Small Hydropower Plants’ Impacts on the Ecological Status Indicators of Urban Rivers

Abstract

Water is the basis of life for living creatures and is used for various purposes, especially in agriculture, industry, municipal services, and energy production. Assessing water quality in terms of its various uses is not without significance. This study investigates the water quality within two small hydropower plants (SHPs) in central European urban areas from an environmental perspective. Wrocław I and Wrocław II SHPs on the Odra River in Poland were selected as case studies. This study presents the results of four-year observations (2017–2020) conducted in different locations located upstream and downstream of the barriers. The following aspects were assessed: physicochemical status, trophic status, assessment of fish living conditions, and water quality indices. The results show that SHPs improved the average physicochemical status of the five-day biochemical oxygen demand (i.e., BOD₅; by 6.19% comparing the results downstream and upstream of the SHPs), dissolved oxygen (3.85%), PO₄-P (3.31%), and electrical conductivity (0.52%); however, they worsened in the case of the pH (by 2.63%) and NO₃-N (by 1.83%). Water near the study cases is classified as mesotrophic or eutrophic. The conditions for salmonids and cyprinids were not met due to the increased concentrations of NO₂ and BOD₅ values; in the case of salmonids, also due to the temperature and dissolved oxygen concentration. The water quality indices differed and indicated the quality from poor to good, depending on the classification. This study provides important insights for policymakers regarding the awareness of the impacts of SHPs on water quality in urban areas and the immediate measure needed to be considered to improve aquatic habitat conditions.

1. Introduction

Water is the most abundant element on Earth. Its resources amount to 1.39 billion m³, of which 96.5% is salt water (the remaining 3.5% is freshwater, and almost 70% is stored in the ice cover and glaciers). A total of 71% of the Earth’s surface is covered by water (i.e., seas, oceans, and freshwaters) and the remaining 29% is land [1,2].

Water has many different functions, including participating in the life processes of organisms [3], shaping geomorphological and hydromorphological conditions [4], and is a living environment for aquatic and water-dependent organisms [5]. Water is also used for socioeconomic purposes, e.g., in industry, agriculture, services, drinking water, households, or recreational purposes [6]. Global freshwater use is regularly growing—from 1901–2014, there was an increase of 494% [7]. As a result, the renewable freshwater per capita is also declining. In in 2017, compared with 1962, these resources shrank by 57% [8]. North Africa has the smallest freshwater resources per capita—256 m³ according to FAO AQUASTAT data from 2015, while South America and Oceania have the highest—30,428 m³ and 29,225 m³, respectively [9]. Globally, in 2014, 82.56% of water was used for agricultural purposes, 8.54% for industry, 4.96% for municipal purposes, 2.14% for primary energy production, and 1.78% for power generation [10].

Regarding the use of freshwater water for energy purposes, run-of-river hydropower plants (i.e., using the difference in the levels of river water flowing through the turbines), impoundment (located on reservoirs formed on watercourses), pumped storage (the energy of the water drop between the upper and lower reservoirs, which is then pumped back to the upper reservoir), and tidal (using the phenomenon of the tide in seas and oceans) [11,12] are among most common energy generation schemes. In 2020, the total installed hydropower capacity was 1330 GW, of which China (370.2 GW), Brazil (109.3 GW), and the United States (102.0 GW) had the largest share. Hydropower installed capacity is constantly growing—in 2016–2020, it increased by 6.39% [13]. In 2021, the global share of hydropower in electricity production was 15.28%. The countries with the largest electricity production from hydropower plants per capita were Iceland, Norway, and Bhutan (38,564 kWh, 25,368 kWh, and 11,599 kWh, respectively) [14].

The development of hydropower as a renewable energy source is also important due to international obligations related to adaptation to climate change and limiting the effects of related environmental hazards [15]. An example of such actions is the commitment or consideration by almost 100 countries to adopt net zero greenhouse gas emissions targets, which is in line with the Paris Agreement to limit global warming to a target of 1.5 °C compared with the pre-industrial era [16,17]. The second example is the Sustainable Development Goals 2030 adopted by a resolution of the United Nations General Assembly (Agenda for Sustainable Development 2030), which constitutes an action plan for the transformation and transformation of the world in which the needs of the present generation can be met sustainably, with respect for the environment and taking into account the needs of future generations. In the case of the development of hydropower, the most important goal is Sustainable Development Goal 7, i.e., affordable and clean energy [18,19]. An example of adaptation to climate change, increasing the efficiency of using various energy sources (including hydropower), and limiting greenhouse gas emissions is energy harvesting, e.g., by modulating the operating parameters of syngas engines [20], sensor-based wireless sensor-based pipeline monitoring networks [21], functionalized composite membranes of graphene oxide or polyvinylidene fluoride [22], water–solid triboelectric nanogenerators in hydropower [23], or nanotechnology in solar energy, cell hydrogen, biofuels, and wind and ocean energy [24].

The study aims to assess water quality within two small hydropower plants in a central European urban area: the Wrocław I and Wrocław II small hydropower plants on the Odra River in the city of Wrocław in southwest Poland. The assessment considered the impact of hydropower plants on water quality mainly from an environmental perspective (i.e., assessment of living conditions for fish and trophic status) but also a socioeconomic one (i.e., water supply, suitability for consumption and recreation, and industrial use). The presented studies provide a background for the analysis of the current water quality of the Odra River, in which, in July 2022, an ecological disaster occurred as a result of pollution of the watercourse, manifested by a mass extinction of aquatic organisms (especially fish) and serious damage to aquatic and water-related ecosystems.

2. Materials and Methods

2.1. Study Area

The research was carried out in the city of Wroclaw, located in the Lower Silesia region in southwest Poland (Central Europe; geographical coordinates: 51°06′28″ N 17°02′19″ E) [25]. It is the fourth largest city in Poland (as of 31 December 2021: 674,300 people) and the third largest city in Poland (as of 31 December 2021: 292.82 km²), with a population density of 2192 people per km² (as of 31 December 2021) [26]. The structure of use in agglomeration is as follows (as of 31 December 2021): built-up and urbanized areas—46.83%, agricultural land—38.35%, forest, woody, and bushy land—5.39%, miscellaneous land—4.64%, lands under surface waters—3.46%, wasteland—1.31%, and ecological arable lands—0.02% [27]. The second largest river in Poland, the Odra (catchment area in Poland: 106.056 km², i.e., about 33.9% of the country’s area), flows through Wrocław, as well as its tributaries—the largest of them are Widawa, Bystrzyca, Oława, and Ślęza [28,29]. The research objects described in the article are located in the middle course of the Odra River, i.e., the Wrocław I and Wrocław II hydropower plants.

