Comparative Analysis of Water Quality in Major Rivers of Türkiye Using Hydrochemical and Pollution Indices

Abstract

This study provides a comprehensive analysis of the water quality in five major rivers in Türkiye: Sakarya, Yeşilırmak, Kızılırmak, Seyhan Rivers, and Niğde Creek. Utilizing hydrochemical diagrams and the River Pollution Index (RPI), we assess the temporal and spatial variations in water quality over the past three decades. The hydrochemical characteristics reveal a dominant calcium-bicarbonate (Ca-HCO₃) type water, indicating strong geological control primarily influenced by carbonate weathering. Seasonal variations and anthropogenic influences, particularly agricultural runoff and industrial discharge, contribute to significant changes in ion concentrations, especially in the Kızılırmak and Yeşilırmak Rivers. The RPI results classify these rivers as ‘Severely polluted’ to ‘Moderately polluted’, driven by high levels of suspended solids and biochemical oxygen demand. This study underscores the urgent need for tailored pollution control measures and sustainable water management practices in order to mitigate the impacts of anthropogenic activities and protect the ecological health of these vital water resources. The findings provide a robust framework for future research and policymaking to enhance water quality monitoring and management strategies in the region.

1. Introduction

Water is an essential resource for all living beings, and its quality plays a crucial role in determining its suitability for various purposes. Rapid urbanization and industrialization have had a detrimental impact on the water quality of rivers around the world. As a result, there is an urgent need to assess and monitor the water quality of different rivers to ensure the health and well-being of both humans and the ecosystem [1].

The water quality of river systems can vary geographically and seasonally, as indicated by research conducted on various rivers. According to the World Health Organization (WHO), more than 80% of diseases are caused by consuming contaminated water [2]. Further studies have shown that annual averages of water quality parameters in these river systems suggest poor water quality and pose a high ecological risk to aquatic life [3]. For example, in the urban areas, the concentrations of pollutants such as cadmium and lead were found to be significantly higher in river water samples than in non-urban areas [4]. Furthermore, the impact of anthropogenic activities, such as land use and pollutant load, on water quality is a driving factor in understanding the overall water quality of rivers. Seasonal variations also play a significant role in the water quality of rivers. The availability of water quality data for different rivers has allowed for trend analysis to be conducted, revealing both improvements and concerns. There are numerous studies on this topic, highlighting the importance of research in maintaining and improving water quality. These studies conducted in various countries and river systems contribute to a better understanding of the factors affecting water quality and the measures that can be taken to mitigate these issues [5,6,7,8,9,10,11,12,13,14,15].

To assess and compare the water quality of different rivers, various diagrams such as the Piper [16,17], Schoeller [16,17,18], and Gibbs diagrams [19,20,21,22,23,24] have been widely used in numerous studies [16,17,18,19,20,21,22,23,24,25]. These diagrams help in visualizing the chemical characteristics of water samples and in identifying the processes influencing water quality. The Piper diagram, also known as the Piper trilinear diagram, is a widely used tool for analyzing water chemistry data. It provides a graphical representation of the chemical composition of water, including the proportions of major ions such as bicarbonate, sulfate, chloride, and more. By using the Piper diagram, the relative dominance of different cations and anions in river water samples can be compared, offering insights into the overall water quality and the potential sources of contamination [19,20,21,22,23,24].

Taiwan Environmental Protection Administration (TEPA) has established a River Pollution Index (RPI) categorization system for river water quality evaluation based on water consumption and stream section protection [26]. The River Pollution Index (RPI) is a useful tool for assessing and quantifying the pollution level of rivers [27]. This index facilitates the comparison of pollution levels among different rivers, aiding in the prioritization of pollution control and remediation efforts [26,27]. Lai, Tu, Yang, Surampalli, and Kao [27] focused on the Kaoping River Basin, the largest watershed in Taiwan. The researchers developed a water quality modeling system that integrates the RPI calculation with the Water Quality Analysis Simulation Program (WASP). In addition to the study by Lai, Tu, Yang, Surampalli, and Kao [27] that developed an integrated modeling system for river water quality evaluation using the RPI and suspended solid loading, several other studies have applied the RPI to assess water quality in various rivers around the world [28,29,30,31,32].

