Introduction

The quality of river waters results from complex environmental processes influenced by both natural factors and human activities1. Among natural factors, climate change, hydrological conditions, and the geochemical properties of the catchment play a crucial role. Changes in precipitation patterns, temperature, and seasonal biological dynamics can significantly affect water chemistry, determining the concentrations of nutrients and pollutants2. However, anthropogenic factors, such as land-use changes, agricultural expansion, and urbanization, often lead to rapid changes in the chemical composition of rivers3. This results from a combination of local pollution sources, natural self-purification processes, and the presence of riparian vegetation, which can act as a filter for nutrients and contaminants4.

Water from mountains is usually of better quality because these areas are generally less exposed to industrial or agricultural activities due to pressures from human activity. However, they are highly sensitive to anthropogenic transformations, which can lead to water quality degradation and disruptions in the functioning of the entire ecosystem5. Human activities such as tourism development, urbanization, agriculture, land-use changes, and climate change significantly impact the condition of these waters and their ability to self-purify and regenerate6. For example, tourism activities can lead to increased nutrient enrichment and bacterial contamination in river waters, as observed by Lenart-Boroń et al.7 in the Białka River. The intensification of agriculture, which is associated with increased use of fertilizers and pesticides, results in the contamination of surface water from pollution from diffuse sources. Non-point source pollution from livestock and poultry farming contributes to elevated levels of total nitrogen (TN) and chemical oxygen demand (COD) in rivers, as observed in the East River and Baoxing River8. Meanwhile, climate change (rising air temperatures and changes in precipitation patterns) in mountainous regions reduces water availability and increases the concentration of dissolved organic and inorganic carbon in streams, leading to deteriorating water quality9. In mountain lakes and streams, rising temperatures and permafrost thawing cause higher concentrations of dissolved substances such as sulfate and base cations. This happens due to enhanced mineral weathering and increased contact with weatherable minerals10. Changes in precipitation patterns, snowmelt timing, and the ratio of rain to snow also significantly affect both water quality and resources11,12.

Mountains play a crucial role in global water supply, often referred to as “natural water towers” due to their significant contribution to meeting both natural and anthropogenic water demands. Mountains provide water resources for billions of people worldwide. More than 50% of mountain areas have an essential or supportive role for downstream regions, with 7% of global mountain areas providing essential water resources and another 37% delivering important supportive supply13. It is estimated that mountain water is vital for the survival of about 1.4 billion people, particularly in regions such as the Himalayas or the Andes. By the mid-21st century, about 1.5 billion people, or 24% of the world’s lowland population, will be critically dependent on mountain runoff14.

In the Polish Carpathians (Southern Poland), most drinking water intakes come from surface waters such as rivers and retention reservoirs. For example, in the Małopolskie Voivodeship, the share of total drinking water abstraction from surface waters in this region is approximately 75%, one of the highest in Poland15,16. Due to the mountainous nature of the region and the numerous watercourses, surface waters are a key source of drinking water supply for the population. However, surface water intake presents challenges, including the need to protect these waters and purify them from pollutants originating from various sources, such as industrial activities, agriculture, and municipal contamination. In recent decades, significant changes in land use have occurred in the Carpathian region. There has been a noticeable increase in forested areas, particularly due to the abandonment of agricultural land and natural forest succession. Agricultural land has also significantly decreased, often replaced by forests or grasslands17. This trend is evident in various parts of the Carpathians, including the Polish Carpathians and the Apuseni Mountains in Romania, where agricultural activities have declined since the early 1990 s, following the fall of the communist regime18. Urbanization of these areas also impacts land use changes, as there has been growing development of residential and recreational areas. For example, in the Polish Carpathians, there has been a shift from residential/agricultural buildings to residential/recreational buildings. Along with urbanization, there also has been strong development of road networks, facilitating access to remote areas and contributing to further land use changes19. An additional challenge for water quality in the Carpathians is the observed climate changes, including increasingly frequent extreme events, such as droughts and intense rainfall, which may further affect the availability and quality of surface waters20,21,22.

The aims of the study were to:

  1. 1.

    identify long-term trends in the chemical composition of river water and determine the impact of land-use changes and climate change on these trends.

  2. 2.

    analyze the spatial variability of water chemistry along the river course, identifying areas more sensitive to pollution and assessing the influence of point and diffuse pollution sources.

