Ecological risks of dammed reservoir waters using the example of the Turawa Reservoir in southern Poland

doi:10.21203/rs.3.rs-3875802/v1

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Ecological risks of dammed reservoir waters using the example of the Turawa Reservoir in southern Poland

https://doi.org/10.21203/rs.3.rs-3875802/v1

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A study of the ecological potential and the chemical pollution of the Turawa Reservoir (southern Poland), as well as an assessment of the pollution of its recharge waters, was carried out between 2019 and 2021. Four research teams took part in the study. The reservoir was chosen because of the eutrophication process that has been ongoing for several decades. Phytoplankton, phytobenthos and macrozoobenthos were surveyed as part of the ecological potential assessment. Analysed indicators were: BOD₅, COD_Cr, nitrogen concentrations (Kjeldahl nitrogen TKN, ammonium nitrogen NH₄-N, nitrate nitrogen NO₃-N and total nitrogen TN) and phosphorus (orthophosphate phosphorus PO₄-P and total phosphorus TP), as well as conductivity and water pH. Using ion chromatography, the ionic composition (F⁻, Cl⁻, Br⁻, NO₂⁻, NO₃⁻, PO₄³⁻, SO₄²⁻, Na⁺, NH₄⁺, K⁺, Mg²⁺, Ca²⁺) and heavy metal concentrations (Cr, Ni, Zn, Cu, Cd, Pb) of the basin waters and the waters supplying the basin were determined. Coliforms, faecal enterococci and Salmonella spp. were also assessed. The condition of the reservoir water was assessed as poor. The poor quality of the water supplying the reservoir, and the bottom sediments, in which large quantities of, among other things, nutrients and heavy metals have accumulated over the years, have been identified as the cause. A factor related to biomass accumulation was also identified. During the vegetation cycle, due to biochemical processes taking place, the oxygen and nutrient indicators can be significantly influenced. The problem outlined characterises most European dam reservoirs, especially shallow reservoirs with low thermal stratification.

Biological sciences/Biotechnology/Environmental biotechnology

Earth and environmental sciences/Environmental sciences/Environmental chemistry/Environmental monitoring

biological elements

oxygen indicators

nutrient indicators

heavy metals

microbiological parameters

Water pollution has now become a global problem ¹. As a result of pollution in many watercourses and stagnant waters, the capacity for self-purification has been reduced, resulting in an imbalance of ecosystems ². The waters of the seas and oceans are at risk, especially in the estuaries of rivers carrying large loads of pollutants with their waters ³. The global nature of the problem is evidenced by the presence of toxic substances ⁴ and microplastics ⁵ in waters and sediments of Arctic seas, as well as in marine organisms ⁶. In recent decades, the threat to aquatic ecosystems is compounded by climate change and global warming, causing extreme climatic events: flooding and drought ⁷. Sources of pollution entering surface waters are point source (e.g. industrial and municipal wastewater entering waters through sewerage systems) and area source (e.g. uncontrolled leachate from agricultural fields, industrial and post-industrial areas, atmospheric deposition). The well-being of aquatic ecosystems is also affected by environmental failures and ecological disasters. Organic compounds pose a serious threat to aquatic ecosystems: detergents, pesticides ⁸, persistent organic pollutants – POPs ⁹ polycyclic aromatic hydrocarbons – PAHs ¹⁰, organochlorine compounds, such as organochlorine pesticides and polychlorinated biphenyls – PCBs ¹¹, among others. Organic pollutants posing as yet unrecognised risks also include those present in trace amounts in water: pharmaceuticals and chemical breakdown products of plastics ^12,13.

Many countries and international organisations set legal standards to counteract the pollution of the aquatic environment. In European Union countries, the Nitrates Directive ¹⁴ was introduced in 1991 to protect waters from pollution by nitrates of agricultural origin, and in 2000 the WFD – Water Framework Directive ¹⁵ was introduced, which obliges member states to protect and control the quality of surface waters. In Poland, the implementation of the WFD is regulated by the Act of 20 July 2017 – Water Law (Journal of Laws 2017, item 1566), as amended.

A noticeable result of surface water pollution due to anthropopression is eutrophication. As defined by the WFD, eutrophication refers to undesirable effects caused by increased loading of water bodies with nutrients, particularly nitrogen or phosphorus compounds ¹⁶, causing accelerated growth of algae and higher forms of plant life, and resulting in undesirable biological disturbance of the aquatic environment and deterioration of water quality ¹⁵. Agricultural production and municipal and industrial effluents are considered the main sources of nutrients ¹⁷. The problem of eutrophication affects all surface water types, from small watercourses to lakes and artificial reservoirs, rivers and marine waters ¹⁸. Some of the more vulnerable types exposed to eutrophication are shallow reservoirs fed by rivers whose catchments are located in agricultural areas ¹⁹ or exposed to other sources of nutrients, and it is in this context that the ecological and chemical status of the Turawa Reservoir retention waters was investigated. For more than three decades, an excessive bloom of cyanobacteria (Cyanophyta) has been observed in the reservoir, indicating its overfertilisation and progressive eutrophication process. The reason for this, as with many other dam reservoirs, is the pollution brought by the reservoir recharge waters ²⁰.

The aim of the research carried out was to: (1) assess, in terms of seasonal changes also, the quality of the waters of the Turawa Reservoir and the waters feeding the reservoir and (2) identify potential sources of pollution.

The research was conducted between 2019 and 2021 by four research teams using different methods and analytical techniques.

2.1. Characteristics of the study area

The Turawa Reservoir is located about 20 km north-east of the city of Opole, Poland (Fig. 1). It is a retention reservoir, built in 1933–38, with an area of 20.8 km² and a maximum capacity of 100 million m³. The average depth of the resrvoir is approximately 5 m, and the maximum, approximately 13 m ²¹. The reservoir is located on the Mała Panew River, which is its main source of water. Water from a small Libawa River also flows into the reservoir. Below the reservoir, from the Mała Panew River, water is collected for the needs of the Opole Power Plant. The reservoir additionally performs a recreational function.

