Characteristics of Surface Water Quality Affected by Agricultural Non-point Sources in Guigang City

[Objectives]To analyze the influence characteristics of surface water quality by agricultural non-point sources in Guigang City of Guangxi. [Methods]The daily concentration series of water quality indicators at three state-controlled monitoring stations in Guigang City from 2019 to 2021 was analyzed by using Daubechies (db) wavelet, and Morlet wavelet was used to analyze the daily average concentration of water quality indicators. Continuous wavelet transform (CWT) was used to analyze the monthly concentration series of water quality indicators at three state-controlled monitoring stations in Guigang City from 2014 to 2021. [Results]The Daubechies (db) wavelet analysis showed that the concentrations of CODMn, TP, and TN had the maximum values during June-July and October-November, and there were spatial differences among monitoring stations (CODMnconcentration exceeding the standard was the most serious in Shizui, and DO concentration not up to standard was the most in Thermal Power Plant, and NH3-N, TP and TN exceeding the standard was the most in Wulin Ferry). Morlet results showed that principal period of wavelet variance graphs of CODMn, NH3-N, and TP was 340 d, and there was the same sub-period of 140 d, and principal period of wavelet variance graph of DO was 260 d. CWT results showed that CODCrhad similar resonance periods of about 1-2 and 5-7months; BOD5and CODMnwas dominant by the resonance period of 1-4 months (2014-2017); DO had a similar resonance period of about 1-3 months; NH3-Nwas dominant by the resonance period of 1-5 months. [Conclusions]The surface water quality of Guigang City was mainly affected by the residual nitrogen and phosphorus nutrients and pesticide residues from agricultural production activities.