In 2007, monitoring of the quality of water environment of the Three Gorges reservoir area included the monitoring on water quality of both mainstream and tributaries of the Yangtze River and water bloom of primary tributaries. The assessment of water quality complies with Environmental Quality Standard for Surface Water (GB3838-2002). The assessment of comprehensive nutrition status of water bodies complies with the Technical Regulations on Eutrophication Evaluation Method and Grading for Lakes (Reservoirs) developed by China National Environmental Monitoring Center.

5.1 Water Quality Monitoring

In 2007, a total of 13 water quality monitoring sections were established at both mainstream and tributaries of the Yangtze River in the reservoir area. Among them, 6 sections were at the mainstream and located at Zhutuo, Tongguanyi and Cuntan of Chongqing Municipality; Qingxichang in Fuling; Tuokou in Wanzhou; and Guandukou in Badong County. Seven monitoring sections were at primary tributaries of the Yangtze River, they were Beibei section and Linjiangmen section of Jialing River; Wulong section at Wujiang River, mouth of the Yulin River, mouth of Pengxi River, mouth of Daning River and mouth of Xiangxi River respectively. The assessment of water quality had 13 items, including pH, dissolved oxygen, permanganate value, BOD5, ammonia nitrogen, petroleum, TP, Hg, Cd, As, Cu, Pb and Cr6+.

5.1.1 Water Quality of the Mainstream

In 2007, the overall water quality of the mainstream of the Yangtze River was better than that of 2006. Among the 6 sections, Tuokou section and Guandukou section met Grade Ⅱ national surface water quality standard; while Zhutuo section, Tongguanyi section, Cuntan section and Qingxichang section met Grade Ⅲ national surface water quality standard. Among them, water quality of Zhutuo section and Cuntan section went up from Grade Ⅳ in 2006 to Grade Ⅲ. The quality of the remaining 4 sections kept stable.

Water quality was the best during February-May and in October. All sections met Grade Ⅱ or Ⅲ national standard in each month. It was relatively good in January, November and December, 5 sections met Grade Ⅱ or Ⅲ standard in each month except one section falling into Grade Ⅳ due to petroleum pollution. Water quality was relatively poor in June and September. In June, 2 sections only met Grade Ⅴ national surface water quality standard due to petroleum pollution. In September, the water quality of 3 sections was Grade Ⅳ due to the impacts of total phosphorus. Water quality in July and August was the poorest with TP, permanganate value and petroleum as main pollutants.

Table 5-1　Water quality of the mainstream of the Yangtze River in the Three Gorges reservoir area in 2007

Table 5-2　Percent of different grades of water quality of the sections of the mainstream of the Yangtze River in the Three Gorges reservoir area in 2007 (%)

5.1.2 Water Quality of the Tributaries

In 2007, the overall water quality of the tributaries of the Yangtze River in the Three Gorges reservoir area was relatively poor, but slightly better than that of 2006. Among the 7 monitoring sections, Beibei section met Grade Ⅱ water quality standard. Linjiangmen section and Wulong section met Grade Ⅲ standard, whereas the other four sections met Grade Ⅳ standard due to the pollution by total phosphorus. Among them, the water quality of Beibei section improved from Grade Ⅲ in 2006 to Grade Ⅱ, and that of Linjiangmen section improved from Grade Ⅳ in 2006 to Grade Ⅲ in 2007.

At least 2 sections failed to meet Grade Ⅲ national surface water quality standard in each month. The percent of monitoring sections meeting national water quality standard ranged from 28.6% to 71.4%. The failure of the 4 monitoring sections at river mouth to meet national water quality standard was quite common due to the impacts of TP.

Table 5-3　Type of water quality of the sections of primary tributaries of the Yangtze River in the Three Gorges reservoir area in 2007

Section name

River name

Jan.

Feb.

Mar.

Apr.

May

June

July

Aug.

Sep.

Oct.

Nov.

Dec.