The utilization of water quality diagrams, along with the assessment of river water usability for irrigation and the incorporation of the RPI, enables a comprehensive understanding of the water quality of different rivers and aids in the development of strategies for effective water resource management and pollution control measures.

This study focuses on the water quality modeling of the major rivers in various basins across Türkiye. Specifically, it examines the water quality parameters over the past 30 years, including BOD5, Ca²⁺, Cl⁻, COD, DO, EC, Fe, K⁺, Mg²⁺, Mn, Na⁺, NH₄-N, NO₂-N, NO₃-N, o-PO₄, pH, SO₄²⁻, SS, TDS, TH, SO₄²⁻, CO₃²⁻, HCO₃⁻, and pH, SS, TDS, and TH with measurements and analyses conducted at regular intervals in laboratories. The rivers under study include the Sakarya, Yeşilırmak, Kızılırmak, and Seyhan Rivers and Niğde Creek. The analysis results are categorized into three sections: annual, dry season, and wet season, with models developed for each season. The study utilizes the diagrams of Piper [16], Schoeller [18], and Gibbs [17] to determine the hydrochemical characteristics. Additionally, the diagrams of USSL [16] and Wilcox [18] are employed to assess the suitability of river water for irrigation, while pollution potentials are determined using the RPI to establish pollution indices.

2. Materials and Methods

2.1. Study Area

The study area includes five major rivers in Türkiye: the Sakarya, Yeşilırmak, Kızılırmak, and Seyhan Rivers and Niğde Creek. These rivers were selected due to their significant roles in regional water resources, agriculture, and ecological balance. Sakarya River, originating in the northwestern part of Central Anatolia, is one of Türkiye’s longest rivers, stretching over 824 km. It flows into the Black Sea and has a catchment area of approximately 58,000 square kilometers. The river basin experiences a continental climate with hot summers and cold winters, with annual precipitation varying widely across the basin. Sakarya River is vital for agriculture, industry, and domestic water supply in the region [33,34]. Yeşilırmak River, which also originates in Central Anatolia, runs for about 418 km before discharging into the Black Sea. Its catchment area covers around 36,100 square kilometers. The river basin supports diverse agricultural activities and is characterized by a temperate climate with significant seasonal variations in precipitation. Yeşilırmak River is an essential water source for irrigation and hydroelectric power generation [35].

Kızılırmak River is the longest river entirely within Türkiye, with a length of 1355 km and a drainage basin covering approximately 78,000 square kilometers. It originates in the eastern part of Central Anatolia and flows into the Black Sea. The river basin has a varied climate, ranging from semi-arid in the upper reaches to temperate in the lower regions. Kızılırmak River is crucial for agriculture, drinking water, and industrial uses [36]. Niğde Creek, a smaller but significant stream in Central Anatolia, flows through the Niğde province. The creek’s catchment area is characterized by its semi-arid climate, with hot, dry summers and cold, wet winters. It supports local agriculture and small-scale irrigation projects. It is a key component of the regional water management strategy, although it faces challenges from seasonal variations in flow and quality [37]. Seyhan River, located in southern Türkiye, flows from the Taurus Mountains through the Çukurova Plain and into the Mediterranean Sea. It spans approximately 560 km and has a drainage area of about 21,700 square kilometers. The basin experiences a Mediterranean climate with hot, dry summers and mild, wet winters. Seyhan River is vital for irrigation, drinking water, and hydroelectric power, making it a central component of the region’s agricultural and economic activities [38,39].