The Mała Panew catchment area extends over 2,131.5 km² and is 56.8% covered with forests. Agriculture occupies 35.3% of the catchment area, urban areas – 7.3%, and land under standing waters – 0.6%. For many years, Mała Panew waters were classified as eutrophic waters ²². In addition, heavy metals are brought into the reservoir along with the waters of Mała Panew along with biogens, originating, among others from the catchment areas that stretch as far as the industrialised areas of Upper Silesia.

2.2. Assessment of biological elements

The assessment of biological elements was carried out in accordance with the applicable standards set out in Annex V WFD ¹⁵, which included a multimetric phytoplankton index for river and dam reservoir classification (IFPL), a diatom index for river and dam reservoir classification (IO) and a macroinvertebrate index for dam reservoirs (MZB).

Samples were taken at five test sites (Fig. 2b, points 32–36) in the summer months of 2019 and 2020 and at three test sites (Fig. 2b, points 33–35) in 2021.

2.3. Measurements of indicators characterising aerobic and biogenic condition

Determination of the biochemical oxygen demand index (BOD₅) was made in accordance with PN-EN 1899-1:2002. The chemical oxygen demand index (COD_Cr) was determined in accordance with PN-ISO 15705:2005. Chemical forms of nitrogen and phosphorus were determined using the spectrophotometric method (Thermo Scientific, Evolution 220 UV-Visible Spectrophotometer). A detailed description of the analysis is given in ²³. Samples for testing were taken in 2019–2021. The sampling sites are shown in Fig. 2.

Sampling sites in reservoir feed waters are designated on the rivers: Mała Panew (1–14 and 16–18), Libawa (19–23) and Myślina (15).

2.4. Examination of the ionic composition of reservoir waters

A high-resolution Metrohm Profession IC AnCat – MCS ion chromatograph was used to determine ions in the waters of the Turawa Reservoir.

Ions (anions F⁻, Cl⁻, Br⁻, NO²⁻, NO³⁻, PO₄³⁻ and SO₄²⁻, and cations Na⁺, NH₄⁺, K⁺, Mg²⁺ and Ca²⁺) were determined according to the standard for determinations performed by ion chromatography: PN-EN ISO 10304-1:2009/AC:2012 and PN-EN ISO 14911:2002.

Ion concentrations are presented as an average value from three repetitions. Water samples for the determination of ionic composition were taken using the following standard: PN-EN ISO 5667-4:2003. The data on the limits of detection and quantification of the apparatus and the uncertainty of the method measurement results, expressed by standard deviation sm, are summarised by ²⁴.

Samples for the study were taken in 2019–2021. Seasonal changes were evaluated in February–December 2020. Sampling locations are shown in Fig. 3.

2.5 Determination of conductivity, pH and heavy metals (Cr, Ni, Zn, Cu, Cd and Pb) in waters

A conductivity meter CC551, from Elmetron Sp.j., Zabrze (PL), was used to determine the conductivity of water (PN-EN 27888:1999), the absolute error of indication of which was ∆κ = 0.1 µS cm^− 1. A pH meter CP551, from the same company, was used to determine the pH index (PN-EN ISO 10523:2012), whose absolute error was ∆pH = 0.02. Samples were taken at the locations marked on the maps in Fig. 2a and 3. Heavy metals were determined in accordance with PN-EN ISO 1188T:2009 by the authorised laboratory of J.S. Hamilton Sp. z o.o., located in Gdynia.

2.6. Assessment of the sanitary condition of the reservoir waters

The following indicators were selected to assess the sanitary condition of the water: the number of heterotrophic microorganisms (TNB), determined by the serial dilution method (PN-ISO 6222:1999); the number of coliform bacteria (CBN) in 100 ml of water, determined by the membrane filtration method (PN-EN ISO 9308-1:2014); the number of faecal enterococci (FEN), according to PN-EN ISO 7899-1:2002; and Salmonella spp. bacteria (NSB), determined by the membrane filtration method (ISO 19250:2010). Detailed methodology is given by ²⁵. Samples for the study were collected in 2020, in the months of July, September and December, at site No. 5, marked on the map in Fig. 3.

2.7. Method of interpreting the results

The distribution of pollutants in the studied water bodies was interpreted by comparing the relative values of x_i concentrations, determined by the number of n sites/months in which samples were taken, according to the relationship:

$${x}_{i}={{c}_{i} / {\Sigma }}_{i=1}^{n}\bullet {c}_{i}$$

On the basis of the results interpreted in this way, it is possible to point out possible anomalies in the distribution of concentrations and to graphically compare data sets that differ significantly in numerical values, since the range of variation of concentrations for each set is within 〈0, 1〉, and not, as in the case of absolute concentrations, within 〈0, ∞). For concentrations less than the limit of quantification (IQL), the following was used to determine the standard deviation s (%): c = 0.5 - IQL.

The test results were grouped based on the parameters identified and the analytical methods and techniques used.

3.1. Biological elements in the waters of the Turawa Reservoir

The biological elements of the reservoir were determined on the basis of IFPL, IO and MZB indicators, adopting criteria for the type 0 surface water category, namely dam reservoirs ¹⁵. Table 1 collects limit values for individual water quality classes and distinguishes the range of values of marked indicators, with the indication of the extreme values from the entire series of tests in brackets. The table also shows the percentage share of measurement results in a given class.

