Analysis of water quality parameters in foothill rivers in the context of the construction of dam reservoirs

Backwater in retention reservoirs is potentially exposed to various contaminants, such as nutrients or heavy metals. Therefore, actions performed at the construction planning stage for such a facility are crucial; they are aimed at precisely evaluating the river’s physical and chemical potential, forecasting the process of eutrophication in the reservoir, and adopting appropriate engineering solutions to reduce the undesired deterioration in the quality of backwater. The aim of this study was to assess the physical and chemical conditions of selected rivers in the context of planned retention reservoirs and to evaluate the risk of water quality deterioration after impoundment. The study was conducted in the Ścinawka and Włodzica rivers in southwestern Poland, where the construction of storage reservoirs was considered. Water samples were taken at monthly intervals from March 2023 to March 2025 along the entire length of the running waters: from their springs to the location of the planned barrier, where bottom sediments were also examined. The applied methods included the analysis of physical and chemical parameters of water and bottom sediments. All physical and chemical parameters were summarised, and their concentrations were evaluated on the basis of national regulations; correlations of the parameters were analysed; and their variability in space and time was presented. Moreover, the size distribution of the components of bottom sediments and their richness in carbon, nitrogen and heavy metals were identified. The results showed that the contamination of water in the selected rivers results mainly from high concentrations of nitrogen and phosphorus compounds, which causes a high risk of eutrophication in hypothetical reservoirs. According to the Vollenweider model and the Benndorf modification, both reservoirs were classified as eutrophic. The analysis indicates that the hazard related to other physical and chemical parameters (oxygen, organic matter, salinity, mineralisation, suspended matter, metals) is moderate; however, the reservoirs should be designed with regard to the risk of episodic oxygen deficiency and a potential for silting up and load accumulation.

1 Introduction

Retention reservoirs play an important role in integrated management of water resources by allowing the accumulation of water for flood control, drought prevention, and provision of water for ecosystems and people (Połomski and Wiatkowski, 2023). Despite their apparent similarity to lakes, artificial storage reservoirs differ significantly from natural structures; they combine the features of river (in the tributary area) and lake (at the barrier) ecosystems (Hayes et al., 2017). Stagnant water has different physical and chemical properties than water flowing in a river. Therefore, when planning retention, it is necessary to manage both the amount and quality of water in a proper manner. Performing the right actions as early as during the design stage makes it possible to prevent negative effects of further use (Keyvanfar et al., 2021). Thus, surface water quality testing is an indispensable element of planning of retention infrastructure–it provides data to evaluate the condition of the environment and to make decisions mitigating the risk of degradation of water ecosystems (Deng et al., 2023).

The quality of surface water in the context of the planned retention reservoirs depends on numerous environmental and anthropogenic factors. Excessive loads of contaminants (including nutrients) flowing to rivers constitute a direct cause of progressive degradation of water in retention reservoirs. Eutrophication, which is ‘over-fertilisation’ of water caused by, but not limited to, the excess of nitrogen and phosphorus compounds, leads to massive algae growth, a decrease in transparency, and oxygen deficiency, which, in turn, reduce the quality and functionality of the reservoir (Yang et al., 2008; Połomski and Wiatkowski, 2024). This process may happen on a natural basis but is frequently accelerated by human activity, such as due to the outflow of nutrients from farmland or the discharge of wastewater (Akinnawo, 2023). Under the conditions in a retention reservoir, eutrophication is sometimes amplified by lower water flow velocity, longer retention time, and enhanced sun exposure, which favour algal blooms. This issue occurs in both smaller units, such as the Zemborzycki reservoir on the Bystrzyca river in Poland (with a capacity of 6.3 mln m³), and the world’s largest reservoirs, including Lake Nasser on the Nile in Egypt and Sudan, with a capacity exceeding 132 km³ (Goher et al., 2021). It should be emphasised that the changes in the condition and quality of water depend not only on the inflow of external contaminants, but also on the processes within the water ecosystem, e.g., circulation of matter, biological activity (Harvey et al., 2024). As a result, the proper functioning of a water body largely depends on maintaining the good quality of the water, and the susceptibility of the body to degradation is determined by the conditions in the watershed (e.g., method of land use) and the ecosystem’s resilience.

The dynamic nature of the reservoirs causes the water’s physical and chemical parameters to fluctuate significantly in time and space. Seasonal changes in water quality are identified on an annual basis–in summer, higher temperature and enhanced sun exposure favour the growth of algae biomass and the decrease in oxygen content, whereas in periods of increased precipitation, there are inflows of fresh external contaminants and suspended matter (Xu et al., 2022). Zones with different conditions may also develop within a single reservoir, which translates into differences in transparency, concentration of nutrients, or water oxygen supply at individual sections of the reservoir bowl (Woelmer et al., 2023). It is therefore necessary to carry out ongoing monitoring at numerous points and periods to capture this variation. An example of such studies is the analysis performed in relation to the Turawa reservoir (Poland), where measurement series and statistical methods from more than 20 years were used to evaluate the spatial and time variation of the water quality. It was demonstrated that this reservoir is highly susceptible to eutrophication processes, and the water quality indicators used enabled the quality classification of the reservoir water in the period of 1998–2020 as good to moderate, whereas in recent years, a clear trend of deterioration of ecological conditions was noticed (Buta et al., 2023). Such results confirm that the planning and use of the reservoirs have to include local and time-based fluctuations of water quality to properly evaluate the effectiveness of the protective or reclamation actions taken.

In terms of retention water quality studies, particular attention should be paid to the role of bottom sediments. Mineral and organic suspended matter accumulates on the bottom of the reservoir together with numerous contaminants, from nutrients to heavy metals. Reservoirs on rivers may act as nutrient ‘traps’ (using sedimentation and denitrification), but bottom sediments may also become a secondary source of contaminants, e.g., as a result of decomposition of organic matter (Ismukhanova et al., 2022). Under oxygen deficiency, phosphorus absorbed in sediments may be released back into the water to sustain eutrophication. It has been identified that frequent agitation of water down to the bottom (e.g., in shallow reservoirs exposed to wind) causes bottom sediments to drift and increases the phosphorus concentration in the water depths (Siemieniuk et al., 2016). As a result, the composition and properties of sediments (concentration of nutrients, organic matter, and chemical contaminants) form integral elements of water quality evaluation. Bottom sediment studies provide information on the potential accumulation of contaminants in the reservoir and make it possible to forecast potential hazards (e.g., secondary water contamination on changes in the environmental conditions or during hydraulic work).

The legal and institutional context also highlights the importance of water quality during the planning and construction of retention reservoirs. In EU member states, the Water Framework Directive (2000/60/EC) establishes the basis for integrated water policy to protect water and improve its quality (European Parliament and Council of the European Union, 2000). This directive obliges the member states to achieve at least good ecological and chemical conditions of surface waters and to prevent their further degradation. For example, these principles have been implemented in the Polish legislation in the Water Act, which explicitly states that the use of water (including the operation of water-based equipment) must not deteriorate the condition of this water or associated ecosystems. It means that when planning a new retention reservoir, comprehensive environmental impact analyses should be performed, including both the quality of inflowing water and the expected quality of water in the future reservoir. A similar approach is also seen in other parts of the world. In the United States, the Clean Water Act remains the key legal act, which established the basis for federal policy for water quality protection and tools to control contaminant load, and are also used for the purpose of retention reservoir impact assessment. In Asia, important regulations have been implemented in China, where the ‘Three Red Lines’ policy and the ‘10-Point Water Plan’ focus on improving water quality in rivers and lakes and on reducing eutrophication of reservoirs. Compliance with legal requirements (e.g., maintaining or improving the condition of water according to drainage reservoir water management plans) requires zoning decisions to be based on reliable scientific research. Therefore, monitoring and evaluating the quality of surface water are fundamental elements of responsible planning of retention infrastructure to ensure that newly constructed reservoirs meet their functionality without deteriorating the condition of the water environment but instead contribute to its sustainable protection.

The objective of this manuscript is to: 1. identify two rivers where it is reasonably possible to construct retention reservoirs; 2. carry out comprehensive water and chemical studies (complemented with bottom sediment studies) from river springs to the cross-section of the hypothetical barrier at monthly intervals for 2 years; 3. analyse the collected results based on statistical calculations and reference data; 4. draw conclusions related to hazards and risks concerning the quality of water in potential reservoirs; 5. start a discussion on the impact of the interpretation of the results of these water and chemical studies on the need to design appropriate engineering solutions at the construction planning stage for storage reservoirs.

2 Materials and methods

As part of these studies, physical and chemical parameters of water from the Ścinawka and Włodzica rivers in southwestern Poland were analysed, where the construction of storage reservoirs is considered. Water samples were taken on a monthly basis for 25 months from March 2023 to March 2025 at four locations along the Włodzica river (one of which is on the river’s main tributary) and at six locations along the Ścinawka river, which are indicated on the map presented in Figure 1.

