Evaluating Surface Water Nitrogen Pollution via Visual Clustering in Megacity Chengdu

The current standards used for nitrogen pollution evaluation are lacking, and scientific classification methods are needed for nitrogen pollution to improve water quality management capabilities. This study addresses the important issue of assessing surface water nitrogen pollution by utilizing two advanced multivariate statistical techniques: self-organizing maps (SOMs) obtained using the K-means algorithm and the Hasse diagram technique (HDT). The research targets of this study are the rivers of the megacity Chengdu, China. Samples were collected on a monthly basis in 2017-2020 from different sites along the rivers, and their nitrogen pollution parameters were determined. The grouping of nitrogen pollution parameters and the clustering of sampling events using SOMs facilitate the preprocessing required for the HDT, wherein clusters are ordered according to the pre-clustered water sampling events. The results indicate that nitrogen pollution in the Chengdu River Basin, which is prominent and mainly driven by nitrate nitrogen, can be categorized into five levels. The nitrogen pollution in Tuo River is serious. Although the degree of ammonia nitrogen pollution in Jin River is higher, the pollution range is smaller. Furthermore, these results were evaluated by the SOMs and HDT to be clear and reliable. Overall, these findings can provide a basis for local environmental legislation.

Automated summary

The current standards for evaluating nitrogen pollution are insufficient, and scientific classification methods are needed to improve water quality management capabilities. This study focuses on the assessment of surface water nitrogen pollution in the rivers of Chengdu, China, using self-organizing maps (SOMs) obtained with the K-means algorithm and the Hasse diagram technique (HDT). The results indicate that nitrogen pollution in the Chengdu River Basin can be categorized into five levels, with nitrate nitrogen being the main driver. The seriousness of nitrogen pollution in the Tuo River is prominent, while the pollution range of ammonia nitrogen in the Jin River is smaller but with higher degree. These findings, evaluated by SOMs and HDT, provide a reliable basis for local environmental legislation.