Spatial-temporal distribution characteristics and traceability analysis of organic matter in Shahe Reservoir (Beijing, China).

Shahe Reservoir is a key node in the upstream of the North Canal, and the water quality has gradually improved after the implementation of low water operation in 2018. The organic matter(OM) in the sediment decreased from 16.66 to 14.22%.In this study, the FI index and parallel factor method were used to investigate the traceability of OM and dissolved organic matter (DOM) in the Shahe Reservoir before and during low water level operation(LWLO), and the results showed that the terrestrial source fraction of OM in sediments was mainly related to organic-rich terrestrial plant residues carried by tributaries and overflows/outfalls during the rainy season, and the FI index indicated that the organic matter (OM) in the Shahe Reservoir before and during LWLO in each DOM in the area is derived from authigenic sources of autotrophic microorganisms, algae, etc. The parallel factor method shows that more than most of the pollutants in the DOM are input from endogenous sources and a small proportion of pollutants are input from exogenous sources. Nutrients in both sediment and interstitial water increased during the LWLO, with TN and TP levels increasing by 262.38 and 204.45 mg·kg in sediment, NH-N, PO-P, TN and TP in interstitial water increasing by 0.98, 1.36, 2.07 and 4.33 mg·L, respectively. Pearson correlation analysis and principal component analysis showed that OM was significantly correlated with nutrients: OM and TN (p < 0.01) and OM and TP (p < 0.05) in the pre-LWLO; OM and TN and TP (p < 0.01) in the LWLO.The results suggested that organic matter pollution control should be mainly carried out from the perspective of endogenous input, focusing on controlling the release of nutrients in sediments.