Figure 1

Figure 1. Map presenting the Włodzica and Ścinawka rivers and water sampling locations.

Locations no. 6 on the Ścinawka river and no. 3 on the Włodzica river are close to the springs of the rivers. Numbers 4 and 2 on the Ścinawka river mark the start and end of the course of this river in another country (the Czech Republic). Numbers 5 and 3 on the Ścinawka river are located in rural and urban development areas, respectively. The same applies to numbers 4 and 2 on the Włodzica river: no. 4 is located on the largest tributary in a rural area, and no. 2 is in the centre of the biggest city in the river basin (Nowa Ruda). Both numbers 1 are assigned to the locations where the bodies of the dams for the Sarny reservoir (on the Włodzica river) and the Ścinawka Górna reservoir (on the Ścinawka river) are to be theoretically erected. Basic parameters of these reservoirs are summarised in Table 1.

Table 1

Table 1. Basic parameters of the Sarny and Ścinawka Górna reservoirs.

For this study, the concentrations and variation of the following physical and chemical parameters were analysed: electrolytic conductance (EC), total nitrogen (TN), Kjeldahl nitrogen (KN), organic nitrogen (ON), ammonia nitrogen (NH₄), nitrates (NO₃), nitrites (NO₂), phosphates (PO₄), total phosphorus, sulphates (SO₄), chlorides (Cl), dissolved oxygen (DO), 5-day biochemical oxygen demand (BOD₅), chemical oxygen demand (COD), total suspended solids (TSS), total dissolved substances (TDS), sodium (Na), potassium (K), calcium (Ca), magnesium (Mg), and heavy metals: zinc (Zn), copper (Cu), cadmium (Cd), lead (Pb), chromium (Cr), nickel (Ni), manganese (Mn), and iron (Fe). The analyses were complemented with bottom sediment studies performed in location no. 1 on both rivers (in terms of size distribution, carbon-to-nitrogen (C/N) ratio, and heavy metals: Cu, Zn, Pb, Cr, Ni, and Cd). Water samples were collected from the surface layer using a water sampler in accordance with the PN-EN ISO 5667-6:2016-12 standard and transported in 1.5-L plastic bottles. Laboratory analyses were conducted at the Environmental Research Laboratory of the Institute of Environmental Engineering, Faculty of Environmental Engineering and Geodesy, Wrocław University of Environmental and Life Sciences. Detailed information on the applied laboratory procedures for the analysis of selected physicochemical parameters is provided in the Supplementary Material.

Summaries and statistical analyses were performed for the obtained values, and the results were used to draw conclusions in the context of risks related to the maintenance of a proper quality of water in the planned reservoirs.

2.1 Summary and correlation of physical and chemical parameters

Minimum, maximum and mean values, and medians were determined using Excel for all parameters, taking into account the entire river without the division into individual locations. The results were compared to the applicable Polish Regulation assigning rivers to different water quality classes based on individual parameters. The ‘class points’ (C.P.) were also specified: 1 point was given for class I (best), 2 points were given for class II (good), and 3 points were given for a class below II (bad) of concentration measurement. Then, the mean value was calculated based on the points given, and the following classification of the final ecological condition was adopted according to this parameter: 1.00 to 1.66 – class I, 1.67 to 2.33 – class II, 2.34 to 3.00 – below class II. To evaluate the correlations between physical and chemical parameters, a non-parametric Spearman’s rank correlation coefficient (ρ) was used. This method does not require meeting the assumptions of normality of distribution and is particularly useful in environmental studies where the data is often non-linear and outliers may affect the result of traditional correlation methods. The Spearman’s coefficient was calculated according to the following formula:

$\rho =1-\frac{6\sum d_i^2}{n\left(n^2-1\right)}$

where:$d_i$ – difference between value ranks of a given pair of variables;$n$ – number of observations.

2.2 Spatial variability of physical and chemical parameters

The analysis of the spatial variability of the physical and chemical parameters was performed based on the results obtained at the individual measuring points located along the rivers. A single-factor analysis of variance (ANOVA) was used to evaluate the importance of differences. This test allows the comparison of mean values in more than two groups (in this case, measuring stations). The assumptions of normality and homogeneity of variance were verified using the Shapiro-Wilk test and Levene’s test, respectively. The variance was calculated according to the following formula:

$s^2=\frac{1}{n-1}{\displaystyle \sum _{i=1}^n}{\left(x_i-\overline{x}\right)}^2$

where:$x_i$ – observed value;$\overline{x}$ – arithmetic mean;$n$ – number of observations.

The obtained data was presented using box plots that allow the simultaneous evaluation of median, variability range, and outliers for each station.

2.3 Time variability of physical and chemical parameters

The time variability of the tested physical and chemical parameters was analysed based on the data collected during the entire monitoring period, which covered two full years. A trend analysis was performed separately for each measuring point. A linear regression of the least squares was used for each parameter to estimate the trend of changes in time.

$x_i\left(t\right)={\beta }_{0,i}+{\beta }_{1,i}*t+\epsilon$

where:$x_i\left(t\right)$ – parameter concentration at time $t$ at location $i$;${\beta }_{0,i}$ – free term;${\beta }_{1,i}$ – directional factor (trend slope);$\epsilon$ – random component (of the model’s remainder).

The directional factor value was interpreted as the rate of variation in the concentration of a parameter in a time unit. For each match, the coefficient of determination R² was also calculated, which describes the ratio of variability of the observed values explained by the linear model.

$R^2=1-\frac{{\sum }_{j=1}^n{\left(x_j-{\widehat{x}}_j\right)}^2}{{\sum }_{j=1}^n{\left(x_j-\overline{x}\right)}^2}$

where:$x_j$ – observed value;${\widehat{x}}_j$ – value forecast by the regression model;$\overline{x}$ – arithmetic mean of observed values;$n$ – number of observations.

According to the Shapiro-Wilk test, the normal distribution of the model’s remainders $\epsilon$ and their variance homogeneity in the entire range of the independent variable (time) were confirmed. When these conditions were met, the value $p$ was determined based on a bilateral Student’s t-test to specify the statistical importance of the trend of the parameter’s concentration change in time.

2.4 Bottom sediments

The samples were marked in terms of their size distribution using the Bouyoucos areometric method in Casagrande’s and Prószyński’s modifications. They were then divided into respective fractions using screens with different mesh sizes (Tomczyk et al., 2021). The amount of gravel fraction > 2 mm (soil matrix) in the sample was identified, and the percentage share of sands (0.05–2 mm), silts (0.002–0.05) and clays (>0.002 mm) in the remaining part of the sample was specified. The content of heavy metals in the samples, such as Cd, Cr, Cu, Ni, Pb, and Zn, was also determined using the absorption spectrophotometry method. The total carbon (TC) and total nitrogen (TN) were determined, and the C/N parameter was obtained (Feng et al., 2024), by analysing the gases produced during the combustion of samples.

2.5 Quality of water in potential reservoirs

Based on the reference data, the importance of the concentrations of individual physical and chemical parameters was analysed in the context of the construction of hypothetical storage reservoirs and the risk of the deterioration of the quality of the water contained therein. To place the obtained results within the framework of classical concepts of eutrophication assessment, an adapted and simplified approach based on the Vollenweider model was applied (Milstead et al., 2013; Vollenweider, 1968). It should be emphasised that the use of the Vollenweider balance model in this study does not represent a strict calculation of the annual phosphorus load from the entire catchment, as it is based on point measurements of total phosphorus concentrations rather than temporally integrated load data covering a full hydrological year. The model was therefore applied in an indicative and comparative manner, aiming to provide a preliminary assessment of the potential eutrophication risk of the hypothetical reservoirs rather than a quantitative estimation of the actual phosphorus load. In this approach, the relationship between phosphorus input and its anticipated concentration in the reservoir was considered with respect to average depth and water retention time, following the conceptual assumptions of the Vollenweider model. Based on these assumptions, the anticipated concentration of total phosphorus in the reservoir was calculated according to the following formula:

$TP=\frac{\raisebox{1ex}{$L_p$}\!\left/ \!\raisebox{-1ex}{$z$}\right.}{1+\sqrt{\tau }}$

where: $TP$ – anticipated average concentration of total phosphorus in the water [µg/L];$z$ – average depth of the reservoir, expressed as the ratio of the reservoir’s volume to its area [m];$\tau$ – retention time, expressed as a ratio of the reservoir’s volume to the average annual flow [years].

The obtained values (TP) were compared to the limits of trophic states to determine whether a potential reservoir is classified as an oligotrophic, or a eutrophic one.