The Wrocław II hydropower plant is located at km 252.2 of the Odra River (51°06′55″ N 17°01′52″ E) and the Wrocław I hydropower plant is located at 252.4 km (51°06′48″ N 17°01′45″ E). The Wrocław II hydropower plant has a capacity of 1.0 MW, is equipped with 2 Francis turbines, and was built in 1924–1926. The Wrocław I hydropower plant has the following specifications: 4.83 MW, 2 Francis, and 2 Kaplan turbines, built-in 1921–1924. Both are small hydropower plants (up to 5.0 MW), run-of-river, equipped with fish passes, and their damming height is 5.2 m [30,31].

2.2. Field and Laboratory Research

The investigation was carried out at four observation points in the period from June 2017 to May 2020 on the Odra River in Wrocław (Poland), i.e., 1—a point upstream of the Wrocław II hydropower plant, 2—a point downstream of the Wrocław II hydropower plant, 3—a point upstream of the Wrocław I hydropower plant, and 4—a point downstream of the Wrocław I hydropower plant (Figure 1).

Surface water samples for field tests were collected monthly with scoops and then transported under refrigeration to the Environmental Research Laboratory of the Wrocław University of Environmental and Life Sciences. They were determined within 24 h of sampling [32].

The scope of the analysis included ten parameters, listed in

Table 1. Specification of laboratory methods for physicochemical determinations of water [32,33].

No.	Parameter	Name of the Method	Measurement Range
1.	pH	Potentiometric method	0.00–14.00
2.	Electrical conductivity (EC)	Conductometric method	0.1–2000 µS/cm
3.	Temperature of water (Temp.)	Temperature sensor (in situ tests)	−50.0–199.9 °C
4.	Turbidity (Turb.)	Nephelometric method	0.1–1000 NTU
5.	Ammonium nitrogen (NH4-N)	Spectrophotometric method	0.001–1000 mg/L
6.	Nitrate nitrogen (NO3-N)		0.1–7.0 mg/L
7.	Nitrite nitrogen (NO2-N)		0.001–1.2 mg/L
8.	Phosphate phosphorus (PO4-P)		0.001–0.5 mg/L
9.	Dissolved oxygen (DO)	Electrochemical sensor	0.00–20.00 mg/L
10.	Biochemical oxygen demand (BOD5)	Dilution method	0.1–2000 mg/L

. The measurement error was 5% for each method [33].

2.3. Data Analysis

The data were then analyzed from an environmental, social, and economic perspective, taking into account the following aspects:

• Assessment of the physicochemical status of elements of water was conducted based on the regulation in force in Poland on the classification of ecological status [34] and implementing the assumptions resulting from the Water Framework Directive concerning the monitoring of surface water bodies [35]. The assessment was carried out for nine physicochemical parameters. For the river type 21, i.e., great lowland river [36], classification details are presented in

  Table 2. Classification of the state of physicochemical elements for the tested research points of the Odra River (Poland).

  	Classification	1st Class (Very Good Status)	2nd Class (Good Status)	3rd Class (Below Good Status)
  Parameter (Unit)
  Temperature (°C)		≤22.0	22.1–24.0	>24.0
  DO (mg/L)		≥8.2	7.4–8.1	<7.4
  BOD5 (mg/L)		≤3.0	3.1–4.9	>4.9
  EC (μS/cm)		≤753	754–850	>850
  pH (-)		7.7–8.4	7.5–8.4	<7.5, >8.4
  NH4-N (mg/L)		≤0.760	0.761–0.843	>0.843
  NO3-N (mg/L)		≤2.0	2.1–2.2	>2.2
  NO2-N (mg/L)		≤0.01	0.02–0.03	>0.03
  PO4-P (mg/L)		≤0.065	0.066–0.101	>0.101

  . After the evaluation, the results were transformed, i.e., class I—1 point, class II—2 points, class III—3 points, and then the average physicochemical status for each parameter was calculated using the following scale: 1.00–1.66 points—class I, 1.67–2.33 points—class II, and 2.34–3.00 points—class III. The compliance with the standards [34] was also assessed concerning the limit values of the state of the analyzed physicochemical parameters, expressed as the quotient of the average concentration of a given parameter from the research period to the limit value specified in the regulation mentioned above, and then the results were classified according to the following scale: 0.00–1.00 points—very good quality of the physicochemical parameter, 1.01–2.00 points—good quality of the parameter, 2.01–3.00 points—moderate quality of the parameter, 3.01–4.00 points—poor quality of the parameter, and >4.00 points—bad quality of the parameter.

• Determination of the trophic status of water based on the Carlson index (TSI_P) [37,38,39,40] and the Trophic Level Index (TLI) [41,42,43] was conducted based on the average concentration of total phosphorus (TP) and, in the case of TLI, total nitrogen (TN = NH₄-N + NO₃-N + NO₂-N) from the analyzed research period (TP and TN in µg/L) for each of the points was taken into account, using the following Formulas (1)–(4) [37,41]:

TSI_P = 14.42 log (TP) + 4.15

(1)

TLI = 0.50 (TL_N + TL_P)

(2)

TL_N = −3.61 + 3.01log (TN)

(3)

TL_P = 0.218 + 2.92log(TP)

(4)

• The classification for both trophic status indices is shown in

  Table 3. Classification of the trophic status of reservoirs based on the Carlson index in terms of phosphorus concentration (TSI_P) [37,38,39,40].