Table 1

Limiting values for type 0 surface water categories: extreme values determined for Turawa Reservoir (in parentheses) and percentage (%) from n = 20 measurements indicating a given water class for the 2019–2021 study period (dark boxes); ecological status: HIGH – very good, GOOD – good, MOD – moderate, POOR – poor, BAD – bad
Parameter	Ecological status/potential
Parameter	HIGH	GOOD	MOD	(%)	POOR	(%)	BAD	(%)
IFPL	≥ 0.96	≥ 0.79	≥ 0.47	-	≥ 0.16 (0.16)	30	< 0.16 (0.10)	70
IO	> 0.75	≥ 0.65	≥ 0.45 (0.45)	15	≥ 0.20	35	< 0.20 (0.15)	50
MZB	> 0.60	≥ 0.50	≥ 0.40 (0.42)	35	≥ 0.20	40	< 0.20 (0.12)	25

The data contained in the table indicate that, in terms of IFPL, the ecological status of reservoir waters can be described as poor (70% of the 20 determinations made). Also, IO and MZB indicators in most of the analysed samples indicate bad or poor ecological status.

3.2. Aerobic and biogenic conditions of Turawa Reservoir water and waters feeding the reservoir

Samples for testing were taken in the waters of Mała Panew, Libawa and the Turawa Reservoir, in the places marked in Fig. 2. Table 2 summarises the mean, minimum and maximum values of oxygen and biogenic parameters determined in 2019–2021.

Table 2

Oxygen and biogenic indicators in river waters Mała Panew and Libawa, and in the Turawa Reservoir, in 2019–2021; mean (min.–max.)
Parameter	Unit	Mała Panew River	Libawa River	Turawa Reservoir
BOD₅	mgO₂ L^− 1	1.8 (0.1–6.6)	2.0 (0.4–5.8)	6.0 (1.1–10.5)
COD_Cr	mgO₂ L^− 1	71.9 (1.3–170)	59.7 (1.3–125)	10.9 (3.7–18.8)
TKN	mg L^− 1	2.9 (0.38–13.0)	2.7 (0.38–6.5)	2.1 (0.97–5.2)
NH₄-N	mg L^− 1	0.58 (0.04–5.9)	0.30 (0.10–1.6)	0.19 (0.01–1.0)
NO₃-N	mg L^− 1	2.2 (0.09–8.7)	1.7 (0.1–4.9)	6.1 (1.9–10.7)
TN	mg L^− 1	5.1 (0.53–19.0)	4.5 (1.7–9.7)	8.3 (4.5–13.8)
PO₄-P	mg L^− 1	0.05 (0.001–0.38)	0.04 (0.002–0.17)	-
TP	mg L^− 1	0.15 (0.008–0.65)	0.12 (0.015–0.50)	0.57 (0.05–2.54)*

*In 2020 only.

The data presented in Table 2 show that the maximum values of oxygen indicators

(BOD₅) and biogenic waters of the Turawa Reservoir, in relation to the permissible values of oxygen and biogenic indicators for surface water bodies of type 0, namely dam reservoirs

¹⁵ exceed class II. High values of oxygen and biogenic indicators have been demonstrated in the waters of Mała Panew and Libawa.

The graphs in Fig. 4 show the distribution of relative concentrations of the tested oxygen and biogenic indicators (Dependence 1, n – number of sampling sites) in water samples taken for testing from the places specified in Fig. 2. The dashed line indicates the mean values. Based on the results presented in Fig. 4, the distribution of pollutants in the tested waters can be requested.

The highest values of oxygen indicators in the waters of Mała Panew were recorded in places 8–11 (COD_Cr was also recorded at site 3). In Libawa water, oxygen indicators were evenly distributed, while the waters of the Turawa Reservoir indicate relatively high values of BOD₅ and COD_Cr at 32–38 (north-eastern part of the reservoir – Fig. 2b). In the case of biogenic indicators, a significant increase was recorded in the waters of Mała Panew in places 11–13. In Libawa water, as in the case of oxygen indicators, a similar distribution of biogenic indicators was observed, while in the waters of the Turawa Reservoir places 32 and 34, can be distinguished (northern edge of the reservoir). Figure 4e and 4f also indicate possible sources of pollution. A comparison of indicator values from places 24–31 indicates that the source of pollution is the waters of the Mała Panew River, while the reason for the relatively significant increase in the value of indicators in places 32–38 is biochemical processes.

The graphs in Fig. 5 show the relative values of oxygen indicators (BOD₅ and COD_Cr) and biogenic indicators (TN and TP) in the waters of Mała Panew and the Turawa Reservoir, in terms of seasonal changes (Dependence 1, n – number of months in which samples were taken). Samples were collected in 2020. The dashed line indicates the mean values.

The average concentrations of TN and TP on a quarterly basis are presented in ²⁶.

3.3. The Turawa Reservoir waters ionic composition

The results of the study of the ion composition of the Turawa Reservoir waters against the background of 24 water reservoirs in the Opolskie province were published in ²⁴. This article analyses the average surface distribution of ion concentrations in reservoir waters, and seasonal changes recorded in 2020. Sampling sites are marked in Fig. 3. Table 3 summarises the average, minimum and maximum values of the ionic composition of waters, determined in 2019–2021.

Table 3

Ion composition (mg L^− 1) waters of the Turawa Reservoir in samples taken for testing in 2019–2021
Parameter	Mean (min.–max.)	Parameter	Mean (min.–max.)
F⁻	0.247 (0.095–0.608)	Cl⁻	39.1 (17.4–84.5)
NO₂⁻	< 0.066 (< 0.03–4.75)	Br⁻	0.91 (0,16–3.44)
NO₃⁻	8.47 (0.20–49.0)	PO₄³⁻	< 0.05 (< 0.05–0.17)
SO²⁻	62.5 (44,2–171.8)	Na⁺	20.9 (8.1–47.6)
K⁺	8.47 (3.52–26.98)	NH₄⁺	< 0.07 (< 0.05–2.54)
Ca²⁺	44.2 (15.4–142)	Mg²⁺	7.69 (0.97–22.84)

