Simulation Study On The Water Quality Response In The Tail Of Ganjiang River Based On Comprehensive Land Use Remediation

The purpose of this study was to investigate the hydrological situation and water quality characteristics in the Ganjiang River sink before and after the comprehensive improvement project,which includes the setting of four diversion weirs on the main branch,north branch,middle branch and south branch of the Ganjiang River,and is planned to be completed by the end of 2026.Firstly,the quantitative relationship between land use and water quality response in the Ganjiang river basin was analyzed.Secondly,assuming different land use growth trends in the 20 km radius buffer zone upstream of the Waizhou Hydrological Station from 2021 to 2027,three land use scenarios（a total of five working conditions）were set.Thirdly,the concentration changes of COD_Mn,NH₃-N,TN and TP in Ganjiang River sink（dry year）under different working conditions were simulated and analyzed after the completion and operation of the project（2027）,and corresponding pollutant reduction schemes were put forward.The main conclusions are as follows:（1）In the land structure of Ganjiang River Basin,the area of forest,cropland and grassland accounts for more than 90%.Over the past two decades,the size of cities in the Ganjiang River basin has been on the rise..（2）Upstream of the water quality monitoring point,land use types within a 20-km radius buffer scale had a higher water quality explanation rate of 96.28 percent.（3）Hydrodynamic conditions and sediment contamination levels are two important factors that affect the release of nutrients in sediment.The released fluxes of TN,TP,and NH₃-N in the sediments of the Ganjiang sink show an exponential dependence on the flow velocity.（4）Before the project,the northern branch of the Ganjiang River often broke off.After the project,the lowest water levels of the four downstream branches are all above 15m,and the inflow of the north,middle and south branches of the Ganjiang River has increased,which has a favorable impact on ensuring local water use.（5）After the completion of the project,the concentration of COD_Mn,NH₃-N,TN and TP in the front section of the four sluices in the lower reaches of the Ganjiang River decreased during the dry months.The concentration of COD_Mn changes from Class III before the project to Class I after the project in the front section of the North Branch at the Lianxin hydro-junction;The TP concentration changes from Class IV before the project to Class II after the project in the pre-project section of the South Branch at Gili hydro-junction.（6）In the 20-km radius buffer zone in the upper reaches of the Waizhou Station Basin,the land use types from 2021 to 2027,according to the conversion rate changes of 50%,100%and 150%from 2010 to 2020（three land growth scenarios,a total of five working conditions）.After the project is completed and operated（2027）,the concentrations of COD_Mn,NH₃-N and TP in the front section of sluices of the four branches of the lower reaches of the Ganjiang River can meet the requirements of functional zoning of water environment.With the exception of the south branch of the Ganjiang River,the NH₃-N and TP concentrations in the front section of the sluices of the other three branches can meet the Class II water quality standards throughout the year.（7）When the TN concentration in the inflow water of Waizhou station is not higher than the Class II water quality standard value（0.5mg/L）,and the total amount of pollutants（point source,non-point source）in the region is not higher than the total amount of pollutants into the river in 2020.After the construction and operation of the project,the TN concentration in the Ganjiang River sink can meet the water function requirements.zoning throughout the year.