These rivers were chosen for their diverse hydrological and climatic conditions, which provide a comprehensive overview of the water quality issues and management practices in Türkiye. The study area for these rivers is shown in Figure 1. The locations of the sampling sites along the riverine network are marked in Figure 1 with red dots, providing clear identification of the specific sampling locations along each river. The study analyzed water quality parameters over the past 30 years, which were obtained from the State Hydraulic Works (DSİ). These data encompass a range of water quality parameters: BOD5, Ca²⁺, Cl⁻, COD, DO, EC, Fe, K⁺, Mg²⁺, Mn, Na⁺, NH₄-N, NO₂-N, NO₃-N, o-PO₄, pH, SO₄²⁻, SS, TDS, TH, SO₄²⁻, CO₃²⁻, HCO₃⁻, pH, SS, TDS, and TH, measured regularly and analyzed in laboratories to ensure accuracy and reliability. The key hydrological and environmental characteristics of the major rivers, including discharge, length, catchment area, rainfall, temperature, and land use, are summarized in

Table 1. Key hydrological and environmental characteristics of major rivers in Türkiye.

Streams	Discharge (m3/s)	Length (km)	Catchment Area (km2)	Rainfall (mm/Year)	Temperature (°C)	Land Use (%)
Sakarya	190	824	58,000	500–900	10–15	A: 45; U: 20; F: 35
Yeşilırmak	110	418	36,100	600–1200	12–16	A: 50; U: 25; F: 25
Kızılırmak	150	1355	78,000	400–800	8–14	A: 55; U: 15; F: 30
Seyhan	211	560	21,700	600–1000	15–20	A: 60; U: 20; F: 20
Niğde	50	60	1500	300–500	10–15	A: 70; U: 10; F: 20

Note: Agricultural: A; Urban: U; Forest: F.

. This table highlights the diverse climatic and hydrological conditions across the studied rivers, which are critical for understanding the variations in water quality and the impacts of anthropogenic activities. As seen in Table 1, these rivers exhibit significant differences in their physical and environmental parameters, reflecting the varied regional conditions that influence their hydrochemical characteristics and pollution levels (DSİ).

The analysis results were categorized into annual, dry season, and wet season classifications, with models developed for each season to understand the seasonal variations in water quality. Additionally, various diagrams (Piper, Schoeller, Gibbs) were used to determine the hydrochemical characteristics, while the USSL and Wilcox diagrams assessed the suitability of river water for irrigation. The RPI was used to evaluate pollution levels and potential contamination risks in these rivers.

Water samples were collected in the mid-channel and near shore at fixed intervals: y during the dry and wet seasons over 30 years. As mentioned above, the present study analyzed water quality parameters such as BOD5, Ca²⁺, Cl⁻, COD, DO, EC, Fe, K⁺, Mg²⁺, Mn, Na⁺, NH₄-N, NO₂-N, NO₃-N, o-PO₄, pH, SO₄²⁻, SS, TDS, TH, SO₄²⁻, CO₃²⁻, HCO₃⁻, pH, SS, TDS, and TH, measured regularly and analyzed in laboratories to ensure accuracy, and they were measured by the State Hydraulic Works (DSI). Moreover, different analyses such as standardized methods, including titration for Ca²⁺ and Mg²⁺, spectrophotometry for NO₃-N and PO₄³⁻, and ion chromatography for Cl⁻ and SO₄²⁻, were applied. For each river, 12 samples per year were collected, resulting in a total of 360 samples over the 30-year period. This sampling frequency was consistent across all five rivers to ensure a robust analysis of temporal and spatial water quality variations.

2.2. Water Quality Diagrams

To analyze and compare the water quality of different rivers, several hydrochemical diagrams were utilized. These diagrams help visualize the chemical composition of water samples and understand the processes influencing water chemistry. The Piper diagram is a trilinear plot that categorizes water samples based on their hydrochemical facies. It integrates two ternary plots for cations (Ca²⁺, Mg²⁺, Na⁺ + K⁺) and anions (Cl⁻, SO₄²⁻, CO₃²⁻ + HCO₃⁻), along with a central diamond plot to provide a comprehensive view of the ionic composition [40]. The Schoeller diagram presents a logarithmic comparison of major ion concentrations, making it useful for comparing the chemical composition of water samples from various locations or depths [41].

The Gibbs diagram helps identify the dominant processes controlling water chemistry, such as precipitation, rock–water interaction, or evaporation, based on the ratios of Na⁺/(Na⁺ + Ca²⁺) and Cl⁻/(Cl⁻ + HCO₃⁻) versus TDS [42]. These diagrams were instrumental in assessing the hydrochemical characteristics and water quality variations in these streams over the past 30 years.