Noteworthy are the differences in the distribution of biogenic ion concentrations: NO₂⁻_, NO₃⁻_, PO₄³⁻ and NH₄⁺. During the three-year test period, standard deviation of the results of the measurement of the concentrations of these ions significantly exceeded the determined mean value. The graphs in Fig. 6 show the distribution of relative concentrations of test ions (Dependence 1), in relation to places (Fig. 3), sampling (n – number of sampling sites), and in terms of seasonal changes – tested in 2020 (n – number of months in which samples were taken). The dashed line indicates the mean values. Ions were divided in terms of differences in concentration distribution expressed by standard deviation s from the mean value to s ≤ mean and s > mean. The results shown in Fig. 6 indicate comparable concentrations: F⁻, Cl⁻, Br⁻, SO₄²⁻, Na⁺, K⁺, Ca²⁺ and Mg²⁺ (s ≤ mean), both according to the site from which the samples were taken and the period during which the samples were taken. The concentrations of the ions in question are slightly higher than the mean value in samples taken at where the Mała Panew River joins the reservoir (sites L and I). Significant differences in the concentrations of NO₂⁻_, NO₃⁻_, PO₄³⁻ and NH₄⁺ (s > mean) were recorded. Relative concentrations of biogenic ions, higher than the mean value, were recorded mainly in the northern part of the reservoir and at the tributary of the Mała Panew River. The relative concentrations of NO₃⁻ and PO₄³⁻ in terms of seasonal changes are lowest in the spring-summer season, while the relatively high concentrations of NO₂⁻ (April) and NH₄⁺ (October) may be more incidental.

3.4. Measurements of conductivity, pH and heavy metal concentrations in the tested waters

Measurements of aerobic, biogenic and ionic water composition were supplemented in situ with conductivity and pH measurements. Table 4 summarises the average, minimum and maximum conductivity values κ and pH, determined in 2019–2021 in feed waters (Mała Panew, Libawa) and the Turawa Reservoir.

Table 4

The results of conductivity measurements and pH waters supplying the reservoir – Mała Panew and Libawa – and waters of the Turawa Reservoir, carried out in 2019–2021; mean (min. – max.)
Parameter	Unit	Mała Panew River	Libawa River	Turawa Reservoir
κ	µS cm^− 1	384 (106–833)	300 (171–512)	411 (360–447)
pH	-	7.36* (5.70–8.12)	7.40* (6.32–8.12)	8.79* (7.30–9.21)

* The average pH value was determined from the average hydrogen ion concentration.

From the data presented in Table 4, it can be seen that there were significant differences observed between the minimum and maximum values in the case of waters feeding the reservoir only. The probable reason for the recorded changes is the salinity level of the watercourses feeding the studied waters. Differences in pH values in the watercourses can be interpreted similarly.

However, pH measurements in the waters of the Turawa Reservoir, which, based on the data collected in Table 4, can be considered alkaline waters, require interpretation. For this purpose, pOH values (pOH = 14 - pH) were determined in terms of seasonal changes recorded in 2020, and OH⁻ concentrations were determined on this basis. The relative monthly distribution of pOH and c_OH- hydroxide anion concentrations is shown in Fig. 7. The dotted line indicates the average value.

Barely visible differences in the pOH index, due to the logarithmic scale, are better interpreted graphically using hydroxyl anion concentrations. A clear increase in the relative values of OH⁻ concentrations was recorded in July and August, during the period of intense cyanobacterial blooms.

Table 5 collects data on heavy metal contamination of reservoir and reservoir feed waters. The results in the table indicate relatively high concentrations of Zn in Turawa Reservoir waters and comparable concentrations of Cr and Cu in the reservoir and the waters feeding the reservoir. Significant amounts of Zn are introduced into the reservoir with the waters of the Mała Panew River. Cd concentrations in the feeder waters were below the limit of quantification. Concentrations of Ni, Cd, and Pb were not determined in the reservoir waters.

Table 5

Concentrations of heavy metals (µg L^− 1) in reservoir feed waters and in Turawa Reservoir waters
Metal	Mała Panew	Libawa	Turawa Reservoir *
Cr	7.2 (0.1–53)	7.6 (0.1–43)	4.0 (1.0–12)
Ni	5.0 (1.0–5.6)	6.0 (1.0–5.5)	-
Zn	56 (1.0–340)	15 (2.0–102)	176 (n.d.–422)
Cu	26 (2.0–92)	23 (7.0–68)	23 (10–56)
Cd	n.d.	n.d.	-
Pb	5.0 (1.0–59)	4.0 (1.0–10)	-
*The results were published in ²³

Figure 8 shows the relative average concentrations of heavy metals determined in the reservoir feed waters. The dashed lines indicate average values.

The results shown in the graph (Fig. 8a), indicate an increase in the concentrations of metals in the waters of the Mała Panew River between the sampling site numbered 8 on the map in Fig. 2a and the Turawa Reservoir. The waters of the Libawa River (Fig. 8b) show no significant differences in metal concentrations in samples taken at sites 19–23 (Fig. 2a).

3.5. Assessment of the sanitary condition of the waters of the Turawa Reservoir

In 2019–2021, to assess the sanitary condition of the reservoir waters, microbiological tests were conducted on the probable number of indicators (MPN/100 mL^− 1): heterotrophic microorganisms (TNB), coliform bacteria (CBN), faecal enterococci (FEN) and Salmonella spp. bacteria (NSB). Table 6 shows the minimum and maximum values, detailing the percentage of measurement results (%) indicating that the limit values were exceeded: CBN and FEN, as defined in ²⁷. As a reminder, the Turawa Reservoir also serves a recreational function.

Table 6

Indicators of the sanitary status of the reservoir waters examined in 2019–2021 (MPN/100 mL^− 1), with the percentage (%) of exceedances of the limit values
Year	TNB (22 °C)*	TNB (36 °C)*	CBN	(%)	FEN	(%)
2019	3495–6500	2250–19856	93–240	0	14–23	0
2020	1073–417500	323–522500	240–2400	50	16–290	17
2021**	18358	33250	1745	67	145	0

*No limit value specified.

** In 2021, measurements were made in July only.

No Salmonella spp. were recorded in 2019–2021. The highest number of exceedances of the limit values was recorded against the CBN indicator in 2020 and 2021.