Study area

The Stara River catchment (22.3 km²) is located in the Carpathian Foothills (southern Poland) (Fig. 1), which consists of two levels of the Carpathian Foothills threshold. The higher (southern) level is composed of resistant flysch, while the lower (northern) level consists of flysch and Miocene rocks covered by loess-like formations23. The predominant soil types in the catchment area (80%) are typical clay-illuvial soils (Haplic Luvisol) and stagnogleyic clay-illuvial soils (Stagnic Luvisol). The area is also characterized by the presence of brown soils, as well as alluvial and deluvial soils24,25. The Stara River catchment is located in a temperate climate zone, within the moderately warm submontane climatic belt26. For the study period 2003–2023, the average annual air temperature was 9.3 °C, with an average annual minimum of 4.6 °C and a maximum of 14.4 °C. The mean annual precipitation over the same period was 716 mm. Almost half of the catchment area (mainly the southern and western parts) is covered by forests, consisting mainly of hornbeam, riparian, and beech forests. There are also deciduous and coniferous woodland communities, including fir, pine, and oak27. A quarter of the area is occupied by agricultural land. The field layout is characterized by small farms with long, narrow parcels cultivated along the slope of the land. One-fifth of the catchment area consists of pastures and meadows, while almost one-tenth is covered by urban areas (BDOT10k). In the upper part, the study area is partially located within the boundaries of the town of Bochnia. The remaining catchment area Lies within rural and forested regions. In 1984, a Scientific Station of Jagiellonian University was established in the lower part of the catchment. A meteorological station was set up, and the Stara River was equipped with a water gauge. Since 1993, scientists from the station have been conducting hydrological and meteorological monitoring.

Fig. 1
figure 1

Study area and location of sampling points.

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Methods

This article identifies long-term trends using data from 2003 to 2023 on water chemistry and daily discharge of the Stara River, as well as daily meteorological data from the Scientific Station of the Jagiellonian University in Łazy, which is located within the studied catchment. However, for the analysis of the spatial variability of water chemistry along the course of the Stara River (14 sampling points – Fig. 1), data from 2022 to 2024 were used. The selection of sampling sites along the Stara River was based on hydrological and land-use criteria, with particular attention given to locations most likely to reflect the influence of anthropogenic pressure. Monitoring points were strategically situated near the confluences with major tributaries, as well as in the vicinity of known point sources of pollution, such as wastewater discharges, and within or adjacent to agricultural areas. The meteorological data included daily total precipitation (PP), average daily air temperature (AvgTA), minimum air temperature (MinTA), and maximum air temperature (MaxTA). Water chemistry samples were generally collected once per month, resulting in an average of 12 samples per year, with a higher frequency (twice per month) applied until 2008. The water chemistry data comprised parameters such as pH, electrical conductivity (EC), total suspended solids (TSS), water mineralization (TDS), and the concentrations of 11 ions. Major ions (Ca²⁺, Mg²⁺, Na⁺, K⁺, HCO3−, SO42−, Cl⁻) as well as nitrogen and phosphorus compounds (NH₄⁺, NO3⁻, NO2⁻, PO43−) were determined using a DIONEX 2000 ion chromatograph with an AS-4 autosampler. The method and accuracy of chemical analyses were carried out in accordance with the standards: PN-89 C-0463828, PN-EN ISO 10304−129, and PN-EN ISO 1491130. The detection limits (LOD) for all analyzed ionic species are summarized in Table 1.

Table 1 Limits of detection (LOD) for water chemistry parameters in the Stara River.
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Water mineralization was calculated as the sum of the determined ions, while the total inorganic nitrogen (TIN) concentration was calculated as the sum of NH4-N, NO3-N, and NO2-N concentrations. Total suspended solids (TSS) in water were determined using the gravimetric method.

To illustrate changes in land use, it has been decided that the data will be presented at intervals of five years (2005, 2010, 2015, 2020). Data for 2005 and 2020 was taken from available reliable resources: the calculations made as part of the KNB project from the years 2002–200431 and polish Database of Topographic Objects32. The remaining land-use data were derived from the supervised classification of Landsat 5 (2010) and Sentinel-2 (2015) satellite imagery performed with ArcGIS Pro 3.1.0. To assess interannual variability in atmospheric precipitation, a Relative Precipitation Index (RPI) analysis was carried out for the period 2003–2023.

