Composition, Spatial Distribution Characteristics and Source Analysis of Chromophoric Dissolved Organic Matter in the Lanzhou Reach of the Yellow River

Abstract

Chromophoric dissolved organic matter (CDOM) in riverine systems can be affected by environmental conditions and land-use, and can thus provide important information regarding anthropogenic activities in surrounding landscapes. It can modify the optical properties of waters and affect the balance and availability of dissolved nutrients and metals in water bodies. However, the characteristics of CDOM in the Lanzhou reach of the Yellow River have not yet been reported. In this study, the optical properties of water samples collected at 32 locations during April 2017 across the Lanzhou reach of the Yellow River were examined using UV-VIS and excitation-emission matrix fluorescence spectroscopy-parallel factor analysis (EEM-PARAFAC), to determine CDOM compositional changes, spatial distribution characteristics, and sources. Cluster analysis was used to categorize samples into groups of similar pollution levels within a study area. Results showed that CDOM was primarily comprised of low molecular weight organic substances with aromatic structure belonging to complex "protein-like-humic-like" substances, and dominated by protein-like substances (organism sources). Two humic-like components (C1, C4), one tryptophan-like component (C2), and one non-humic-like component (C3) were identified by PARAFAC. Tryptophan-like substances were predominant in the components of CDOM, accounting for 51.06% of average total fluorescence intensity. Humic-like materials and non-humic-like substances accounted for 36.74% and 12.20%, respectively. Weak correlations were observed between protein-like substances and humic-like substances, indicating different sources of these components. The distribution of total fluorescence intensity showed a distinct spatial pattern; trends in fluorescence intensity were weak-strong-weak along an upstream to downstream continuum, mainly affected by changes in the content of protein-like substances. The spatial variation of the CDOM in the Lanzhou reach of the Yellow River can therefore be assessed based on protein-like materials dynamics. Public spaces along rivers offer opportunities for community gatherings and recreational activities. However, high-intensity anthropogenic activities strongly influence CDOM concentration and composition in this area in different ways; sources include increased residential/commercial wastewater, catering, water recreation facilities pollution, shipping, and a small amount of industrial discharge. In addition, it was concluded that endogenetic pollution may become the main source of internal loading in the Lanzhou reach of the Yellow River, implying that stronger endogenetic pollution control is needed to alleviate CDOM pollution and improve water quality.