Study On The Output Characteristics Of Non-point Source Pollution From Upland In The River Network Polder Area Based On The Process Into The River

The output of nitrogen（N）and phosphorus（P）from upland was the main cause of water eutrophication.In polder areas,besides rainfall intensity,the process of N and P discharging into river were also affected by tide and hydraulic management.Meanwhile,the release of ditch sediments might also increase the N and P load into river.Nevertheless,study on the output behavior of N and P was limited to experiment plot,and there was lack of simultaneous study on the runoff yielding and N and P’s entrance into the river under rainfall events.An upland with an area of 0.1665 hm~2 was chosen in Chongming Island,which was located in the lower reaches of the Yangtze River.An automatic sampling methods was established to conduct high-frequency sampling under rainfall events.Meanwhile,the process of runoff yielding and outflowing into river was also studied.Furthermore,to clarify the nitrate load from farmland,the nitrogen and oxygen isotope composition was monitoring,and the stable isotope model SIAR was used to quantitatively study the contributions of different sources.The main findings are as follows:（1）The runoff volume and runoff coefficient in rainfall events were significantly correlated with rainfall（r=0.940,P<0.01 and r=0.706,P<0.01）.The curve number（CN）for upland ranged from 29to 88 and correlated best with the antecedent moisture condition of the 10 cm soil layer(AMC₁₀),which is more suitable as an indicator of soil water holding capacity.The onset of peak runoff lagged behind the peak of rainfall after the upland began to produce flow.During flow production,the water level of ditches rises significantly faster than that of rivers,which is influenced by the opening and closing of gates in the polder.The runoff velocity at the farm drainage outfalls and ditch inlets was significantly negatively correlated with EC.Rainfall increased the runoff flow and produced a dilution effect,resulting in a decrease in EC,which can be used as a discriminating criterion to exclude flow velocity anomalies measured by the Doppler principle.（2）Turbidity readings of runoff was significantly and positively correlated with sediment concentrations（r=0.869,P<0.01）,and total P（TP）concentrations were significantly and positively correlated with TSS concentrations（r=0.786,P<0.01）.Turbidity readings can be used to indicate sediment and P concentrations in the runoff,and total N（TN）and nitrate-N（NO₃^--N）concentrations had good positive correlations with EC.Rainfall had a dilution effect on TN and NO₃^--N concentrations,while P concentrations had a significantly negative correlation with EC readings.Turbidity readings in runoff increased with the increase of rainfall intensity,while EC readings decreased with the increase of rainfall intensity.Using the hydrograph of turbidity or EC readings versus runoff velocities,it was found that the turbidity were mainly clockwise lagged,while EC were mainly counterclockwise lagged.（3）The duration of the peak phase was mainly influenced by rainfall,and there was a first flush effect of sediment discharge.The concentrations of TN and NO₃^--N decreased with the increase of runoff velocity,and the concentrations of TP and dissolved total P（DTP）were significantly and positively correlated with runoff velocity（P<0.01）.The event mean concentration（EMC）of TN and NO₃^--N showed a significant negative correlation with the time to the last fertilizer application（r=-0.786,P<0.05;r=-0.609,P<0.05）.Besides soil tillage and crop residue return,the EMC of P was significantly influenced by rainfall intensity（r=0.57,P<0.05）.（4）The amount of sediment loss from upland was highest in July,which could be attributed to the highest amount of rainfall.The load of TN and NO₃^--N was mainly influenced by fertilizer application,and the annual load of N from upland was 30.0 kg/hm~2.The P load was mainly influenced by rainfall,and the annual load of P was 1.9 kg/hm~2.The NO₃^--N and particulate P（PP）were the main form of N and P loss from upland respectively.（5）The release of N and P from sediment might cause the increasing concentrations of ammonium-N（NH₄⁺-N）and P along the ditch.When rate of flow in ditch reached a critical velocity,the sediment at the bottom of ditch might be disturbed.The reduction in sediment thickness caused by the first rainfall peak was significantly correlated with the amount of rainfall during that period（r=0.980,P<0.01）.（6）NO₃^--N was the main form of N of streams in Chongming Island.The differences inδ¹⁵N-NO₃^-values of different main streams were more obvious,while there is no significant difference inδ¹⁸O-NO₃^-.Using SIAR model,it was found that the NO₃^--N in the streams mainly came from domestic sewage,and the contribution of N from soil was also increased due to the rainfall-runoff.The contribution of the four sources to nitrate in mainstreams is in the following order:domestic sewage>soil N>fertilizer>rainfall,where the contribution of domestic sewage and soil N can exceed 30%.