Water quality protection and human activity management in Taihu Lake

"The lake is the mirror of human civilization", and the water quality protection of the lake can better reflect the degree of human civilization. As a part of water system, lakes have been the cradle of human culture and civilization since ancient times. Especially the freshwater lake in the lower reaches of the Yangtze River has always been the land of plenty in China's agricultural economy, and a large number of poems and songs praising Dongting, Poyang Lake and Taihu Lake have emerged, which has become an important part of China's water culture.

With the development of modern industrialization, in addition to all kinds of natural organic matter and soil erosion, the water quality of lakes may also be affected, and all kinds of artificial chemicals may lead to sudden changes in the water quality of lakes. Today, rivers have been seriously polluted, and lakes have become the last position to protect human water resources. The deterioration of lake water quality will fundamentally destroy the conditions for the sustainable development of social economy in the basin.

To protect the water quality of lakes, the most important thing is to deal with the relationship between people and lakes in the basin and establish a long-term harmonious relationship between people and lakes.

Second, the basic characteristics of the lake

The basic characteristics of the lake change with the shape and capacity of the lake box, and human activities in the basin will also affect and change the basic characteristics of the lake.

Lakes with large capacity have large environmental capacity and are relatively difficult to be polluted, but once polluted, it is difficult to purify. Shallow lakes are also easy to be polluted and are greatly affected by wind and heat.

Due to the flow in the lake, pollutants or sediments entering the lake are transported to the whole lake. Therefore, in order to master the migration and diffusion of pollutants and sediment in lakes, it is necessary to master the characteristics of lake water flow. The causes of lake water flow can be divided into: ① Throughput, which is caused by the water flowing in and out of the lake channel; (2) Wind-induced flow is caused by the shear force of the wind on the water surface; (3) The heat flow is caused by solar radiation, and the water bodies receiving radiation have different depths and different heat capacities; (4) Coriolis force is caused by the difference between centrifugal force and linear velocity caused by the rotation of the earth. In flood season, the huff and puff flow has a great influence, and in dry season, the wind-driven flow has an influence, while the heat flow only shows its influence when the topography of the lake box changes greatly and the water body is different in depth. For small lakes, the influence of Coriolis force can be ignored. For lakes in the northern hemisphere, due to the influence of south monsoon and Coriolis force, most of them will have counterclockwise circulation. The dominant lake flow in Taihu Lake is also counterclockwise circulation.

When encountering strong wind, the water level near the wind side of the lake will drop, while the water level far away from the wind side will rise, and the lake will tilt. The water level difference in Taihu Lake is high in the north and low in the south, which can reach 0.3 ~ 0.4 meters. If the wind is strong and weak, or the wind direction changes constantly, the lake will produce a static array phenomenon similar to tide, also known as false tide, and its period is related to the shape of the lake basin. The water surface oscillation period of Taihu Lake is about 1 ~ 1.5 hours. In the past, there was a saying among fishermen in Taihu Lake that there were "eighteen tides a day".

For deep lakes, temperature stratification may occur in spring and summer every year, which affects the material exchange between the upper and lower layers. Taihu Lake is a shallow water lake, and generally there will be no stratification.

Judging from the change process of lake water quality, it is a change process from a greedy lake in childhood to a middle-aged lake (poor) in youth, to a mature middle-aged lake (rich) and then to an eutrophic lake in old age. Under natural conditions, this process develops quite slowly, but it may also become very rapid due to the stimulation of human activities. Freshwater red tide often occurs when the lake water quality enters youth, or from youth to maturity, while "water bloom" often occurs in old age. Therefore, some people call red tide "zit" of the lake, and water bloom "senile plaque". Once the lake enters the eutrophic state, it means entering the old age. With the occurrence of water bloom, it will also be swamped, and the water depth in the coastal lake bay area will become shallow, and the lake area will gradually shrink. Once the lake is eutrophic, it is difficult to reverse it and restore its youth, but various measures can be taken to delay its aging process.

Three. Water quality evaluation of Taihu Lake

In the early 1980s, Taihu Lake first appeared a large area of water bloom in the northern bay area, and then developed to the center of the lake. In July, Wuxi Meiyuan Waterworks 1990, 1994 and 1995 were blocked by algae, resulting in abnormal water taste. At present, Taihu Lake presents typical senile symptoms. Namely, large-scale bloom and swamping of East Taihu Lake.

The amount of sediment entering Taihu Lake is not large, roughly estimated to be 2 million m3/ year, which is equivalent to the average annual sedimentation1m m/m, but because the lake is shallow, it is picked up whenever there is wind and waves, and flows with the lake to East Taihu Lake and other lake bay areas. In some places of East Taihu Lake, the annual siltation can reach 0. 1 m, and the water depth gradually decreases.

