table 2. The quality standards for surface waters specified in Directive 2006/44/EC [11] and in the Regulation of the Minister of Infrastructure [9].

Directive 2006/44/EC
Indicator Name, Unit	Salmonid Waters		Cyprinid Waters
pH	6.0–9.0
TSS mg·dm−3	≤25
DO mgO2·dm−3	50% ≥9.0		50% ≥7.0
BOD5 mgO2·dm−3	≤3.0		≤6.0
TP mg·dm−3	≤0.2		≤0.4
NO2-N mg·dm−3	≤0.01		≤0.03
NH4-N mg·dm−3	≤1.0		≤1.0
The Regulation of the Ministry of Infrastructure
Indicator name, unit	The threshold value for water quality class
	I	II	III	IV	V
Indicators characterizing oxygen conditions and organic pollution
DO mgO2·dm−3	≥8.9	≥7.6	Not classified—nc
BOD5 mgO2·dm−3	≤2.3	≤3.5
TOC mg·dm−3	≤8.2	≤10.0
Indicators characterizing salinity
Conductivity in 20 °C µS·cm−1	≤420	≤690	Not classified—nc
Indicators characterizing biogenic conditions
NH4-N mg·dm−3	≤0.14	≤0.40	Not classified—nc
NO3-N mg·dm−3	≤1.10	≤2.00
TN mg·dm−3	≤2.00	≤3.30
TRP mg·dm−3	≤0.06	≤0.09
TP mg·dm−3	≤0.17	≤0.33

). In accordance with the guidelines specified in the aforementioned Regulation of the Minister of Infrastructure, the Sąpólna River was classified as a lowland river (RzN) [9].

2.3. Statistical Analysis

Statistical analysis was conducted using Statistica 13.3 software from TIBCO Software Inc. (Palo Alto, CA, USA). Variance analysis (ANOVA, p < 0.05) and the Duncan test (p < 0.05) were employed to determine statistically significant differences in water quality indicators among the various research sites.

3. Results and Discussion

3.1. General Comparison of Research Sites

In general, significant variations (p < 0.05) in physicochemical water conditions were observed between the individual research sites (

Table 3. Average annual values, minimum and maximum values, and standard deviation (±SD) of the analyzed indicators at selected points on the Sąpólna River.

