5:15 PM - 7:15 PM
[AHW28-P20] Characteristic of river water chemistry and its formation factors in the Onga River basin from the viewpoint of major dissolved components
Keywords:Major Dissolved Components, Sulfate ion, Cluster analysis, Ongagawa river, Chikuho Coalfields
The Onga River, which flows through the Chikuho region of Fukuoka Prefecture into the Hibikinada Sea, provides water resources to surrounding municipalities, including Kitakyushu City. The Onga River basin faces several challenges that negatively affect water quality: (1) low sewerage coverage and limited sewage treatment capacity, despite having the highest population density among first-class water systems in Kyushu; (2) algae growth due to stagnant river water at weirs, such as river mouth weirs; (3) nutrient runoff from agricultural lands, primarily paddy fields; and (4) the presence of numerous abandoned coal mining pits and piles of low-quality coal waste (known as “Bota-yama”) distributed throughout the basin.
To improve the water environment and enhance river management, it is essential to understand the characteristics of water quality and its formation processes. However, these aspects have not yet been fully elucidated for the Onga River basin.
Therefore, in this study, we analyze the concentration, composition, and spatial distribution of major dissolved solids. We then estimate the factors influencing the chemical composition by considering land use in each sub-watershed at the observation points.
A cluster analysis using the Wade equation was conducted based on the equivalent concentration values of eight major dissolved solids. The analysis results classified the water quality into three distinct groups. The first group, characterized by low concentrations of dissolved solids, is predominantly of the calcium-bicarbonate type (Group A). The second group, which has a higher concentration of dissolved solids than Group A, is further subdivided into two compositional patterns: calcium-bicarbonate and sodium-bicarbonate (Group B). The third group, with the highest concentration of dissolved solids, is primarily of the calcium (or sodium)-sulfate type, though sodium-chloride and sodium-bicarbonate types were also observed at individual sites (Group C).
The calcium-bicarbonate type with low concentrations (Group A) is a common river water composition and is more prevalent in the upper reaches of the river, suggesting minimal anthropogenic influence. Group B is more abundant downstream, including urban areas. Cluster C is not distributed in tributaries on the Hikosan River side.
Sulfate ions are often detected in areas with volcanic activity and hot springs; however, such geological features are scarce in the Onga River basin. Yamashita (1989) reported that water discharged from abandoned mining pits in the Chikuho coalfield exhibited high sulfate concentrations similar to those in Group C. This suggests that the impact of coal mines may persist nearly 50 years after the decline of the coal industry. However, if coal mines are indeed influencing water chemistry, it raises the question of why Group C is absent on the Hikosan River side, despite the presence of numerous coal mines there.
To improve the water environment and enhance river management, it is essential to understand the characteristics of water quality and its formation processes. However, these aspects have not yet been fully elucidated for the Onga River basin.
Therefore, in this study, we analyze the concentration, composition, and spatial distribution of major dissolved solids. We then estimate the factors influencing the chemical composition by considering land use in each sub-watershed at the observation points.
A cluster analysis using the Wade equation was conducted based on the equivalent concentration values of eight major dissolved solids. The analysis results classified the water quality into three distinct groups. The first group, characterized by low concentrations of dissolved solids, is predominantly of the calcium-bicarbonate type (Group A). The second group, which has a higher concentration of dissolved solids than Group A, is further subdivided into two compositional patterns: calcium-bicarbonate and sodium-bicarbonate (Group B). The third group, with the highest concentration of dissolved solids, is primarily of the calcium (or sodium)-sulfate type, though sodium-chloride and sodium-bicarbonate types were also observed at individual sites (Group C).
The calcium-bicarbonate type with low concentrations (Group A) is a common river water composition and is more prevalent in the upper reaches of the river, suggesting minimal anthropogenic influence. Group B is more abundant downstream, including urban areas. Cluster C is not distributed in tributaries on the Hikosan River side.
Sulfate ions are often detected in areas with volcanic activity and hot springs; however, such geological features are scarce in the Onga River basin. Yamashita (1989) reported that water discharged from abandoned mining pits in the Chikuho coalfield exhibited high sulfate concentrations similar to those in Group C. This suggests that the impact of coal mines may persist nearly 50 years after the decline of the coal industry. However, if coal mines are indeed influencing water chemistry, it raises the question of why Group C is absent on the Hikosan River side, despite the presence of numerous coal mines there.