Analysis Of Non-point Source Pollution Loading Characteristics For Polders In Plain River Network Area

In China, large amounts of polders of all sizes are widely distributed in the plain river network. With seepage-proof dykes blocking the free water exchange between inside and outside, polders are relatively closed units. Only at the time of drainage or irrigation, water exchange is enabled through the culvert, sluice, and pumping station manually so that water in polders can be discharged outwards or outer water can be inlet inwards. The unique hydrology structure and water management methods in polders result in the distinct characteristics of non-point source pollution.This paper chooses Shangxia West Polder in West Taihu Lake as a typical research sample. Analyzes the sources of polder pollution based on the combined consideration of the polder agricultural management practices and special water exchange process. Through the simulation and calculation of the NPS in the typical polder, as well as the continuous monitoring of the quantity and quality of exchanging water, researches on the temporal and spatial characteristics of the internal NPS nitrogen and phosphorus output and their discharging amount.Main contents and achievements in this paper are as follows:(1)Based on the field investigation and literature analysis, the characteristics of the spatial structure and water allocation are identified. Also, the types, outputing path,and outputing mechanism of NPS are summarized. The NPS from farmland runoff, livestock breeding,aquaculture and rural life are collected in the ditches, swags, and rivers in polders firstly. In the non-drainage period, the NPS is accumulated in the polders with long-term. Only at the time of irrigation, the NPS in polders can be discharged outwards with drainage through the culvert, sluice, and pumping station manually.(2)Through monitoring and calculating of the exchanging water quantity and water quality inside and outside Shangxia West Polder in 2015. The researches about the water flow allocation and about changes of the polders NPS are conducted. The conclusions are as follows: the quantity of irrigation water is slightly higher than that of drainage water during the rice season. The drainage water and the inflowing water are all at level of eutrophication. In addition, the average concentration of both nitrogen and phosphorus indicators in the drainage water is higher than that in the inflowing water, which indicates that the NPS in the polder flows into the receiving water at the time of drainage,thus causing further pollution in outside waters. The annual concentration of nitrogen and phosphorus pollution load is basically the same in the four drainage and irrigation stations,despite some differences in spatial distribution. The total nitrogen (TN) and total phosphorus (TP) concentration is closely related to the polder agricultural management practices. The concentration will significantly increase after people planting crops and top dressing fertilizers,whereas it will fall to the lowest point between the planting interval of two crops.From the comparison of water quality inside and outside the polder, it is demonstrated that the TN and TP concentration inside is higher than outside in summer. At the time of drainage in summer, the polluted water in the polder will be discharged to the outside rivers with NPS,and the polder becomes a "source" of pollution to the outside rivers. In the other three seasons, the TN and TP concentration in outside rivers is higher than that in the polder. But since there is no water exchange during this period,the quality of water in the polder is affected by the pollutants inside.(3) By applying the SWAT model based on the process, the output of NPS load in typical polders is estimated and analyzed. According to the four drainage and irrigation station control area, Shangxia West Polder is divided into four sub watersheds. Based on the geography, meteorology, agriculture management practices and so on in research region, model parameter are extracted. The NPS in Shangxia West Polder is simulated by distributed model, the results basically reflected the trend of nitrogen and phosphorus load output in the polder. However, for plain river network area containing large area of polders, the simulated output of NPS will be far higher than the actual value without considering the effects of polders.