Process Simulation And Watershed Management For Non-point Source (NPS) Pollution In Tidal Plain With Dense River Networks, China

As point pollution sources like industry pollution and domestic pollution have been under control, non-point sources pollution (NPS) has become the main factor on urban water quality, and received great attention from governments and researchers. In this context, process simulation and management and management of NPS have turned into a hotspot issue. Urbanization remarkably makes a great impact on the process of NPS in tidal plain with dense river network because of its special hydro-geomorphologic characteristics. Therefore, Lingang New City (LGNC) of Shanghai was taken as an example in this dissertation. From an intersection point of environment, ecology, hydrology and management science, combining theoretical research and empirical study, laboratory simulation and site monitoring, mathematical model and computer simulation, the dissertation takes the sub-watershed division as the breakthrough point, focuses on two main lines of non-point pollution, namely "source-process-sink" and "experiment-simulation-management". The dissertation illuminates the feature of NPS process, constructs a watershed-based management model and proposes methods, principles and key strategies for management of NPS in urban watershed. The main results are as follows,1. Based on typical water conservation zone management system in tidal plain with dense river network, the dissertation analyses characteristics of urban watershed in this area, and discusses the NPS-control-oriented zoning system from three dimensions of spatial scale, runoff path and controlling intensity. Zoning system is formed by four hierarchies:first-level watershed management is to balance relationship between study area and water conservation zones from large-scale and high-intensity by paying attention to main rivers:second-level watershed management is to regulate drainage system and function division from median scale and intensity by focusing on median rivers; third-levelwatershed management is to adjust the relation between small topographic area and watershed outlets from small-scale and low-intensity by concerning regional rivers; forth-level watershed management is for data support and mechanism exposition of NPS progress. As for LGNC, there are 1 first-level watershed,13 second-level sub-watersheds and 137 third-level sub-watersheds. Second-level watersheds are mainly in accord with the existing function zones, which may guarantee implement of management strategies.2. The dissertation discusses division method and process simplification method. On this basis, the dissertation represents process and features of NPS of urban watershed in tidal plain with dense river network using data by laboratory simulation and site monitoring.(1) The dissertation proposed the concept of Effective Pervious Areas (EPA) from aspect of NPS management compared to classical concept of Effective Impervious Area (EIA). EPA indicates the pervious area that receives and stores runoff from other areas. On this basis, the dissertation brings up the concept of Effective Green Area (EGA).(2) The dissertation analyses first flush effect of urban impervious area. Total level of NPS output of LGNC is lower than that of inner city of Shanghai. First flush effect of particulate pollutants is significant, but not dissolved pollutants. The concentration of particulate pollutants is much high than dissolved pollutants. It may because that LGNC is a newly-developed area. Land use type and rainfall intensity have great affect on first flush effect.(3) The dissertation illuminates the "source-sink" effect of urban pervious area. In theory, urban pervious area is the source as well as the sink of NPS. However, existing studies mainly focus on its sink effect.a) In terms of"source":surface runoff pollutant concentrations from existing green spaces in downtown are low, better than Grade V of national standard; however, surface runoff pollutant concentrations from newly-developed green spaces in suburban area are higher, inferior to national standard of Grade V, even reach up to 3.56 times of Shanghai Sewage Discharge Standard. Thus, pervious surface areas including urban green spaces can be regarded as potential pollution source, and pollution intensity is related to the location, completion time and soil origin.b) In terms of'sink'":urban green spaces are efficient for surface runoff pollution removal. Removal rate of COD(71.01±1.75%) is higher than TP(69.00±1.78%) and NH4+-N(64.39±1.84%). Removal effect of urban existing green spaces is more significant than suburban newly-developed green spaces (p<0.05). For different covers, effect of Ophiopogon japonicas is best, then Zoysia japonica, and exposed soil. Removal of pollutants happens mainly in soil horizon of 60cm to 80cm underground, and physical process is predominant. Moreover, pollution load, hydraulic load, detention time and timing of drainage have affect on pollution removal rate.c) Transformation of source and sink:In theory, urban pervious area acts as both source and sink. Non-effective pervious area functions as source, while effective pervious area functions as sink. Concretely, green spaces which have curbstones and cannot take in external runoff are sources, and green spaces which absorb and