Parameter Point	Temp. °C	pH	Conductivity µS·cm−1	TSS mg·dm−3	DO	BOD5	CODCr	TOC	Alkalinity mgCaCO3·dm−3	Total Hardness mgCO32·dm−3
					mgO2·dm−3			mgC·dm−3
1	8.8 ± 5.9 a	7.1 ± 0.1 a	708 ± 145 a nc	50 ± 45 a	4.0 ± 2.3 a nc	4.3 ± 0.7 ab nc	83.2 ± 34.8 b	24.8 ± 4.1 a nc	325.6 ± 78.9 c	365.1 ± 63.8 b
	min max 1.4–16.6	min max 6.9–7.3	min max 438–912	min max 17–170	min max 0.9–7.0	min max 2.9–5.2	min max 36.8–119.6	min max 19.90–31.11	min max 205.5–457.5	min max 292.5–472.8
2	13.6 ± 5.6 a	7.2 ± 0.2 ab	1991 ± 720 c nc	620 ± 1602 a	6.0 ± 1.2 a nc	4.6 ±1.1 b nc	137.8 ± 103.9 b	41.4 ± 42.9 a nc	235.8 ± 50.7 b	297.2 ± 48.6 a
	min max 6.0–22.1	min max 6.8–7.7	min max 1116–3662	min max 15–5150	min max 4.4–7.4	min max 2.2–6.0	min max 31.9–369.1	min max 18.4–166.07	min max 165.0–330.0	min max 243.3–399.3
3	11.6 ± 6.0 a	7.4 ± 0.2 b	1449 ± 542 b nc	750 ± 2329 a	4.0 ± 2.2 a nc	3.7 ± 1.2 ab nc	99.0 ± 80.9 b	78.6 ± 195.4 a nc	231.0 ± 36.8 ab	311.9 ± 56.0 ab
	min max 4.1–23.2	min max 7.1–7.7	min max 804–2520	min max 12–8140	min max 0.0–7.8	min max 1.1–5.5	min max 34.3–319.7	min max 17.5–699.1	min max 178.5–300.0	min max 275.8–440.3
4	9.6 ± 4.8 a	7.7 ± 0.2 c	465 ± 103 a II	69 ± 55 a	8.8 ± 1.3 b II	3.1 ± 1.1 a II	27.5 ± 20.5 a	14.3 ± 3.1 a nc	179.5 ± 25.8 a	285.6 ± 33.3 a
	min max 2.3–16.5	min max 7.7–7.9	min max 198–578	min max 28–232	min max 6.9–11.4	min max 0.2–4.4	min max 1.9–62.3	min max 7.1–18.8	min max 140.0–220.0	min max 243.5–328.3
5	10.1 ± 4.9 a	7.6 ± 0.2 c	689 ± 145a II	179 ± 407 a	8.4 ± 1.7 b II	3.5 ±1.6 a II	31.4 ± 23.6 a	27.9 ± 40.0 a nc	183.5 ± 30.3 a	288.5 ± 49.3 a
	min max 3.7–17.2	min max 7.3–7.9	min max 458–874	min max 21–1465	min max 6.2–11.6	min max 0.9–6.9	min max 6.3–76.5	min max 8.8–153.9	min max 140.0–235.0	min max 209.8–367.3
6	9.9 ± 4.8 a	7.7 ± 0.3 c	619 ± 92 a II	53 ± 32 a	9.3 ± 1.3 b I	3.3 ± 1.5 a II	22.1 ± 9.9 a	14.6 ± 3.1 a nc	183.0 ± 27.0 a	290.4 ± 54.5 a
	min max 2.9–17.1	min max 7.2–8.1	min max 494–796	min max 21–130	min max 7.2–11.1	min max 0.5–5.3	min max 3.2–36.8	min max 7.9–19.3	min max 136.5–220.0	min max 227.3–440.3
p	0.299	0.000	0.000	0.504	0.000	0.051	0.000	0.438	0.000	0.005

Note: Explanations: The averages in columns marked with different lower case letters indices are statistically significantly different at p < 0.05 (Duncan’s test). Classification of water according to the Regulation of the Minister of Infrastructure dated 25 June 2021—Class I, II, III, nc—no class. Classification of water according to Directive 2006/44/EC of the European Parliament and of the Council of 6 September 2006—blue color represents waters meeting the conditions for the life of salmonid fish, green represents waters meeting the conditions for the life of cyprinid fish; red indicates waters not meeting the requirements for the life of cyprinid fish. Source: own study.

and

Table 4. Average annual values, minimum and maximum values, and standard deviation (±SD) of biogenic indicators at selected points on the Sąpólna River.