In addition to the classical Vollenweider model (Vollenweider, 1968), the approach modified by Benndorf was also considered (Wiatkowski and Paul, 2009). In this modification, the emphasis is placed not on total phosphorus (TP, which also includes forms that are difficult to absorb biologically) but on the fraction of soluble reactive phosphorus (SRP), which can be derived from PO₄ concentrations and represents the most bioavailable form for phytoplankton. The method employs the same hydraulic parameters (mean depth z and water residence time τ), but the phosphorus load is expressed as the annual areal SRP loading (L’_SRP), according to the following formula:

$L_{SRP}^{\prime }\hspace{1em}=\frac{C_{SRP}*Q}{A}$

where: $L_{SRP}^{\prime }$ – areal loading of soluble reactive phosphorus [g P/m²*year];$Q$ – mean annual discharge [m³/year]; $A$ – reservoir surface area [m²] calculated as the ratio of reservoir volume to its mean depth $C_{SRP}$ – mean SRP concentration in the inflow [g P/m³], calculated as the product of PO₄ concentration and 0.326 (the ratio of the molar mass of phosphorus M_P to the molar mass of the orthophosphate ion M_PO4):

$C_{SRP}={PO}_4*\frac{M_P}{M_{{PO}_4}}={PO}_4*0.326$

where: $M_P$ = 30.97 [g/mol]; $M_{{PO}_4}$ = 94.97 [g/mol].

The obtained values of areal loading (L’_SRP) were compared with the critical loading curves developed by Vollenweider and adapted by Benndorf for dam reservoirs (Wiatkowski and Paul, 2009). These curves account for the ratio $z/\tau$, i.e., reservoir depth relative to water residence time. The plot was constructed using a logarithmic scale, which allows a power-law relationship to be represented as straight lines. The boundaries between trophic states (oligotrophic, mesotrophic, eutrophic, hypereutrophic) were approximated using the relationship $L_{SRP}^{\prime }=k*{\left(\frac{z}{\tau }\right)}^{0,5}$, where the coefficient k takes values of 0.02 (boundary between oligotrophic and mesotrophic), 0.2 (boundary between mesotrophic and eutrophic), and 2.0 (boundary between eutrophic and hypereutrophic).

Assuming the construction of a pre-reservoir for both sites, its capacity to remove inorganic nitrogen can be determined as a function of the N:P ratio in the inflow (defined as the ratio of the sum of NH₄, NO₂, and NO₃ concentrations to the PO₄ concentration) and the mean water residence time (calculated as the ratio of reservoir volume to mean discharge). Functional relationships describing this process have been presented in the literature (Pütz and Benndorf, 1998) and serve as the basis for estimating the potential retention of biogenic compounds in pre-reservoirs. In this study, a nomogram based on these relationships was used, onto which the obtained results were plotted.

3 Results

3.1 Summary and correlation of physical and chemical parameters

The minimum, maximum, and mean values of all physical and chemical parameters analysed in the study, without the division into individual locations, are summarised in Table 2 (the Włodzica river) and Table 3 (the Ścinawka river). The tables also include the results’ medians, standard deviations and ‘class points’ (C.P.).

Table 2

Table 2. Physical and chemical parameters of the Włodzica river.

Table 3

Table 3. Physical and chemical parameters of the Ścinawka river.

Although there are differences related to the classification of both rivers based on individual parameters, they are slight or arise from a different class assigned to these water bodies. Pursuant to the Polish regulations (Ministry of Infrastructure, 2021), the Włodzica river was classified as ‘Sudetes stream (PGS)’, and the Ścinawka river was classified as ‘small upland river on a silicate bed (RW_krz)’. Irrespective of the above, it was observed for both rivers that in the case of most parameters the lowest identified values would allow the identification of at least good water quality (class II or above). The exception is the concentration of chlorides (Cl) that, irrespective of the sampling date, was too high, which is also the case for the Włodzica river for total dissolved substances (TDS), calcium (Ca), and magnesium (Mg). Different conclusions should be drawn, however, for the majority of the maximum and average values, which in particular refers to nutrients; that is, various forms of nitrogen and phosphorus. For these parameters, maximum values always indicated bad water quality, whereas the average values allowed the classification of the rivers as class II water bodies. The above exceedances may be caused by local (including illegal) discharges of wastewater or surface run-offs contaminated by fertilisers (nitrates and phosphates) originating from the Włodzica and Ścinawka river basins, which are intensely active farmland. No elevated amounts of particularly hazardous heavy metals were identified in either river, and cadmium and lead, which are not included in the tables, were not identified in any study. Locations were found with one-off exceedances of the assumed standard concentrations of copper (Cu) and chromium (Cr), but they were the results deviating significantly from the average values. Figure 2 presents the Spearman’s correlation matrices developed for nutrients (various forms of nitrogen and phosphorus), dissolved oxygen (DO) and 5-day biological oxygen demand (BOD₅).

Figure 2

Figure 2. Spearman’s correlation matrix for the Włodzica (A) and Ścinawka (B) rivers.

It should be emphasised that the correlations presented below represent statistical co-variation between parameters, while the proposed links to hydrochemical and biogeochemical processes constitute process-based interpretations supported by existing literature rather than direct causal evidence. In the Włodzica river, there is a highly positive relationship between nitrates and total nitrogen (ρ = 0.74), indicating a substantial contribution of the nitrate form to the total nitrogen pool. Such an association is commonly observed in catchments influenced by agricultural activity, where nitrate often represents the dominant fraction of inorganic nitrogen. A clear positive relationship between the biochemical oxygen demand and the dissolved oxygen demand (ρ = 0.54) is atypical and deviates from the pattern usually observed under conditions of elevated organic pollution. When BOD₅ is high, a decrease in DO is generally seen, which results from intensive decomposition of organic matter by microorganisms. In this case, the observed relationship may reflect the co-occurrence of seasonal changes in temperature, oxygenation, and photosynthetic activity. For instance, when there is a high inflow of organic matter, there may be a heavy supply of oxygen due to the production of oxygen by phytoplankton (Jargal et al., 2021). Dissolved oxygen is in turn highly negatively correlated with NO₂ (ρ = −0.55) and PO₄ (ρ = −0.47), suggesting a co-occurrence of elevated nutrient concentrations and reduced oxygen availability. This pattern may be associated with local inputs of nutrient-rich waters, such as municipal wastewater discharges or other anthropogenic sources, which can promote biomass production and subsequent oxygen consumption during its decomposition. Electrolytic conductance shows a positive relationship with NO₃ and TN but a negative relationship with PO₄ (ρ = −0.26), which may indicate differing source characteristics or transport pathways for nitrogen and phosphorus compounds. In particular, phosphate concentrations may be influenced by internal river processes, such as the release of phosphorus from bottom sediments under low-oxygen conditions. PO₄ is poorly and negatively correlated with TN (ρ = −0.21), whereas the PO₄ – TP correlation is moderate (ρ = 0.35), suggesting that a considerable fraction of total phosphorus may occur in organic forms or be associated with suspended particulate matter.

In the Ścinawka river, the pattern of correlations differs markedly. Electrolytic conductance is positively correlated with both nitrates (ρ = 0.54) and phosphates (ρ = 0.42), which may indicate their co-occurrence due to shared sources of contamination, such as combined household and agricultural wastewater inputs or runoff from fertilised areas. PO₄ shows a moderately strong positive correlation with total phosphorus (ρ = 0.48), which confirms that PO₄ has an important share in the TP pool. The correlation between NO₃ and TN is even higher (ρ = 0.90), which indicates that nitrates dominate the total nitrogen. PO₄ is positively correlated with NO₂ (ρ = 0.61), which may reflect their joint occurrence under conditions of increased nutrient loading, potentially related to anthropogenic pressures within the catchment. Dissolved oxygen has a highly negative correlation with PO₄ (ρ = −0.59) and NO₂ (ρ = −0.47), which is consistent with eutrophication-related processes, where elevated nutrient concentrations tend to co-occur with deteriorating oxygen conditions (Lin et al., 2021).

3.2 Spatial variability of physical and chemical parameters

Based on the spatial analysis of the data, the parameters were isolated for both rivers for which a statistically relevant difference (ρ < 0.05) between individual measuring stations was observed. The distributions of concentrations of these parameters (box plots with division into locations) are presented in Figure 3 (the Włodzica river) and in Figure 4 (the Ścinawka river). Prior to the application of analysis of variance, the assumptions of data normality and homogeneity of variances were verified. The Shapiro–Wilk test indicated that for the Włodzica River approximately 47.5% of the analysed datasets met the assumption of normal distribution, whereas the remaining 52.5% showed statistically significant deviations from normality. For the Ścinawka River, the respective proportions were 41.66% and 58.33%. Homogeneity of variances among the sampling sites was assessed using Levene’s test. For the Włodzica River, this assumption was satisfied for 80% of the analysed parameters, while for the Ścinawka River it was met for 70% of the parameters. Detailed results of the statistical analyses are provided in the Supplementary Material.

Figure 3

Figure 3. Distribution of parameter concentrations with a statistically relevant spatial difference–the Włodzica river. (A) – Distribution of NH₄. (B) – Distribution of PO₄. (C) – Distribution of EC.