In the study, the Mann-Kendall (MK) method33,34 and Sen’s slope estimator35 were used to analyze trends in water chemistry, as well as meteorological and hydrological data. Trend calculations were performed both on an annual scale and separately for seasons, as well as for the growing and non-growing seasons. The Mann-Kendall method is one of the most commonly used statistical methods for detecting trends in time series of hydrological, meteorological, and environmental data. It is a non-parametric test, making it particularly useful for analyzing highly variable data and studying long-term trends in water chemistry, temperature, precipitation, and river flows. Sen’s slope estimator, also known as Sen’s method, is a non-parametric technique used to determine the magnitude of trends in time series data. It is often applied alongside the MK test to quantify the rate of change over time in environmental data. In the study, Principal Component Analysis (PCA) and the Kaiser criterion were applied to identify the most important factors shaping the water chemistry of the Stara River. PCA is a widely used multivariate statistical technique that helps identify key factors influencing water chemistry by reducing the dimensionality of complex datasets. This method transforms a set of correlated variables into a smaller number of uncorrelated principal components (PC), which explains the maximum variance within the dataset. The Kaiser Criterion is a widely used method for selecting the most significant principal components in PCA. According to this criterion, only components with an eigenvalue greater than 1 should be retained for further analysis. For PCA, data on water chemistry, discharge, and land use in the Stara River catchment area were used. To examine the relationship between land use and the physicochemical parameters of the Stara River’s water, Pearson correlation analysis was used. The existence of statistically significant relationships was tested at a significance level of p = 0.05. The strength of correlation coefficients was interpreted according to the scale proposed by Overholser and Sowinski36. Calculations were performed using Statistica 13 software.

Results

Spatial variability of water chemistry along the course of the Stara river

Along the course of the Stara River, an overall increase in the values of most physicochemical parameters is observed (Table 2). Electrical conductivity (EC), total dissolved solids (TDS), and the concentrations of major ions gradually increase downstream, although the magnitude of this increase does not exceed 50% over the studied section. In contrast, nutrient compounds – particularly nitrogen and phosphorus species – exhibit a substantially higher rate of increase. Specifically, the concentrations of NO3-, NH4+, and PO43- rise approximately 3-fold, 9-fold, and 90-fold, respectively. These sharp increases in nutrient concentrations are primarily associated with wastewater inputs and are most pronounced at sampling locations situated downstream of lateral tributaries and drainage canals that receive stormwater and domestic sewage from urbanized areas, as well as in the vicinity of intensive agricultural holdings. Additionally, a separate point source of nutrient enrichment is linked to the discharge from a small wastewater treatment plant located near the river channel. This study identifies both point and diffuse sources of pollution and emphasizes their influence on the longitudinal variability of water chemistry in the Stara River. In contrast to the notable increase in nutrient concentrations, only minor changes in SO42- concentrations are observed along the river course. The structural composition of dissolved ions along the river shows a somewhat different pattern. While the relative contributions of Ca2+, Mg2+, HCO3-, and SO42- decrease downstream, the proportions of Na+, Cl⁻, and K+ increase significantly – by 74%, 116%, and 131%, respectively (Fig. 2). A similar trend is observed in the relative shares of nutrient compounds: the proportions of NO3-, NH4+, and PO43- increase approximately 2-fold, 6-fold, and 40-fold, respectively. Nevertheless, it is important to emphasize that the absolute concentrations of nitrogen and phosphorus compounds in the Stara River remain low throughout the study period.

Table 2 The mean values and coefficients of variation (CV) of physicochemical water parameters along the longitudinal profile of the Stara river in 2022–2024.
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Fig. 2
figure 2

Changes in the structure of chemical composition, concentrations of water mineralization (TDS), nitrogen compounds (NH4-N, NO3-N, NO2-N) and concentrations of inorganic nitrogen (TIN) along the longitudinal profile of the Stara River. Values represent average concentrations and mean percentage contributions of chemical components from the years 2022–2024.

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Land use changes in the Stara river catchment

In the Stara River catchment, agricultural areas predominate, consisting mainly of arable land, which accounts for approximately 31%, and meadows and pastures, which make up 17%. A relatively large share of the catchment is covered by forests, around 45%. Built-up areas constitute the smallest share (Table 3; Fig. 3). During the study period, the share of agricultural land decreased by approximately 10%, mainly due to a decline in arable land (by about 12%). Over the years, the area of meadows and pastures increased. Additionally, the shares of built-up areas and forests grew by 3.6% and 6.3%, respectively (Table 3).