  	Reference	Vollenweider (1965) [23]	Sakamoto (1966) [24]	EPA Survey (1974) [25]	Carlson and Simpson (1996) [22]
  Trophic Status
  Oligotrophic		<10	<20	<10	<6
  Oligo-mesotrophic		10–20	-	-	6–12
  Mesotrophic		20–50	20–50	10–20	12–24
  Mesoeutrophic		50–100	-	-	24–48
  Eutrophic		>100	>50	>20	48–96
  Hypertrophic		-	-	-	>96

  and

  Table 4. Classification of the trophic status using the Trophic Level Index—TLI [41,42,43].

  TLI Value	Trophic Status	Description
  0.0–2.0	Microtrophic	Very clean water, with very low levels of nutrients and algae, often glacial water, very good water quality
  2.01–3.0	Oligotrophic	Low levels of nutrients and algae, clear and blue water, good water quality
  3.01–4.0	Mesotrophic	Average levels of nutrients and algae, moderate water quality
  4.01–5.0	Eutrophic	High amounts of nutrients and algae, cloudy water, poor water quality
  5.01–9.0	Supertrophic	Very large amounts of phosphorus and nitrogen, significant algae blooms, poor water transparency, usually not meeting the standards for recreation, very poor water quality

  .

• Fulfillment of the conditions for the life of salmonids and cyprinids was based on the requirements set out in the Polish regulation on the requirements to be met by inland waters being the living environment of fish in natural conditions [44]. The results for temperature, DO, pH, BOD₅, TP, NO₂, and NH₄-N were taken into account (

  Table 5. The requirements of the conditions for the life of salmonids and cyprinids according to Polish regulations [44].

  Parameter	Compliance Rate of the Results (Percentile)	A Group of Fish	Requirement
  Temperature	98%	Salmonids	max 21.5 °C and ∆ 1.5 °C
  		Cyprinids	max 28.0 °C and ∆ 3.0 °C
  DO	100%	Salmonids	min 7 mg/L
  		Cyprinids	min 5 mg/L
  pH	95%	Salmonids, cyprinids	6–9 and max ∆ 0.5
  BOD5	95%	Salmonids	max 3 mg/L
  		Cyprinids	max 6 mg/L
  TP	95%	Salmonids	max 0.2 mg/L
  		Cyprinids	max 0.4 mg/L
  NO2	95%	Salmonids	max 0.01 mg/L
  		Cyprinids	max 0.03 mg/L
  NH4-N	95%	Salmonids, cyprinids	max 0.78 mg/L

  ).

• Assessment of water quality was conducted with the use of water quality indices providing information on both the general water quality and the possibility of using water for various purposes (i.e., water supply, recreation, living conditions for fish fauna, agriculture, and industry), i.e., Oregon Water Quality Index (OWQI) [45], Overall Index of Pollution (OIP) [46], Dinius Water Quality Index (DWQI) [47], Indian Central Control Board Water Quality Index (CPCB WQI) [48], Universal Water Quality Index (UWQI) [49], and The National Sanitation Foundation Water Quality Index (NSF WQI) [50,51,52,53]. The results were obtained by analyzing raw physicochemical data and then calculating the sub-index values for individual parameters based on formulas and determining the final value of the index based on the arithmetic or weighted mean of sub-indexes (detailed specification in

  Table 6. Basic information on determining the water quality indexes described in the article (OWQI, OIP, DWQI, CPCB WQI, UWQI, and NSF WQI)—equations converted to a scale of index values from 0 to 100 points [33,45,46,47,48,49,50,51,52,53,54].

  Index Name	Data for the Index Calculation	Parameters Taken into Account	Final Index Value
  			Equation	Explanation of Symbols
  Oregon Water Quality Index (OWQI) [33,45,46,54]	Median value	DO, pH, BOD5, NH4 + NO3, TP, temperature	$OWQI=11.1\sqrt{\frac{n}{{{\displaystyle \sum }}_{i=1}^n\frac{1}{S{I}_i^2}}}-111$	OWQI—the final index value, SIi—the sub-index value for each parameter, and n—the number of parameters considered in the calculations.
  Overall Index of Pollution (OIP) [33,46,54]	Maximum value *	Turbidity, pH, BOD5, NO3	$OIP=-6.25 \frac{{{\displaystyle \sum }}_i{P}_i}{n}+100$	OIP—the final index value, Pi—the sub-index value for each parameter, and n—the number of parameters considered in the calculations.
  Dinius Water Quality Index (DWQI) [33,46,47,54]	Maximum value *	pH, BOD5, temperature, NO3	$DWQI={\displaystyle \sum _{i=1}^n}{I}_i{w}_i$	DWQI—the final index value, Ii—the sub-index value for each parameter, wi—the weight value of each parameter (pH = 0.226, BOD5 = 0.284, temperature = 0.226, NO3 = 0.264), and n—the number of parameters considered in the calculations.
  Indian Central Pollution Control Board Water Quality Index (CPCB WQI) [33,46,48,54]	Maximum value *	pH and BOD5	$CPCB WQI={\displaystyle \sum _{i=1}^n}{I}_i{w}_i$	CPCB WQI—the final index value. Ii is the sub-index value for each parameter, wi—the weight value for each parameter (pH = 0.537, BOD5 = 0.463), and n—the number of parameters considered in the calculations.
  Universal Water Quality Index (UWQI) [33,46,49,54]	90th percentile **	DO, pH, BOD5, TP, NO3	$UWQI={\displaystyle \sum _{i=1}^n}{I}_i{w}_i$	UWQI—the final index value, Ii—the sub-index value for each parameter, wi—the weight value for each parameter (DO = 0.332, pH = 0.085, BOD5 = 0.166, TP = 0.166, NO3 = 0.251), and n—the number of parameters considered in the calculations.
  The National Sanitation Foundation Water Quality Index (NSF WQI) [50,51,52,53,54]	Raw data	pH, EC, NO3-N, PO4-P, DO, BOD5	NSF WQI =$\frac{1}{100}{\left[{\displaystyle \sum }_{i=1}^n{q}_i·w_i\right]}^2$	NSF WQI—the final index value, qi—the percentage of samples that fall within the limit values of the parameters, wi—the weight value for each parameter (pH = 0.04, EC = 0.13, NO3-N = 0.11, PO4-P = 0.20, DO = 0.28, BOD5 = 0.24), and n—the number of parameters considered in the calculations.