Parameter Point	TRP	TP	NO2−-N	NO−3-N	NH4+-N	TON	TN
	mg·dm−3
1	0.477 ± 0.337 cd nc	0.617 ± 0.452 b nc	0.018 ± 0.022 a	0.331 ± 0.233 a I	0.187 ± 0.370 ab II	2.658 ± 1.735 a	3.194 ± 1.865 a II
	min max 0.160–1.227	min max 0.195–1.712	min max 0.004–0.074	min max 0.037–0.641	min max 0.015–1.237	min max 1.427 ± 6.134	min max 1.316–6.475
2	0.674 ± 0.283 c nc	0.919 ± 0.350 c nc	0.316 ± 0.955 a	2.422 ± 1.532 b nc	0.095 ± 0.156 a I	7.780 ± 11.323 a	10.613 ± 10.776 a nc
	min max 0.228–1.078	min max 0.347–1.385	min max 0.013–3.346	min max 0.641–5.490	min max 0.023–0.578	min max 0.740 ± 38.215	min max 3.425–39.335
3	0.688 ± 0.385 c nc	0.927 ± 0.449 c nc	0.238 ± 0.539 a	1.350 ± 1.002 b II	0.290 ± 0.331 b II	10.308 ± 24.821 a	12.187 ± 24.398 a nc
	min max 0.165–1.331	min max 0.283–1.580	min max 0.016–1.938	min max 0.121–3.199	min max 0.048–1.230	min max 1.414 ±88.910	min max 3.121–89.180
4	0.076 ± 0.057 a II	0.154 ± 0.175 a I	0.019 ± 0.009 a	1.377 ± 1.427 b II	0.032 ± 0.019 a I	1.970 ± 2.527 a	3.398 ± 2.900 a nc
	min max 0.014–0.201	min max 0.03–0.674	min max 0.006–0.041	min max 0.240–4.891	min max 0.012–0.058	min max 0.414 ± 8.939	min max 1.004–9.921
5	0.326 ± 0.316 bd nc	0.457 ± 0.386 b nc	0.066 ± 0.096 a	1.459 ± 1.413 b II	0.125 ± 0.090 ab I	3.626 ± 5.366 a	5.276 ± 5.222 a nc
	min max 0.021–0.988	min max 0.028–1.206	min max 0.007–0.354	min max 0.260–4.854	min max 0.021–0.359	min max 0.491 ± 18.967	min max 1.481–19.797
6	0.141 ± 0.097 ab nc	0.202 ± 0.121 ab II	0.036 ± 0.022 a	1.447 ± 1.132 b II	0.066 ± 0.034 a I	1.909 ± 2.187 a	3.457 ± 2.350 a nc
	min max 0.025–0.239	min max 0.048–0.414	min max 0.010–0.077	min max 0.436–4.151	min max 0.036–0.133	min max 0.029 ± 8.228	min max 1.352–9.123
p	0.000	0.000	0.046	0.012	0.048	0.380	0.217

Note: Explanations: The averages in columns marked with different lower case letters indices are statistically significantly different at p < 0.05 (Duncan’s test). Classification of water according to the Regulation of the Minister of Infrastructure dated 25 June 2021—Class I, II, III, nc—no class. Classification of water according to Directive 2006/44/EC of the European Parliament and of the Council of 6 September 2006—blue color represents waters meeting the conditions for the life of salmonid fish, green represents waters meeting the conditions for the life of cyprinid fish; red indicates waters not meeting the requirements for the life of cyprinid fish. Source: own study.

). Due to large fluctuations in the values of some analyzed parameters throughout the year (e.g., TSS, COD_Cr, TOC, TP, TRP, NO₂-N, NH₄-N), very high standard deviations were obtained (Table 3 and Table 4).

Therefore, the statistical analysis did not reveal significant differences between sites for TSS, TOC, and NO₂-N, even though there were highly variable values, sometimes up to nine times greater, e.g., TSS at sites S1 and S3 (Table 3). Such a high degree of variation in hydrochemical indicators is often observed in river waters due to locally concentrated pollution, such as wastewater treatment plant discharges (as in the case of the river we studied) [26], but it can also be due to inputs from intensive fish farms [27]. It is also significant whether these pollutant inputs are constant or pulsating. Each of these types of impacts (locally concentrated, diffuse, pulsating, and constant) will be neutralized differently by the ecosystem. According to Niemi et al. [28], it can generally be assumed that concentrated and pulsating impacts have a limited and known duration of action. These include, for example, discharges of rapidly diluted chemical effluents. Permanent pollution sources, on the other hand, are those caused, for example, by the regulation of a watercourse.