Table 3 Land use changes in the Stara river catchment.
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Arable land is primarily concentrated in the central and north-eastern parts of the catchment, characterized by gentler slopes; however, its area has significantly declined during the study period. The greatest increase in forest cover is observed in the highest-lying parts of the catchment, particularly in its southern section, where processes of secondary succession and afforestation contribute to improved water retention and reduced erosion. In contrast, the most substantial expansion of built-up areas occurred in the north-central part of the catchment, associated with the growth of the town of Bochnia and the urbanization of its suburban zone (Fig. 3).

Fig. 3
figure 3

Land use changes in the Stara River catchment.

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Long-term trends in water chemistry of the Stara river

The waters of the Stara River have a slightly alkaline pH, with an average electrical conductivity of 479 µS·cm⁻¹ and an average TDS value of 364 mg·dm⁻³ (Table 4). In the chemical composition of the water, HCO3− ions dominate among anions, while Ca2+ ions prevail among cations. Among nitrogen compounds, NO3− ions reach the highest concentrations. During the study period, nitrogen and phosphorus compounds exhibited the highest variability in water chemistry (CV > 200%). For major ions, variability was low to moderate, as the CV of concentration values generally did not exceed 35%, except for K⁺ ions.

Table 4 Descriptive statistics and coefficient of variation (CV) for physicochemical parameters and water discharge at the gauging station on the Stara river in the period 2003–2023.
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The results from Table 5; Fig. 4A show the long-term trends in water chemistry, hydrological, and meteorological parameters for the Stara River on an annual scale, as well as seasonally and during the growing and non-growing seasons. The results indicate a significant increase in air temperature in the Stara River catchment, with the most pronounced increase observed in MaxTA, rising by nearly 1 °C per decade. However, annual precipitation totals remain unchanged, and there are no significant trends in the discharge of the Stara River. nevertheless, the variability of these parameters during the study period and between individual years is often substantial, as indicated by the Relative Precipitation Index (Fig. 4B) and the hydrograph of the Stara River discharge (Fig. 4C). Significant annual trends in physicochemical parameters include pH, Ca2+, Na+, HCO3−, SO42− and Cl−. Water pH and SO42− concentrations show a significant decline, while Ca2+, Na+, HCO3−, and Cl− concentrations are increasing (Fig. 5). The most substantial rise is observed for HCO3−, with an increase of nearly 24 mg·dm−3 per decade.

Fig. 4
figure 4

Climatic and hydrological variability in the Stara River catchment in the period 2003–2023. A – Trends in air temperature (AvgTA, MinTA, MaxTA) and precipitation (PP); B – Relative Precipitation Index (RPI); C – Hydrograph of the Stara River discharge.

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Table 5 Trend statistics for annual and seasonal water chemistry, hydrological and meteorological parameters.
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Summer exhibits the most significant changes in water chemistry, with increasing TDS, EC, dissolved ion concentrations (Ca2+, Na+, Cl⁻, HCO3−) and rising NO3− and TIN levels. The highest increase is observed for TDS – approximately 57 mg·dm⁻³ per decade. The least significant trends in water chemistry are observed in autumn (SO42− – decreasing trend and NO2− – increasing trend). In spring, significant increasing trends were found for TDS, Ca2+, Na+, HCO3−, and Cl−, whereas K+ concentrations in the Stara River decrease. In winter, an increase in the concentrations of Ca2+, Na+, HCO3−, and Cl− ions is also observed over the studied period. Additionally, an increase in NO2− concentrations and a decrease in SO42− were noted. Among the seasons, the greatest increasing trends in TDS and ion concentrations are observed in summer, with the exception of HCO3−, which shows the highest increase in spring. Spring stands out from other seasons due to the absence of significant air temperature trends. In the remaining seasons, a significant increasing trend is observed for AvgTA and MaxTA, while in winter, MinTA also shows a significant rising trend. Winter also exhibits the largest increases in air temperature values, with AvgTA rising by nearly 2 °C per decade. No significant trends are observed for PP and Q in any season. During the growing season, more significant trends in the water chemistry of the Stara River are observed compared to the non-growing season, but there are no significant trends in meteorological parameters. In contrast, during the non-growing season, air temperature, TDS, and the concentrations of Ca2+, Na+, HCO3−, and Cl− increase significantly.