  Designations in the table: * DO—minimum value; pH—minimum and maximum value (a less favorable result); ** DO—10th percentile; pH—10th and 90th percentile (a less favorable result); NH₄—ammonia (NH₄ = 0.78125NH₄-N); NO₂—nitrites (NO₂ = 0.304NO₂-N); and NO₃—nitrates (NO₃ = 0.2257NO₃-N).

  ). The final result was expressed on a scale from 0 to 100 points on a 5-point scale, i.e., 0–24.9 points—bad water quality (class V), 25.0–49.9 points—poor water quality (class IV), 50–74.9 points—moderate water quality (class III), 75.0–94.9 points—good water quality (class II), and 95.0–100 points—very good water quality (class I). A detailed description of the research methodology for the mentioned water quality indices (especially in terms of determination of sub-index values and methods of classification of results calculated for these indices) can be found in other articles [33,45,46,47,48,49,50,51,52,53,54].
• This study also analyzed descriptive statistics and checked the obtained results regarding their statistical significance. For this purpose, a non-parametric Wilcoxon rank test was performed for data with a distribution not corresponding to the linear distribution, in which the null hypothesis is that two groups of variables (i.e., in this case, data upstream and downstream of individual hydropower plants) do not differ in the median [55]. The analyses were performed using the SPSS Statistics 26 software.

3. Results

3.1. Descriptive Statistics

The greatest differences were noted in the case of NO₃-N. The median between points differed by 27.82% for the Wrocław II hydropower plant and 10.83% for the Wrocław I hydropower plant. For PO₄-P, these changes were 5.56% and 6.25%, respectively, within the hydropower plant, for turbidity—2.16% and 8.11%, and NH₄-N—6.25% and 0.00%. In the case of the remaining parameters, the differences in the medians between the points upstream and downstream of the hydropower plants were less than 5.00% (

Table 7. Descriptive statistics at analyzed research points.

	Point	1 (Median ± SD)	2 (Median ± SD)	3 (Median ± SD)	4 (Median ± SD)
Parameter
pH		8.00 ± 0.48	7.90 ± 0.50	8.00 ± 0.51	8.00 ± 0.45
EC (µS/cm)		1087 ± 358	1103 ± 363	1103 ± 359	1082 ± 358
Temp. (°C)		13.70 ± 7.59	13.80 ± 7.47	13.70 ± 7.48	13.70 ± 7.35
Turb. (NTU)		3.70 ± 7.30	3.78 ± 9.44	3.70 ± 7.02	3.40 ± 7.13
NH4-N (mg/L)		0.16 ± 0.17	0.15 ± 0.17	0.14 ± 0.17	0.14 ± 0.16
NO3-N (mg/L)		1.33 ± 0.91	1.70 ± 0.92	1.57 ± 0.93	1.40 ± 0.93
NO2-N (mg/L)		0.026 ± 0.019	0.024 ± 0.019	0.021 ± 0.020	0.021 ± 0.019
PO4-P (mg/L)		0.090 ± 0.037	0.085 ± 0.037	0.080 ± 0.039	0.085 ± 0.036
DO (mg/L)		9.20 ± 1.89	9.10 ± 1.83	8.85 ± 2.09	9.00 ± 1.90
BOD5 (mg/L)		2.50 ± 2.19	2.45 ± 1.85	2.70 ± 1.72	2.80 ± 2.67

Designations in the table: SD—standard deviation.

).

3.2. Wilcoxon Rank Test

The analysis of the results in pairs showed the statistical significance of the results for EC and PO₄-P within the Wrocław II hydropower plant and the pH at the Wrocław I hydropower plant (i.e., in each of these cases, p was less than 0.05;

Table 8. Wilcoxon rank test results for research points.

Test Statistics Parameter	Point 1/2		Point 3/4
	Z	p	Z	p
pH	−1.118	0.264	−2.138	0.033 *
EC	−2.763	0.006 *	−1.890	0.059
Temperature	−0.989	0.323	−1.598	0.110
Turbidity	−0.009	0.993	−0.721	0.471
NH4-N	−1.580	0.114	−0.055	0.956
NO3-N	−1.304	0.192	−1.753	0.080
NO2-N	−0.317	0.751	−1.837	0.066
PO4-P	−2.032	0.042 *	−0.548	0.584
DO	−0.039	0.969	−1.924	0.054
BOD5	−1.017	0.309	−0.187	0.852

*—statistically significant value (for p < 0.05); Z—normalized test statistic; and p—test probability.

). The highest significance was demonstrated for EC within the Wrocław I hydropower plant. Values close to statistical significance were observed in the Wrocław I hydropower plant for DO (p = 0.054) and EC (p = 0.059).

3.3. Physicochemical Status

The general physicochemical status at each research point is below good (class III). The parameters that worsen this status are EC, NO₂-N, and PO₄-P (class III, II, and II for individual parameters; Figure 2). The smallest exceedances of the norms [19] for the analyzed parameters occurred in the case of NH₄-N, BOD₅, NO₂-N, and NO₃-N (respectively: 0.13, 0.21, 0.22 and 0.29), and the biggest occurred for EC, temperature, and DO (respectively: 1.00, 0.94, and 0.83).

By analyzing the differences between the results of upstream and downstream hydropower plants, the physicochemical status is improved for BOD₅, DO, PO₄-P, and EC (by: 6.19%, 3.85%, 3.31%, and 0.52%, respectively) and worsened for pH and NO₃-N (by 2.63% and 1.83%). It should be noted that the Wrocław I hydropower plant improved the physicochemical status for seven out of nine assessed parameters (from 1.03% to 7.41%). In two cases, no changes were noted. The Wrocław II hydropower plant improved its status in two out of nine cases (from 5.13% to 7.41%), worsened in three out of nine cases (1.33% to 7.55%), and in four out of nine cases, there was no difference between the results. When analyzing all the results of the average physicochemical status obtained for the parameters upstream and downstream of the Wrocław I and II hydropower plant (

Table 9. Detailed summary of the results of the average physicochemical status within the Wrocław I and Wrocław II hydropower plants.

