Determination of pollution status of Nilüfer river by water and bottom sediment analysis

Water quality is one of the major environmental determinants that affect the ecosystem, agricultural production and socio-economic development of a country (Kundu & Ara 2019). Monitoring and evaluation of water quality is a very important sustainability issue for surface waters, especially rivers, which are vital for humans (Boyd 2015; Wu et al. 2018).

Rivers' water is widely used for irrigation in the regions where they flow basins. The greatest concern in agricultural production is the presence of excess salts in water and soil, which degrades water and soil quality and consequently reduces crop yields (Tsado et al. 2014). Main anions and cations such as CO⁼₃, HCO⁻₃, CI⁻, SO⁼₄, B, Na⁺, Ca²⁺ and Mg²⁺ are significant ions that in high concentrations can alter the suitability of irrigation water for use (Choudhary et al. 2007). At low amounts, some cations are beneficial for crops while at high amounts they can reduce the irrigation water quality and soil which exerts toxic effects to plant.

Hence, irrigation water quality criteria developed by the US salinity laboratory have been followed in many countries to evaluate the usability of water in agricultural production (Richards 1954).

The rivers become polluted from rapid population growth, urbanization and industrial activities. In recent years, the polluted and chemically degraded river also poses a significant threat to its ecosystem (Su et al. 2011; Srebotnjak et al. 2012; Islam et al. 2014). Industrial wastewater discharge into water bodies is one of the constant anthropogenic polluting sources. As a result, water resources are polluted in terms of heavy metals. Also, the heavy metal content of irrigation water affects the soil ecosystem and plant growth. The effect of heavy metals on the food cycle is known to everyone (Wantasen et al. 2019).

Pollution of river bottom sediments with heavy metals (HMs) has emerged as a main environmental issue related to intensive anthropopressure on the water environment (Jaskuła et al. 2021). Sediment is an integral and dynamic component of aquatic environments and streams. Bottom sediments are important indicators of pollution status of rivers. Sediments serve as a reservoir for heavy metals, chemical and organic compounds. Heavy metals and other compounds absorbed and transported by sediments can significantly affect the properties and quality of the environment. In particular, it is important to assess the total amounts and availability of heavy metals are indicators of potential pollution in the river ecosystem (Kabala & Singh 2001; Pueyo et al. 2003; Caeiro et al. 2005).

Environmental and health problems have been observed due to pollution occurring in rivers used for irrigation purposes (Malvandi 2017; Ustaoğlu & Tepe 2019; Jie et al. 2021). The lack of irrigation water resources of sustainable quality due to population growth and water quality degradation is becoming a major challenge for agricultural production (Winpenny et al. 2010).

The Nilufer river is of strategic importance to the people of Bursa in Turkey. From its source near Mount Uludağ and flowing past Bursa, the river tends to the northwest along its course of 203 km. It is also used for irrigation along its course. As it passes through the city, different types of wastewater from domestic and industrial activities are discharged into the Nilüfer river. Therefore, it is known that the water quality deteriorates. However, there is a lack of information regarding the relationship between the status of metal pollution in river sediment and the content of metal in water. The Nilüfer river basin contributes a significant amount of agricultural production to the national food system, including various vegetables, tomatoes, onions, wheat and maize.

We think that the Nilufer river has been polluted significantly during the 30–40 year industrialization and urbanization period. It is necessary to pay attention to the environmental and health threats that may be caused by its use in irrigation in the Nilüfer river, which has polluted water and bottom sediment. This study was an attempt to assess the status of irrigation water quality and bottom sediment pollution from Nilüfer river in Bursa city, with a view to determining its ability to suitability for use in vegetable crop irrigation. The total and DTPA-extractable heavy metal contents in sediment samples were determined. This study can be considered the first attempt to evaluate the heavy metals pollution in sediments in Nilüfer river sediment by using sediment quality guidelines (SQGs).

The main objectives of this study were (1) to evaluate the Nilüfer river in terms of irrigation water quality parameters and (2) to estimate toxicity for bottom sediments collected in the Nilüfer river in terms of the consensus-based SQGs.

Nilufer river flows through a valley on the southern slopes of Uludag mountain and enters the Bursa plain from the west side of the Bursa city. It passes the Bursa plain from south to north in a stream bed and combines with the Susurluk stream and finally flows into the Marmara Sea. During this flow, domestic (urban and rural) and industrial wastewater is discharged into the stream in some regions. Kestel textile industrial zone in the western parts of the city, illegal factories in the eastern and western industrial zones can be given as examples. As a result of these discharges, the water quality of the stream deteriorates day by day. In terms of agriculture, the crucial point here is that the Nilüfer stream is generally used for irrigation, especially in sites outside the city center.