The water quality status of the Turawa Reservoir was assessed, taking previous studies and the current status into account. Biological elements and the chemical and sanitary status of the reservoir waters and feeder waters were considered. Pollution sources were also identified.

4.1. Water quality in the Turawa Reservoir and feed waters

Studies of the reservoir water quality by Polish scientific research centres date back several decades. The reservoir has also aroused the interest of foreign scientists ^28,29. Their main goal was to assess the biological and chemical threat to the ecosystem and search for methods to revitalise the reservoir ^30,31. Water quality studies of the Mała Panew River and the waters of the Turawa Reservoir are also carried out regularly by the Regional Department of Environmental Monitoring in Opole. For the waters of the Mała Panew River, control points have been set at locations described by geographic coordinates: 50.621246 N, 18.463813 E (Zawadzkie, site no. 12, Fig. 2a) and 50.70047 N, 18.18794 E (Jedlice, site no. 18, Fig. 2a). For the waters of the Turawa Reservoir, the measuring point was determined at the place described by the coordinates: 50.73584 N, 18.09328 E (site no. 37, Fig. 2b) ²².

In terms of biological elements, the condition of Turawa Reservoir waters was classified as poor (2010–2012 and 2018–2019) and moderate (2013–2016) ²². The results of our research, conducted in 2019–2021 (Table 1), indicate that, in terms of biological elements, the condition of the reservoir waters varied from moderate to poor, depending on the indicator, with the majority of measurements indicating poor condition in terms of IFPL and IO indicators. Our research covered the waters of the littoral zone of the northern shore of the reservoir (Fig. 2b, sites 32–36). It can be concluded that the condition of the waters, in terms of biological elements, has deteriorated over two decades, with our research covering the littoral zone, while the research conducted by the Regional Department of Environmental Monitoring in Opole covered the bottom, at the outflow of the Mała Panew River from the reservoir (site 37, Fig. 2b).

Tests conducted in 2019–2021 on oxygen indicators BOD₅ and COD_Cr in Turawa Reservoir waters (Table 2) indicate that the average values remain in Class I (COD_Cr ≤ 25 mgO₂ L^− 1) and Class II (BOD₅ > 3.0–6.0 mgO₂ L^− 1) relative to the limit values set for river water bodies of surface water type 0, namely dam reservoirs ¹⁵, with exceedances of the limit value registered for BOD₅. Much higher COD_Cr values were registered in waters of the Mała Panew River (average 71.9 mgO₂ L^− 1) and the Libawa River (average 59.7 mgO₂ L^− 1). In comparison, in the waters of the Mała Panew River (Jedlice, site No. 18, Fig. 2a) in 2005–2018, average values of BOD₅ in the range of 1.5–3.3 mgO₂ L^− 1and COD_Mn = 6.5–24 mgO₂ L^− 1were recorded ²². Other studies conducted in 2002–2005 indicated average values of BOD₅ = 2.16 mgO₂ L^− 1and COD_Cr = 21.4 mgO₂ L^− 1at the inflow to the reservoir and BOD₅ = 3.58 mgO₂ L^− 1and COD_Cr = 34.1 mgO₂ L^− 1 in the waters of Turawa Reservoir ³². In studies conducted in 2010–2015, the average value of BOD₅ was set at 2.2 mgO₂ L^{− 1 33}. Analysing data from the last two decades, it can be concluded that BOD₅ rates are comparable.

Disturbingly high COD_Cr values were recorded in the waters of the Mała Panew River during the implementation of our research, influenced by measurements made at sites 2–4 and 8–11 (Fig. 4a), and relatively high values of both indicators were recorded in June 2020 (Fig. 5a). In the waters of the reservoir, relatively high values of BOD₅ and COD_Cr were recorded in September 2020, which could be caused by increased oxygen demand due to the decomposition of organic matter (algae and cyanobacteria). Positive correlations of the increase in BOD₅ and COD_Cr values with increasing organic matter are indicated by other authors ³⁴. In terms of total organic carbon (TOC), the waters of the Turawa Reservoir were classified in Class II: >10–15 mgC L^{− 1 15}: the average in 2009, 2011 and 2014 was set at 11.4 mgC L^{− 1 35}. In 2016 the yearly mean was 11.2 mgC L^{− 1 22}.

Studies of nutrient parameters carried out in 2019–2021 (Table 2) indicate that Class II was exceeded concerning the limits of NO₃-N and TP set for river water bodies of surface water type 0, namely dam reservoirs (limits for Class II: NO₃-N ≤ 5.0 mg L^− 1, TP ≤ 0.4 mg L^− 1) and that Class II was maintained concerning TN: 5 < TN ≤ 10 mg L^{− 1 15}. Higher average NH4-N concentrations were recorded in the reservoir recharge waters compared with the reservoir waters, with the results of measurements at sites 11–13 having a decisive influence on the average value (Fig. 4b). There was also an increase in other nutrient indicators at sites 11–13 (Fig. 4b). Differences in the mean values of NO₃-N from Tables 2 and 3) reach 30% and are due to different sampling locations and periods. In the reservoir waters, the highest values of biogenic indicators were recorded in the southern part of the reservoir (Fig. 4f) at sites 32–36 (Fig. 2b).