Parameter	Comparison of the Results of Upstream and Downstream HPs (Average Physicochemical Status)
	Wrocław II Hydropower Plant (HP)			Wrocław I Hydropower Plant (HP)
	Improvement	Deterioration	No Changes	Improvement	Deterioration	No Changes
pH	8.33%	2.78%	88.89%	0.00%	2.78%	97.22%
EC	2.78%	2.78%	94.44%	0.00%	2.78%	97.22%
Temperature	0.00%	0.00%	100.00%	0.00%	0.00%	100.00%
NH4-N	0.00%	0.00%	100.00%	0.00%	0.00%	100.00%
NO3-N	11.11%	0.00%	88.89%	8.33%	11.11%	80.56%
NO2-N	2.78%	0.00%	97.22%	2.78%	5.56%	91.67%
PO4-P	8.33%	19.44%	72.22%	19.44%	16.67%	63.89%
DO	5.56%	8.33%	86.11%	2.78%	11.11%	86.11%
BOD5	5.56%	16.67%	77.78%	8.33%	13.89%	77.78%

), it can be seen that in most cases, there were no changes in the average physicochemical state after passing through hydropower plants (Wrocław II hydropower plant—from 72.22% to 100%; Wrocław I hydropower plant—from 63.89% to 100.00%).

Comparing the percentage of improvement and deterioration in the average physicochemical status for the Wrocław II hydropower plant, an improvement was noted for NO₃-N, pH, and NO₂-N (by 11.11%, 5.56%, and 2.78%), and a deterioration was noted for BOD₅, PO₄-P, and DO (by 11.11%, 11.11%, and 2.78%). For the Wrocław I hydropower plant, an improvement was noted for PO₄-P (by 2.78%), while a deterioration was noted for DO, BOD₅, pH, EC, NO₃-N, and NO₂-N (respectively by: 8.33 %, 5.56%, and 2.78% in the other six cases). The differences were, therefore, small and usually below 5%. Convergent results for both hydropower plants were obtained for DO and BOD₅. A deterioration in the average physicochemical status was noted for both of these parameters.

In terms of the results against the limit values (Figure 3), there was an improvement for four out of the eight assessed parameters (BOD₅, NH₄-N, DO, water temperature; changes from −2.21% to −60.00%), a deterioration for three out of eight parameters (PO₄-P, NO₃-N, EC; changes from 0.70% to 16.67%), and no change for one parameter, NO₂-N.

When analyzing the results separately for the Wrocław I and Wrocław II hydropower plants (

Table 10. Detailed summary of the results of the meeting standards against parameter limit values within the Wrocław I and Wrocław II hydropower plants.

Parameter	Comparison of the Results Upstream and Downstream of HPs (Meeting Standards—Limit Values)
	Wrocław II Hydropower Plant (HP)			Wrocław I Hydropower Plant (HP)
	Improvement	Deterioration	No Changes	Improvement	Deterioration	No Changes
EC	22.22%	72.22%	5.56%	52.78%	36.11%	11.11%
Temperature	30.56%	38.89%	30.56%	44.44%	22.22%	33.33%
NH4-N	52.78%	30.56%	16.67%	38.89%	30.56%	30.56%
NO3-N	44.44%	55.56%	0.00%	58.33%	36.11%	5.56%
NO2-N	27.78%	25.00%	47.22%	30.56%	13.89%	55.56%
PO4-P	47.22%	19.44%	33.33%	22.22%	38.89%	38.89%
DO	47.22%	38.89%	13.89%	58.33%	27.78%	13.89%
BOD5	55.56%	41.67%	2.78%	44.44%	44.44%	11.11%

), a greater percentage of improvements and deterioration of the calculated indicators related to the limit values of physicochemical parameters was observed than for the average physicochemical status. For the Wrocław II hydropower plant, deterioration of values was noted for three out of eight parameters (EC, NO₃-N, temperature, deterioration by 50.00%, 11.11%, and 8.33%, respectively, comparing the percentage of improvements and deteriorations) and an improvement was noted for five out of eight parameters (PO₄-P, NH₄-N, BOD₅, DO, and NO₂-N by: 27.78%, 22.22%, 13.89%, 8.33%, and 2.78%, respectively). For the Wrocław I hydropower plant, deterioration was noted for one of eight parameters (PO₄-P, by 16.67%), improvement was noted for six of eight parameters (changes ranging from 8.33% to 30.56%), and no change (same number of improvements and deteriorations) was noted for one of eight parameters (BOD₅). Convergent results were obtained in both hydropower plants for NH₄-N, NO₂-N, and DO. There were more cases of improvement than deterioration of values. However, it should be emphasized that for NO₂-N, no change was observed in 47.22% of cases for the Wrocław II hydropower plant and 55.56% for the Wrocław I hydropower plant.

3.4. Trophic Status

The calculated Carlson index indicates that the water within the Wrocław I and Wrocław II hydropower plants are mesotrophic or eutrophic due to the average total phosphorus concentration in the analyzed period (

Table 11. Trophic status at the analyzed research points on the Odra River (Poland)—Trophic Status Index.

Point	Carlson Index (TSIP)	Classification
		Vollenweider (1965) [23]	Sakamoto (1966) [24]	EPA Survey (1974) [25]	Carlson and Simpson (1996) [22]
1	69.47	Mesotrophic	Eutrophic	Eutrophic	Eutrophic
2	68.31
3	68.90
4	68.49

) [22,23,24,25]. The differences between the points are not large. Downstream of the Wrocław II hydropower plant, the Carlson index value was lower by 1.67% than the point upstream of the hydropower plant. For the Wrocław I hydropower plant, the downstream value was lower by 0.60% than the point upstream of the hydropower plant. Downstream of the hydropower plants, less enrichment with phosphorus compounds were found than upstream.

The TLI (

Table 12. Trophic status at the analyzed research points on the Odra River (Poland)—Trophic Level Index.

Point	TLIP	TLIN	Trophic Level Index (TLI)	Classification
1	5.05	5.96	5.51	hypertrophic
2	5.11	5.86	5.48
3	5.01	5.91	5.46
4	5.08	5.88	5.48

) [26,27,28] noted hypertrophic status at each point due to high phosphorus and nitrogen concentrations. Higher concentrations of total nitrogen had a greater negative effect on the final result. Downstream of the Wrocław II hydropower plant, the trophic status improved, and downstream of the Wrocław I hydropower plant, it worsened (by 0.38% and 0.30%, respectively). Considering the overall results, the hydropower plants marginally improved the trophic status expressed in TLI, i.e., by 0.04%.