River bottom sediment and water samples were collected from six sampling sites along the Nilüfer river as shown in Figure 1. Water and sediment samples were collected during non-irrigation (November, January and March: first sampling) and irrigation season (May, July and September: second sampling) for the 2015 year.

Water samples were collected in 1 L PE (polyethylene) bottles and brought to the research laboratory for analysis. Water samples were filtered through Whatman 42 filter paper before analysis. Sediment samples were collected in polyethylene drums using a plastic scoop. The samples were used in the analysis after drying in an oven at 65 °C. Samples were sieved 2 mm before metal analysis. In the study, a total of 110 samples of 54 water and 46 sediments were collected.

The filtered water samples were immediately acidified (pH 2.0) with HNO₃ and refrigerated at 4 °C till their use for heavy metal analysis. pH and EC values were determined in situ. Water samples were analyzed for irrigation water classes such as anions (CO⁼₃, HCO⁻₃, CI⁻, SO⁼₄), cations (Na⁺, K⁺, Ca²⁺, Mg²⁺), the concentration of N-NH₄-N, NO₃-N, P and B according to Estefan et al. (2013).

Heavy metals in water and sediment samples were determined after microwave-assisted digestion with HNO₃. Also, bioavailable heavy metals were extracted using 0.05 M DTPA, 0.01 M CaCl₂ and 0.1 M TEA buffered solution at a pH of 7.3. Four grams of sediment sample was mixed with 40 mL of extraction solution in 100 mL. After 2 h of agitation, the filtrate was obtained. The metals in the filtrates were determined by the inductively coupled plasma optical emission spectroscopy method.

Determination of the total heavy metal content in the river bottom sediment gives information about the contamination status. However, in order to determine the ecological risk, it is necessary to determine the content of available heavy metals (Sundaray et al. 2011). In this study, total and DTPA-extractable heavy metals were determined in sediment samples. Different SQGs have been published for aquatic environments using a variety of approaches (Muller 1979; Tomlinson et al. 1980; Ahdy & Khaled 2009; Islam et al. 2015). In the study, heavy metal analysis results in sediment samples were compared with TEC (below which harmful effects are unlikely to be observed) and PEC (above which harmful effects are likely to be observed) values (MacDonald et al. 2000; Table 2).

A sampling of the Nilüfer river and its tributaries was carried out in six different sampling sites for one year (2015–2016). The pH value, electrical conductivity (EC), anions (CO⁼₃, HCO⁻₃, CI⁻, SO⁼₄), cations (Na⁺, K⁺, Ca²⁺, Mg²⁺), the concentration of NH₄-N, NO₃-N, P and B ion analyzes were done to determine the quality of irrigation water. Analysis results of water samples are given in Table 3.

The pH values of the water samples in the sampling sites were determined between 6.21 and 8.18 depending on the sampling time. pH values are between acceptable limit values for irrigation water specified by FAO. The toxicity of many pollutants increases with changes in the pH of the waters (Bouslah et al. 2017). The pH value in water is an indicator of anions and cations. Clogging of drippers in irrigation systems is related to the pH and calcium carbonate value of the water used (Kadyampakeni et al. 2018).

The EC value of the water samples ranged from the lowest value of 295 μS cm⁻¹ at 1st sample site (S1) to the highest, of 1,204 μS cm⁻¹ in 4th sample site (S4). The EC value of the water samples in the S4, S5 and S6 sample sites were higher than the FAO permissible limits of higher than 1.000 μS cm⁻¹ and therefore pose threat to crops, especially during the irrigation time in summer. The EC value is a good indicator of the total dissolved salts in irrigation water. This may be related to the decrease in the amount of river water or the increase in the discharged water during the summer period.

The content of bicarbonate in water samples varied between 0.64 and 6.73 me L⁻¹. No carbonate was detected in water samples. CI⁻ ions varied between 0.74 and 7.60 me L⁻¹. SO⁻₄ ions varied between 0.22 and 1.05 me L⁻¹. Nitrogen forms (NH₄-N, NO₃-N), phosphorus (P) and boron (B) ions were also analyzed in water samples. The amounts of NH₄-N, NO₃-N, P and B varied between the lowest trace 0.043 and 0.012 mg L⁻¹ and the highest trace 3.95, 4.42, 1.20 and 0.31 mg L⁻¹, respectively.

