Introduction

Water pollution is one of the most important problems in the world and environmental concerns today. Determining water quality is of particular importance in water resources management (Zamani et al., 1401). The chemical composition of water is an important factor that must be considered before domestic use or irrigation purposes (Hosseinkhani and Najib, 2014). Factors such as rainfall, erosion, agricultural and urban activities, increased urbanization and industrialization affect the water quality of rivers. Therefore, proper prediction of possible changes in water quality is very important (Zamani et al., 1401). One of the very simple methods, devoid of any mathematical and statistical complexity, that can describe water quality conditions and be used as a powerful advanced tool for relevant decision-making is the use of the Water Quality Index (WQI). Since a large part of river water is allocated for agricultural purposes, their quality should be evaluated to protect public health and the environment (Ildermi et al., 1402). A study of the quality indicators of the Tajan River showed that this river is in the medium class, mainly due to the discharge of urban wastewater, industries and agricultural farm effluents (Delbari et al., 1401). In a study of the quality indicators of the Ghaen River in the period 2007-2016, it was found that the water quality has decreased from upstream to downstream, which could be due to the increase in water salinity and the inflow of agricultural, industrial and urban drainage. (Khadempour et al., 1396). The water quality indicators of the Gorgan-Rud River were also in the poor quality class due to the presence of urban wastewater and agricultural drainage in urban areas (Gholizadeh et al., 1399). Water for irrigation canals in Lahijan city is supplied from Manjil Dam. Along the irrigation route, due to the incomplete completion of the urban sewage collection network and its lack of connection to the Lahijan city sewage treatment plant, the lack of implementation of the sewage collection network in marginal areas, and unsanitary garbage disposal and leachate leakage from the landfill, raw sewage and leachate have entered the irrigation canals directly and indirectly at various points, drastically changing the water quality of these canals. Therefore, it is necessary to have water quality information for the effective management of irrigation systems. So that any management decision that directly or indirectly affects the canal water can be made with more awareness. The aim of this study is to determine the water quality of agricultural canals in Lahijan city using the NSFWQI quality index and the sodium absorption ratio SAR and compare their results with international standards. This study allows for the description of physicochemical properties and assessment of the suitability of surface canal water for irrigation and agriculture based on the analysis of quality parameters.
Materials and methods
Based on field visits, the main points of entry of wastewater and leachate into streams and agricultural networks were determined, and based on these points, 10 sampling stations were selected from various main and secondary canals of the farms. Considering the seasonality of irrigation of the farms, which are mainly rice cultivation, sampling was carried out in the irrigation months of May, June and July, and the samples were transferred to the laboratory for microbial testing of fecal coliform (FC) and measurement of biochemical oxygen demand (BOD), nitrate, phosphate, total suspended solids (TSS), sodium, calcium and magnesium, observing the cold chain, and other parameters including temperature, pH, dissolved oxygen (DO), turbidity and total dissolved solids (TDS) were measured using portable equipment at the sampling site. According to the results of the parameters, quality indices were calculated and examined at different sampling stations, and statistical analyses were performed based on NSFWQI and SAR quality indices, Wilcox plots, Kruskal-Wallis statistical tests, and Dunn's post-test. Quality comparison was performed based on the grading of canals, primary irrigation source, and irrigation standards.
Results and discussion
Based on the NSFWQI quality index, the irrigation canals of Lahijan city were ranked in the average to bad category. Station one had the best quality, which is probably due to the higher water discharge and the constant water flow in the stream, as well as the lower inflow of urban sewage and the inflow of leachate after treatment. Also, the worst quality was related to the two stations in the village of Tamoshel, which indicates the effects of direct leachate discharge to these canals and the diversity of the type and amount of pollutants discharged in these two stations. Evaluation of the canal quality based on the SAR irrigation indices showed that station one has the best quality and the other stations were ranked in the excellent, good and average category according to the month of sampling. According to the Wilcox diagram, only station one was ranked slightly saline and suitable for irrigation, and the other stations were rated saline and with the necessary measures for proper irrigation. The study of parameters between stations showed that parameters such as BOD, phosphate, turbidity and coliforms differed significantly between stations, which statistical tests such as Kruskal–Wallis and Dunn's post-test also confirmed these differences significantly. Also, the study of the status of parameters according to the degree of the channels revealed that with increasing degree of the channel, some pollution parameters such as nitrate, phosphate, BOD and coliforms increased, while parameters such as DO (dissolved oxygen) decreased. Comparing the status of parameters at the sampling stations of the city with the initial water outlet from the Sefidrood Dam indicated a significant decrease in the quality of most important water parameters, including an increase in organic load, nitrate, phosphate and turbidity at the sampling stations. Comparing the parameters at ten stations with irrigation standards shows that important parameters such as phosphate, organic load (BOD) and turbidity have significantly exceeded the permitted standards in several stations.
Conclusion
The irrigation quality of the sampling stations was examined based on NSFWQI and SAR quality indices and Wilcox plot, which showed that the sampling station, which is the upper Temoshel village stream, had the best quality, while the quality of the irrigation canals decreased sharply after entering the urban area due to various factors, including urban sewage, waste leachate, and agricultural runoff. The examination of parameters between the stations showed that parameters such as BOD, phosphate, turbidity, and coliforms differed significantly between the stations. These differences were mostly due to the entry of various pollutants into water sources in areas close to farms and urban areas. While the main stations had relatively better water quality. Statistical tests such as Kruskal–Wallis and Dunn’s post-test also confirmed these differences significantly. The status of the parameters according to the degree of the channels showed that with the increase in the degree of the channel, from the main channel to degree 3 and degree 4, which are closer to the farms and more polluted areas, some pollution parameters such as nitrate, phosphate, BOD and coliforms increased, while parameters such as DO decreased. This trend shows that the side channels are directly affected by agricultural and human activities and the water quality in them is declining. Therefore, the degree of the channel is an indicator of the level of pollution and the quality of water usable for irrigation. Considering the comparison of the status of the parameters in the sampling stations of the city with the initial water outlet from the Sefidrood Dam, the results indicated a significant decrease in the quality of most important water parameters, including an increase in organic load, nitrate, phosphate and turbidity in the city compared to the water outlet from the dam. This indicates that after the water exits the dam and passes through different paths, pollutants enter the water and its quality decreases. Also, high levels of fecal coliform in some stations indicate significant microbial contamination that threatens the health of the ecosystem and agricultural products. Findings from comparing the results of quality parameters of ten stations with irrigation water standards showed that while some parameters such as magnesium, calcium and sodium are within acceptable limits, some pollution parameters such as phosphate, BOD, turbidity and coliforms have exceeded the permissible limits of irrigation water standards in several stations. This situation can lead to problems such as the accumulation of pollutants in the soil, reducing agricultural productivity, creating unsuitable conditions for plant growth and threatening human health and the environment. Therefore, it is essential to implement management programs to control pollutants, proper water treatment and continuous monitoring of the quality of water resources.