2.3. River Pollution Index (RPI)

The RPI is a comprehensive tool used for assessing and quantifying the pollution levels in rivers. It incorporates several key water quality parameters, including dissolved oxygen (DO), biochemical oxygen demand (BOD), suspended solids (SS), and ammonia nitrogen (NH₃–N). Each parameter is assigned a score that reflects the water quality, ranging from non-polluted to grossly polluted. We have reviewed and corrected the notation for all ionic charges and their respective mass indices. For example: Calcium (Ca²⁺), Bicarbonate (HCO₃⁻), Sulfate (SO₄²⁻), and Sodium (Na⁺). All these corrections have been made to ensure clarity and accuracy in the chemical representations throughout the manuscript. The S_i scores mentioned were calculated using the following method:

$$S_i={\displaystyle \frac{Measured value}{Threshold value}}$$

(1)

This formula is applied to each water quality parameter individually, and the results are averaged to obtain the overall River Pollution Index (RPI) score. This method allows for a standardized comparison of pollution levels across different rivers and time periods. The overall RPI is calculated by averaging these scores, providing a single value that represents the river’s pollution level. This index is particularly useful for comparing pollution levels among different rivers and identifying areas that require pollution control and remediation efforts [26,27].

The RPI is calculated using the following formula:

$$\mathrm{R}\mathrm{P}\mathrm{I}={\displaystyle \frac{1}{4}}\sum _{i=1}^4{S}_i$$

(2)

where S_i represents the individual scores for each water quality parameter (DO, BOD5, SS, NH₃–N). The scores for each parameter are categorized as follows.

Table 2. RPI standards for a single parameter (p.; polluted) based on TEPA [26], WHO [43], EPA [44], and WFD [45].

Parameters	Units	Non/Mildly p.	Lightly p.	Moderately p.	Severely p.
DO	mg/L	DO ≥ 6.5	6.5 > DO ≥ 4.6	4.5 ≥ DO ≥ 2.0	DO < 2.0
BOD5	mg/L	BOD5 ≤ 3.0	3.0 < BOD5 ≤ 4.9	5.0 ≤ BOD5 ≤ 15.0	BOD5 > 15.0
SS	mg/L	SS ≤ 20.0	20.0 < SS ≤ 49.9	50.0 ≤ SS ≤ 100.0	SS > 100.0
NH3–N	mg/L	NH3–N ≤ 0.50	0.50 < NH3–N ≤ 0.99	1.0 ≤ NH3–N ≤ 3.0	NH3–N > 3.0
Index Score	-	1	3	6	10

shows the criteria for the rivers of Türkiye adapted from established guidelines from the World Health Organization (WHO) [43], the United States Environmental Protection Agency (EPA) [44], and the European Union Water Framework Directive (WFD) [45].

The sub-index scores for each parameter are summed and then averaged to determine the RPI. The resulting S value is then used to classify the overall water quality: Non/mildly polluted; S ≤ 2.0, Lightly polluted; 2.0 < S ≤ 3.0, Moderately polluted; 3.1 ≤ S ≤ 6.0, Severely polluted; S > 6.0.

3. Results

Piper plots were scattered to display individual data points for each of the 360 samples collected over the 30-year period for each river, and they are not mean values of the mentioned rivers. This approach was chosen to capture the full range of hydrochemical variability. The seasonal variations are reflected in the spread of the data points rather than being averaged out. Piper diagrams for the Sakarya, Yeşilırmak, Kızılırmak, and Seyhan Rivers and Niğde Creek reveal consistent hydrochemical characteristics across different seasons. The calcium-bicarbonate facies of the Sakarya and Seyhan Rivers indicate strong geological control due to significant carbonate weathering, which is consistent across annual, dry, and wet seasons. Yeşilırmak River’s mixed magnesium-bicarbonate composition suggests a combination of natural and anthropogenic influences, varying slightly with seasonal changes. Kızılırmak River’s sodium-chloride and sodium-sulfate facies reflect salinity issues, which are exacerbated during the dry season due to reduced flow and evaporation and persist during the wet season due to higher discharge. Niğde Creek’s mixed facies indicate diverse geological influences with no single dominant process, consistent across all seasons (Figure 2).

