Effect Of Dredging On Water Environmetnal Quality Of Ditch System Inside Of Nanhan Embankment In The Dongting Lake Region

The Dongting Lake region plays an important role in safeguarding national food security,of which the multiple ditches is densely distributed inside of the embankments for agricultur-al irrigation and drought and drainage resistances.However,due to the increasing production and daily life activities of local residents,ditch siltation has gradually becoming a serious pro-blem inside of the embankments in recent years.Since 2016 Hunan Water Conservancy Depa-rtment have centrally organized and implemented the ditch dredging project in the Dongting Lake region.The ditch dredge is commonly carried out by water dredging and sediment exca-vation method,which could greatly change water environment factors and even fundamentall-y affect the water environment quality of the ditch system.With supporting by science and te-chnology major project of Hunan Water Conservation Department,we selected a typical regi-onn-Nanhan Embankment of Nanxian County as the target area.In this study,through field survey and sampling collection,laboratory analysis and model simulation,the main sources of pollution and the current status of water environment quality of the ditch system in Nanhan Embankment were identified.Then,the effects of ditch dredging on its surface water quality,water environmental capacity of a typical ditch system and eutrophication indicators were an-alyzed.Finally,the relationships between ditch dredge and nitrogen content and speciations e-xisted in the sedimentation and theirs migration patterns were investigated.The main results are as follows:(1)The main water environment compartments of the Ditch system inside of Nanhan Embankment are surface water(underground water neglected),sediments,aquatic biotics,river bank and riverbank plants.Generally,the branch ditches have a width of 3?5 m and a depth of 1?1.5m,while main trunks are 5?10 m in width and 2?4 m in depth.As no river slope in the plain terrain,the ditch system usually has a small flow velocity of almost 0 except for working time for flood discharge and water diversion.At the same time,fast-growing weed and ditch siltation were easily appeared in the ditch system,which was easily affected by the production and daily life activities of local residents.Overall,water environment quality of the ditch system was relatively poor.Results show that over 52%ditches in Nanhan Embarkment have been seriously polluted with water hyacinth or algae,and 92%of them have severe problems such as fast-growing weed or siltation.Livestock and poultry husbandry aquaculture,and crop farming were main production activities as well as sewage discharge and garbage disposal were typical daily life activities of local residents,of which all could greatly affected the water environment quality of the ditch system inside of Nanhan Embankment.(2)According to Water Environment Quality for Surface Water(GB3838-2002),most of the ditch surface water were inferior to Grade V,especially for the sections affected by resident activities including livestock and poultry husbandry,aquaculture and sewage discharge,which could easily fell into moderate or severe level of eutrophication.Ditch-dredging could rapidly promote the water quality of the surface water to Grade III(GB3838-2002),and subsequently water quality indexes including total nitrogen(TN),total dissolved nitrogen(TSN),permanganate index(CODMn),chlorophyll a(Chla),total phosphorus(TP),nitrate nitrogen(NO3--N),ammonia nitrogen(NH4+-N)gradually increased while dissolved oxygen(DO)and water transparency(SD)gradually decreased.Model calculation results show that ditch-dredging could significantly promote the water environmental capacity of the ditch surface water,of which the CODMn,TN and TP were increased from 13.99 t/m to 38.89 t/m,from 2.76 t/m to 7.73 t/m,and from 0.55.t/m to 1.47 t/m,respectively.(3)The average TN content in the ditch sediments was 1036.4 mg/kg,and the content of each nitrogen speciation follows the order:acid hydrolysable nitrogen(HN,59.05%)>residual nitrogen(RN,21.46%)>exchangeable nitrogen(EN,19.08%)>free nitrogen(FN,0.41%).Meanwhile,the TN could be increased to 1457.8 mg/kg under resident activities including livestock and poultry husbandry,aquaculture and sewage discharge,but could be decreased to 342.6 mg/kg by the ditch-dredging implementation.When implemented ditch-dredging project,the diffusion fluxes of NH4+-N and NO3--N in interstitial water of the sediment were decreased,and their migration directions were also changed.