Seasonal changes in TN and TP in the waters of the Mała Panew River (Fig. 5b) and reservoir waters (Fig. 5d) are difficult to interpret. A detailed analysis of the data for 2019–2021 indicates that the high variability of TN and NO₃-N concentrations in the waters of the Mała Panew River is influenced by surface runoff from agricultural land ²⁶. Agricultural land, as a source of nitrogen compounds in surface waters, is also indicated by other authors ^36,37. In the reservoir waters the concentrations of biogenic ions show a decreasing trend during the growing season (Fig. 6d), contrary to the values of TN, for which the highest values of concentrations were registered in June and August (Fig. 5d). This may be influenced by the increase in the concentration of ammonium and organic nitrogen (Kjeldahl nitrogen – TKN) and the intensity of the nitrification and denitrification processes ³⁸. The average values of nutrient indicator concentrations in the reservoir waters, determined in 2002–2005, were: NO₃-N 2.99 mg L^− 1, NO₂-N 0.13 mg L^− 1, NH4-N 0.33 mg L^− 1 and PO₄-P 0.20 mg L^− 1. At the Mała Panew River tributary to the reservoir, they were: NO₃-N 10.11 mg L^− 1, NO₂-N 0.14 mg L^− 1, NH₄-N 0.45 mg L^− 1 and PO₄-P 0.30 mg L^{− 1 32}. The average values of TN and TP indicators from 2010–2015 in the waters of the Mała Panew River were, respectively: 3.9 mg L^− 1 and 0.135 mg L^{− 1 33} and were comparable to the average values determined in 2019–2021 (Table 2).

According to ²² the water chemical condition of the Turawa Reservoir and the Mała Panew River in 2013–2015 was assessed as being below the “good” status, unlike the waters of the Libawa River, whose condition, with the exception of NH₄-N content, was determined as Class I. A significant improvement in the quality of the waters of the Mała Panew River in terms of NO₃-N and PO₄-P contents can be found compared with the results of the 2002–2005 study. The ionic composition of Turawa Reservoir waters shows significant differences in the distribution of concentrations of biogenic ions: NO²⁻, NO³⁻, PO₄³⁻ and NH₄⁺ (Fig. 6c), depending on the sampling location. The relative concentrations of biogenic ions, which were higher than the average value, were recorded mainly in the north-eastern part of the reservoir and at the tributary of the Mała Panew River. The distribution of concentrations of the other analysed ions: F⁻, Cl⁻, Br⁻, SO₄²⁻, Na⁺, K⁺, Ca²⁺ and Mg²⁺ are within the range: s ≤ mean, indicating a relatively uniform distribution of these ions in the reservoir waters. In 2013, a study of the ionic composition of 24 water reservoirs in the Opole Province (PL) was conducted, including the Turawa Reservoir ²⁴. In comparison with other dam reservoirs located in Opole Province (Otmuchów Reservoir and Nysa Reservoir), Turawa Reservoir had higher concentrations of most of the ions studied: F⁻, Cl⁻, Br⁻, NO₃⁻, SO₄²⁻, Na⁺, K⁺ and Ca²⁺, as well as higher values of water conductivity and pH.

The conductivity of Turawa Reservoir water (Table 4) was within Class I of the water quality of surface water type 0, namely dammed reservoirs (κ ≤ 1000 µS cm^{− 1 15}). In the feed waters, the average conductivity values were comparable, and the maximum values did not exceed 1000 µS cm^− 1.

The issue of concern has become the alkaline reaction of the reservoir waters (Table 4). The average pH value exceeds the value designated for water quality class I of surface water type 0, namely dammed reservoirs ¹⁵. The alkalinity of the reservoir waters is also indicated by previous studies. In the years 2002-05 the pH in the range 7.60–9.95 was determined ³². In 2013 pH higher than 10 was observed ²⁴. The studies carried out in 2011-14 indicate the pH values in the range 7.70–9.90 ²⁰.

Alkaline water pH values, as a factor promoting cyanobacterial blooms, have been pointed out by other authors ^39–41. However, it should be noted that a large increase in pH values in the reservoir waters was observed during the summer, the season of intense algal blooms (Fig. 7) and, in the reservoir feed waters, determined pH values were contained within the limits 5.70–8.12 (Table 4). Therefore, it can be concluded that the increase in pH during summer is not influenced by the quality of the feed water, but is caused by biochemical processes occurring in the reservoir waters.

Concentrations of heavy metals Cr, Zn, Cu, determined in the waters of the Turawa Reservoir and in the feed waters (Table 5), do not exceed the limits set for surface waters: Cr ≤ 50 µg L^− 1, Zn ≤ 1000 µg L^− 1, Cu ≤ 50 µg L^{− 1 15}. The average values of Ni and Pb in the Mała Panew and the Libawa River waters exceed the values of average annual concentrations of chemical status indicators for surface water bodies (Ni ≤ 4 µg L^− 1, Pb ≤ 1.2 µg L^− 1) while maintaining the quality standards for maximum allowable concentrations (Ni ≤ 34 µg L^− 1, Pb ≤ 14 µg L^− 1). Cd concentrations were below the analytical method’s limit of quantification. However, analysis of ²² data from the last two decades indicates significant Cd concentrations in 2011 and 2014 in the waters of the Mała Panew River, above and below the Turawa Reservoir, based on which the chemical status of the waters was classified as being below the “good” status. In 2015, in the waters of the Mała Panew River above the reservoir, Cd concentrations also contributed to poor water assessment (below the “good” status). In 2018, the waters of the Mała Panew River were classified as Class I for concentrations of Cr, Ni, Zn, Cu, Cd and Pb. Cadmium concentrations in the waters of the Turawa Reservoir, measured in 2018, were (µg L^− 1): mean < 0.092, min. < 0.04, max. 0.25.

Investigations into the sanitary condition of Turawa Reservoir water, conducted in 2019–2021, indicate a significant probable number of exceedances of coliform bacteria (CBN) in 2020–2021 (Table 6). Exceedances of faecal enterococci (FEN) and large numbers of heterotrophic microorganisms (TNB) were also recorded in 2020. In 2019, there were no exceedances of CBN and FEN. The problem occurring in the last two decades is evidenced by local announcements and press reports based on the results of tests of Turawa Reservoir waters conducted by the District Sanitary and Epidemiological Station in Opole, with examples: July 2005 – CBN exceedance; July 2010 – CBN and FEN exceedances; July 2015 – CBN exceedance.