Fig. 5
figure 5

Significant annual trends in physicochemical parameters.

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The relationships between the physicochemical parameters of water and land use for annual scale and for different seasons are summarized in Table 6. The strength of the relationships between catchment land use and river water chemistry is weak to moderate, with approximately half being statistically significant. The most negative correlations are observed between meadows and pastures and water chemistry. Only in the spring period do positive, though very weak (r < 0.2), correlations between water chemistry and meadows and pastures dominate.

On the other hand, the most positive correlations are found between water chemistry and forests. Only in spring do negative correlations prevail, and they are weaker. Positive correlations between biogenic compounds (N, P) and land use are observed in forested and urban areas, whereas agricultural land mostly shows negative correlations with the concentrations of these ions. It is noteworthy that SO42− is positively correlated only with arable land and total agricultural area. Most major ions, especially Ca2+, Mg2+, and HCO3−, as well as TDS, show a weak relationship with land use. The strength of this relationship slightly increases during winter and autumn.

Table 6 Pearson correlation between the physicochemical characteristics of water and land use in the Stara river catchment.
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In bold - statistically significant correlation coefficients (p = 0.05). Interpretation of Pearson correlation coefficients is based on the following scale: |r| < 0.2 – very weak or no correlation; 0.2 ≤ |r| < 0.4 – weak corelation; 0.4 ≤ |r| < 0.7 – moderate correlation; 0.7 ≤ |r| < 0.9 – strong correlation; |r| ≥ 0.9 – very strong correlation.

Factors influencing water chemistry of the Stara river

The principal component analysis (PCA) identified three main factors shaping the water chemistry of the Stara River, collectively explaining 73.6% of the variability. Factor 1 accounted for 37.1% of the variability, factor 2 for 23.6%, and factor 3 for 12.9% (Fig. 6; Table 7). In factor 1, a relationship was observed between discharge, total suspended solids concentration, major ion concentrations, electrical conductivity, and total dissolved solids in the Stara River. A pattern emerged where higher discharge corresponded to increased total suspended solids in river water but lower concentrations of major ions. Factor 2 highlighted the relationship between land use, Q, and the concentrations of NO3-, PO43-, and total inorganic nitrogen. The higher the share of urbanized and forested areas, the higher the values of biogenic compounds (NO3-, PO43-, TIN), while Q was lower and there was less agricultural land. Factor 3 revealed a correlation between the share of agricultural and urbanized areas and the concentrations of NH4+, NO2-, and PO43-. A higher share of agricultural land in the catchment was associated with a lower share of built-up areas and with increased concentrations of nitrogen (NH4+, NO2-) and phosphorus compounds in the river water.

Fig. 6
figure 6

Principal Component Analysis of water chemistry and land use: the projection of factor loadings for PC 1 × 2 and PC 1 × 3.

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Table 7 Principal component analysis (PCA) of water chemistry and land use: eigenvalues, explained variance and cumulative explained variance.
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Disscusion

As water travels downstream, its chemistry can change due to various factors, such as tributary inflows, groundwater interactions, and anthropogenic inputs. These changes may manifest as either smooth gradients or abrupt shifts, depending on specific conditions and influences. Nevertheless, the study by Hensley et al.37 also demonstrated that longitudinal profiles of different physicochemical water parameters exhibit distinct patterns, resulting from spatial and temporal variations in the factors and processes that shape them. Along the longitudinal profile of the Stara River, it is evident that nutrient compounds (N and P) are influenced not only by natural processes but also predominantly by anthropogenic factors, as indicated by abrupt, stepwise changes in both their concentrations and shares. Among the major ions, Na+ and Cl- are also subject to anthropogenic influences, as their concentrations and relative contributions to the chemical composition increase significantly, whereas the proportions of Ca2+, Mg2+, and HCO3- decrease.

The impact of land use changes within the catchment is also noticeable. In the upper course of the river, within the forested area, the increase in TDS and ion concentrations in the river water is relatively small. However, as land use transitions to agricultural and urban areas, these parameter increases become markedly more pronounced. Many studies show that human activities, such as agriculture, industrial discharges, and urbanization, contribute to the chemical composition of river water. These activities can introduce pollutants such as heavy metals, nutrients, and organic matter, which can degrade water quality38,39,40.