The mean sodium concentration of samples ranges from 0.32 to 5.97 me L⁻¹. The highest value was observed in sample sites S4, S5 and S6 as between 5.70 and 7.30 me L⁻¹, while the lowest value was found to be 0.21 and 0.29 me L⁻¹ in sample sites S1 and S2 (Table 3). The mean K⁺ content of samples varied between 0.001 and 0.26 me L⁻¹. The Ca²⁺ concentration of samples varied between 0.87 me L⁻¹ recorded at sample site S2 and 3.71 me L⁻¹ at sample site S6. The mean Mg²⁺ concentration of water samples varied between 0.31 and 1.68 me L⁻¹.

Na⁺ and CI⁻ contents in water samples were found to be above the limit values. Some of the water samples (sample sites S3, S4, S5 and S6) exceeded the threshold set by Ayers & Westcot (1985), which considered CI⁻ contents > 10 meq L⁻¹ and Na⁺ content > 3–9 meq L⁻¹, a serious problem, and the water is classified as ‘slight moderate and severe’ degree of restriction on use irrigation in terms of Na⁺ and CI⁻ concentrations. Generally, higher values were determined in the summer months in the second sampling period. It was seen that there were discharges in sample sites S3, S4, S5 and S6 along the Nilüfer river.

The Cd, Cr, Ni, Pb, Fe, Cu, Mn, Zn, Co and Al analyses were made in water samples. Heavy metals were found in varying amounts in the samples. Heavy metal analysis results in water samples are given in Table 4.

Heavy metal concentrations were determined below the limit values for irrigation water. However, Fe, Cu, Ni, Mn and Pb contents are above the limit values specified in the ‘Surface Water Quality Regulation’ in Turkey (Anonymous 2015; Table 4). Relatively high levels of heavy metals and other elements in the Nilüfer river have been determined in various studies conducted at different times (Aydinalp et al. 2005; Üstün 2011; Dorak & Çelik 2017).

In the study, anion and cation amounts were analyzed and important parameters were also calculated in terms of agricultural irrigation (Vasan & Raju 2007). The SAR, TDS, Na%, RSC, MAR, KR and PI parameters are also given in Table 5 for the evaluation of the selected study sites according to the irrigation water class.

The average SAR values of the water samples ranged from 0.22 to 4.21. The SAR value of the water samples was below 10. It is suitable for irrigation in terms of SAR value. High SAR values indicate the danger of sodium displacing adsorbed calcium and magnesium ions, which will damage soil structure and plant roots.

The mean TDS values of the water samples ranged from 154.3 to 672.0 mg L⁻¹. The highest TDS value was determined as 1,003.5 in the sample site S6 during the summer season. The TDS values of water samples were between the irrigation quality permissible limit ‘middle’ 500 and 1,000 mg L⁻¹. These water quality values indicated that it could have detrimental effects on sensitive plants. A plant variety is important, especially when the Nilüfer river is used during the irrigation period.

The mean Na% values of the water samples ranged from 8.92 to 60.68 (Table 3). The water samples were included in the ‘permissible’ and ‘doubtful’ irrigation water classes (Table 1). The high percentage of sodium (>60%) in irrigation water has adverse effects on the soils. Na negatively affects the soil structure and causes a decrease in the product. High Na⁺ concentration in irrigation water tends to be absorbed by clay particles replacing Mg²⁺ and Ca²⁺ ions. This exchange process from soil reduces permeability and ultimately results in soil with poor internal drainage (Ayers & Westcot 1985).

The RSC values ranged from −2.45 to 2.70 (Table 3). According to the results of the analysis, it was observed that the water samples did not contain RSC. RSC values were higher in the second sampling period. The value for safe irrigation is below 1.25 meq L⁻¹. Sample sites S4, S5 and S6 were not suitable for irrigation purposes. Although the RSC method has been used to assess potential infiltration problems, waters with high concentrations of HCO⁻₃ tend to cause precipitation of Ca² and Mg² ions as the water in the soil becomes more concentrated. As a result, the relative ratio of Na⁺ ions in water increases in the form of sodium bicarbonate (Salifu et al. 2017).