Figure 4

Figure 4. Distribution of parameter concentrations with a statistically relevant spatial difference–the Ścinawka river. (A) – Distribution of NH₄. (B) – Distribution of NO₃. (C) – Distribution of NO₂. (D) – Distribution of TN. (E) – Distribution of EC. (F) – Distribution of BOD₅. (G) – Distribution of PO₄.

The analysis of concentration distribution of ammonia nitrogen (NH₄), phosphates (PO₄), and electrolytic conductance (EC) at four measuring points located along the Włodzica river from the spring (Site 4) to the potential location of the barrier at the river mouth (Site 1) reveals distinct differences. The highest NH₄ concentrations were recorded in the spring section of the river and gradually decreased downstream, which is consistent with commonly reported longitudinal patterns in river systems. This trend may be attributed to the combined effects of nitrification and ammonium sorption onto suspended particles and bottom sediments occurring under progressively changing hydrological and oxygen conditions along the river course (Hamdhani et al., 2023). Phosphates show a different pattern: the highest median at Site 3 suggests a local inflow of nutrients, whereas lower levels at the spring and the mouth reflect biotic and abiotic processes of long-term phosphorus removal related to its flow down the river (Kozyrev et al., 2023). The electrolytic conductance is significantly higher both at the spring and at the mouth, which may be justified by the presence of mineralised groundwater at the river’s upper course (richer in Ca²⁺, Mg²⁺, HCO₃⁻) and the accumulation of dissolved ions, such as NO₃⁻, Cl⁻, SO₄^2- or Na⁺ at the river’s lower course, which come from farming and wastewater. In natural streams, the conductance generally increases from the spring to the mouth. The observed pattern–high EC at Site 4, minimum EC at Site 3, and its re-increase at Site 1 – indicates that the analysed river combines the features of mineral water at the spring with local inflows with a lower conductance and higher salinity at the lower course.

The analysis of the concentration distribution of total nitrogen (TN), ammonia nitrogen (NH₄), nitrites (NO₂), nitrates (NO₃) and phosphates (PO₄) at four measuring points located along the Ścinawka river from the spring (Site 6) to the potential location of the barrier (Site 1) reveals a spatial arrangement of changes. The highest concentrations of NH₄ were observed at the river’s central and upper course (Sites 4–5), whereas at the lowest and highest courses they were significantly lower. The maximum concentration of NO₂ was also recorded at the same Sites 4–5, which may reflect the combined effect of local inputs of reduced nitrogen and in-stream ammonium production through organic matter mineralisation, together with temporarily limited nitrification efficiency, leading to the accumulation of nitrite as an intermediate product of nitrogen oxidation. NO₃ and TN exhibited the highest concentrations in the lower course of the river (Sites 1–2) and decreased upstream, which may reflect spatial differences in nitrogen transformation processes and source distribution along the river continuum. Higher nitrate concentrations in the lower section are consistent with more advanced nitrification under favourable oxygen conditions, whereas the upstream decrease may be associated with enhanced biological uptake and denitrification, as well as reduced external nitrogen inputs (Meng et al., 2018). An opposite trend was observed for electrolytic conductance–the highest values were recorded at the location of the potential barrier, and the lowest values were at the spring, which may result from the inflow of saline municipal water at the river’s lower section and the inflow of water with low mineralisation into the river’s spring section. Phosphate concentrations (PO₄) also showed significant spatial variability, with elevated values observed at selected midstream sites, which may be associated with local point and diffuse sources of nutrient input as well as reduced phosphorus retention capacity in these river sections. Similarly, BOD₅ values differed significantly along the river course, indicating spatial differences in the load of biodegradable organic matter. Higher BOD₅ levels at selected sites suggest local inputs of organic pollution and intensified microbial activity, which may further influence oxygen conditions and nutrient transformations in the river system. In total, these results underscore that the quality of water in the river is determined by local sources of contamination and the self-cleaning processes associated with the nitrogen cycle and ion transfer.

3.3 Time variability of physical and chemical parameters

Particularly important parameters that present the quality of water (TN, TP, DO, EC) were isolated for both rivers, and their variability in time was analysed in the context of identifying possible trends or seasonal relationships. Plots for these parameters are presented in Figure 5 (the Włodzica river) and in Figure 6 (the Ścinawka river).

Figure 5

Figure 5. Changes in time of selected parameters of quality of water in the Włodzica river. (A) – TN variation chart. (B) – TP variation chart. (C) – DO variation chart. (D) – EC variation chart.

Figure 6

Figure 6. Changes in time of selected parameters of quality of water in the Ścinawka river. (A) – TN variation chart. (B) – TP variation chart. (C) – DO variation chart. (D) – EC variation chart.

For all the analysed parameters, a statistically relevant (ρ < 0.05) trend on the Włodzica river was identified only for total nitrogen, with 43% and 27% (R² = 0.43 and R² = 0.27, respectively) of the increase in concentration at Site 2 and Site 4, respectively, being explained by the factor that determines these changes and intensifies over the course of time. A slight increase in TN over time (slope 0.0022 for Site 2 and 0.0072 for Site 4) may be related to the rising nitrogen pressure at the river basin (e.g., intensified fertilisation or changes in land use). For total phosphorus, despite a single peak episode at Site 3 (during winter 2024), the trend lines for all locations are flat, which means that no significant change occurred in time. At river sections, phosphorus is absorbed by plants and is subject to sorption at sediments, which may account for the stable TP level in the longer term. For TN and TP, no clear, repetitive seasonal pattern was recognised. Dissolved oxygen has an increasing trend at each station, but no trend is statistically relevant. The increase in DO may be related to the lower load of organic substances or, primarily, the temperature decrease, since there is a clear increase in this parameter during winter months. Changes in the electrolytic conductance also show no important trends, but the fluctuations of this parameter may reflect both the dilution of river water at freshet and the concentration of ions: HCO₃⁻, Ca²⁺, Cl⁻, NO₃⁻ i SO₄²⁻ in periods of low flow (Çetin et al., 2020).

Similar to the Włodzica river, a statistically relevant trend on the Włodzica river was identified only for total nitrogen, whose 18% and 22% (R² = 0.18 and R² = 0.22, respectively) increase in concentration at Site 2 and Site 5, respectively, may be explained by the factor that determines these changes and intensifies over the course of time. A moderate increase in TN over time (slope 0.0353 for Site 2 and 0.0173 for Site 5) may be related to the rising nitrogen pressure at the river basin (e.g., intensified fertilisation or changes in land use). For total phosphorus, despite a single peak episode at Site 5 (during autumn 2024), the trend lines for all locations are flat, which means that no significant change occurred in time. At river sections, phosphorus is absorbed by plants and is subject to sorption at sediments, which may account for the stable TP level in the longer term. For TN and TP, no clear, repetitive seasonal pattern was recognised. Dissolved oxygen has an increasing trend at each station, but no trend is statistically relevant. Particularly low DO levels were recorded in July and August 2024, which corresponds to the periodic drought in the region when there was no water at Site 6 (the river was completely dry). The increase in DO may be related to the lower load of organic substances or, primarily, the temperature decrease, since there is a clear increase in this parameter during winter months. Changes in the electrolytic conductance also show no important trends, but the fluctuations of this parameter may reflect both the dilution of river water at freshet and the concentration of ions: HCO₃⁻, Ca²⁺, Cl⁻, NO₃⁻ i SO₄²⁻ in periods of low flow (Çetin et al., 2020).

3.4 Bottom sediments

Figures 7, 8 and Table 4 below present the results of bottom sediment testing, for which samples were taken at the locations of potential dams on both analysed watercourses.

Figure 7

Figure 7. Grain size distribution for bottom sediments at the location of a potential barrier (Site 1) on the Włodzica river.

Figure 8

Figure 8. Grain size distribution for bottom sediments at the location of a potential barrier (Site 1) on the Ścinawka river.

Table 4

Table 4. Heavy metal content and C/N ratio of the bottom sediments of the Włodzica and Ścinawka rivers.

The Włodzica river bottom sediments have a diverse composition. Large stones (>20 mm) comprise approximately 30% of the gravel fraction. Globally, however, sand fractions dominate, in particular fine and very fine sand (0.5–0.25 mm) and medium sand (1–0.5 mm). The sediments also contain a small share of sludge and clay (<0.063 mm). Such a distribution indicates moderate flow energy: high water surface elevations allow the transportation and accumulation of gravel, and when the main stream is weaker, it also stops finer sand. According to the principle that when the river transport capacity decreases, coarse grains are the first to sediment, and finer grains are conveyed farther, the mixed fraction in the Włodzica river sediments indicates periodic changes in flow velocity. The presence of sludge and clay fractions suggests that there are sections of the bottom with low energy where the finest particles may accumulate; fine fractions are generally deposited in still water where the energy of the carrier is too low to keep them suspended (Levenson and Fonstad, 2022).