reduce pollution of external runoff are sinks. In summary, setting of curbstones is key point to change source to sink. By constructing curbstones at proper sites, disposing appropriate plant cover, soil type and soil depth, leading external runoff into green spaces in a proper velocity and volume, installing enough detention time can improve the reduction efficiency of urban pervious area.d) The dissertation takes soil salinity as an example to analyze leaching effect of precipitation to key element of urban pervious area. Soil salinity of urban pervious area has spatial and temporal variation in small scale, and the leaching effect differs from urban to suburban. In this study, salinity of existing urban green spaces is low. and salinity values of all 704 samples are under moderate salinity. In contrast, newly-developed green spaces are in highly salinization. Of all 240 samples, forty percent samples are saline soil, and plaque salinization is notable. Average variation coefficients of urban green spaces and suburban green spaces are 37.7%and 67.9%. Result of variance analysis shows that seasonal variance of four green-spaces is significant (p<0.05), the average value of topsoil salinity is spring>winter>fall>summer, and there is significant correlation between seasons of topsoil (p<0.01). Leaching effect of existing green spaces in center city is not significant, but situation of newly-devolped green spaces in suburban area is significant. Effluent of the former is plain water, while effluent of the latter is mixohaline. Rainfall intensity and precipitation are main factors.e) Three main non-point pollution sources are surface runoff (pollution from land use), rainfall and disperse rural domestic sewage. In recent decades, pollution load and transform flux are in significant increase. Change of pollution load and transform flux can be divided into three phases:the first phase (from 1965 to 1994) when load produced was not many and grew at low speed, the second phase (from 1995 to 2003) when problem of non-point pollution appeared, and the third phase (from 2004 to 2008) when the problem of non-point pollution emerged as a major issue. Rapid and large-scale development in LGNC results in change of land use structure and more pollution to be produced. On the other side, function of green space as sink should be promoted (average removal rate of 2.07%).3. The dissertation builds a management model combining systematic dynamics, multi-objective optimization, scenario planning, stakeholder survey and GIS, and takes LGNC as a case.(1) The dissertation points out that there are five aspects of uncertainty in management of NPS in urban watershed:Spatial and temporal uncertainty, source management uncertainty, process management uncertainty, sink management uncertainty, uncertainty of stakeholders and decision maker's preference and uncertainty of models themselves.(2) For newly-developed zone, to manage NPS by changing land use structure is really difficult. In consequence, the dissertation proposes a thought of management by raising ecological function of urban pervious area as "sink" and using sewer net to receive first flush.(3) The dissertation investigates coupling mechanism of systematic dynamics, multi-objective optimization, scenario planning, stakeholder survey and GIS, and proproses the four steps coupling process:sub-watershed and sub-system analysis, sub-watershed development scenarios setting, spatial and temporal simulation and prediction of watershed non-point pollution, and optimal proposal determination.(4) The dissertation sets five typical scenarios and applies them into simulation of LGNC. The five scenarios are current situation SI, function promotion situation S2 and S3 (considering effective green space and effective wetland), and comprehensive reduction situation S4 and S5 (considering combination of effective green space, effective wetland and sewer construction). The main results are as follows,a) Through limited function promotion without sewer construction, management effect is not satisfied. For example, compared to current COD transform flux of 15564t/a and runoff coefficient of 0.42, in 2020, COD transform flux and runoff coefficient decrease by 7.5% and 7.8% by S2 and 1.3% and 2.6% by S3.b) Through function promotion and sewer construction, NPS transform flux is drastically cut, and runoff coefficient realizes "zero growth". Compared to SI, in 2020, COD transform flux and runoff coefficient decrease by 33.3% and 80.3% by S4 and 26.2% and 50.6% by S3.c) Relative error between land use structure of LGNC simulated in 2020 and current planning is tiny (p>0.05). Thus, to adjust land use structure planning fractionally is operable for NPS management.(5) The dissertation constructs a system for NPS management of urban watershed from'high-median-low" management levels, "source-process-sink" management procedures, and key strategies based on system theory. In accordance with different urban scales, the dissertation raises differentiated non-point pollution management strategies:take Shanghai as a case to show rainfall utilization is essential at at the city or regional level, take inner-city of Shanghai as a case to show effective green area is essential for the developed area, take LGNC of Shanghai as a case to show zero-growth of runoff is essential for the newly-developed area.