Developing new spectral indices for identifying DOM sources in Liaohe River in a large-scale river basin by fluorescence spectroscopy and random forest model

Human activities can have a tremendous impact on rivers. The leaching of riparian soil, discharge of industrial wastewater, agricultural activities, and domestic sewage discharge can all introduce new dissolved organic matter (DOM) into rivers, thereby altering the structure of the original DOM in the river. In recent years, fluorescence traceability technology, especially three-dimensional fluorescence spectroscopy (3D-EEM) combined with parallel factor analysis (PARAFAC), has become an important tool to study the source and change of DOM. However, there are differences in the definition and calculation methods of the traditional fluorescence index, which leads to its limited applicability in different water bodies, so it is unable to accurately distinguish the sources of pollutants. Here, by combining the random forest model with three-dimensional to two-dimensional, taking the Liaohe River Basin as the research object, a new fluorescence traceability model is constructed. Divide it into three sources, i.e., originating from urb sewage, agricultural wastewater and soil organic matter, and thus defined as U-DOM, A-DOM, T-DOM, respectively. Based on the random forest model, special nodes of three-dimensional fluorescence spectrum can be identified, which can be tracked from three sources. It was closely related to the key peaks and nodes of EEM from three different sources, i.e., URB sewage (U-SoI: U-1/U-2 =(Ex: 245, Em: 440)nm/((Ex: 230, Em: 345)nm), agricultural wastewater (A-SoI: A-1/A-2 =( Ex: 245, Em: 325)nm/( Ex: 260, Em: 430)nm) and soil organic matter (T-SoI: T-1/T-2 =( Ex: 260, Em: 430)nm/( Ex: 285, Em: 365)nm). It is found that when U-SoI is greater than 0.9, it can be recognized as a typical U-DOM; When A-SoI is between 0.2 and 0.3, it indicates that it is A-DOM; When T-SoI is less than 0.5, it is displayed as T-DOM. Compared with the traditional fluorescence index traceability method, the new traceability model shows that the operation is more convenient and efficient, can provide more accurate traceability results, and can flexibly distinguish the source of pollutants according to the specific environment and pollution characteristics, so as to better adapt to the traceability requirements in different scenarios.