Allocation For Total Mass Control Of Nitrogen Pollutants In Tiaoxi River Estuary

With the increasingly serious water pollution in China, total mass control of water pollutants has become the fundamental guarantee for sustainable development. A scientific and reasonable scheme for total mass control of water pollutants should give due to consider three principles, namely fairness, efficiency and feasibility. In this study, a framework of the water pollutants total mass control based on "environmental capacity, total mass control, effect response" was established, which combined the three principles, and Tiaoxi River Estuary of South Taihu Lake was selected as the study area. Water quality simulation based on WASP model was taken as a key technique in this framework. The results were shown as following:(1) Water environmental pollution restricts the sustainable socio-economic development of the Taihu Lake basin, and nitrogen is the critical limiting factor for the eutrophication problems in Taihu Lake. Nitrogen analysis for the cross sections of the estuary area showed that the main nitrogen species in overlying water was inorganic nitrogen, while organic nitrogen was dominated in sediment. Source appointment of total nitrogen (TN) showed that agricultural non-point source pollution was a major pollution source, accounting for47.10%of the TN, while domestic pollution and industrial pollution accounted for28.93%and23.97%, respectively. Among the agricultural non-point sources, the farming contributed relatively more emissions of TN than the livestock and aquaculture.(2) The water environmental capacity of TN was calculated by the simulation based on WASP model for different water quality goals. The concentrations of TN in the Tiaoxi River Estuary were higher than the water standard of Grade Ⅴ for most of the time, and the TN load was2152.8t·a-1. The concentrations of TN meeting the standard of Grade Ⅴ, Grade Ⅳ and Grade Ⅲ were set as the short-term, mid-term and long-term goals, respectively. To reach these goals, the corresponding TN loads should be reduced to1444.5t·a-1,1190.8t·a-1and937.1t·a-1with the reducing rates of32.90%,44.70%and56.50%, respectively. Those results indicate high pressure in emission reduction of TN and that alleviating water pollution will be a progressive and gradual process.(3) The TN load of Tiaoxi River Estuary was allocated to the regions under the principle of fairness. Based on the Gini coefficient model which involved the factors of industrial, agricultural (included farming, livestock, and aquaculture), and domestic pollution, the allocation of TN achieved the fairness goal with all the Gini coefficients of factors less than0.4. Meanwhile, the allocation scheme had been adjusted based on the regional pollution sensitivity, so that the reducing rate for short-term goal declined from32.90%to31.12%.(4) The concentration of TN was simulated by WASP model in different reducing rates. The results showed that the compliance rate of TN meeting Grade V, Grade IV and Grade III would reach the goals steadily after phases in the short-term, mid-term and long-term in the current reducing rate. And the results of economic feasibility test showed that the economy index for allocation reduced from0.266to0.230, indicating that the economic efficiency increased slightly. In conclusion, the allocation scheme had reached the standard in environmental and economic feasibility.