The Schoeller diagrams for the streams reveal consistent hydrochemical characteristics across dry, wet, and annual seasons. Each river exhibits a dominant presence of bicarbonate and carbonate ions, indicating significant weathering of carbonate rocks, which is typical for regions with abundant limestone and dolomite. In all five streams, the low and stable concentrations of sodium, potassium, sulfate, chloride, nitrate, phosphate, aluminum, manganese, and iron suggest minimal pollution from anthropogenic sources such as agriculture, industry, or urban runoff. However, Kızılırmak River shows relatively higher and stable concentrations of sodium, chloride, and sulfate, suggesting the influence of both natural geological processes and anthropogenic activities, such as industrial discharge and agricultural runoff (Figure 3).

The Gibbs diagrams were selected to show the separate application of Gibbs diagrams for each river and present a deliberate choice to better understand the distinct hydrochemical processes governing each river system. While this might appear as an overuse, it is essential for accurately capturing the geological and climatic influences specific to each river basin. The detailed analysis provided by these diagrams helps in understanding the complex interactions that affect water quality. The Gibbs diagrams for the streams collectively reveal that rock–water interaction is the dominant hydrochemical process influencing the water chemistry across all seasons. This consistency suggests that the geological formations in these basins, which include abundant limestone and dolomite, play a significant role in determining the hydrochemical characteristics of the rivers. The clustering of data points within the rock dominance field in the annual, dry, and wet season diagrams underscores the importance of mineral dissolution from these geological sources (Figure 4, Figure 5, Figure 6, Figure 7 and Figure 8).

However, there are some notable variations. Kızılırmak River shows a dual influence of rock–water interaction and evaporation, particularly during the dry season. This is evident from the data points extending into the evaporation dominance field, indicating increased salinity issues. These issues are likely exacerbated by reduced water flow and higher evaporation rates, as well as potential anthropogenic activities such as agriculture and industrial discharge (Figure 6).

For the other rivers—Sakarya, Yeşilırmak, Niğde, and Seyhan—the diagrams consistently show rock–water interaction as the primary controlling process, with minimal influence from evaporation or precipitation dominance. This uniformity across different seasons highlights the strong geological control over the hydrochemistry of these rivers, with seasonal variations having a relatively minor impact compared to the dominant geological processes (Figure 7 and Figure 8).

According to the USSL diagram across all five streams, a common theme is the high salinity hazard (C3), with most rivers falling within the low sodium hazard (S1) category. However, Kızılırmak River stands out, with some data points indicating higher sodium hazards (S3) and salinity levels (C4), suggesting a more complex hydrochemical environment possibly influenced by both natural and anthropogenic factors (Figure 9).

The overall suitability of these rivers for irrigation is contingent upon effective management practices addressing the high salinity levels. For Kızılırmak River, additional measures for mitigating sodicity are also required. This comparative analysis highlights the importance of continuous monitoring and adaptive management to ensure the sustainable agricultural use of these river waters.

The Sakarya, Yeşilırmak, and Seyhan Rivers and Niğde Creek exhibit similar hydrochemical characteristics, with data points falling within the ‘Excellent’ and ‘Good’ categories. As seen from Figure 10 via the Wilcox diagram, the low %Na and moderate EC values suggest that the water quality is generally suitable for irrigation, with minimal management required to mitigate salinity and sodicity risks.

Kızılırmak River shows a distinct pattern with higher %Na and EC values, leading to data points in the “Permissible” and “Doubtful” categories. The higher salinity and sodicity risks necessitate careful management to ensure sustainable agricultural use (Figure 10).