The analysis indicates that climate change is occurring in the Stara River catchment, evident in rising air temperatures, both annually and in specific seasons, including the non-growing season. However, there are no significant trends in total precipitation, either on an annual scale or in individual seasons. Despite increased evaporation from rising air temperatures, the lack of significant changes in annual or seasonal precipitation has resulted in no notable changes in river discharge. However, most studies suggest that climate change affects river chemistry primarily through discharge variations driven by changes in precipitation2. A decrease in river discharge, caused by reduced precipitation and rising air temperatures, increases the concentration of most ions in river water due to reduced dilution capacity2,41. Conversely, an increase in river discharge induced by climate change creates a dilution effect, reducing the concentration of chemical substances, particularly major ions42. Nevertheless, higher discharge can also lead to increased transport of suspended solids and higher concentrations of biogenic substances (N, P, C). This results from more intense and frequent surface runoff, particularly from agricultural areas or impervious urban surfaces43. Climate change accelerates also chemical weathering of rocks, which releases main ions into river systems. This process is driven by increased temperatures and changes in precipitation patterns, which enhance the dissolution of minerals. The study by Gong et al.44 found that between 1980 and 2020, the largest proportion of ions in river waters derived from rock weathering consisted of HCO3⁻ and Ca2⁺, and the rate of weathering has increased by 30%. Similarly, in the Stara River catchment, the highest annual increasing trend is observed for the concentration of these ions, which may originate from the thick loess deposits present in the area.

However, climate change associated solely with a significant increase in air temperature also influences the chemistry of surface waters. Rising air temperatures affect water temperature, which in turn alters biogeochemical balances in aquatic ecosystems. This impacts gas solubility, the intensity of biological processes, and the transport and retention of chemical components. For example, studies by Ducharne45 on the Seine River (France) showed that a warming climate leads to increased oxygen deficits in water and higher phytoplankton biomass. Additionally, climate change-induced temperature increases significantly affect nitrogen dynamics. Higher temperatures can enhance denitrification processes, which reduce nitrate concentrations in water. This was demonstrated in studies by Gervasio et al.46where an increase in average air temperature by approximately 3 °C over 30 years led to a 30% reduction in total nitrogen loads in the Po River. This resulted from boosted denitrification capacity of river sediments along the lowland reaches, particularly during the summer period.

Land use changes significantly impact river water chemistry, with various thresholds indicating when these impacts become important. For example, a minimum of 45% natural vegetation cover is required to maintain nitrogen concentrations within acceptable levels in river waters47. On the other hand, studies by Kim et al.48 indicate that not exceeding 10% of impermeable surfaces in a catchment is crucial to mitigate the degradation of water quality. Increased residential and urban land use strongly correlates with deteriorating water quality, particularly in terms of higher concentrations of pollutants such as nitrogen, phosphorus, and carbon compounds49,50,51. In the Stara River catchment, a deterioration in water quality is also observed as the built-up area expands. This is evident in the spatial variation of water chemistry along the river course, particularly through point-source wastewater discharges, which cause abrupt changes in nitrogen and phosphorus compounds. This impact is also reflected in the increasing trends of Na+ and Cl- ion concentrations. Rising river water salinity is a significant issue observed worldwide52. The increase in water salinity associated with urbanization is influenced by urban expansion, the increase in impervious surfaces, the application of road salts, and the impact of wastewater treatment plants53. Also agricultural activities contribute significantly to nutrient loading in rivers, particularly nitrates and phosphates, which can lead to eutrophication in surface waters54,55. The decline in arable land and the increase in meadows and pastures in the Stara River catchment have a positive effect on water quality, as evidenced by the mostly insignificant long-term trends for N and P compounds (Table 5). A decreasing trend in SO42- concentrations is also observed in the Stara River catchment, which results from the reduction in agricultural land use and the decrease in fertilizer application, as fertilizers are an important source of sulfur in river waters. The impact of agricultural land use changes on SO42- concentrations is most pronounced in winter, as confirmed by trends and correlation coefficients (Tables 5 and 6). Nevertheless, long-term observations also indicate that the concentrations of ions such as SO42- and NO3⁻ in river water have decreased in some regions due to reduced acidic deposition. The decline in acid deposition is also observed in the Carpathians; for example, the study by Minďaš et al.56 showed a downward trend in S-SO4 concentrations in precipitation at Chopok, with a rate of − 0.0567 mg·dm⁻³ per year.

