Study of Temporal Variations of Phosphorus Pollution along Siahroud River in Guilan Province

Phosphorus (P) is one of the essential elements for plant growth, which is considered as one of the potential sources of water pollution due to its excessive use as chemical fertilizers, and also due to discharge of municipal and industrial wastewater into water resources. P has high spatial and temporal variations because of the affecting factors, including rainfall intensity, land use, slope, and soil erosion. The aim of this study was to investigate the temporal variations of phosphorus pollution at various regions along Siahroud river, located in Guilan province. The study has been conducted in Siahroud watershed of Rasht with different land uses including forest (Jokolbandan and Saravan regions) as well as agricultural (Jokolbandan region), industrial (Saravan and University regions), and urban area (Gil and Golsar regions). Water samples were collected from the various regions of the river with different land uses during ten months. The contents of total, dissolved and suspended solids, along with total, soluble and particulate P were measured in water samples. Total P was measured by potassium persulfate digestion method. Nitrogen content was also determined according to Kjeldahl method in the samples taken on December, January, February and March. Results showed higher level of P pollution in the urban (Golsar with the value of 0.261 mg.L-1) and industrial regions compared to other regions of the river. In the winter, most of soluble P discharge was observed from the agricultural areas, while it revealed higher discharges from the urban areas in the summer. The maximum P pollution (0.296 mg.L-1) was related to the Golsar region. In addition, the high rate of total P in Jokolbandan (0.188 mg.L-1) can be due to the destruction of forest and the slope steepness of this region. Most of the solid particles discharged from the watershed were also in the form of suspended solids (annual average, 503 mg.L-1). The results of the mean comparison showed that there was a significant difference in the spatial variations of the concentration of various forms of P including total, soluble and particulate P, and the highest amount of P was observed at the end of the river. Furthermore, the results showed that there was no significant difference between soluble and particulate P in the study area, while a power relationship was observed between soluble P and soluble solids (R2=0.541). Results demonstrated that the output of P in various forms is a function of time. In general, the rate of P pollution was increased from the upstream to the downstream and showed high temporal variations. While soil erosion was recognized as the reason for high levels of P pollution in agriculture regions such as Sangar in rainy season, the discharge of municipal and industrial wastewater into the river was the reason for P pollution in the urban areas. Therefore, control of soil erosion in agricultural lands and prevention of wastewater discharge of municipal and industrial into the river could reduce effectively P pollution.