The observed average PO₄-P level for the multi-year period is higher for the presented results (0.086–0.093 mg/L) than for the estuary section of the Odra River (0.070 mg/L) [56].

3.5. Living Conditions for Salmonids and Cyprinids

The assessment of compliance with the requirements [29] in the context of the living conditions of salmonids and cyprinids is presented in

Table 13. List of results in the analyzed test points concerning fulfilling the conditions for fish.

Parameter	Group of Fish	Point
		1	2	3	4
Temperature	Salmonids (max value)	−	−	−	−
	Cyprinids (max value)	+	+	+	+
	Salmonids (max ∆)	+		+
	Cyprinids (max ∆)	+	+	+	+
DO	Salmonids	−	−	−	+
	Cyprinids	+	+	+	+
pH	Salmonids and cyprinids (values)	+	+	+	+
	Salmonids and cyprinids (max ∆)	+		+
BOD5	Salmonids	−	−	−	−
	Cyprinids	−	−	−	−
TP	Salmonids	+	+	+	+
	Cyprinids	+	+	+	+
NO2	Salmonids	−	−	−	−
	Cyprinids	−	−	−	−
NH4-N	Salmonids and cyprinids	+	+	+	+

Blue background—standards are met; red background—standards are not met.

. It shows that assumptions for five out of seven parameters for cyprinids were met, and for salmonids, assumptions for three out of seven parameters were met. High concentrations of NO₂ worsened the assessment and BOD₅ in water for both groups of ichthyofauna (i.e., 0.053–0.055 mg/L and 6.48–6.95 mg/L), as well as too low concentrations of DO (i.e., except for the point downstream of the Wrocław I hydropower plant) and too many high temperatures for salmonids (respectively: 6.30–6.95 mg/L and 24.00–24.18 °C).

3.6. Water Quality Indices

The overall water quality ranged from poor (CPCB WQI, Wrocław II hydropower plant; 45.34 and 49.23 points) to good (OIP, Wrocław II hydropower plant and upstream Wrocław I hydropower plant; Wrocław II—77.85 and 78.33 points, Wrocław I—79.95 points; NSF WQI, downstream Wrocław I hydropower plant—75.50 points), and the average water quality was moderate (62.83 points). The detailed results are presented in Figure 4. Considering the interpretation of the results for the DWQI, this water would require treatment before consumption; however, its quality is acceptable for most water sports (i.e., including bathing), and the water quality is suitable for fish less sensitive to pollution. It allows normal industrial production not requiring high-quality water [41].

Hydropower plants, on average, deteriorated the value of the indices by 1.35%; however, these values differ depending on the indicator and the sites. For the Wrocław II hydropower plant, for five out of six indices, an improvement in their value downstream was observed (from −5.65% to 8.58%; mean = 0.88%). For the Wrocław I hydropower plant, for three out of six indices, an improvement in their value downstream was observed (from −21.75% to 4.18%; mean = −3.54%).

4. Discussion

4.1. Descriptive Statistics

For comparison, studies conducted in Spain by Valero in 2012 [57] indicate that the largest changes in the values of parameters within the tested hydropower plant concerned EC and DO (7.50% and 7.12%); the Fantin-Cruz and other studies from 2015 in Brazil [58] concerned turbidity, TP, and NO₃ (respectively: 38.00%, 28.00%, and 14.00%); the studies conducted in Lithuania by Vaikasas and others from 2015 [59] concerned TP and TN (26.97% and 15.49%); the studies conducted by Luo and others in China concerned TP and DO (366.7% and −35.09%) [60]; the studies conducted by Xu and others in China concerned EC and DO (−7.50% and 7.12%) [61]; and the studies conducted by Tomczyk and Wiatkowski in Poland concerned NH₄-N (from −38.89% to 22.86%) and turbidity (from −7.14% to 5.95%) [62]. Detailed results are summarized in

Table 14. Comparison of the results obtained for physicochemical parameters within different hydropower plants (difference in medians at points downstream and upstream of hydropower plants) [57,58,59,60,61,62].

	Reference	Vaikasas et al., 2015 [59]		Luo et al., 2019 [60]	Xu et al., 2022 [61]	Valero, 2012 [57]	Fantin-Cruz et al., 2015 [58]	Tomczyk and Wiatkowski 2021 [62]	This Research
Parameter
pH						0.17%		0.93% to 2.16%	−1.25 % to 0.00%
Temperature						2.73%		−0.10% to 2.14%	0.00% to 0.73%
EC						−7.50%		−2.02% to −1.17%	−1.90% to 1.47%
Turbidity							−38.00%	−7.14% to 5.95%	−8.11% to 2.16%
DO				−35.09%		7.12%		0.56% to 14.04%	−1.09% to 1.69%
TP		0.00%	−26.97%	366.7%	−85.00% to 0%		−28.00%	−7.22% to -0.19%	−5.56% to 6.25%
TN		−2.08%	15.49%	−13.33%				−5.06% to 3.15%	−9.82% to 23.61%
NO3-N							−14.00%	−4.97% to 5.50%	−0.83% to 27.82%
NH4-N					−72.72% to 8.33%			−38.89% to 22.86%	−6.25% to 0.00%
Installed capacity		0.095 −0.6 MW	0.15 −1.3 MW	900 MW	1800 −3000 MW	11.82 MW	210 MW	0.06 −0.385 MW	1.0 −4.83 MW
Damming height		<5 m	5–15 m	105 m	71–161 m	70 m	243 m	1.8–3.75 m	5.2 m
Type of hydropower		impoundment (storage)				run-of-river
River		Virvytė, Venta, Obelis, Šušve, Varduva		Lancang	Jinsha	Lérez	Correntes	Bystrzyca	Odra
Country		Lithuania		China		Spain	Brazil	Poland

. Analyzing the results in more detail, especially for TP, for which there is an almost full cross section of case studies, it is noted that the size of changes within hydropower plants is influenced by the damming height and the type of hydropower. The higher the damming height, the greater the impact usually, and storage hydropower plants have a stronger impact than run-of-river hydropower plants on the parameters of water quality. No convergent relationships were found in the changes in parameter values within hydropower plants.