One of the most important criteria for irrigation water evaluation is considered to be Mg content. Water with a MAR value of less than 50% is suitable for irrigation. MAR results from this study show that all the samples are well below 50% and as such are suitable for irrigation (Tables 1 and 3). Raghunath (1987) and Gupta & Gupta (1987) suggest that a high Mg²⁺ hazard value (50%) harms the crop yield as the soil becomes more alkaline. Crop yields are adversely affected by excess Mg²⁺ in irrigation water as the saline soil (Joshi et al. 2009).

KR is an important parameter used in the evaluation of water quality for irrigation. Sodium measured against calcium and magnesium is evaluated to classify waters for irrigation purposes. KR greater than 1 indicates an excess Na level in the water. Therefore, water with a KR of less than 1 is suitable for irrigation. The KR of the water samples in the Nilüfer river ranges between 0.09 and 1.65. In the first sampling period, water samples were found suitable for irrigation. However, the samples taken during the summer period (sample sites S3–S6) were not found suitable in terms of KR.

The PI values also indicated the suitability of water for irrigation, as the soil permeability is affected by the long-term use of irrigation water, as influenced by Na⁺, Ca²⁺, Mg²⁺ and HCO⁻₃ contents of the soil (Selvam et al. 2018). Class I and Class II water types are suitable for irrigation with 75% or more maximum permeability and Class III type of water with 25% of maximum permeability. The average PI values for all water samples vary between 37.2 and 85.6% (Table 1). According to PI values, the water samples were in Class I and II categories in all the sample sites (S1–S6); indicating that the waters are suitable for irrigation purposes in all sample sites (S1–S6).

The SAR and EC values show that all water samples were between the C₂S₁ and C₃S₁ categories according to Wilcox's diagram. All the water samples are under the low sodium hazards (S₁) class according to Table 1. Irrigation water of C₂S₁ class water is the preeminent quality because of its low value of EC and SAR. However, irrigation water of the S2–S3–S4–S5 and S6 sample sites was considered as C₃S₁ particularly harmful according to irrigation water quality standards. In these waters with high salt content, washing and special soil cultivation are required in order not to experience salinity problems in the long term (Trisha et al. 2017).

Minimum, maximum and mean concentrations of total and DTPA-extractable heavy metals were measured in this study. Results are shown in Table 6. Heavy metal concentrations vary over a wide range. The average concentration of heavy metals in sediments was in the decreasing order of Fe > Mn > Cr > Zn > Ni > Cu > Pb > Cd.

As shown in Table 6, the concentrations of heavy metals at sample sites (S4–S6) were much higher than at other sites because these sites were located downstream of the river and extensive discharging of wastewater. Heavy metal concentrations in sediment were higher in the second sampling time than in the first sampling time due to the lower water flow during the second time (summer, irrigation time) which could help to accumulate the heavy metals in sediment (Islam et al. 2014). It has been reported that the mobility of heavy metals depends not only on the total concentration in the sediment but also on the sediment properties, metal properties and environmental factors.

Heavy metals determined in the bottom sediments in the study were compared with the limit values (Table 2). If a heavy metal concentration in the bottom sediment was less than the TEC or TEL values, the bottom sediments were considered to be clean to marginally polluted. No effects on the majority of sediment-dwelling organisms were expected below the TEC or TEL concentration values. If the concentration of the toxic element in the bottom sediment was greater than the PEC or PEL values, the sediments were to be considered heavily polluted. Adverse effects on the majority of sediment-dwelling organisms were expected when the concentrations exceeded PEC or PEL of SQGs values (MacDonald et al. 2000).

Fe concentration in Nilüfer river sediments was found between 2.39 and 5.68% according to the sampling site. The DTPA-extractable Fe content of the sediment samples varied from 9.8 to 142.1 mg kg⁻¹. The higher DTPA-Fe values were determined in the second sampling period (Table 2). The values of the recommended sediment quality directive for the metals to be used in the sediment quality assessments and the relevant concern levels were examined. The determined values appear to be a risk in terms of Fe in all sample sites (Persaud et al. 1993). Total Mn content in sediment samples varied between 468 and 1,053 mg kg⁻¹. The DTPA-extractable Mn content of the sediment samples varied from 3.6 to 29.8 mg kg⁻¹. In terms of sediment quality, the Mn content was evaluated between 460 and 780 mg kg⁻¹ (TEC, threshold effect concentration) with 780 and 1,100 mg kg⁻¹ (PEC, probable effect concentration) by Persaud et al. (1993). According to these results, there was a risk in terms of Mn in the flow basin of Nilüfer river in the sample sites S1, S2 and S3.

