Spatial variation of water quality across sequential stations in a perennial river, Northwest Iran

Abstract

Water quality assessment is essential for understanding the impact of pollution on aquatic ecosystems, particularly in rivers that cross diverse land uses and population centers. This study compares water quality parameters of a large perennial River at three consequent monitoring stations in Northwest Iran, to assess changes in physical, chemical, and biological parameters using statistical and graphical methods. After testing the assumptions for statistical comparisons, the Mann-Whitney U test was used to compare the distributions of water quality parameters at the monitoring stations. Graphical techniques such as Raincloud plots and Density plots were used for visual representation of the data. The results indicate overall increases in TSS, turbidity, and COD as the river flows downstream. For instance, the mean concentration of TSS increased from 18.09 mg/L at Arena-Village (upstream) to 26.80 mg/L at Pol-Mohammadyar (downstream). Similarly, turbidity levels rose from 4.50 NTU at Arena-Village to 17.75 NTU at Pol-Naghadeh (middle station). The results indicate that most water quality parameters did not differ significantly between the three monitoring stations. However, turbidity (Turb) showed a statistically significant difference between the upstream (Arena-Village) and downstream (Pol-Mohammadyar) stations (p = 0.0477), suggesting localized impacts of human activities, particularly sand mining. In general, TSS, turbidity, and COD tended to increase downstream, while many chemical parameters remained relatively stable. This suggests that anthropogenic activities, particularly sand mining, contribute to the degradation of water quality as the river progresses. Excessive sand and gravel extraction from the riverbed, confirmed during field visits, may be a primary cause of increased turbidity and TSS at downstream stations. This activity disrupts the riverbed and releases suspended particles into the water column, negatively affecting the physical quality of the water. The study emphasizes the need for monitoring and regulating sand mining activities to prevent further deterioration of water quality. The observed increase in pollutants downstream emphasizes the cumulative effect of multiple pollution sources, which can adversely affect long-term aquatic ecosystems and human health.