The correlation heatmaps for the streams reveal geochemical reactions in water quality parameters. Across all rivers, strong positive correlations are observed between Total Dissolved Solids (TDS), Electrical Conductivity (EC), and major ions such as calcium (Ca²⁺), magnesium (Mg²⁺), sodium (Na⁺), potassium (K⁺), bicarbonate (HCO₃⁻), sulfate (SO₄²⁻), and chloride (Cl⁻). These correlations suggest that the ionic composition of the river waters is primarily influenced by similar geochemical processes, such as mineral dissolution and rock–water interactions, which are consistent across different river systems (Figure 11, Figure 12 and Figure 13).

TDS and EC; both parameters show strong positive correlations with most major ions, indicating that higher ion concentrations contribute significantly to the overall salinity and conductivity of the river waters.

Ca²⁺ and HCO₃⁻: A significant positive correlation between these ions suggests that carbonate rock dissolution is a major contributor to the river chemistry. SO₄²⁻ and Cl⁻: These ions also display strong positive correlations with TDS and EC, reflecting their contribution to the total ionic content of the waters.

For Kızılırmak River, Cl⁻ shows a particularly strong positive correlation with Na⁺ and EC, which may indicate additional sources of chloride, such as agricultural runoff or industrial discharges, contributing to the ionic composition. Seyhan River: DO displays a negative correlation with BOD5, suggesting higher organic pollution levels affecting the oxygen dynamics in this river. This pattern is less pronounced in the other rivers.

4. Discussion

The hydrochemical analysis of the streams reveals significant insights into the water quality and the controlling processes. The consistent presence of calcium-bicarbonate (Ca-HCO₃)-type water across most rivers suggests that carbonate weathering is a dominant process, typical in regions with abundant limestone and dolomite formations. This geological control is further supported by the stable concentrations of major ions such as calcium (Ca²⁺) and bicarbonate (HCO₃⁻) across different seasons, indicating minimal seasonal variation.

The Schoeller diagram is a valuable tool for evaluating the hydrochemical facies and the origin of water samples. This diagram aids in differentiating water types based on their chemical composition, allowing for comparisons between rivers and the identification of potential pollution sources [41]. For instance, the Schoeller diagrams in our study showed a dominant presence of bicarbonate and carbonate ions across the Sakarya, Yeşilırmak, Kızılırmak, and Seyhan Rivers and Niğde Creek, indicating the substantial weathering of carbonate rocks. However, the Kızılırmak River exhibited higher concentrations of sodium, chloride, and sulfate, suggesting that both natural geological processes and anthropogenic activities like industrial discharge and agricultural runoff are influencing its water quality.

The Gibbs diagram further assists in assessing the geochemical evolution of water and identifying the controlling processes that govern the chemical composition of river water [42]. Our findings showed that rock–water interaction is the dominant hydrochemical process across all seasons for most rivers, consistent with the presence of abundant limestone and dolomite formations. However, Kızılırmak River exhibited dual influences of rock–water interaction and evaporation, particularly during the dry season, leading to increased salinity levels.

These diagrams collectively underscore the geological processes dominating the water chemistry across different river systems, aligning with findings from previous studies. For example, studies utilizing the Schoeller and Piper diagrams have similarly identified the significant role of geological processes and anthropogenic activities in influencing river water quality [19,23]. Furthermore, our use of the Gibbs diagram parallels the findings of previous research, highlighting the critical influence of rock–water interactions in determining water chemistry [46].

Anthropogenic impacts are more pronounced in certain rivers. Kızılırmak River, for instance, exhibits higher salinity and sodium hazards compared to the other rivers. This is evident from the USSL diagram, which places some data points of Kızılırmak River in the higher salinity (C4) and sodium hazard (S3) categories. Such conditions necessitate careful management to mitigate salinity and sodicity risks for sustainable agricultural use. Similarly, Yeşilırmak River shows signs of anthropogenic influence with increased sodium and chloride concentrations during the dry season.