The presented results indicate that, over the past two decades, the quality of Turawa Reservoir waters has improved in terms of biogenic indicators and cadmium concentration, which for many years was one of the chemical indicators that classified the chemical status of the reservoir waters as being below the “good” status. Still of concern are the biological indicators: IFPL and IO, as well as pH, indicating frequent exceedances of the limit values for water quality class II of surface water type 0, namely dam reservoirs. This problem is reported by ²². Frequent exceedances of indicators of sanitary status are also of concern. The ionic composition of F⁻, Cl⁻, Br⁻, SO₄²⁻, Na⁺, K⁺, Ca²⁺ and Mg²⁺ and the salinity level of the reservoir are not a problem.

Extensive data on, among other things, the natural characteristics of the catchment area, the reservoir and its surroundings, GPR studies of the reservoir basin, studies of the distribution and composition of bottom sediments, groundwater investigation, and studies of the ecological status of the reservoir carried out in 2003 and 2004 were collected in a monograph, published in Polish, titled “The ecological status of Turawskie Reservoir: Assessment of the ecological condition of Turawskie Reservoir in order to develop measures for its improvement” ⁴².

4.2. Identification of water pollution sources in the Turawa Reservoir

Analysis of literature data indicates two main sources of pollution of the reservoir waters: polluted waters of the Mała Panew River ⁴³ and bottom sediments accumulated since the 1940s in the basin of the Turawa Reservoir ^44,45. For many years the Mała Panew River basin water has had exceedances of limit values for many indicators, including nutrients and heavy metals. The catchment area includes forested, urbanised, agricultural and industrial areas ²⁶, and extends, as mentioned, to the Upper Silesia area. The maps in Fig. 9 show examples of the results of measurements of eutrophic indicators carried out in 2004–2007, as well as exceedances of limit values in the assessment of the ecological status in 2013–2015. It is important to note the pollution of the Graniczna Woda watercourse, whose waters through the Stoła watercourse enter the Mała Panew River. Numerous exceedances of oxygen indicators, biogenic indicators and heavy metal concentrations were recorded in the Border Water in 2013–2015 ⁴⁶.

Our research carried out in 2019–2021 indicates sites where there was an increase in the relative concentrations of oxygen and nutrient indicators in the waters of the Mała Panew River (Figs. 4a and 4b): an above average increase in the COD_Cr indicator near Kalety (Fig. 2a, site 3), an above average increase in the BOD₅, COD_Cr, PO₄-P and TP indicators at the inflow of the Table into the Mała Panew River (Fig. 2a, site 8), and an increase, well above average, in the concentrations of various forms of nitrogen near Zawadzkie (Fig. 2a, sites 11–13). Higher concentrations of heavy metals, especially Zn and Pb, were also recorded in the waters of the Mała Panew River, following the inflow of the waters of the Stoła River towards the reservoir (Fig. 8a). The Stoła River, or rather its tributary Graniczna Woda, was identified as a major source of pollution in the waters of the Mała Panew River in 2013–2015 (Fig. 9). A study was conducted in 2000–2002, which showed that the bottom sediments (surface fraction < 1 mm) of the Stoła River and its tributary Graniczna Woda contained up to 3 mg g^− 1 Cd, 0.9 mg g^− 1 Cu, 20 mg^− 1 Pb and 30 mg^− 1 Zn ⁴⁷. Based on an analysis of the results of studies of heavy metal concentrations in sediments carried by the Mała Panew River into the floodplain, supported by dendrochronological studies, a conjecture was made that, in 1960–1985, the waters of the Mała Panew River may have been among the heaviest metal-polluted waters in Europe ⁴⁸.

The presented data indicate that, for many decades, pollution carried with the waters of the Mała Panew River has been a serious burden on the Turawa Reservoir ecosystem. However, own studies conducted in 2019–2021 indicate that the quality of the waters of the Mała Panew River has significantly improved. Based on the results of the studies, it can be concluded that, in the waters of the Mała Panew River, the average values of the studied indicators (Tables 2, 4 and 5), with the exception of COD_Cr and NH₄-N, are comparable or lower than those of the waters of the Turawa Reservoir. The reservoir is not burdened by the waters of the Libawa River, which reached a good ecological status in 2013–2015 ²². Our studies (Fig. 4e, 4f and 6) indicated the relatively good quality of Libawa waters.

The second mentioned source of pollution of the reservoir waters is the accumulation of sapropel bottom sediments, and the pollutants and toxic substances accumulated in them. In a study conducted in 2004 ⁴⁴, pesticides, polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs) and a number of toxic heavy metals, such as Cd, Hg and Pb, were identified in the bottom sediments of Turawa Reservoir. The presence of heavy metals in the bottom sediments of the reservoir is also confirmed by other authors. In 2003–2004, the range of concentrations (min.–max.) of heavy metals in sapropel sediments that had accumulated in the eastern and north-eastern parts of the reservoir were: Cd 0.35–0.40 mg g^− 1, Pb 0.60–0.70 mg g^− 1, Zn 7–8 mg g^{− 1 42}, average concentrations of heavy metals were determined in the bottom sediments in the sapropel zone as: Cd 0.280 mg g^− 1, Cr 0.038 mg g^− 1, Cu 0.105 mg g^− 1, Mn 0.940 mg^− 1, Ni 0.037 mg g^− 1, Zn 4.77 mg g^{− 1 45}. Large amounts of TP were also identified in the sapropel zone in 2008 in sapropel sediments: 1.8 ± 0.6 mg g^{− 1 49}. Studies of the radioactive distribution of 137Cs in the bottom sediments of the reservoir have also been conducted ⁵⁰. The Turawa Reservoir is located in the area of the so-called Opole Anomaly, where, after the Chernobyl nuclear power plant accident, a relatively large deposition of this radionuclide was recorded for the area of Poland ⁵¹.