Total Zn content in sediment samples varied between 43.1 and 2,076.0 mg kg⁻¹. The DTPA-extractable Zn content of the sediment samples varied from 0.43 to 176.0 mg kg⁻¹. The determined Zn concentrations showed that there was a risk in the sample sites S3, S4, S5 and S6, especially in the second sampling period. The Zn concentration in sediment samples was higher than PEL, SEL, TET and ERM values (Table 2). Total Cu content varied between 24.2 and 125.2 mg kg⁻¹ in the sample sites. DTPA-Cu of the sediment samples varied from 1.09 to 16.9 mg kg⁻¹. The determined value in the sample site S6 was within PEC values in the second sampling period. Nilüfer river sediment samples were evaluated within TEC values (Table 2). This has shown that the risk of contamination for copper has not yet occurred. However, DTPA-Cu values should be taken into account. The Cd content varied between 0.068 and 0.214 mg kg⁻¹ in the sample sites. DTPA-Cd of the sediment samples varied from 0.006 to 0.132 mg kg⁻¹. Nilüfer river sediment samples were evaluated within TEC values in terms of Cd content (Table 2).

Total Cr concentration in Nilüfer river sediments was found between 66.5 and 351 mg kg⁻¹ according to the sampling sites. The DTPA-extractable Cr content of the samples varied from 0.011 to 0.063 mg kg⁻¹. The Cr concentration in sediment samples was higher than PEL, SEL, TET and ERM values (Table 2). According to the results of Cr, it had shown that the Nilüfer river was in a ‘very polluted’ state in all flow regions.

Total Ni content varied between 36.8 and 204.0 mg kg⁻¹ in the sample sites. DTPA-Ni of the sediment samples varied from 0.19 to 11.3 mg kg⁻¹. The Ni concentration in sediment samples was higher than PEL, SEL, TET and ERM values (Table 2) also. According to the results of Ni analysis, the Nilüfer river was in a ‘very polluted’ state in all flow basins. The total Pb content varied between 3.82 and 19.7 mg kg⁻¹ in the sample sites. DTPA-Pb of the sediment samples varied from 0.42 to 2.50 mg kg⁻¹. Nilüfer river bottom sediments are considered to be ‘clean’ to ‘marginally polluted’ in terms of Pb and Cd concentrations (Table 2).

In the study, the pollution status of the Nilüfer river, which has been under industrialization and urbanization for a long time, was determined by water and bottom sediment analysis. As a result of the analysis of water samples, the regions where the river was used for irrigation purposes were classified as C₂S₁ and C₃S₁ irrigation water. According to the KR quality parameter to be considered in the use of irrigation water, Nilüfer river water was not found suitable for use as irrigation water.

Heavy metal analyses were evaluated in bottom sediment samples taken from the sampling site in the study and results were compared with the sediment quality limit parameters.

In this study, the total Zn, Cr and Ni content of Nilufer river were higher than those of the SQGs PEL value and Consensus-SQGs PEC value. These indicated that the Nilüfer river sediments were polluted with Zn, Cr and Ni and this may cause adverse effects on the majority of sediment-dwelling organisms.

Heavy metal concentrations in the water of Nilüfer are low, but heavy metal transfer occurs from the bottom sediment to the water. Consequently, prolonged use increases the possibility of accumulation of contamination in soil and transfer of metals to plants.

Heavy metals cause toxic effects on plants such as inhibition of growth and photosynthesis, chlorosis, altered water balance and nutrient uptake, and senescence resulting in plant death. In addition to adverse impacts on plants, heavy metals pose a threat to human health due to their permanence in nature. Accumulation of heavy metals in irrigated soils and plants causing contamination of the food supply can be dangerous for humans and animals. Total and extractable Fe and Mn concentrations in the bottom sediment were also determined. Although iron and manganese are not heavy metals, it is necessary to pay attention to their concentrations as their excess causes nutrient imbalance in the soil.

The resulting data showed that one should be careful in the use of water resources for irrigation purposes. The results of this study will be shared with the authorities and efforts will be made to take the necessary measures for a solution. Otherwise, significant pollution problems may occur in the cultivated soils in case of continued use as irrigation water.

The authors are grateful to the field and laboratory assistance rendered by Mrs Saliha Dorak, Duygu Özsoy and Mr Samet Vaiz.

This work was supported by the Scientific and Technical Research Council of Turkey (TOVAG 114 O 713).

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.