EXTENDED ABSTRACT

Introduction
One of the main challenges in water quality management is the large volume of data from monitoring various physical, chemical, and microbial parameters. In this regard, the use of Water Quality Indices (WQI) as an efficient and simple tool has attracted the attention of researchers and responsible institutions. In response to the need for a localized index, the Iranian Department of Environment developed the national surface water quality index (Iran Water Quality Index for Surface Water Resources-Conventional Parameters; IRWQIsc) based on 11 key parameters. The Gorganrud River, with a length of about 300 km and a watershed area of 10,250 km², is one of the most important rivers in Golestan Province and the Caspian basin. Cultivation of various crops, high consumption of chemical fertilizers and pesticides, along with the discharge of urban and industrial wastewater, have imposed significant pollution pressure on this water resource. This study aimed to investigate the seasonal quality of Gorganrud River water using the IRWQIsc index, identify critical stations, and determine the most important parameters affecting water quality reduction.

Materials and methods
Six sampling stations were selected along the river to cover upstream (minimal pollution), midstream (affected by urban and agricultural pollutants), and downstream (accumulation of pollutants) areas: (1) Pol-e Shahr-e Kallaleh, (2) Gonbad (Pol-e Gedam Abad), (3) Voshm Gir Dam outlet, (4) Pol-e Kamarbandi-e Aqqala, (5) Pol-e Khajeh Nafas, and (6) Gorganrud Estuary. Eleven water quality parameters were measured: pH, BOD₅, COD, dissolved oxygen (DO), electrical conductivity (EC), ammonium (NH₄), nitrate (NO₃), phosphate (PO₄), total hardness, turbidity, and fecal coliforms. pH, temperature, EC, and DO. Fecal coliforms were determined by the multiple tube fermentation method (MPN) according to APHA. Data were analyzed using SPSS version 26. One-way ANOVA followed by Duncan's HSD post-hoc test was used to compare water quality indices among stations and seasons.

Results and discussion
The IRWQIsc values ranged from 15 to 48.4 across all stations and seasons. The Gorganrud Estuary showed the highest quality (48.4, "moderate"), while Pol-e Kamarbandi-e Aqqala showed the lowest (15, "very bad"). The highest BOD (53 mg/L), COD (245 mg/L), EC (10,990 µS/cm), and fecal coliforms (996 MPN/100mL) were recorded at downstream stations. Spring: IRWQIsc ranged from 15 to 47.5. Pol-e Kamarbandi-e Aqqala (15, "very bad") and Gorganrud Estuary (47.5, "moderate") represented the extremes. Summer: IRWQIsc ranged from 20.8 to 42.3. Most stations showed "relatively bad" quality. Water quality decreased compared to spring due to reduced river flow and increased pollutant concentrations. Autumn: IRWQIsc ranged from 27 to 46.6. Voshm Gir Dam outlet (27) and Pol-e Kamarbandi-e Aqqala (28.1) showed "bad" quality, while Pol-e Shahr-e Kallaleh (46.6) showed "moderate" quality. Winter: IRWQIsc ranged from 32.1 to 48.4. The Estuary showed the highest quality (48.4, "moderate"), and most stations were classified as "relatively bad". Water quality decreased from upstream to midstream, reaching its lowest point at Pol-e Kamarbandi-e Aqqala, then showed partial recovery at downstream stations. This pattern indicates significant pollution input from Gonbad-e Kavus and Aqqala cities, followed by natural self-purification processes.

Conclusion
The water quality of the Gorganrud River is unfavorable, classified as "relatively bad" to "very bad" in most stations and seasons. Pol-e Kamarbandi-e Aqqala was identified as the most critical pollution hotspot. Analysis of spatial variations showed that water quality decreased from upstream to downstream until Aqqala, then improved at downstream stations due to natural self-purification processes. Temporal variations demonstrated better quality in wet seasons (winter and spring) due to dilution effects, although increased erosion in winter can offset this benefit at some stations. Considering the vital role of this river in agricultural water supply and feeding the International Wetland of Gorgan Bay, urgent actions are recommended: continuous water quality monitoring, proper operation of wastewater treatment plants, optimization of chemical fertilizer use, establishment of riparian buffer zones, and public awareness raising. Future studies should measure heavy metals, pesticides, and persistent organic pollutants, and extend the monitoring period over several years.