Overall, site S1 had lower concentrations of most analyzed indicators compared to the other sites. This could be attributed to the infiltration of water through the earthen embankment. The results suggest that the inflow of this water did not contribute to additional pollution in the drainage channel, where treated sewage from the municipal treatment plant is discharged. Sites S2 and S3, located on the channel receiving treated sewage, had elevated values of almost all water quality indicators compared to S4, situated on the Sąpólna River. The differences were particularly significant for NO₂-N (over 16-fold increase) and conductivity, TSS, and TRP (increases of over eight times) (Table 3 and Table 4). These results coincide with the observations of Rutkowska et al. [26], who attributed the high concentrations of the identified forms of nitrogen, phosphorus, and organic matter to the impact of a sewage treatment plant on the river. As a result, the waters of the Raszynka River studied in this section did not meet the standards of even Class II surface water quality. The Raszynka River is small in length (16.84 km), and its catchment area of 75.9 km² is mainly agricultural. A sewage treatment plant is located in the middle section of the Raszynka River. The aforementioned factors (length of the river, nature of the catchment area, sewage inflow) are very similar to the conditions of the Sąpólna River, which we studied, making it an excellent reference point for the results obtained [26].

3.2. Indicators Characterizing Oxygen Conditions and Organic Pollution

At sites S1, S2, and S3, the average annual values of dissolved oxygen in water were very low, ranging from 4.0 to 6.0 mg O₂·dm⁻³ (with total absence of oxygen at site S3 in April 2022) (Table 3 and

Table 5. Values of selected indicators on the Sąpólna River in the month of sudden sewage discharge (April) compared to average annual values.

Parameter Point	Conductivity µS·cm−1		TSS mg·dm−3		DO		CODCr		TOC		TP		TON		TN
					mgO2·dm−3				mgC·dm−3		mg·dm−3
	April	average	April	average	April	average	April	average	April	average	April	average	April	average	April	average
1	606	708	64	50	5.4	4.0	78.1	83.2	22.9	24.8	0.525	0.617	1.427	2.658	1.541	3.194
2	1810	1991	228	620	5.1	6.0	85.2	137.8	166.1	41.4	1.136	0.919	38.215	7.780	39.335	10.613
3	1068	1449	8140	750	0.0	4.0	191.8	99.0	699.1	78.6	1.472	0.927	88.910	10.308	89.180	12.187
4	420	465	66	69	9.2	8.8	11.8	27.5	13.6	14.3	0.176	0.154	1.319	1.970	2.049	3.398
5	848	689	1465	179	6.2	8.4	59.2	31.4	153.9	27.9	1.206	0.457	18.967	3.626	19.797	5.276
6	650	619	130	53	9.8	9.3	17.8	22.1	14.0	14.6	0.199	0.036	1.677	1.909	2.693	3.457

). Waters at these sites did not meet the requirements of Directive 2006/44/EC [11] and the regulation of the Ministry of Infrastructure 2021 [9]. In the Sąpólna River (S4, S5, and S6), dissolved oxygen gradually increased, and BOD₅ decreased with the flow. Site S6 recorded the highest DO values, meeting the requirements for Class I water quality (suitable for salmonid fish habitat) (Table 2 and Table 3) [11]. The low DO concentrations and elevated BOD5 values at S1, S2, and S3 found in the study indicate an influx of pollutants, probably contained in the wastewater discharged from the wastewater treatment plant. They contain nutrients and organic matter, and their mineralization leads to a decrease in dissolved oxygen concentrations in the receiving water body, which in our case is first the drainage canal and then the Sąpólna River. This is in agreement with the observations of, e.g., Ahmed [29] or Dębska et al. [30], who explained such processes by oxidoreductive reactions and increased microbial activity, further enhanced by rising surface water temperatures as an ecosystem response to ongoing climate warming. This phenomenon also leads to a general trend of decreasing dissolved oxygen concentrations in river waters [30,31]. Noteworthy is the extremely high concentration of COD recorded at sites S2 and S3, 369.1 and 319.1 mgO₂·dm⁻³, respectively (more than 100 times higher than the concentrations recorded at sites S4, S5, and S6) (Table 3). In contrast, these values were similar to the COD concentrations in the raw wastewater. For example, in wastewater discharged to a treatment plant near Warsaw, COD was in the range of 178.3–415.5 mgO₂·dm⁻³ [32]. In parallel, elevated COD values are also accompanied by elevated concentrations of total phosphorus and ammoniacal nitrogen, as observed in the Utrata River, a river with a length of 76.5 km and a catchment character very similar to that of the Sąpólna River (catchment area of 702 km²) [30]. As in the case of the Sąpólna River, the Utrata River is polluted by tributaries from agricultural land, urban areas, and sewage treatment plants [30].