Among the phenomena that concern the impact of hydropower plants on physicochemical parameters, it is possible to note, e.g., supersaturation (occurring in cascade reservoir hydropower plants during higher water flows and manifested by a reduction in DO content—this is reflected in the DO results from Luo and other studies from 2019) [63,64,65], resuspension of sediments rich in nutrients (release of nitrogen and phosphorus compounds contained in the sediments into the water in periods of higher flows downstream of hydropower plants; during periods of low and medium flows, accumulation of sediments rich in nutrients upstream of hydropower plants occurs) [66,67,68], changes in thermal and oxygen conditions (mainly in storage hydropower plants due to the effect of the swirling movement of water below the damming) [69,70,71]. Future research should consider the operating cycles of hydropower plants as well as hydrological conditions that greatly impact the physicochemical properties of water.

4.2. Wilcoxon Rank Test

Described results are consistent with the analyses carried out earlier in hydropower plants on the Bystrzyca River in Poland, e.g., Sadowice, Skałka, and Marszowice hydropower plants [62]. In these studies, statistical significance for ANOVA was also found for EC, pH, and TP, but also for NO₃-N and DO on the scale of the Bystrzyca River (effect size: 0.924, 0.310, 0.178; 0.541 and 0.322; where 0—the effect is absent, 1—the effect is always present). An ANOVA was also performed within the hydropower plants, comparing only the points upstream and downstream of the hydropower facilities. The significance was noted in the following cases: Marszowice hydropower plant—NH₄-N (effect size 0.442), DO (0.408), pH (0.405), and NO₃-N (0.408); Skałka hydropower plant—EC (0.623) and pH (0.338); and Sadowice hydropower plant—no statistically significant variation.

4.3. Physicochemical Status

Research carried out on the hydropower plants on the Bystrzyca and Ślęza rivers (tributaries of the Odra River) shows that the general physicochemical status did not change noticeably within the hydropower plants [62,72]. In the case of the Sadowice, Skałka, and Sadowice hydropower plants on the Bystrzyca River, a very good physicochemical condition was noted in terms of temperature, which was good for EC, NH₄-N, and TP. This status was below good for pH, NO₃-N, NO₂-N, DO, and BOD₅ (exception: the condition within the Marszowice hydropower plant was below good for EC; in this case, there are pressures related to the urbanization processes as this hydropower plant is located in the city of Wrocław, while the others are located in forest–agricultural and rural areas) [62,73].

Concerning the hydropower plant on the Ślęza River located in Wrocław, the physical and chemical condition was below good for DO, BOD₅, EC, pH, NH4-N, NO₃-N, NO₂-N, and TP, and very good for the water temperature. Attention was drawn to a large percentage of exceedances of EC (95.8%) and NO₂-N (45.8%) and their values (maximum by 204–220% and 865–923%). It was indicated that the EC exceedances result from the characteristics of the catchment area, geological structure, and anthropogenic interactions. The hydropower plant itself has little effect on changes in the value of this parameter (the median change was 0.25% compared with the hydropower plant’s lower and upper positions) [72]. Increased concentrations of NO₂-N occurred in April and May 2020 caused by heavy rainfall, which carried large loads of pollution from the upper part of the agricultural catchment (fertilizers and plant protection products). The impact of the hydropower plant was visible only in 1 out of 24 examined months and the reason was most likely the release of sediments accumulated upstream of the damming of the hydrotechnical structure and the resuspension of the sediments due to a hydraulic jump on the lower side and the subsequent release of accumulated NO₂-N [74]. Similar cases occurred, for example, in the area of an impoundment hydropower plant in the French Alps [75] or on hydraulic models imitating individual technical elements of hydropower plants (e.g., pressure sand traps) [76]. A similar phenomenon also happened within the analyzed Wrocław II hydropower plant in June 2018. The NO₂-N concentration increased by 0.018 mg/L downstream of the hydrotechnical structure, while the average and median change between the upper and lower stations for the entire period was 0.00 mg/L.

4.4. Trophic Status

The calculated trophic indexes for other studies within hydropower plants (

Table 15. Calculated results of trophic indices for other hydropower plants (HPs).

Trophic Index	Point Location	References
		Vaikasas et al., 2015 [59]		Luo et al., 2019 [60]
TSIP	Upstream HP	58.05	58.05	53.20
	Downstream HP	59.67	64.34	75.41
TLIP	Upstream HP	4.96	4.96	4.53
	Downstream HP	5.10	5.51	6.48
TLIN	Upstream HP	6.19	7.60	5.95
	Downstream HP	6.19	7.76	5.76
TLI	Upstream HP	5.57	6.28	5.24
	Downstream HP	5.64	6.64	6.12
Information about hydropower plants
Location of hydropower plants		Lithuania		China
Type of hydropower plants		storage
Hydropower plants’ capacity		0.095–0.6 MW	0.15–1.3 MW	900 MW
Damming height		<5 m	5–15 m	105 m

) indicate that the greater the damming height and capacity, the greater the impact on the trophic status of the water [59,60]. In the case of hydropower plants with a damming height of fewer than 5 m and a capacity of 0.095–0.6 MW, the change in value was respectively: TSI_P—2.79% and TLI—1.26%; for a damming height of 5–15 m and a capacity of 0.15–1.3 MW, the change in value was respectively:10.84% and 5.73%; and for a damming height of 105 m and a capacity of 900 MW, the change in value was respectively: 41.75% and 16.79%. Contrary to the research presented here, storage hydropower plants deteriorated in the points downstream compared with the upstream structures. Overall results indicate either mesotrophic or eutrophic (taking TSI_P into account) or supertrophic (taking TLI into account). In five out of six cases, the factor deteriorating the result of the calculated TLI was TN (consistent with the results of these studies), and in one case the factor was TP.