The bottom sediments of the Ścinawka river frequently include large stones (>20 mm) that comprise more than 70% of the gravel fraction composition, and the sand fraction primarily contains relatively large grains (1–2 mm). The share of sludge and clay fractions is very low (<0.063 mm). Such a dominant position of gravel indicates a high-energy hydrodynamic environment in the region where a potential barrier is to be constructed. Large, heavy grains may only be deposited, whereas sand and sludge is conveyed farther.

The C/N ratio of the bottom sediments of both rivers indicates different sources and natures of organic matter. The C/N = 14 for the Włodzica river suggests that autochthonous matter, originating from primary production in the riverbed (phytoplankton, periphyton), and processed organic residues predominate, which is typical of environments with a higher share of nitrogen in organic matter. The C/N = 22.8 for the Ścinawka river is a relatively high value, characteristic for land-based matter rich in lignin and cellulose (vascular plants), which degrades slower (Gao et al., 2023). The presence of this fraction in the sediments may restrict the rate of mineralisation and facilitate the accumulation of organic matter at the bottom and, under anaerobic conditions, increase the risk of release of associated heavy metals and phosphorus. The concentrations of heavy metals in tributary sediments of the Włodzica and Ścinawka rivers are generally low in view of applicable standards. For instance, the Polish Regulation of the Minister of Environment of 2002 (currently repealed) allowed the following parameters in sediments: 150 mg Cu/kg, 1,000 mg Zn/kg, 200 mg Pb/kg, 75 mg Ni/kg, 200 mg Cr/kg, and 7.5 mg Cd/kg (Ministry of the Environment, 2002). Compared to these limits, the measured values are lower by an order of magnitude, which signifies a lack of heavy contamination. Furthermore, international environmental guidelines confirm that the sediments are below the concentrations considered potentially toxic. Thresholds were established according to the following international overview of sediment quality guidelines (Wenning, 2005): TEC (Threshold Effect Concentration) and PEC (Probable Effect Concentration). None of the metals analysed in the sediments of the Włodzica and Ścinawka rivers exceeds the lower thresholds (TEC), and the majority of the values is significantly below these limits, which suggests that the risk of negative impact on the benthos biota is low. A relatively higher concentration of Zn in the Włodzica river (approx. 99 mg/kg) is, however, close to the TEC (121 mg/kg), which may indicate moderate enrichment and require further monitoring. Elevated concentrations of Zn and the presence of Cd in the sediments of the Włodzica river suggest local sources of contaminants, likely related to the anthropogenic activity in the river basin (e.g., past mining activity in the Nowa Ruda region or municipal run-off) because these metals are frequently released as a result of mining operations, metallurgical industry, or the use of phosphate fertilisers. In general, this data demonstrates a moderate metal load of sediments, while the values remain within acceptable limits according to both national and international criteria.

3.5 Quality of water in potential reservoirs

The water in the Włodzica and Ścinawka rivers features relatively high concentrations of total nitrogen (TN) and total phosphorus (TP). The average concentrations of TN of 2.5–3.6 mg/L, and TP of 0.26–0.36 mg/L indicate significant nutrient enrichment. To provide an indicative assessment of the trophic potential of the potential reservoirs, the Vollenweider model was applied to both rivers, and the results of the model were presented for minimum, average, and maximum TP values for the entire section of the river (Table 5) and for the cross-section being the direct location of the reservoir (Table 6). The results obtained signify that there is a major difference whether the calculations include the TP values from a particular location or from the entire section of the watercourse. Nevertheless, it should be emphasised that these results represent comparative and orientational estimates rather than formal trophic classifications. According to the anticipated concentration of total phosphorus, the average and maximum values corresponded to conditions typical of hypereutrophic systems, whereas only the minimum values were consistent with mesotrophic conditions.

Table 5

Table 5. Results of the Vollenweider model, assuming minimum, average, and maximum values for the entire river.

Table 6

Table 6. Results of the Vollenweider model, assuming minimum, average, and maximum values for the location of potential reservoirs.

Based on the Vollenweider model in the Benndorf modification, the calculations were performed using the concentration of soluble reactive phosphorus (SRP), which represents the fraction of phosphorus directly available to phytoplankton (Wiatkowski and Paul, 2009). The obtained results were generally consistent with the conclusions drawn from the classical model, suggesting a high eutrophication susceptibility of both reservoirs. At maximum SRP concentrations, the predicted areal phosphorus loading fell within ranges commonly associated with eutrophic (Sarny Reservoir) to hypereutrophic (Ścinawka Górna Reservoir) conditions, indicating a potentially elevated risk of intense algal blooms and oxygen deficits in the hypolimnion. At mean SRP concentrations, the classification shifted toward conditions characteristic of eutrophic (Ścinawka Górna Reservoir) or mesotrophic (Sarny Reservoir) systems, corresponding to the Vollenweider balance results for minimal total phosphorus values. Thus, the Benndorf modification supports the comparative findings of the classical model, while providing a process-oriented indication of short-term eutrophication risk by focusing on the phosphorus fraction that is directly bioavailable to phytoplankton. The results are presented in Figure 9.

Figure 9

Figure 9. Diagram of critical phosphorus loads according to Vollenweider, modified by Benndorf, showing the trophic classification of planned reservoirs.

The concentration ratio TN:TP for both rivers oscillates around 10, which means that no element is extremely limiting, and both are in high excess. Such a stoichiometry favours periodic common or alternate limitation, and may promote the dominance of cyanobacteria capable of assimilating atmospheric nitrogen at the deficiency of mineral nitrogen. Significantly, the analysis of time trends indicated a statistically relevant increase in concentrations of TN in recent years (e.g., in the Włodzica river: R² to 0.43) with no clear trend for TP. It suggests a rising nitrogen pressure in river basins (e.g., due to intensification of the use of fertilisers) at a stable but high level of phosphorus. If this trend were maintained, it would mean a further increase in the TN:TP ratio and a potential change to the state of predominance of phosphorus limitation, although at such high concentrations of both nutrients, the overall risk of eutrophication would remain high regardless of moderate changes in their proportions (van Wijk et al., 2024).

Based on the water residence time and the ratio of mean concentrations of mineral nitrogen to phosphates in the Włodzica and Ścinawka rivers, an approximate scale of inorganic nitrogen reduction was determined, assuming the construction of pre-reservoirs at both planned sites. Readings from the nomogram (Pütz and Benndorf, 1998) indicate that, in the case of the Sarny and Ścinawka Górna reservoirs, nitrogen reduction could reach approximately 20% and 21%, respectively. Despite the shorter residence time in the Ścinawka Górna reservoir, the potential efficiency of nitrogen removal is similar to that obtained for the Sarny reservoir, which may be related to the lower N:P ratio that promotes more intensive uptake of nitrogen compounds. The results are illustrated in Figure 10.

Figure 10

Figure 10. The impact of potential pre-reservoirs on the process of inorganic nitrogen removal from the Sarny and Ścinawka Górna reservoirs.

Parameters related to dissolved oxygen and oxygen demand allow the evaluation of the degree of organic contamination and the risk of oxygen deficiency. Average values of BOD₅ in the Włodzica river (approx. 1.68 mg/L) and the Ścinawka river (approx. 2.61 mg/L) indicate a relatively low load of organic matter in both rivers. Extreme trends reveal, however, some contamination episodes. The maximum recorded concentration of BOD₅ in the Ścinawka river amounted to 28.8 mg/L (in the Włodzica river, it was max. 4.2 mg/L), which greatly exceeded a typical range for clean water and indicated incidental heavy organic contamination. Correspondingly, the maximum COD in the Ścinawka river (11.9 mg/L) also exceeded the maximum values recorded in the Włodzica river (6.8 mg/L), which confirmed that the Ścinawka river sporadically experienced higher loads of organic and chemical contamination than the Włodzica river. Moreover, there were episodes of a complete oxygen deficiency in the upper course of this river. It should be taken into account when designing the reservoir–at flooding and slower flow, there is a risk of zones developing with insufficient oxygen supply, particularly in the event of an inflow of organic contaminants or algal bloom (Jargal et al., 2021).

The concentration of total suspended solids (TSS) translates directly to water turbidity and a potential silting up of the reservoir through sedimentation of flowing sediments. The average values of TSS in both rivers are moderate and amount to approx. 30 mg/L. There is, however, a great difference between average and peak conditions–it particularly applies to the Ścinawka river, where the maximum value of TSS was as high as 354 mg/L, whereas in the Włodzica river it amounted to 96 mg/L. Such high values probably resulted from episodic, more intense erosion run-offs and are potentially harmful for river organisms. It should be noted, however, that the interpretation of total suspended solids (TSS) concentrations in rivers should account for the complex nature of sediment transport hydrodynamics, including flow velocity, turbulence, channel morphology, bed and bank erosion, and the mobilisation of bottom sediments. These factors may exhibit pronounced spatial and temporal variability along the river course. Consequently, observed increases or decreases in TSS concentrations may reflect changes in hydrodynamic conditions rather than solely differences in sediment inputs or anthropogenic pressure. Therefore, the results obtained indicate a real risk of silting up of the reservoirs, in particular on the Ścinawka river. Even though the average turbidity is not alarmingly high, the rivers at freshet convey sediments in sufficient amounts to allow quick accumulation of deposits at the retention site (Upadhayay et al., 2021).