However, the average annual values of TSS and TOC at all research points did not meet the requirements of Directive 2006/44/EC [11] (for TSS) and the regulation of the Ministry of Infrastructure 2021 [9] (for TOC) (Table 2 and Table 3). The highest annual average TSS value at site S3 was 750 mg·dm⁻³, exceeding the permissible limit of 25 mg·dm⁻³ by 30 times (Table 2 and Table 3, Figure 5). During high-flow periods in the channel (floodlike conditions) and simultaneous high discharge of sewage from the treatment plant, this indicator reached 8140 mg·dm⁻³ (over 300-fold exceeding the permissible values) (Table 5).

Suspended solids in the drainage channel water ranged from 15 to 5150 mg·dm⁻³ (S2) and from 12 to 8140 mg·dm⁻³ (S3), indicating significant pollution caused by sewage discharge from the treatment plant into the drainage channel (Table 3 and Table 5; Figure 5). The effect of this activity is still visible at site S5, where the average annual value of this indicator is more than seven times higher than the requirements for waters suitable for salmonid and cyprinid fish habitat (Table 2 and Table 3) [11]. The TSS value at this site was very high in April 2022, reaching 1465 mg·dm⁻³ (Table 5). Elevated annual average TSS values at S6 (also very high maximum values, up to 130 mg·dm⁻³, Table 5) indicate a significant danger to the spawning grounds created above this point. Suspended solids can settle between the stones on the artificially created spawning grounds, leading to oxygen deficits [33]. Oxygen deficiency can disrupt embryonic development, reducing fertilization rates and embryo survival in fish [12,13,14,34]. The accumulation of suspended solids can also shorten the usefulness of the artificial spawning grounds [21]. Increased suspended solids are also dangerous for the fauna in the river and can disrupt the habitats of benthic organisms [35].

In contrast, the average annual TOC value at S3 was 7 times higher than the threshold value for Class II water quality, which is 10 mg·dm⁻³ (regulation of the Ministry of Infrastructure from 2021) (Table 2 and Table 4). The maximum value of this parameter (699.1 mg·dm⁻³) was recorded in April 2022 (Table 5). As indicated by the studies, the concentration of organic substances measured as TOC decreased along the river, reaching an annual average value of 14.6 mg·dm⁻³ at site S6 (Table 3). Similarly, COD_Cr showed high values, four to six times higher on average at S2 and S3 compared to site S6 (Table 2 and Table 3). In the case of sudden pollution, the value of this indicator reached as high as 191.8 mg O₂·dm⁻³ (Table 5).

This also indicates that the reason for such high average annual COD_Cr values is probably the high pollutant load discharged from the municipal wastewater treatment plant, analogous to the results of studies conducted by, for example, [26,30,32].