Whole year

Beibei

Jialing River

Ⅱ

Ⅱ

Ⅱ

Ⅱ

Ⅱ

Ⅱ

Ⅱ

Ⅱ

Ⅱ

Ⅲ

Ⅱ

Ⅱ

Ⅱ

Linjiangmen

Jialing River

Ⅲ

Ⅲ

Ⅳ

Ⅱ

Ⅱ

Ⅱ

Ⅳ

Ⅱ

Ⅱ

Ⅳ

Ⅲ

Ⅲ

Ⅲ

Wulong

Wujiang

Ⅲ

Ⅲ

Ⅱ

Ⅱ

Ⅱ

Ⅳ

Ⅲ

Ⅲ

Ⅲ

Ⅲ

Ⅲ

Ⅲ

Ⅲ

Mouth of Yulin River

Yulin River

Ⅳ

Ⅲ

Ⅳ

Ⅳ

Ⅲ

Ⅳ

Ⅳ

Ⅳ

Ⅳ

Ⅴ

Ⅳ

Ⅲ

Ⅳ

Mouth of Pengxi River

Pengxi River

Ⅳ

Ⅳ

Ⅳ

Ⅳ

Ⅳ

Ⅳ

Ⅳ

Ⅳ

Ⅲ

Ⅲ

Ⅳ

Ⅳ

Ⅳ

Mouth of Daning River

Daning River

Ⅳ

Ⅳ

Ⅳ

Ⅳ

Ⅳ

Ⅳ

Ⅳ

Ⅳ

Ⅳ

Ⅳ

Ⅲ

Ⅳ

Ⅳ

Mouth of Xiangxi River

Xiangxi River

Ⅳ

Ⅳ

Ⅳ

Ⅳ

Ⅲ

Ⅳ

Ⅳ

Ⅳ

Ⅲ

Ⅳ

Ⅳ

Ⅳ

Ⅳ

 Note: The assessment of water quality of river month sections shall comply with the lake and reservoir standard　 specified in Environmental Quality Standard for Surface Water (GB3838-2002).

Table 5-4　Percent of different grades of water quality of the sections of the primary tributaries of the Yangtze River in the Three Gorges reservoir area in 2007 (%)

5.2 Monitoring of Water Bloom and Nutrition Status of Primary Tributaries

5.2.1 Early Warning Monitoring for Water Bloom

During March-October of 2007, 82 early warning monitoring sections for water bloom were established at the primary tributaries of the Yangtze River affected by impoundment. Five indicators including chlorophyll-a, TP, TN, permanganate value and transparency were employed to assess the comprehensive nutrition status of the water bodies.

The findings showed that 9.5%-25.6% of the sections of the primary tributary waters of the Yangtze River in the Three Gorges Project area were under eutrophication during March - October with monthly average of 15.9%, evidently better than that of the last year. 4.9%-11.0% of the sections were under oligotrophic condition and 69.5%-84.4% under mesotrophic status with monthly average at 6.5% and 77.6% respectively.

Table 5-5　Nutrition status of primary tributary waters of the Yangtze River in the Three Gorges reservoir area during March-October of 2007

The eutrophication of tributary backwater zones of the Three Gorges reservoir area was evidently heavier than that of non- backwater zones due to the impact of impoundment. 14.0%-33.3% of their sections were under eutrophication with monthly average of 21.0%, about 13.2 percentage points higher than that of the non-backwater zones .

5.2.2 Site Monitoring of Water Bloom

In 2007, water blooms occurred at 7 primary tributaries of the Yangtze River in the Three Gorges reservoir area, which were the Ruxi River, Huangjin River, Pengxi River, Modao River, Meixi River, Daning River and Xiangxi River. The dominant algae species were Cyclotella of Bacillariophyta, Peridinaceae of dinophyta, Eudorina of Chlorophyta, Cryptomonas of cryptophyta and Microcystis flos-aquae (Wittr) Kirchner of Cyanophyta etc.. In general, the dominant water-bloom algae species of some tributaries in the Three Gorges reservoir area showed a trend turning from river type (diatom, dinophyceae etc.) into lake type (green algae, cryptophyceae and blue-green algae etc.).

Monitoring results showed that the concentrations of nitrogen and phosphorus in tributary waters of the Three Gorges reservoir area were relatively high, falling into the range of mesotrophic – eutrophication with relatively high eutrophication in some waters in some period. In water bloom areas, pH value was 8.4-9.4 with chlorophyll-a concentration at 42.4-291.7 μg/L and algae density at 2.1×106-1.3×108 /L.