Water salinity and mineralisation affect the usability of water in the retention reservoir (e.g., potable water quality, irrigation capacity) and chemical attack on infrastructure (risk of corrosion). The average concentrations of chlorides (Cl), sulphates (SO₄), electrolytic conductance (EC), and total dissolved substances (TDS) determined for both rivers indicate water with moderate mineralisation. In general, the water chemistry of both rivers poses no risk of corrosion or salinity to the planned reservoirs. The water potentially intended for retention may be considered suitable for most purposes (after possible treatment–also for drinking), and in terms of ecology, this level of salinity causes no harm to freshwater organisms (Miltner, 2021).

Taking into account the size distribution and the nature of the load of the individual rivers, it should be noted that the construction of a barrier will also reduce the flow velocity and may cause silting up of the reservoir bowl. However, due to a high share of coarse grain fractions (gravel, sand), it can be assumed that under normal conditions, there is a low potential for suspended matter transport, and at moderate velocities, the majority of the material remains in the riverbed. The load may, however, be set in a more intense motion at freshet, and silting up may apply to the reservoir’s inlet zone. The relatively high C/N ratio in the sediments of the Ścinawka river may point to a risk of higher accumulation of organic material at the bottom of the reservoir, and under anaerobic conditions, it may lead to the release of associated contaminants: both phosphorus and heavy metals. As far as the latter are concerned, the identified concentrations were not dangerously high (both in the water and in the sediments), which may lead to the conclusion that the reservoir is not going to be loaded with significant internal metal emissions (Sojka et al., 2023).

4 Discussion

The aim of this study, conducted on the example of the Włodzica and Ścinawka rivers, was to address the relatively rarely discussed issue of water quality assessment in foothill rivers, carried out at the stage of planning the construction of retention reservoirs. The original contribution of this study is to demonstrate that the appropriate selection of analysis methods (both technical and statistical or model-based) and accurate identification of the spatial and temporal variability of hydrochemical parameters are crucial for a reliable assessment of the risks associated with the pollution of stored water. In discussing the obtained results, several aspects are highlighted. They include the differences between the rivers, the importance of sampling locations and the seasonal nature of measurements, the translation of hydrochemical data into practical conclusions concerning the planning of reservoir construction, and any other studies that might elaborate on the water quality evaluation. In terms of comparison of both watercourses, despite their different size and flow dynamics, the concentrations of individual parameters are at similar levels, with several exceptions, such as episodic maximum values of BOD₅ in the Ścinawka river (up to 28.8 mg/L), which were considerably higher than in the Włodzica river (approx. 4.2 mg/L), and positive correlations of phosphates with conductance and with total nitrogen in the Ścinawka river (unlike the Włodzica river), which suggests a domination of soluble mineral load (Janicka et al., 2022). In view of the studies of bottom sediments, it was claimed that both watercourses contain low concentrations of heavy metals, with the size distribution featuring a slight diversity, as the Włodzica river features a more balanced material transport and deposit, which reflects the variable flow energy and the presence of calmer microhabitats. On the other hand, the Ścinawka river has a particularly coarse bed, which is typical of rivers with high flow energy, where smaller grains flow farther. These differences are not significant and may result from different bed slopes, geological makeup, and the nature of the river reservoirs of both watercourses (Sojka et al., 2023). It shows that the adjacent rivers likely have similar values of physical and chemical parameters, and the analyses performed may constitute an essential reference in the context of studies performed on other watercourses in the region. Assuming a more detailed approach to the results obtained, however, the essence of the selection of subsequent measuring points should be emphasised. The comparison of both watercourses shows that when the highest concentrations of ammonia nitrogen in the Włodzica river occurred in the source section and decreased down the course, the maximum values of NH₄ and nitrites in the Ścinawka river were recorded in the middle section, which suggests a different arrangement of sources of contaminants and a different course of nitrification processes. When planning the construction of retention reservoirs, water sampling at the location of the barrier is important; however, to identify the sources of contaminants and take reasonable actions, it is required to increase the number of measuring points along the entire river. Site 5 on the Ścinawka river may be provided as an example–the concentrations of nutrients were particularly high, and the determination and elimination of the root cause (e.g., illegal discharge of contaminants) would further positively influence the mitigation of the risk of accumulating nitrogen and phosphorus compounds in the reservoir (Buta et al., 2023). Although the statistical analyses applied in this study do not allow for direct inference of cause–effect relationships, the observed spatial patterns of nutrient concentrations may be interpreted in the context of well-established hydrochemical and biogeochemical processes operating in river ecosystems (Dębska et al., 2021). For example, a study on the spatial distribution of nitrogen in the Pingjiang River demonstrated that elevated concentrations of NH₄ and NO₂ in upstream reaches were associated with local inflows and limited nitrification under low dissolved oxygen conditions, illustrating the combined influence of anthropogenic inputs and biogeochemical constraints on the spatial patterns of nitrogen forms (Luo et al., 2025). In particular, the contrasting longitudinal distributions of ammonium nitrogen, nitrites, and nitrates along the Włodzica and Ścinawka rivers indicate differences in dominant nitrogen transformation pathways as well as in the spatial arrangement of pollution sources (Wang et al., 2023). Elevated NH₄ and NO₂ concentrations in the middle reach of the Ścinawka River may reflect the influence of local point or diffuse inputs combined with a reduced intensity of nitrification processes, whereas higher NO₃ and total nitrogen concentrations in the lower reaches are consistent with more advanced nitrification under improved oxygen conditions (Aissa-Grouz et al., 2015). Similarly, the negative relationships observed between dissolved oxygen and phosphate and nitrite concentrations are consistent with feedback mechanisms characteristic of eutrophication processes, in which increased nutrient availability enhances biological production followed by intensified oxygen consumption during the decomposition of organic matter (Bouwman et al., 2013). Based on data from stable nitrogen isotope experiments conducted in 72 mountainous and sub-mountainous streams across different regions, it has been shown that total biotic nitrate uptake and denitrification increase with rising nitrate concentrations in streams; however, the efficiency of these processes decreases at higher concentrations, thereby reducing the proportion of nitrate removed from downstream transport (Mulholland et al., 2008). Quantitative tracing of nitrogen sources in river systems using δ¹⁵N and δ¹⁸O isotopes further indicates that different anthropogenic sources may dominate in different sections of the river network, which has important implications for the interpretation of spatial variability in TN and NO₃ concentrations (You et al., 2025). Consequently, the differences observed between the studied rivers may result not only from contrasting hydrological conditions and catchment characteristics, but also from varying intensities of anthropogenic pressure and the effectiveness of self-purification processes along the river continuum (Irani et al., 2025). Although the proposed interpretations remain hypothetical, they are consistent with established concepts of nutrient cycling and provide a process-based framework for the interpretation of the obtained statistical results (Bouwman et al., 2013).