The RPI results indicate that all five rivers are frequently classified as “Severely polluted,” with occasional periods of “Moderately polluted” status. This severe pollution is primarily driven by high levels of SS and BOD5, reflecting significant particulate matter from soil erosion, urban runoff, and agricultural activities. Niğde Creek shows a higher frequency of severe pollution in recent years, driven by elevated NH₃-N and SS levels, whereas Kızılırmak River shows variability in pollution status with temporary improvements in water quality. This finding is consistent with the study, which focused on the Kaoping River Basin, Taiwan’s largest watershed [27]. This integration allowed for real-time water quality assessment and better management strategies for the Kaoping River. Several other studies have similarly applied the RPI to assess water quality in various rivers worldwide, emphasizing the utility of the RPI in evaluating and managing river pollution levels [28,29,30,31,32].

Comparisons with previous studies provide additional context for these findings. For instance, a study on the Ravi River in Punjab, Pakistan, found it highly polluted due to residential and industrial wastewater being directly discharged into the river through drainage systems [47]. This highlights the impact of anthropogenic activities on water quality, which is similarly observed in the Kızılırmak River’s increased salinity due to industrial discharge and agricultural runoff. By comparing our results with prior studies, we can confirm the significant geological control over water quality in these rivers while also recognizing the considerable impact of human activities, particularly in the Kızılırmak River. These insights are crucial for developing effective water management strategies tailored to each river’s unique characteristics and seasonal variations.

In comparison to neighboring countries and other regions in Europe, Turkish rivers exhibit higher levels of pollution due to less stringent wastewater treatment and agricultural practices. For example, studies in Greece have shown lower levels of NH₃-N and SS due to better-managed agricultural runoff [48]. Similarly, a study on the rivers Topolnitsa and Luda Yana in Bulgaria highlights that better wastewater treatment and stricter agricultural practices contribute to lower pollution levels in Bulgarian rivers [49]. In Italy, recent efforts in wastewater treatment and agricultural runoff management have significantly improved water quality in major rivers, reducing nitrate and phosphate pollution [50].

These findings highlight the urgent need for comprehensive pollution control measures across all rivers. Continuous monitoring, improved wastewater treatment, and sustainable agricultural practices are essential to mitigate pollution and protect the ecological health of these vital water resources. The unique pollution patterns of each river suggest that tailored interventions are necessary to address specific sources of pollution effectively. For instance, Kızılırmak River requires strategies for managing salinity and sodium hazards, while Niğde Creek needs focused efforts to reduce NH₃-N and SS levels.

5. Conclusions

This comprehensive study on the water quality of the Sakarya, Yeşilırmak, Kızılırmak, and Seyhan Rivers and Niğde Creek in Türkiye underscores the critical interplay between natural geological processes and anthropogenic activities. The dominant calcium-bicarbonate (Ca-HCO₃) water type across these rivers, as revealed by hydrochemical diagrams, indicates significant geological control due to carbonate weathering. However, seasonal variations and human influences, particularly agricultural runoff and industrial discharges, notably impact water quality, especially in the Kızılırmak and Yeşilırmak Rivers.

The findings demonstrate that while geological processes predominantly govern the hydrochemical characteristics, human activities can also exacerbate pollution levels. The RPI results classify these rivers from ‘Severely polluted’ to ‘Moderately polluted’, primarily driven by high levels of suspended solids and biochemical oxygen demand. These results highlight the need for effective pollution control measures and sustainable water management practices tailored to the specific conditions of each river.

Comparative analysis with rivers in neighboring countries such as Greece, Bulgaria, and Italy reveal that Turkish rivers suffer from higher pollution levels due to less stringent wastewater treatment and agricultural practices. Implementing advanced wastewater treatment protocols and sustainable agricultural practices, as demonstrated by these countries, could significantly improve water quality in Turkish rivers.

The findings show that the water quality of rivers has importance for the health, and it requires continuous monitoring and adaptive management strategies to mitigate the impacts of anthropogenic activities and protect the ecological health of these vital water resources to analyze the results. Future research should focus on enhancing water quality monitoring precision and addressing emerging contaminants and climate change impacts to ensure the sustainability of these river systems.

In conclusion, this study provides an application to understand the factors influencing river water quality in Türkiye and offers valuable guidelines for developing targeted interventions and policymaking to enhance water quality and management practices in the region. Moreover, water quality trends can be analyzed and studies can be expanded via different water quality indexes.