3.3. Indicators Characterizing Salinity and Water Acidity

The conductivity value of water in the drainage channel was relatively high, increasing from site S1 (780 µS·cm⁻¹) to S2 (1991 µS·cm⁻¹) and then decreasing at site S3 (1449 µS·cm⁻¹). These results classified these waters as nonclassified [9]. Lower values of this parameter were recorded in the waters of the Sąpólna River, averaging from 465 (S4) to 689 (S5) and 619 (S6) µS·cm⁻¹ (Table 2 and Table 3). This classifies the waters of the Sąpólna River as Class II water quality [9]. The detected conductance values should be considered high and influenced by the incoming wastewater from the Nowogard wastewater treatment plant (regardless of whether it was treated or discharged when the operating capacity of the plant was exceeded during heavy rainfall). This is because in our study, we found conductance values that were even three times higher than in the studies of, for example, Rutkowska et al. [26]. On the other hand, they were similar to the conductance in rivers with high anthropopressure, analogous to what was shown, e.g., by Puczko et al. [16]. Only the waters of the Sąpólna River at site S4 had conductance values similar to those recorded in the river studied by Rutkowska et al. [26] (these were sites outside the direct impact of the wastewater treatment plant).

The pH of the water was lower at sites S1, S2, and S3 (pH between 7.1 and 7.4), while at S4, S5, and S6, it increased to values of 7.6–7.7 (statistically significant differences at p < 0.05) (Table 3). According to normative values, pH between 6.0 and 9.0 characterizes waters suitable for fish life [11] (Table 2 and Table 3). Alkalinity and water hardness at the drainage channel sites (S1, S2, and S3) had elevated values compared to the other sites (S4, S5, and S6), with statistically significant differences (p < 0.05) (Table 3). These differences may be due to the different chemical composition of the soils in the immediate watershed, which enriches the waters of the drainage channel with alkalizing compounds (bicarbonates) more than the waters of the Sąpólna River.

It should also be noted that the overall alkalinity of both the drainage channel and river water indicates resistance to acidification (Table 3). As reported by Dickinson [36], alkalinity for such waters exceeds 75 mg CaCO₃·dm⁻³. The salinity indicators observed in the waters of the Sąpólna River (S4, S5, and S6) are characteristic of rivers in Western Pomerania [37].

3.4. Indicators Characterizing Biogenic Conditions

The highest values of TRP and TP were recorded at the drainage channel sites (S1, S2, and S3), while the lowest values were found in the river waters (S4 and S6), with statistically significant differences (p < 0.05) (Table 4). Regarding TP, in accordance with the regulation of the Ministry of Infrastructure from 2021 [9] and Directive 2006/44/EC [11], these waters are mainly nonclassified, except for sites S4 and S6, where the waters meet the requirements for Class I and Class II water quality, suitable for salmonid and cyprinid fish habitat (Table 2 and Table 4). Elevated TRP values at site S3 (over 11 times higher) and similarly high concentrations at other sites (excluding. site S4) indicate continuous pollution appearing along the drainage channel and river (Table 2 and Table 4). This can be explained by pollutant discharges, as was the case in April 2022 at S3 (Table 5). Site S4 is located above the confluence of water from the drainage channel carrying water from the treatment plant into the Sąpólna River, and therefore, it has better water quality (Figure 1). In the case of S5, located below the channel’s confluence into the river, both TRP and TP values remained high in the water, both as annual averages and during specific incidents (April 2022) (Table 4 and Table 5). Only at S6 is there a significant decrease in their concentrations (Table 4). As reported by Meybeck [38], the concentration of total phosphorus in rivers not subjected to anthropopressure is below 0.025 mgP·dm⁻³, indicating that the studied water bodies are subject to strong anthropogenic pressure.

Our results should be considered very high in the area of wastewater discharge (sites S2 and S3), as TP concentrations were more than six times higher than those recorded by Rutkowska et al. [26]. Only at site S4 (no impact from the wastewater treatment plant) were TP concentrations similar to those found by Rutkowska et al. [26] at river sites located in areas of intensive agricultural activity. Similar correlations between the degree of river degradation and the intensity of agricultural land use and the influence of wastewater treatment plants have also been found in other studies [39,40]. On the other hand, the extreme phosphorus concentrations recorded in April 2022 may have been caused by emergency inflows of wastewater after heavy rainfall. This could have led to the overloading of the bioreactors of the Nowogard treatment plant and the emergency discharge of highly polluted wastewater into the canal (sites S2 and S3) and then into the Sąpolna River (sites S5 and S6). However, it should be noted that spring snowmelt, and in our case, heavy rainfall, washes phosphorus from agricultural land and urban areas. As a result, such waters contribute elevated phosphorus loads to the river in agricultural areas but much higher loads in urban areas [41]. The relationships described above apply analogously to nitrogen compounds. This is because waters on agricultural land are characterized by high concentrations of nutrients (including NO₂⁻-N, NO₃⁻-N, and NH₄⁺-N) whose sources are organic and mineral fertilizers [42,43].