Although the statistical analyses applied in this study enabled the identification of key patterns of spatial variability in water quality parameters, the interpretation of the results should take into account certain methodological limitations. Spearman’s rank correlation analysis was used to identify statistical relationships between water quality parameters; however, it does not imply cause–effect relationships, and the obtained correlation coefficients reflect only the co-occurrence of variables within the analysed dataset. Therefore, the reported relationships should be interpreted as potential associations resulting from common pollution sources, similar environmental mechanisms, or interacting hydrochemical processes, rather than as evidence of direct causal effects between individual parameters (Bocianowski et al., 2024). A detailed assessment of spatial variability in water quality parameters was conducted using one-way analysis of variance (one-way ANOVA), which, although one of the most widely applied statistical methods in environmental studies, may be considered controversial when not all datasets meet the assumption of normality (Włodzica: 47.5%; Ścinawka: 41.66%) or when complete homogeneity of variances among sampling sites is not confirmed (Włodzica: 80%; Ścinawka: 70%). Nevertheless, one-way ANOVA has been shown to be robust to moderate violations of the normality assumption, and numerous simulation studies have demonstrated that departures from normality do not lead to a substantial increase in error rates provided that certain conditions are met, such as sufficiently large group sizes (Schmider et al., 2010). Similarly, with respect to homogeneity of variances, minor differences in variability among sampling sites do not automatically preclude the application of one-way ANOVA. In environmental studies, heterogeneity of variances across locations is a common phenomenon and often reflects real differences in hydrological conditions or pollutant inputs. When variance inequalities are moderate and not accompanied by large differences in sample sizes, one-way ANOVA retains its ability to reliably detect differences between mean values. It should also be noted that the Shapiro–Wilk test for normality and Levene’s test for homogeneity of variances are particularly sensitive for larger datasets and often indicate statistically significant deviations from assumptions even when such deviations have limited practical relevance for result interpretation. Therefore, these tests should be treated as diagnostic tools rather than as strict criteria determining the choice of statistical method (Glass et al., 1972). Despite partial violations of the normality assumption, one-way ANOVA was applied instead of non-parametric alternatives such as the Kruskal–Wallis test, because the primary objective of the analysis was to compare mean values of water quality parameters among sampling sites rather than differences in rank distributions. Moreover, under balanced data designs and moderate assumption violations, analysis of variance generally exhibits higher statistical power than non-parametric tests, which is particularly important in environmental studies characterised by high natural variability (Smeeton et al., 2025). Regardless of this, in order to additionally verify the robustness of the obtained results with respect to the choice of statistical method, a non-parametric Kruskal–Wallis test was also applied to the analysed parameters. The outcomes of this sensitivity analysis were largely consistent with those obtained using one-way ANOVA for both rivers, confirming the presence or absence of spatial differences for the same set of parameters. A minor discrepancy was observed only for NO₃ in the Włodzica river, for which the Kruskal–Wallis test indicated statistical significance at the p < 0.05 level (p = 0.03), while one-way ANOVA did not. This difference likely reflects the higher sensitivity of the rank-based test to distributional heterogeneity and episodic variability rather than a strong or systematic spatial effect, because significance was not achieved at the level of p < 0.01. The application of linear regression to trend analysis in relatively short time series of water quality parameters, characterised by strong seasonality and high natural variability, is also associated with certain methodological limitations. Linear models assume, among others, homoscedasticity, independence of observations, and normality of residuals, which are often difficult to satisfy in environmental datasets, particularly in the presence of extreme events such as floods, droughts, or incidental pollution discharges (Kocsis et al., 2017). Seasonal fluctuations in concentrations may mask or weaken the signal of long-term trends, leading to low coefficients of determination (R²), even when real temporal changes are present. Moreover, linear regression is sensitive to outliers, which may disproportionately influence the slope of the fitted trend line and the assessment of its statistical significance (Liu et al., 2022). Both parametric methods, such as linear regression, and non-parametric trend tests (e.g., the Mann–Kendall test) exhibit limited statistical power and sensitivity to data structure under such conditions (Ali et al., 2019). In the present study, linear regression was therefore applied as an exploratory tool, allowing for the assessment of the direction and general character of temporal changes, while avoiding prognostic interpretations and overemphasis on the values of the coefficient of determination. The Mann–Kendall test was not applied due to the relatively short observation period and limited number of measurements, which, when combined with the pronounced seasonality of the data, could result in reduced test power and ambiguous outcomes. Consequently, the trend analysis was intentionally restricted to a cautious interpretation of the direction of change.

An extremely important element of hydrochemical studies at the stage of planning the construction of retention reservoirs is to include seasonal variations. A one-off water sampling would not reflect the actual distribution of concentrations of physical and chemical parameters during the year, which is presented in Figures 5, 6 that illustrate the variability in time of the selected indicators. Similary, the results of the Vollenweider model demonstrate a wide range of possible outcomes, where it was possible to determine a moderate potential concentration of phosphorus in the reservoir (41–85 µg/L) and conditions consistent with mesotrophy for the lowest measurements, whereas for maximum values, the concentration amounted to 4,708–25,553 µg/L and values characteristic of hypereutrophic systems were obtained. Similarly, in the case of the Vollenweider model modified by Benndorf, the use of mean or maximum values resulted in markedly different outcomes, highlighting the sensitivity of trophic assessments to input data and reinforcing the orientational character of such classifications. It should be emphasised that the application of classical empirical models, such as the Vollenweider model, to analyses based on point hydrochemical data is associated with important methodological limitations. The model was originally developed using annual phosphorus loads from the entire catchment and assumes data integration over a temporal scale encompassing a complete hydrological cycle. Although, in the present study, total phosphorus concentrations were derived from measurements conducted regularly throughout different seasons under comparable hydrological conditions, close to mean flow values and excluding flood events, it should nevertheless be noted that individual measurements are not able to fully capture the temporal variability of nutrient loads. The results obtained using the Vollenweider models highlight the importance of long-term, regular monitoring programmes and continuous discharge data for a reliable assessment of nutrient pressure and eutrophication risk, while simultaneously indicating that interpretations based on point measurements should be regarded as supportive and indicative rather than definitive. Carrying out extensive and long-term hydrochemical studies at the stage of planning of the construction of retention reservoirs is still a non-standard approach. It is popular to draw conclusions on the basis of one-off measurements or approximation of publicly available data. Hydrological episodes observed during the study, such as drought periods, zero-flow conditions, and short-term but pronounced increases in BOD₅, nutrient concentrations, and total suspended solids, are of particular importance in the context of retention reservoir planning. Under riverine flow conditions, their impacts are usually short-lived; however, following reservoir impoundment, such events may lead to persistent deterioration of water quality. For example, in Delavan Lake (USA), the long-term mean total phosphorus (TP) concentration for the period 1984–2022 was 0.044 mg/L, whereas in selected years TP values several times higher were recorded (0.111 mg/L), accompanied by substantially elevated chlorophyll-a concentrations (59.9 µg/L). This illustrates how short-term or interannual extremes can significantly increase the risk of water quality degradation under retention conditions. During periods of low flow or water stagnation, increased water residence time promotes oxygen depletion, intensification of organic matter mineralisation, and secondary release of nutrients from bottom sediments. Conversely, episodic inflows of high suspended sediment and nutrient loads during flood events may result in rapid sediment accumulation and pulsed nutrient inputs to the reservoir, triggering long-term eutrophication processes. Consequently, short-duration extreme events, despite their relatively small contribution to the annual load balance, may exert a disproportionate influence on reservoir performance, oxygen regime stability, and long-term functionality, and should therefore be explicitly considered during the design and management of such facilities (Robertson et al., 2023).

As the characteristics of the construction of a hydro-engineering structure, including a retention reservoir, are significantly different, for this infrastructure, precisely designed and adequate engineering solutions should be adopted, including in terms of water quality maintenance. There are various technologies aimed at improving the physical and chemical parameters of retention reservoirs. They include equipment limiting the inflow of nutrients from the river basin (primary reservoirs, buffer zones) and activities within the reservoir bowl. The latter may include physical and technical activities (aeration, destratification, dredging), chemical processes (precipitation or bonding of phosphorus, liming), or biomanipulation (introducing or supporting populations of specific species of fauna and flora) (Połomski and Wiatkowski, 2023). It is essential that the adoption of specific solutions results directly from the studies, which facilitates reasonable balancing of investment costs and taking economically justified activities. In summary, if the analysis of inflowing water quality indicates elevated concentrations of nutrients (nitrogen and phosphorus) and a risk of eutrophication, it is justified to apply measures aimed at reducing nutrient loads from the catchment, such as pre-reservoirs, buffer zones, constructed wetlands/vegetated filters or other elements of green infrastructure, as well as targeted catchment-scale actions (e.g., limiting runoff from agricultural areas). When elevated phosphorus concentrations persist, it is particularly important to reduce both external inputs and potential secondary loading from bottom sediments, which supports the design of pre-treatment zones and solutions that enhance sediment retention upstream of the main reservoir basin. If reduced dissolved oxygen (DO) concentrations or elevated indicators of organic load (BOD₅, COD) are observed, measures minimizing the risk of oxygen deficits after reservoir impoundment should be considered. These include limiting the inflow of organic pollutants, appropriate hydraulic design of the structure (avoiding stagnant zones), and the potential implementation of operational measures such as aeration, destratification, or water mixing. Under such conditions, it is also essential to consider low-flow scenarios and extended water residence times, which promote the deterioration of oxygen conditions. Where elevated concentrations of total suspended solids (TSS) or a high risk of sediment transport during flood events are identified, it is crucial to design effective solutions limiting sediment inflow into the reservoir basin (e.g., settling basins, pre-reservoirs, sedimentation zones, debris traps, inlet barriers) and to account for their periodic maintenance. Reducing sediment inflow decreases the rate of siltation and limits the transport of particle-bound contaminants, thereby improving the long-term stability of water quality. If salinity and mineralization parameters (e.g., electrical conductivity, chlorides, sulfates, TDS) are elevated, their potential impact on reservoir uses (e.g., irrigation, drinking water supply) and infrastructure durability (corrosion risk) should be assessed. Where necessary, measures limiting the inflow of saline waters from the catchment (e.g., control of discharges, improved water and wastewater management) should be implemented, along with monitoring programs enabling early detection of adverse changes. In the case of elevated concentrations of heavy metals in water or sediments, priority should be given to source-oriented actions within the catchment (identification and elimination of point sources, reduction of erosion and surface runoff, protection of riparian zones), as the accumulation of metals in bottom sediments may result in long-term loading of the reservoir ecosystem. In each of the above cases, the selection of design and management measures should be linked to the identified type of pressure (catchment-wide, point-source, or internal) and to the operational parameters of the reservoir, particularly water residence time and flow dynamics, as these largely determine the stability of water quality after impoundment (Vallero, 2024). For the Włodzica and Ścinawka rivers, the analyses confirmed the high probability of the occurrence of eutrophication. Thus, the designer should anticipate solutions that mitigate this risk, e.g., by assuming the construction of an additional primary reservoir overgrown with special vegetation that purifies the water by depriving it of nutrients (Wiatkowski, 2011). The results based on water residence time and the ratio of mean concentrations of mineral nitrogen to phosphates confirmed that, for both sites, the reduction of inorganic nitrogen in the pre-reservoirs would be approximately 20%, which would have a significant impact on the quality of the impounded water. Similar retention and purification mechanisms are observed in constructed wetlands, where sorption processes, sedimentation, and macrophyte activity may lead to reductions of total nitrogen by approximately 40%–55% and total phosphorus by approximately 40%–60% relative to the incoming load. This highlights the potential of analogous retention-based solutions for nutrient reduction in small retention reservoirs (Vymazal, 2007).