4.5. Living Conditions for Salmonids and Cyprinids

Fish have different requirements as to the physicochemical parameters of the water, for example, for fish production, the optimal pH for freshwaters is assumed at different levels, e.g., Kenya—6.5–9.0 [77], Australia—5.0–9.0 [78], and Philippines—6.5–8.5 [79]. Lawson suggested [80] that fish have different pH tolerances, i.e., <4.0 and >11.0—point of death, 4.0–5.0 and 9.0–11.0—slow growth or no production, and 6.5–9.0—the optimum for fish production. In the context of these requirements, the recorded pH points were in most cases within the optimal range for fish life, i.e., it changed as follows: point 1—6.8–8.6, point 2—6.5–8.6, point 3—6.5–8.6, and point 4—6.8–8.7. Exceedances of the limit values against the Philippine requirements occurred in points 1–3 in 5.56% of the cases and 4–2.78% of the cases.

In the context of optimal DO contents, different values are given for the proper existence of salmonids and cyprinids [81]. In the case of salmonids, these include the following values: >5.0 mg/L [82,83] and >5.5 mg/L for fish, and >7.0 mg/L for eggs [84] or >9.0 mg/L and >7.0 mg/L for at least 50% of the time [85]. For cyprinids, these values are lower, i.e., a minimum of 5.0 mg/L and 7.0 mg/L for at least 50% of the time [85] and a minimum of 2.9 mg/L for carp [86]. DO concentrations measured at the test points changed as follows: point 1—6.2–13.1 mg/L, point 2—6.4–13.3 mg/L, point 3—5.0–13.1 mg/L, and point 4—6.4–13.10 mg/L. All the requirements mentioned above were met. Considering the highest limit values, i.e., 9.0 and 7.0 mg/L, the standards were met in the ranges from 44.44% to 58.33% and from 83.33% to 94.44% of cases, respectively.

Various studies suggest taking into account parameters such as water temperature [87], total alkalinity [88], various forms of nitrogen—NH₄-N, NO₃-N, NO₂-N [89], phosphorus [90], and heavy metals, e.g., Hg, Pb, Cd, and Ni [91].

4.6. Water Quality Indices

Other studies indicate an ambiguous influence of hydropower plants on the shaping of the values of water quality indicators. In some, there was an improvement downstream of hydropower plants (Malaysia; Ling et al., 2016 [92]), and in others, there was a deterioration of water quality (Brazil; de Oliveira et al., 2021 [93]). Other studies do not provide a clear answer regarding the impact of hydropower plants on water quality (China; Luo et al., 2019 [60], Poland; Tomczyk et al., 2021 [94], and Tomczyk and Wiatkowski 2021 [62]). The differences in the values of the indices fluctuated in the cited studies ranging from −69.09% to +24.26% downstream of hydropower plants compared with the points upstream of the hydropower plants. A detailed summary of the described results is presented in

Table 16. Comparison of water quality indices (WQIs) results within selected hydropower plants.

Percentage Difference in the Values of Water Quality Indices Downstream and Upstream of Hydropower Plants
	Reference	Ling et al., 2016 [92]	de Oliveira et al., 2021 [93]	Luo et al., 2019 [60]	Tomczyk et al., 2021 [94]	Tomczyk and Wiatkowski, 2021 [62]	This Research
WQIs
ATI		14.25%	−9.13%	−1.48%	−2.22%	−0.49%	n/a	n/a
OWQI		24.26%	−39.10%	5.15%	−69.09%	2.40%	−5.65%	2.18%
OIP		1.62%	−60.35%	n/a	−0.63%	0.44%	0.61%	−21.75%
DWQI		2.55%	−29.41%	n/a	0.74%	2.07%	1.64%	−1.04%
CPCB WQI		4.93%	−37.38%	n/a	−3.13%	1.72%	8.59%	−3.69%
UWQI		n/a	n/a	n/a	5.40%	6.21%	1.35%	2.96%
Median		4.93%	−37.38%	1.83%	−1.42%	1.90%	1.35%	−1.04%
Average		9.52%	−35.07%	1.83%	−11.49%	2.06%	1.31%	−4.27%
Information about hydropower plants
Installed capacity		2400 MW	399 MW	900 MW	0.045 MW	0.07 MW	1.0 MW	4.83 MW
Damming height		205 m	208 m	105 m	2.5 m	2.2 m	5.2 m	5.2 m
Type of hydropower		impoundment (storage)				run-of-river
River		Balui	Jequitinhonha	Lancang	Widawa	Bystrzyca	Odra
Country		Malaysia	Brazil	China	Poland

n/a—data not available.

.

5. Conclusions

This study shows that the influence of hydropower plants in urban areas on the shaping of water quality from the natural, social, and economic perspectives is ambiguous. The following conclusions can be drawn from the analyses carried out:

• Differences in the median values of physicochemical parameters within the hydropower plants were small; they amounted to a maximum of 27.80% (NO₃-N at the Wrocław II hydropower plant) and usually did not exceed 10%;
• The changes in concentrations for EC and PO₄-P (Wrocław II hydropower plant) and pH (Wrocław I hydropower plant) were found to be statistically significant;
• Water in the area of the Wrocław I and Wrocław II hydropower plants was classified as mesotrophic or eutrophic;
• The conditions for the existence of cyprinids and salmonids were not met, respectively, for two out of seven assessed parameters (BOD₅, NO₂) and for four out of seven parameters (BOD₅, NO₂, temperature, DO in three out of four points);
• Hydropower plants, on average, deteriorated the value of water quality indices by 1.35%; the overall water quality ranged from poor to good, and the average water quality was moderate. This water would require treatment before consumption; however, its quality is acceptable for most water sports, and the water quality is suitable for fish less sensitive to pollution and allows normal industrial production not requiring high-quality water. The values of these indices varied depending on the index and sites;
• The authors plan to continue and expand this research on the analysis of the migration of aquatic organisms through water steps with hydropower buildings in urban areas and investigate the exact characteristics of the capacity achieved by these hydropower plants along with hydrological conditions. The scope of these studies should include not only the impact on the water and land environment, but also the impact on hydrological conditions, landscape, water relations within the hydropower plant, and economic and social issues (e.g., the profitability of this type of project, compliance with the international energy and climate policy and sustainable development goals, and the location of hydropower plants).