EXTENDED ABSTRACT

Introduction
One of the main challenges in water quality management is the large volume of data from monitoring various physical, chemical, and microbial parameters. In this regard, the use of Water Quality Indices (WQI) as an efficient and simple tool has attracted the attention of researchers and responsible institutions. In response to the need for a localized index, the Iranian Department of Environment developed the national surface water quality index (Iran Water Quality Index for Surface Water Resources-Conventional Parameters; IRWQIsc) based on 11 key parameters. The Gorganrud River, with a length of about 300 km and a watershed area of 10,250 km², is one of the most important rivers in Golestan Province and the Caspian basin. Cultivation of various crops, high consumption of chemical fertilizers and pesticides, along with the discharge of urban and industrial wastewater, have imposed significant pollution pressure on this water resource. This study aimed to investigate the seasonal quality of Gorganrud River water using the IRWQIsc index, identify critical stations, and determine the most important parameters affecting water quality reduction.

Materials and methods
Six sampling stations were selected along the river to cover upstream (minimal pollution), midstream (affected by urban and agricultural pollutants), and downstream (accumulation of pollutants) areas: (1) Pol-e Shahr-e Kallaleh, (2) Gonbad (Pol-e Gedam Abad), (3) Voshm Gir Dam outlet, (4) Pol-e Kamarbandi-e Aqqala, (5) Pol-e Khajeh Nafas, and (6) Gorganrud Estuary. Eleven water quality parameters were measured: pH, BOD₅, COD, dissolved oxygen (DO), electrical conductivity (EC), ammonium (NH₄), nitrate (NO₃), phosphate (PO₄), total hardness, turbidity, and fecal coliforms. pH, temperature, EC, and DO. Fecal coliforms were determined by the multiple tube fermentation method (MPN) according to APHA. Data were analyzed using SPSS version 26. One-way ANOVA followed by Duncan's HSD post-hoc test was used to compare water quality indices among stations and seasons.

Results and discussion
The IRWQIsc values ranged from 15 to 48.4 across all stations and seasons. The Gorganrud Estuary showed the highest quality (48.4, "moderate"), while Pol-e Kamarbandi-e Aqqala showed the lowest (15, "very bad"). The highest BOD (53 mg/L), COD (245 mg/L), EC (10,990 µS/cm), and fecal coliforms (996 MPN/100mL) were recorded at downstream stations. Spring: IRWQIsc ranged from 15 to 47.5. Pol-e Kamarbandi-e Aqqala (15, "very bad") and Gorganrud Estuary (47.5, "moderate") represented the extremes. Summer: IRWQIsc ranged from 20.8 to 42.3. Most stations showed "relatively bad" quality. Water quality decreased compared to spring due to reduced river flow and increased pollutant concentrations. Autumn: IRWQIsc ranged from 27 to 46.6. Voshm Gir Dam outlet (27) and Pol-e Kamarbandi-e Aqqala (28.1) showed "bad" quality, while Pol-e Shahr-e Kallaleh (46.6) showed "moderate" quality. Winter: IRWQIsc ranged from 32.1 to 48.4. The Estuary showed the highest quality (48.4, "moderate"), and most stations were classified as "relatively bad". Water quality decreased from upstream to midstream, reaching its lowest point at Pol-e Kamarbandi-e Aqqala, then showed partial recovery at downstream stations. This pattern indicates significant pollution input from Gonbad-e Kavus and Aqqala cities, followed by natural self-purification processes.

Conclusion
The water quality of the Gorganrud River is unfavorable, classified as "relatively bad" to "very bad" in most stations and seasons. Pol-e Kamarbandi-e Aqqala was identified as the most critical pollution hotspot. Analysis of spatial variations showed that water quality decreased from upstream to downstream until Aqqala, then improved at downstream stations due to natural self-purification processes. Temporal variations demonstrated better quality in wet seasons (winter and spring) due to dilution effects, although increased erosion in winter can offset this benefit at some stations. Considering the vital role of this river in agricultural water supply and feeding the International Wetland of Gorgan Bay, urgent actions are recommended: continuous water quality monitoring, proper operation of wastewater treatment plants, optimization of chemical fertilizer use, establishment of riparian buffer zones, and public awareness raising. Future studies should measure heavy metals, pesticides, and persistent organic pollutants, and extend the monitoring period over several years.