Hundreds of water and sediment samples collected at the end of 2006 have now been analysed by Eawag. The results are remarkable: although pollutant loads are in some cases very heavy, concentrations in the river known as China’s main artery are in the same range as worldwide known from other large rivers.
In the autumn of 2006, as part of the Yangtze Freshwater Dolphin Expedition, Eawag researchers collected water and sediment samples along a 1500-kilometre stretch of China’s longest river. These samples have now been analysed, and the results were presented by the scientistes Michael Berg, Eawag and Wang Ding, China today in Bern at an event organized by the Zurich-based baiji.org Foundation.
Baiji considered extinct
At the media conference, the leaders of the joint Chinese/Swiss expedition explained the background to, and the significance of, this unique mission. A documentary directed by Florian Guthknecht showed the research team at work during the survey, which confirmed what had already been feared: the team declared the Yangtze River baiji, or white-flag dolphin, “functionally extinct” – a conclusion that caused dismay worldwide and especially in China. However, based on the latest findings, the disappearance of the baiji is not attributable to toxic chemicals in the river – pollution is, at least, certainly not the main factor.
Concentrations comparable to other rivers
In general, concentrations of man-made pollutants in the Yangtze are comparable to those found in other large rivers around the world. Although high concentrations of certain elements and organic compounds were measured at some points, these substances were mostly diluted further downstream. Concentrations of several toxic elements such as arsenic, thallium and antimony increased along the course of the river. Heavy metal concentrations in the Yangtze are currently about two to eight times lower than in the Rhine 30 years ago, when pollution levels peaked. European Union guidelines for several heavy metals are all higher than the concentrations found in the Yangtze, suggesting that even today pollution levels are still significantly higher in many European rivers.
While the trend has been widely reversed in Europe, with levels of pollutants declining, they are generally still increasing in China. For example, nitrogen concentrations have approximately doubled over the past 20 years. In Shanghai, concentrations of dissolved nitrogen were twice as high as at the Three Gorges Dam, reflecting the increasing use of mineral fertilizers in agriculture, while phosphate concentrations remained constant at a relatively low level along this section of the river. Of the 236 organic chemicals studied, only a few were found locally at high levels. Many of the persistent substances used in agriculture occur only seasonally and were detected in trace concentrations.
4.6 tonnes of arsenic per day
The fact that concentrations of most pollutants appear low by comparison with other major rivers is party due to the vast quantities of water discharged by the Yangtze (yearly average at the mouth = 39 000 m3/second; for comparison, yearly average discharge of the Rhine at Basel = 1050 m3/second). Accordingly, dilution of the anthropogenic chemical inputs means that there is no immediate risk of damage to the ecosystem. However, where the river enters the East China Sea, the huge pollutant loads are expected to have devastating effects: each day, 1500 tonnes of nitrogen is discharged, causing eutrophication and growth of blue-green algae in the coastal waters, while toxic metals such as arsenic (4.6 tonnes discharged per day, despite the low concentrations) and persistent organic compounds accumulate, entering the food chain of the productive shelf areas.
Growing pressure on the Yangtze
Although it is not possible to rule out synergistic effects between different chemicals, long term effects or impacts of endocrine disruptors, which could not be demonstrated by this study, the results provide no direct evidence of a link between chemical water quality and the disappearance of the Yangtze River baiji or the decline of the Chinese sturgeon or the endemic finless porpoise. These losses must rather be attributable to a variety of circumstances, among which the deterioration of chemical water quality may however be a contributory factor. Other factors include the destruction of habitats, heavy canalization, the cutting-off of tributaries (“hatchery” areas for many fish species) by dams, the drainage of lakes and wetlands for agriculture, overfishing and unselective fishing methods, and heavy shipping traffic. In general, pressure on the Yangtze is growing as a result of industrialization, rising living standards, artificial irrigation and increasing power generation. The planned large-scale diversion of water to the Yellow River valley in the North would also have serious consequences. Water shortages in the Yellow River have become so severe that it fails to reach the sea for several months each year.
In the Yangtze, concentrations of nitrogen, metals and organic compounds are increasing, as shown by comparisons with earlier measurements in the literature. This will further increase the pressure on the river ecosystem – and especially on the coastal waters of the East China Sea – and also affect the quality of groundwater and drinking water supplies. It is therefore essential to monitor these developments and take “at-source” measures as soon as possible so that current trends can be reversed.
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