Lake Taihu (太湖), also called Tai Lake, is the third largest freshwater lake in China. Although it began receiving widespread coverage in the western media after a major algal bloom that covered 1/3 of the lake’s area in 2007, killing fish and disrupting surrounding areas potable water supply, the lake has been experiencing a decline in water quality for the past 20 years. The lake is located on the southern part of the Yangtze in southeastern China and is administered jointly by Shanghai Municipality, Jiangsu Province and Zhejiang Province, serving as a floodwater basin, irrigation supply, drinking water source, aquaculture base and tourist attraction. It is the major source of drinking water for the municipalities of Wuxi, Suzhou and Shanghai. The algal outbreak in 2007, called a “natural disaster” by government officials, caused a noticeble drop in water quality for local residents, who said that they could smell the stench of the algae on their bodies after showering, and were forced to rely on bottled water for drinking.

During the last 20 years, the rapid urbanization of the areas surrounding Taiju, coupled with ineffective management and technical support, have not only caused the eutrophication of the lake, but also its contamination with organic substances and ecological destruction. Now, many doubt the lake’s water quality to ensure safety of the millions who rely upon it as a drinking water source. The water quality of the lake was rated I or II according to China’s National Surface Water Standard up until the 1970s. By the late 1980s the quality had fallen mostly to class III, white in some parts, it reached IV and V. In the late 1990s the lake rated an overall class IV, with approximately one-third of the lake as class V. Lake Tai has become a symbol of water environmental protection, which is a high-priority issue for government at all levels.

Government Five-Year Plans have continued to make Lake Tai an issue of major importance. The Ninth Five-Year Plan (1996-2000) proposed 54 domestic wastewater treatment plants and sewage conduits were planned to be located in the basin. Later however, only 29 plants were completed or partially completed by the end of the period. The Tenth Five-Year Plan (2001-2005), 81 domestic wastewater treatment plants were expected to be built or explanded by 2005. The goal was to be reach over 70% treatment of domestic wastewater. In addition to building more wastewater treatment plants in the Taihu Basin, other measures have also been taken in an attempt to abate the spread of noxious blue-green algal blooms, including releasing algae-eating fish into the lake, physically hauling algae out of the lake, and crackdowns on government corruption in enforcement of effluent standards.

While good intentions obviously exist for the future of Lake Tai, implementation has been difficult and the water quality in the lake has not risen significantly. The main problem of Lake Tai’s pollution issue is the algal blooms. Overgrowth of algae is caused by water that is rich in nutirents (nitrogen and phosphorous), that are usually the limiting factors in algal growth. With nitrogen and phosphorous existing in surface water in excess, algae growth becomes almost limitless, just waiting for the right temporal conditions to cause an extensive bloom. And, when algae goes into respiration conditions at night and when dead algae is digested by microorganisms, the amount of dissolved oxygen in the surfacw water can be virtually depleted, causing fish kills and decrease in biodiversity. According to reports, the major sources of nitrogen and phosphorous in surface water are industrial effluent, domestic wastewater treatment plant effluent, and agriculture. Each type of effluent into surface water has different characteristics; for example, household wastewater is a greater contributor of phosphorous and ammonium (NH4+), while agricultural runoff is a greater contributor of nitrate (NO3-). Below, we can see the contributions of pollution sources Industry (Ind.), Household (Hou.), and Agriculture (Agr.) to the chemical oxygen demand (COD), total nitrogen (TN), and total phosphorous (TP) in Tai Lake. The figure shows that the main contributors of the pollution of Tai Lake have become household discharges and agriculture.

In response to industrial and municipal contributions of nitrogen and phosphorous to surface waters, China’s Five-Year Plans are right to regulate large industries and build more, or retrofit, wastewater treatment facilities (most current wastewater treatment facilities in China do not include tertiary treatment processes that remove nutrients nitrogen and phosphrous from the effluent). Recent banning of phosphorous-containing detergents is another example of effectively reducing the household discharge contribution to the Tai Lake pollution problem.

In my next post, I’ll address agricultural runoff (fertilizers) contribution to the condition of the Tai Lake Basin.

?Source:  Wang Q et al. 2006. “Profile: Toward Integrated Environmental Management for Challenges in Water Environmental Protection of Lake Taihu Basin in China” Environmental Management Vol. 37, No. 5, pp 579-588.

Tags: eutrophication, Tai Lake, Taihu
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