Figure 10a shows the results of our studies of the horizontal distribution of concentrations of oxygen indicators (Fig. 4e), biogenic indicators (Fig. 4f) and biogenic ions determined by ion chromatography (Fig. 6c). Based on the results ⁵², the distribution of increments of heavy metal concentrations in samples of biosorbents exposed in the waters of Turawa Reservoir in 2019–2021 is shown in Fig. 10b. The images do not reflect the scale of absolute concentration. They indicate the approximate location of sites where the measured concentrations were higher than the average values determined for the measurement points indicated in Fig. 2b (oxygen indicators BOD₅ and COD_Cr and biogenic indicators TN and NH4-N) and Fig. 3 (biogenic ions NO₂⁻, NO₃⁻, PO₄³⁻ and NH₄⁺ and heavy metals Ni, Zn, Cd and Pb). Isolines with the percentage of sapropel sediments in the total volume of accumulated bottom sediment are also marked on the reservoir plan (the study was conducted in 2003–2004 ⁴².

The results presented here confirm the impact of two sources of pollution of the reservoir waters: the primary source, through the waters feeding the reservoir, and the secondary source, which is the sapropel bottom sediments accumulated over many decades. However, it is important to note the unfavourable oxygen indicators, which are mainly found in the north-eastern part of the reservoir. It can be assumed that they are the result of biochemical processes involving living and dead organic matter. This is observed by the large increase in the north-eastern part of the reservoir in the indicators BOD₅ and COD_Cr, that took place in September, during the final period of the cyanobacteria bloom (Fig. 5c). Such a phenomenon was not observed in the feed waters (Fig. 5a). Moreover, in the part of the reservoir in question (Fig. 6c), there was an increase in the concentration of nutrient ions during late summer and early autumn (Fig. 6d). Also, such changes did not occur in other parts of the Turawa Reservoir or in the waters feeding the reservoir. In the second half of summer, a significant increase in the concentration of hydroxyl anions was observed in the reservoir waters (Fig. 7).

The outlined problem, observed in many dam reservoirs, especially shallow reservoirs with low levels of thermal stratification, has been addressed by many authors ^53–55. These authors agree that the main sources of pollution in this type of reservoir are recharge water and leachate from adjacent areas. The deposition of atmospheric pollutants is also mentioned. The consequence is the accumulation of sapropel deposits in the reservoir basin, accumulating nutrients and other contaminants, and, as a result, progressive eutrophication. Conducted studies of the Turawa Reservoir waters and the waters feeding the reservoir confirm this pattern. The state of the reservoir waters has been classified as poor for many years, influenced by indicators of biological elements, negative biogenic parameters and exceeded permissible values of heavy metal concentrations.

For decades, the main source of water pollution in the Turawa Reservoir was the waters of the Mała Panew River, which feeds the reservoir. The consequence was the accumulation of large amounts of bottom sediments in the basin in the reservoir, constituting a secondary source of pollution. Another secondary source affecting changes in the chemical composition of the waters was also indicated, related to the large accumulation of nutrients in the reservoir waters and excessive biomass development.

The effect of biochemical processes is evidenced by significant changes in oxygen and nutrient indicators during the growing season. Not insignificant is the increase in the pH index, which is not observed in the waters feeding the reservoir. Analysis of data from the last two decades indicates an improvement in the quality of the waters of the Mała Panew River in terms of most biogenic indicators and heavy metal concentrations, which may result in an improvement in the quality of the reservoir waters over several years.

Funding

Financial support for this work was provided by the National Centre for Research and Development, programme BIOSTRATEG (ZBIORTUR, project 3/343733/15/ NCBR/2018). The APC is co-financed by Wrocław University of Environmental and Life Sciences

Author Contribution

A.K.: Conceptualization, Methodology, Formal analysis Writing - Original Draft, Visualization; M.W.: Conceptualization, Methodology, Formal analysis, Project administration, Funding acquisition; W.S.: Investigation, Formal analysis; M.S.: Investigation, Formal analysis; R.P.: Investigation, Formal analysis; B.W.: Investigation, Formal analysis; J.D.: Investigation, Formal analysis; C.R-D.: Writing - Review & Editing article; A.S-W.: Investigation, Formal analysis, Writing - Review & Editing; A.D-Ś.: Formal analysis; Z.Z.: Formal analysis; Ł.G.: Investigation, Formal analysis data; T.K.: Investigation Formal analysis; S.W.: Formal analysis; M.P-W.: Formal analysis, Resources; A.C.: Investigation, Formal analysis; M.H.: Investigation, Formal analysis; P.W.: Writing - Review & Editing.

Acknowledgements

For their assistance in the project, the authors would like to thank the Regional Water Management Board in Gliwice and the Water Catchment Board in Opole, which are part of the State Water Management Company Wody Polskie, as well as the Voluntary Water Rescue Service of the Opole Voivodeship. They also express their gratitude for their commitment to the project, to Ms: Krystyna Woźniakowska, Joanna Szczypta, Kamila Hamal and Elżbieta Trośniak from the Environmental Research Laboratory of the Wrocław University of Life Sciences.

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No competing interests reported.

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Ecological risks of dammed reservoir waters using the example of the Turawa Reservoir in southern Poland

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Abstract

Figures

1. Introduction

2. Materials and methods

2.1. Characteristics of the study area

2.2. Assessment of biological elements

2.3. Measurements of indicators characterising aerobic and biogenic condition

2.4. Examination of the ionic composition of reservoir waters

2.5 Determination of conductivity, pH and heavy metals (Cr, Ni, Zn, Cu, Cd and Pb) in waters

2.6. Assessment of the sanitary condition of the reservoir waters

2.7. Method of interpreting the results

3. Results

3.1. Biological elements in the waters of the Turawa Reservoir

3.2. Aerobic and biogenic conditions of Turawa Reservoir water and waters feeding the reservoir

3.3. The Turawa Reservoir waters ionic composition

3.4. Measurements of conductivity, pH and heavy metal concentrations in the tested waters

3.5. Assessment of the sanitary condition of the waters of the Turawa Reservoir

4. Discussion

4.1. Water quality in the Turawa Reservoir and feed waters

4.2. Identification of water pollution sources in the Turawa Reservoir

5. Conclusions

Declarations

Funding

Author Contribution

Acknowledgements

References

Additional Declarations

Status:

Version 1