Abstract
The Ziarat River is one of the tributaries of the Qarah-su catchment, and a part of this river flows through Gorgan City. To investigate the environmental hydrogeochemistry of the river, 11 water samples were collected. Physicochemical parameters, the concentrations of major ions and metals, and the biological parameters were analyzed. The obtained results show that pH and EC of the water samples vary from 7.2 to 7.9, and from 701 to 1422 μS/cm, respectively. The highest pH and EC values were observed near the discharging point of cowshed effluents and the Ziarat hot spring, respectively. Based on hydrogeochemical studies, the chemical composition of water samples is mainly controlled by the dissolution process. The water type changed from Ca-SO4 upstream to Na-Cl downstream, probably because of natural and anthropogenic factors.
Regarding the major ion concentrations, all water samples are in the acceptable range for drinking usage. Based on the Wilcox diagram, and regarding the calculated residual sodium carbonate and sodium percentage values, the studied water samples are suitable for drinking and agricultural purposes; however, regarding the values of the magnesium hazard index, most of the samples are not appropriate for agricultural usage. Metal pollution indices show that studied samples are highly polluted, especially those samples that were collected at the wastewater discharge points. The values of dissolved oxygen in all samples are within the permissible limits. However, in some stations, BOD and COD levels are higher than permissible values, and the highest amount of these parameters is observed near the discharging point of the cowshed and agricultural effluents. All water samples are microbially polluted, and the lowest microbial pollution is observed in upstream stations. The values of Water Quality Indices indicate that all studied samples are classified as low and very low quality.
 

INTRODUCTION

Rivers are one of the most important sources of freshwater supply, holding approximately 2000 km3 of water worldwide (Panneerselvam et al., 2021). According to the United Nations report, population
growth in urban areas worldwide will be duplicated by 2050, which is associated with negative environmental impacts, including adverse chemical, biological, and physical changes in river water quality (Zhu et al., 2021). Therefore, assessing and monitoring the hydrogeochemical characteristics of rivers are important steps in environmental management. River water quality may change through natural and anthropogenic factors (Hiruy et al., 2022). Wherever rivers pass through large urban areas, severe water contamination is possible due to the input of untreated wastewater as well as urban runoff discharges (Sekharan et al., 2022).
The Ziarat River, with a length of about 42 km, is one of the tributaries of the Qarasu watershed in Golestan Province, about 8 km of which flows within the city of Gorgan. The Qarasu River discharges into the Caspian Sea. Ziarat River receives various amounts of untreated sewage and wastewater during its flow path, therefore a significant reduction in water quality is likely. The aims of the present study are: to evaluate the hydrogeochemical quality of the Ziarat River water, to compare the hydrogeochemical characteristics of samples collected upstream and downstream, and to investigate the impact of the pollutant discharges on the water quality.
 

MATERIALS AND METHODS

11 sampling stations were selected based on the geology of the area, land uses, and location of potential pollutant sources. pH, EC, TDS, DO and temperature values were measured in situ. The collected water samples were immediately transported to the laboratory and stored at 4˚C until analysis. The turbidity of the samples was measured by a turbidimeter. The concentrations of Na and K were measured by a flame photometer. Mg, Ca, Cl, and HCO3 values were determined by titration, and the concentrations of SO4, NO3, and PO4 were measured by a spectrophotometer. The concentration of potentially toxic elements in the water samples was measured by an atomic absorption spectrometer (AAS) device. BOD was measured by manometry and COD was determined using a spectrophotometer. A nine-tube MPN 9 method was applied to investigate bacteriological water contamination.
 

RESULTS

The turbidity value of the studied samples varied between 3 and 529 (average value of 163.63) NTU, and except for one sample, the value of this parameter in all samples exceeded the World Health Organization standard (5 NTU). The pH and EC of the samples changes from 7.20 to 7.9, and from 701 to 1422 µS/cm, respectively. Based on pH and EC values, all studied samples were within the permissible limit for drinking and agricultural usage. The TDS values varied between 443 and 898 (average value of 621) mg/L. All studied samples were within the permissible range for drinking in terms of TDS values. The water samples were classified as very hard and hard. The concentrations of major ions in most of the studied samples were within the permissible limit for drinking. Based on the statistical analyses and Saturation Index values, the water chemistry was dominantly controlled by dissolution. While the facies-type of water samples in upstream was Ca-SO4, it changed to Na-Cl downstream. Based on the standard values provided by WHO and regarding the calculated RSC and Na % values, the samples were suitable for drinking and irrigation purposes; however, based on the MH index values, most of the samples are unsuitable for irrigation. HEI index values showed that 64% of the samples were categorized as low level of pollution, 27% as medium level of pollution, and 9% as high level of pollution. The HPI index of water samples also showed that 36% of the studied samples were polluted by potentially toxic elements. The values of Contamination Degree (Cd) showed that 64 % of the studied samples were classified as low level of pollution, 9% as medium level pollution, and 27% as high level of pollution. Overall, the obtained results indicated that the highest level of metal pollution was observed in urban areas. The dissolved oxygen (DO) content of all samples was lower than the permissible value of 5 mg/L. BOD and COD values were higher than the acceptable limits (5 and 10 mg/L, respectively) in about 50% of the studied samples. Moreover, the highest BOD and COD values were measured at the sampling sites adjacent to the discharge of dairy farm effluent and agricultural runoff. All the studied water samples were microbially contaminated. The National Sanitation FoundationWater Quality and Oregon Quality Indices values showed that all samples were of very bad (50>NSFWQI>26) and bad (60>OWQI>0) quality.
 

DISCUSSION AND CONCLUSION

The facies and type of water samples were significantly changed because of both natural and anthropogenic factors. The obtained results showed that the water chemistry was mostly controlled by dissolution. Most sampling sites are polluted with potentially toxic elements, and the highest level of metal pollution was detected in sampling sites that were affected by a discharge of urban wastewater. The values of BOD and COD exceeded the permissible limits in about 50% of the sampling sites and their highest values were observed at the discharge site of dairy wastewater and agricultural runoff. All water samples were microbially polluted. The obtained results in the present study show that controlling and managing the input of anthropogenic pollutants into the Ziarat River is of crucial importance to protect the health of water consumers and to reduce the pollutant load entering the Caspian Sea.
 

REFERENCES

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