The risk of the reservoirs silting up is also of some importance, and since the bottom sediment size distribution testing showed the presence of mainly larger fractions, sudden freshets may cause the transportation of considerable amounts of large-size load. As a result, it would be required to consider load-preventing barriers or special debris traps (sedimentation units) in the design upstream of the potential reservoirs (Schwindt et al., 2018). Sediments are not merely a passive component of riverine systems but actively participate in nutrient cycling through sorption, desorption, and redox exchange processes occurring at the water–sediment interface. Under conditions of variable oxygenation, which are typical of rivers with high flow dynamics, bottom sediments may periodically act both as a sink and as a secondary source of phosphorus and reduced forms of nitrogen. For example, synthesis studies reported in the literature (Bouwman et al., 2013) indicate that in river systems a substantial fraction of phosphorus can be temporarily stored in bottom sediments under oxic conditions and subsequently released into the water column as a result of changes in redox conditions or flow deceleration, leading to secondary enrichment of waters with nutrients. These mechanisms may amplify the observed variability in nutrient concentrations, particularly in river reaches characterised by limited turbulence or in zones of potential retention. In the context of planning the construction of retention reservoirs, this implies that even when external nutrient inputs are reduced, accumulated bottom sediments may constitute a long-term internal source of phosphorus and nitrogen, significantly affecting water quality and the rate of eutrophication under conditions of increased water residence time. On a global scale, it has been estimated that in the year 2000 water reservoirs retained approximately 12% of the total phosphorus load transported by rivers, and this proportion may increase to about 17% by 2030, illustrating the substantial role of phosphorus retention in the biogeochemical processes of bottom sediments (Maavara et al., 2015). It should also be emphasised that the described studies do not exhaust all methods of evaluating the quality of water in rivers. To ensure a more complete evaluation, it is possible to extend monitoring with biological elements and advanced analyses of sources of contaminants. As part of the environmental impact assessment, it is recommended to study phytoplankton/zooplankton and benthos, which is consistent with the Water Framework Directive currently functioning in the European Union. The analysis of the composition of phytoplankton biomass makes it possible to assess the actual level of eutrophication and the dominant forms of algae, whereas the biotic indicators reflect long-term effects of organic and nutrient loads. For example, the Plankton Trophic Index (PTI), developed on the basis of phytoplankton data from 1,795 lakes across 20 European countries, showed values that were strongly correlated with total phosphorus concentrations and the trophic status of lakes, thereby confirming the usefulness of phytoplankton as an indicator of the actual degree of eutrophication (Phillips et al., 2013). Another method involves isotopic analyses of the nitrogen and carbon in the water and sediments, which provide an understanding of the source of organic matter and contaminants in water ecosystems; that is, the differentiation of whether the inflow of nutrients arises mainly from field fertilisation or from untreated wastewater, and the evaluation of the rate and form of these compounds when they are incorporated into the cycle of matter in a river or reservoir (European Parliament and Council of the European Union, 2000; Atique and An, 2020). The final elements of a comprehensive evaluation of the quality of water are hydrobiological studies covering the species composition of hydrophytes and ichthyofauna as well as microbiological studies aimed at determining the presence of various bacteria strains as sanitary indicators, e.g., E. coli (Augustyn et al., 2016). This set of analyses allows for a complete evaluation of the quality of the water in the river, but in terms of the construction of retention reservoirs, properly prepared (in view of their location and seasonal nature) hydrochemical studies, which are the subject of this paper, are essential.

5 Conclusion

In both rivers, it was claimed for the majority of the physical and chemical parameters that the lowest identified values would allow the rivers to be classified as very clean watercourses. The average and maximum values indicate, however, significant contamination, in particular with nutrients, and the Ścinawka river is noticeably richer in nitrogen (TN) and phosphorus (TP) than the Włodzica river. The average concentration of TN was 3.6 mg/L (2.5 mg/L for the Włodzica river), and the average concentration of TP was 0.36 mg/L (0.26 mg/L for the Włodzica river). Maximum episodic levels of TP in the Ścinawka river amounted to 6.24 mg/L (2.71 mg/L for the Włodzica river). The results of the Vollenweider model based on the TP and the Benndorf modification based on PO₄ indicate a potentially eutrophic nature of the planned reservoirs on both rivers, which in combination with high nitrogen concentrations identifies a significant risk of eutrophication of such structures. The BOD₅ and COD values were generally low (average BOD₅: 1.7 mg/L for the Włodzica river and 2.6 mg/L for the Ścinawka river), which shows a moderate organic matter load. In the Włodzica river, there was a strongly positive correlation between NO₃ and TN (ρ = 0.74), whereas phosphates had a weak correlation with TN, which suggests that the phosphorus cycle in the ecosystem is associated with the dynamics of nitrogen compounds to a smaller extent. In the Ścinawka river, the correlations were generally stronger, in particular between NO₃ and TN (ρ = 0.90) and between PO₄ and TN (ρ = 0.48). The most important negative correlations included the correlations between DO and all of NO₂, PO₄ and BOD₅, which prove that the increase in the nutrient load leads to oxygen deficiencies. The average level of dissolved oxygen exceeded 10 mg/L but an episode of anoxia (DO = 0 mg/L) was recorded in the Ścinawka river, which was correlated with an incidental peak of BOD₅ (28.8 mg/L). The distribution of parameters along the river courses indicates local sources of contaminants. The highest concentrations of NH₄ in the Włodzica river were recorded in the upper river course, whereas the maximum levels of NH₄ and NO₂ in the Ścinawka river were observed in the middle river course, and the levels of NO₃ and TN increased down the river. The time analysis did not show a clear seasonal nature of TN and TP, whereas a statistically relevant TN increase trend was identified in both rivers, which might have been related to the intensification of fertilisation or changes in land use. A high share of gravel fractions (>20 mm) was recorded in the bottom sediments of both rivers, and the C/N ratio of the bottom sediments indicated that the Ścinawka river contained more land-based organic matter (C/N = 22.8), whereas the Włodzica river mainly contained matter originating in the riverbed (C/N = 14). The heavy metal content (identified in both water and sediments) was at a moderate level, which indicates a low risk of toxicity of water potentially stored in the retention reservoir. In view of these results, both rivers pose a challenge for the construction of retention reservoirs, and it would be necessary to include systems for reducing the inflow of nutrients, such as primary reservoirs (which would allow for an approximately 20% reduction in inorganic nitrogen) and buffer zones, and to engineer solutions that prevent the accumulation of load within the reservoir bowl. The obtained results show that water quality and the potential functioning of retention reservoirs are determined not only by the mean values of physicochemical parameters, but primarily by their spatial and temporal variability, resulting from the combined effects of anthropogenic pressures and natural biogeochemical and hydrodynamic processes. In the studied rivers, a key risk factor is the simultaneous presence of persistently elevated nutrient loads and their episodic intensification, which may lead to a rapid deterioration of the quality of impounded water. Consequently, the planning of reservoirs on such watercourses requires a systemic approach that takes into account both long-term trends in water quality changes and the mechanisms responsible for short-term, yet ecologically significant, disturbances.

Data availability statement

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author.

Author contributions

MP: Conceptualization, Data curation, Formal Analysis, Funding acquisition, Investigation, Methodology, Project administration, Resources, Software, Visualization, Writing – original draft, Writing – review and editing. MW: Conceptualization, Formal Analysis, Funding acquisition, Methodology, Project administration, Resources, Supervision, Validation, Writing – review and editing. PT: Methodology, Resources, Writing – review and editing.

Funding

The author(s) declared that financial support was received for this work and/or its publication. Financed by the Wrocław University of Environmental and Life Sciences and Ministry of Education and Science.

Acknowledgements

As Maksymilian Połomski is PhD student in the 6th edition of the implementation doctorate programme, the authors would like to express their gratitude to the Ministry of Education and Science for its support in carrying out their research.

Conflict of interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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The author(s) declared that generative AI was not used in the creation of this manuscript.

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Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fenvs.2026.1755007/full#supplementary-material

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