Characteristics And Source Analysis Of Organic Matter Components In Ulansuhai

Ulansuhai is typical of agricultural irrigation receding lakes in semi-desert areas throughout the world.The lake is the only location for receding water from agricultural fields in the river-loop irrigation area,which is important for controlling the salinization of the land in the river-loop irrigation area and ensuring the water quality safety of the Yellow River downstream.In recent years,with the implementation of many treatment measures,the eutrophication problem of Ulansuhai has been greatly improved,but the COD_Cr still far exceeds the limit of surface water category V water.In this study,the composition and structural characteristics of organic matter were analyzed by spectroscopic techniques,and the degradation performance of organic matter in the Ulansuhai and its influencing factors were further investigated by natural degradation simulation experiments.The environmental behavior of different organic matter is further investigated by natural degradation simulation experiments.The environmental load,structural characteristics and degradation performance of organic matter are considered to further reveal the environmental behavior of different organic matter.From 2011 to 2021,the overall TN and TP in the Ulansuhai decreased from Class V water to Class IV water,with concentrations decreasing by 78.2%and 53.3%.The COD_Crvalue,though reduced by 54%,still belongs to the inferior class V water.The results of the comprehensive water quality index（CWQI）show that the water quality has obvious spatial and temporal characteristics,with higher pollution level in winter and summer,and a decreasing trend of pollution level from northwest to southeast of the lake,in which COD_Cris the most important pollution characteristic factor,indicating the serious problem of organic pollution in Ulansuhai.The contribution of dissolved organic matter（DOM）in the water column of the Ulansuhai was 76%-90%,with tyrosine-like,fulvic acid-like and humic acid-like fractions as its major fluorescent fractions.DOM in the water column has low molecular mass and is not highly humified.Humic-like fractions dominate in autumn,and protein-like fractions account for a relatively high proportion in winter,spring and summer.The humic acid-like fraction and the fraction related to microbial metabolism accounted for a higher proportion in the drained water column DOM,and the fluorescence intensity of DOM was higher in the ice-covered period than in the non-ice covered period.According to the PCA results for spectral indices and physicochemical water environment factors,the degradation and transformation process of organic matter was influenced by TN,SAL,p H and NH₄⁺-N,the metabolic activities of microorganisms had a strong regulatory effect on water body organic matter.The degradation of organic matter is low in the Ulansuhai.Due to the difference of organic matter composition,the degradation rate of organic matter in Ulansuhai is:sediment（38.7%）>inlet drained water（33.85%）>lake water（27.3%）>receding drained water（23%）.Organic matter degradation rate is higher in the northern and central lakes than in the southern Ulansuhai.The sediment organic matter mainly comes from the interior of the lake,with larger relative molecular mass and higher degree of humification.The organic matter in the water column of the Ulansuhai is strongly influenced by endogenous release than exogenous input.The endogenous characteristics are obvious in winter,spring and summer,and the exogenous characteristics are obvious in autumn,and the exogenous signal of organic matter in the total drained water body is stronger.Since 2016,the lake body for the retention of exogenous organic matter has been significantly reduced,the exogenous input organic matter has been initially controlled.Organic matter in Ulansuhai is influenced by the accumulation of exogenous input for many years,the total amount of endogenous organic matter is large,and the organic matter degradation performance of the water body is poor.