Most research indicate that forest land acts as a natural water purifier. Catchment with higher forest cover usually have better water quality57,58. The observed increase in forested area in the Stara River catchment is an important factor positively influencing river water quality. As confirmed by the spatial variability analysis, the upper section of the Stara River is characterized by the lowest TDS and concentrations of most ions, including nutrients. Additionally, the presence of forests in the uppermost section of the river plays a crucial role in hydrological regulation, erosion control, water retention, and the improvement of surface water quality59. By stabilizing slopes in the highest parts of the Stara River catchment, forests reduce water erosion, thereby decreasing the amount of transported sediments. This is further supported by the lack of significant trends in suspended solids concentrations.

Principal Component Analysis (PCA) revealed that hydrometeorological conditions are the most important factors shaping the concentrations of major ions, TSS, and TDS in the waters of the Stara River. As discharge increases, the concentrations of major ions decrease due to the dilution effect. In contrast, during drought periods with low discharge, concentration effects occur, leading to an increase in major ion concentrations. The opposite relationship is observed for total suspended solids (TSS) – during flood events, TSS values increase, whereas during droughts, they decrease. However, factors 2 and 3 are associated with the impact of land use on nutrient concentrations in the river. Factor 2 indicates that the observed increase in urban and forested areas within the catchment leads to higher nitrogen (N) and phosphorus (P) concentrations in river waters. Studies confirm that the expansion of residential and urban land deteriorates water quality, partly due to the increased input of nitrogen and phosphorus compounds60. A similar trend is observed in the Stara River catchment, where the built-up area is expanding. Additionally, the impact of human activities on rising nutrient concentrations has been confirmed by the spatial variability analysis of water chemistry along the river course. However, most studies indicate that a larger forested area generally improves surface water quality61. Nevertheless, during the autumn-winter period, when trees shed leaves and nitrogen uptake by plants slows down, NO3⁻ runoff into rivers may increase62. These processes are particularly intensive after heavy rainfall and during spring snowmelt, when water leaches nitrogen accumulated in the soil during winter63. Indeed, higher correlation coefficients between forested areas and nitrogen compounds are observed in the Stara River waters during winter and autumn (Table 6). In Turn, Factor 3 highlights the impact of agriculture on water chemistry, primarily on the concentrations of nutrient compounds (NH4+, NO2−, and PO43−). The analysis confirmed the observed trend in the Stara River catchment, showing that as forest and built-up areas expand, the share of agricultural land decreases, leading to a reduction in nitrogen (N) and phosphorus (P) concentrations in river water.

Conclusion

Ongoing climate change, combined with changes in land use within the catchment area, has a significant impact on the chemical composition of river water. These changes affect both long-term trends in water quality and the spatial variability of the chemical composition within the catchment. The combined influence of these two factors leads to significant modifications in the chemical composition of the water, which is reflected both in the temporal scale (long-term changes in the concentrations of individual compounds) and the spatial scale (differences in water chemistry in various parts of the catchment). The rise in air temperature in the Stara River catchment, especially during the winter period, leads to the intensification of biogeochemical processes in the catchment and alters the weathering rate. This, in turn, results in an increase in mineralization and the concentration of major ions, particularly bicarbonates and calcium, in the river water. Changes in land use have a significant impact on the spatial variability of water chemistry within the catchment. The diversity of land use in the Stara River catchment, which includes agricultural, forested, and urbanized areas, leads to distinct differences in water quality in different parts of the catchment. Nevertheless, long-term trends in water chemistry also indicate the influence of land use changes, primarily through the increasing salinity of the Stara River water. The reduction in agricultural land in favor of forested areas and permanent grasslands has had a positive effect on the water quality of the Stara River, resulting in decreased concentrations of nitrogen, phosphorus, and sulfates. In contrast, the expansion of built-up areas and anthropogenically transformed land has had a negative impact, contributing to local deterioration in water quality, as indicated by increased electrical conductivity and elevated concentrations of chlorides and nutrient-related ions. Ongoing changes in the water chemistry of the Stara River are similarly influenced by both rising temperatures driven by global warming and land cover changes within the catchment.