The Onga River basin plays an important role as the main source of water for the Kitakyushu urban area. However, there are concerns about the impact on water quality, such as the low sewerage maintenance rate and water retention due to estuary weirs, despite the fact that more than 600 000 people live in the basin population. This study aims to characterise the water environment of the basin through the collection of river water samples in the field and the analysis of their water quality. In the past, seasonal and spatial considerations of field measurements have revealed characteristics such as lower pH values during the summer months, but this study will also include a discussion of the major dissolved constituents. Although the population of the area is declining with the decline of the coal industry, the area is located between two government-designated cities, and still has a bedroom town character, with many company factories and a large population, so there is concern about anthropogenic impacts on the river. Continuous water quality surveys and discussion of the results should be used to estimate the specific causes of pollution.