The water quality of Taihu Lake is seriously polluted. From 65438 to 0995, there was no Class II water, and the area above Class IV water accounted for 36% of the total area. The biomass of algae in the East Taihu Lake and the central area of the lake is 3.5 ~ 4.5 mg/L, and Wuli Lake and Meiliang Lake are the worst, exceeding Grade V water all year round, and the biomass of algae is 3.5 ~ 4.5 mg/L. 。

The rapid deterioration of water quality in Taihu Lake is mainly due to improper human activities in the basin. If strict and scientific management measures are not taken, the water quality and ecology of Taihu Lake will be difficult to develop in a good direction.

According to the recent survey, the annual pollutant load into Taihu Lake is 2.829 million tons.

T-N 177900 tons, T-P 16900 tons. Industrial emissions account for 83% of CODCr; In T-N, industry accounts for 43.7%, non-point source accounts for 26. 1%, domestic sewage accounts for 14.6%, and livestock production accounts for 12.4%. In T-P, industry accounts for 55.9%, domestic sewage accounts for 23. 1%, and animal husbandry production accounts for 15.8%.

At present, according to the water quality and environmental conditions of Taihu Lake, the main source of organic pollution is industrial discharge. From the point of eutrophication, nitrogen is surplus and phosphorus is the controlling condition. Industrial sewage accounts for more than half of the source of phosphorus, while domestic sewage accounts for nearly a quarter.

Four. Comprehensive control measures of Taihu Lake

As a big lake, Taihu Lake has three major functions, namely, resource function, environmental function and ecological function. The function of resources is mainly to improve the abundant water resources for the surrounding areas; Environmental functions include self-purification, climate adjustment to surrounding areas, and cultural entertainment and leisure functions related to space landscape; Ecological functions include the functions of maintaining biodiversity and protecting the species it owns, as well as the functions of fishery production.

As the goal of comprehensive management, there is only one goal to improve the water quality of Taihu Lake in order to improve its three functions of resources, environment and ecology. Moreover, the three are interrelated, so when considering the comprehensive management of Taihu Lake, to avoid a single management plan, we must first determine a reasonable comprehensive management goal.

The comprehensive control measures of Taihu Lake should include three aspects:

(1) Pollution source control measures;

(2) Lake ecological restoration measures;

(3) Measures for the management of human activities in river basins.

Pollution source control measures

Point source control measures include: (1) sewage control in industrial and mining enterprises and large-scale tourist facilities; ② Town

Domestic sewage treatment plant; (3) River pollutant precipitation facilities; (4) purification facilities of underground pebble trough; ⑤ Pre-storage; ⑥ Biogas facilities for human and livestock manure and urine in rural areas.

Non-point source control measures include: ① developing and utilizing soil purification resources; ② Biological treatment measures in low-lying areas. Studies at home and abroad show that the phosphorus and nitrogen removal rate of polluted natural water can reach more than 70% if it stays in reed areas or aquatic plants for more than 70 hours. ③ Soil and water conservation measures. There are various nutrients in the soil and water lost from sloping farmland, so it should be avoided to enter the lake directly.

Ecological restoration measures of lakes

The organic pollution and eutrophication of lakes are the manifestations of lake aging and ecological environment deterioration. The changes of ecological environment are manifested as the decrease of biological population, the decrease of water transparency, the gradual disappearance of submerged plants, the decrease of phytoplankton, the increase of shellfish and fish with strong pollution resistance, the decrease of small fish with high economic value and the increase of large fish. Due to the abnormal proliferation of algae, the organic matter in the water increases, resulting in stench, which consumes a lot of oxygen in the process of its death and decomposition, which reduces the concentration of dissolved oxygen in the water and even leads to a large number of fish deaths.

While the pollution sources are gradually controlled, the ecology of the lake may be restored to a certain extent. Ecological restoration measures include:

Build an aquatic belt by the lake. Under natural conditions, the lake and the land are continuous, and the wetlands around the lake will be formed with the rise and fall of the lake water level. These wetlands are the most important areas to maintain the biodiversity of lakes. From shallow to deep, a large area of reeds or aquatic plants and aquatic organisms will be formed. It is a place where many kinds of fish and shellfish lay eggs and inhabit, and it is also a place where many kinds of insects and birds nest. At the same time, it is the natural barrier of the lake, which plays a role in shielding, precipitating and absorbing pollutants entering the lake around the lake, and various fungi parasitic on the roots of aquatic plants can also decompose organic matter. However, after the completion of the levee around the lake, the continuity of land and water was destroyed, wetlands disappeared, reeds and aquatic plants suitable for shallow water growth disappeared. For Taihu Lake, if the aquatic zone around the lake is restored, it is necessary to trim the topography around the lake and create conditions suitable for the growth of reeds, aquatic plants and emergent plants.