| Total load control of pollutants discharge is one of the fundamental measures for water quality protection and restoration in China. In this paper, the author gave an overview of the update international concepts and technologies of water pollution total load control, and then a case study of upstream of East Tiaoxi River in Taihu watershed based on Load Duration Curve (LDC) model was carried out. The water flow and quality data in upstream of east Tiaoxi river from 1999 to 2008 were used to set up LDC curve, total maximum daily load (TMDL) on total nitrogen (TN), total phosphorus (TP) and chemical oxygen demand (COD) were calculated, comparing with the results of the total pollutants discharge loads calculated by 1-D steady model as a reference case. The results indicate that TMDL was an updating approach as a total load control technology of pollutants in watershed scale. The main conclusions were elaborated as follows:(1) Constructing LDC for upstream of East Tiaoxi river and determining the flow duration interval (FDI). The author constructed the LDC for upstream of East Tiaoxi river using flow data and water quality data from 1999 to 2008, then divided it into five FDIs by flow characteristic analysis:high flows (0-10%)characterized as flow>66.7 m3/s, which reflected the range of flow when storm happened; moist conditions (10-40%) characterized as66.7 m3/s>flow>21.1 m3/s which reflected the range in high flow situation; mid-range flows (40-60%), characterized as 21.1 m3/s>flow>8.4 m3/s which reflected the range in normal flow situation; dry conditions (60-90%), characterized as 8.4 m3/s>flow>1.4 m3/s which reflected the range in dry flow situation and finally low flows (90-97%), characterized as 1.4 m3/s>flow>0 m3/s which reflected the range in extreme low flow situation.(2) Calculating TMDL on pollutants and improving the accuracy of water pollution simulation. The author predicted the TMDL on TN, TP and COD in East Tiaoxi river based on LDC, then compared with the water loading capacities calculated by LDC and 1-D steady model. The results indicated that water capacity was significant different in different hydrologic years, in dry year and extremely dry year, the TMDLs calculated by LDC was lower than results given by 1-D steady model, However, in other hydrologic years, the results were opposite and the gap existed. Under the condition of designed flow (i.e.90% flow guarantee), the TMDLs on TN, TP and COD calculated by LDC was 18.76%,15.05% and 37.87% of that by 1-D steady model. On the other hand, the loading capacities calculated by 1-D steady model equal to TMDLs calculated by LDC under the condition of 62%,50% and 73% flow guarantee respectively. Therefore, the TMDLs on pollutants calculated by LDC was available to reflect the various of loading capacity inner year and more practical for pollution control for the river whose flow was intensively changing inner year.(3) Analyzing mechanism of pollution discharge, setting margin of safety (MOS) and proposing the method of load allocation. By analyzing the mechanism of pollution discharge using historical water quality data in upstream of East Tiaoxi river, the results indicated that:TN load discharge of East Tiaoxi Watershed was under the water quality level, mainly because of combined pollution from all kinds of pollution sources. In low flow patterns, point sources pollution was the primary source of TN load discharge; in other patterns, point and nonpoint sources pollution both contributed a lot, where nonpoint sources pollution was dominant mainly through agriculture runoff. TN load discharge should be cut by 97.97%. In addition, the author proposed the pattern of load allocation through seasonal and monthly variations of TMDL analysis, the results of TN load allocation were as follows:in spring the parameter was 232.10 kg/d, whereas in summer was 389.97 kg/d, in autumn was 109.81 kg/d and in winter was 24.74 kg/d, monthly load allocation was in range of 3.0-17.61 kg/d, with the maximum in Jun and the minimum in Oct. Moreover, the result of TP was only one fifth of TN, and COD was twenty times of TN. |
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