Among the measured forms of nitrogen in the waters at the research sites, elevated concentrations of NO₂⁻-N were observed. They were up to 10 times higher (at S2) than the normative value for salmonid fish, which is 0.01 mg·dm⁻³ [11] (Table 2 and Table 4). After the water flowed out of the drainage channel carrying water from the treatment plant into the river, a decrease in NO₂⁻-N concentration was observed, reaching a value of 0.036 only at S6. Such a concentration does not meet the requirements for the habitat of cyprinid fish, as it is 0.03 mg·dm⁻³ [11]. However, the maximum values at sites S5 and S6 are concerning (Table 4). Increased NO₂⁻-N concentration can affect the health, survival, and fertility of fish [44,45].

In the case of NH₄⁺-N and NO₃⁻-N, there were no elevated values of these indicators (Table 2 and Table 4). Regarding TON, very high annual average values, as well as maximum values, were recorded at S2 and S3 (Table 4). In April 2022, at S3, during the discharge of untreated water into the channel, the TON concentration was over eight times higher than the annual average value (10,308 mg·dm⁻³) (Table 5). Such high concentrations of TON are recorded when groundwater flows into rivers from peatland catchments or as wastewater or rainwater from urban areas [16,46].

However, the results concerning the values of total nitrogen classified the studied waters (except for site S1) as nonclassified waters [9]. Similarly, TN concentrations decreased along the river (Table 4). The reason for this can be attributed to sudden discharges of untreated water into the drainage channel, during which TN values can increase to 89,180 mg·dm⁻³ (S3, April 2023, Table 5). The maximum values of this indicator recorded at all sites pose a threat to fish life (Table 4). At the most distant site from the drainage channel, site 6, in January 2022, the TN value was 6128, and in February, it was 9123 mg·dm⁻³ (over two times higher than the allowable value for Class II water quality, Table 2) [9]. Above this point are spawning grounds for salmonid fish, and especially during this period, when eggs of salmonid fish may be deposited in the nests, the water should have the appropriate quality for proper embryonic development and hatching.

4. Conclusions

The results of the research allow us to conclude that the Sąpólna River is largely influenced by the same factors that determine the composition and changes in hydrochemical indices observed in comparably small rivers with similarly developed basins.

The water quality at the different points of the studied watercourses is highly variable and under strong anthropogenic pressure. The main concentrated source of pollution of the Sapólna River in the investigated section is the discharge from the sewage treatment plant in Nowogard. In general, the inflow of this wastewater causes a permanent deterioration of the water quality in the drainage channel and then in the Sąpólna River. Episodic discharges of untreated wastewater from the treatment plant during extremely heavy rainfall are very dangerous for the ecosystem. Such events should be considered pulsating disturbances, which are particularly dangerous for the biocoenoses of the Sąpólna River because they cause rapid changes in water quality parameters, resulting in the extreme values of all hydrochemical indicators.

Chronically poor water quality in the river, with episodic extreme increases in pollutant concentrations during periods of heavy rainfall, adversely affects the biodiversity of this ecosystem. It is particularly dangerous for valuable fish species because it limits their range.

In order to protect and improve the water quality in the studied section of the Sąpólna River, it is necessary to take measures to organize wastewater management—first of all, to renovate and expand the sewage treatment plant in Nowogard.