| Water Source Protection Zone of Upper Huangpu River, as the important source of drinking water, has the area of 1058 km2, which accounts for 15% of the total area of Shanghai. It provides more than 50% of drinking water supply of the whole city, whose water quality would directly influence the safety of citizens' drinking water. In this area, the plantation and aquaculture are the primary agricultural activities, because of the much stricter standards for water environment and the limitation or prohibition of activities with higher pollution risk, such as industry and animal husbandry. For the purpose of higher agricultural benefits, the agricultural production method of higher multiple cropping index and higher chemical fertilizer input was adopted, and caused severe agricultural non-point source pollution (ANPSP), whose discharge amounts of nitrogen and phosphorus accounted for more than 60% of the total discharge amounts. The ANPSP was probably the main cause of frequent eutrophication in local water body and sporadic blue algae bloom in Dianshan Lake, which threatened the water environment in the Water Source Protection Zone of Upper Huangpu River. Therefore, it's meaningful to study the ANPSP and its influence to the water environment in this area.Take the Dianshan Lake area (Zhujiajiao Town, Jinze Town and Liantang Town in Qingpu District) in the Water Source Protection Zone of Upper Huangpu River as the target area, this study implemented spot monitoring for nitrogen and phosphorus losses from main agricultural production modes, based on the investigation and survey on the characteristics of agricultural activities. Accordingly, the characteristics of ANPSP and its influence on water quality of Dianshan Lake were studied, using the category analysis method and equivalent pollution load method.The results of the investigation and survey on the characteristics of agricultural activities indicated that, the main agricultural production modes were paddy-wheat rotation, paddy-rape rotation, paddy-green rotation, paddy monoculture, paddy-water oat rotation, water oat monoculture, vegetables rotation, fruits plantation and aquaculture.The results of the site monitoring for nitrogen and phosphorus losses showed that, the runoff coefficients of paddy-wheat rotation, paddy-rape rotation, paddy-green rotation, paddy monoculture, paddy-water oat rotation and water oat monoculture varied from 15% to 25% with no significant correlation with rainfall, which were influenced by not only rainfall, but also water demands of crops. The runoff coefficients of vegetables rotation and fruits plantation varied from 5% to 15% with positive correlation with rainfall, which were dominated by rainfall. The water discharge from aquaculture had no correlation with rainfall, because the water discharge was caused by water change in the period of frequent disease occurrence and pond cleaning at the end of the year. From the point view of the loss concentrations of nitrogen and phosphorus, the loss concentrations of nitrogen and phosphorus were nearly similar from the paddy plantation modes of paddy-wheat rotation, paddy-rape rotation, paddy-green rotation and paddy monoculture, while a little higher from vegetables rotation and fruits plantation. The loss concentrations were the highest from paddy-water oat rotation and water oat monoculture, because of large quantities of fertilizer inputs in water oat plantation. From the point view of the loss loads of nitrogen and phosphorus, the loss loads of total nitrogen (TN) from paddy-water oat rotation and water oat monoculture were the highest and reached around 90 kg/hm2, while it was more than 40 kg/hm2 from paddy wheat, and the fruits plantation had the lowest load which was 16 kg/hm2. The loss load of total phosphorus (TP) from aquaculture was the highest and reached 4 kg/hm2, while they were close to 2 kg/hm2 from water oat monoculture and paddy-water oat rotation, and the vegetables rotation had the lowest load which was 0.12 kg/hm2.The results of the study on characteristics of ANPSP appeared that, the total discharge amounts of TN and TP were 387.13,15.57t respectively, and the discharge amount in equivalent standard reached 1.40×109 m3/a. The load ratios in equivalent standard of TN and TP were 55.42%,44.58% respectively, and the load ratios in equivalent standard of aquaculture was 39.06%. The main source of ANPSP was aquaculture, and the main pollutants of ANPSP need to consider both TN and TP.The results of the correlation analysis between ANPSP and water quality of Dianshan Lake indicated that, because there were lots of other impact factors, the ANPSP had on significant impact on the TN and TP concentrations and stock in Dianshan Lake, and had positive correlation in some seasons. The discharge amounts of TN and TP from ANPSP only accounted for 6.65% and 4.49% of the input amounts of TN and TP from upper reaches, which indicated that the pollutant inputs from upper reaches was the dominating factor for the water quality of Dianshan Lake. However, the correlation between pollutant stocks in Dianshan Lake and pollutant inputs from upper reaches and agricultural non-point sources was more significant than the correlation between pollutant stocks in Dianshan Lake and pollutant inputs from upper reaches, which appeared that, the influence of ANPSP on water quality of Dianshan Lake still could not be neglected, especially under the circumstance of the slow improvement and difficult controlling of water quality from upper reaches. Therefore, the control of local ANPSP was the most urgent task in the Dianshan Lake area in Shanghai. |
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