Geochemical characteristics of sediments in Chaohu Lake, Anhui Province

(Anhui Geological Survey Institute, Hefei 23000 1)

Abstract: The contents of nutrient elements and heavy metals in Chaohu Lake sediments are low, and they are rich in cerium, strontium and Na2O. The distribution of elements forms three geochemical regions, which are mainly controlled by the geological background and sedimentary environment of lakes. The sedimentary sequence of elements carried by rivers is As follows: Cd, P, Pb, Zn, As and S are deposited near the mouth of the lake; Ni, Cr, Cu and N are deposited on both sides of incoming water and at the first 3 ~ 8 km. Zr is deposited on the edge of the lake, and Ti is deposited in the center of the lake. Zhongmiao-Laoshan Uplift limits the spread of sewage to the east half lake.

Keywords: Chaohu Lake; Sediments; Geochemical characteristics

1 Introduction

The eco-geochemical survey of Jianghuai Basin in Anhui Province spans the Yangtze River and Huaihe River, and there are many lakes in the region, including Wabu Lake, Heyao Lake, Jiaogang Lake and Anfengtang Lake in Huaihe River Basin, Chaohu Lake, Longgan Lake, Daguan Lake, Bohu Lake, Wuchang Lake, Pogang Lake, Caizi Lake, Baidang Lake, Fengsha Lake, Huangpi Lake, Jinsheng Lake and Shijiu Lake in Yangtze River Basin. Due to the influence of industrial and agricultural pollution, Chaohu Lake is seriously polluted and has become one of the "three lakes and three rivers" under the state's key control. In this investigation system, 633 0-20cm sediment samples and 168 deep sediment samples (below 1.5m or 20cm above the basement) were collected from the above-mentioned16 lakes, and 54 indexes were determined, and the basic geochemical survey data of Chaohu lake sediments were obtained, which provided a basis for the comparative study of elemental geochemical characteristics of lake sediments under different geological backgrounds.

2 Chaohu Lake hydrological environment and geological background characteristics

Chaohu Lake has many functions such as urban water supply, aquaculture, farmland irrigation, tourism, shipping and so on, and it is also an important water diversion source in the planned project of diverting water from the Yangtze River to the Huaihe River. According to the water depth and sediment thickness of the lake, the whole lake can be divided into three sedimentation (subsidence) centers in the west, with Zhongmiao Town-Beizha Town as the boundary in the west and Huatang Town-Gaolinqiao Town as the boundary in the east (figure 1). Chaohu Lake is replenished by eight rivers, including Nanfeihe River, and the main polluted rivers are Nanfeihe River and Shiwuli River, which flow through Hefei City, and the water quality is worse than Grade V; Paihe River flowing through Feixi County is Class II, and the water quality is Class IV ~ V; Other rivers are generally of Grade III ~ IV water quality. The water pollution area of Chaohu Lake is mainly distributed in the west, and the water quality is mostly V ~ worse than V. The indexes exceeding the standard are mainly TN, TP and CODMn, and the water quality in the east is obviously better than that in the west.

Chaohu Lake is a faulted lake. The Tanlu fault zone runs through Chaohu Lake along Zhongmiao-Nanbeimen on the north shore, and the sedimentary basement of the lake is mostly Tertiary-Quaternary strata. Characteristics of geological background around Chaohu Lake: The coastal lakeside zone from Chaohu City to Sanbing Town in the southeast is mainly composed of Cambrian-Permian carbonate rocks; Jurassic and Cretaceous clastic rocks, Proterozoic shallow metamorphic rocks and metamorphic magmatic rocks are distributed in the coastal lakeside zone of He Changlin-Zhongmiao in the north. The coastal lakeside zone of He Changlin-Luodaying in the north and west is mainly Quaternary river alluvium, and other areas are mostly late Pleistocene loess (Figure 2). Geological background characteristics of the source areas of rivers entering Chaohu Lake: Nanfeihe River and Shiwuli River in the northwest are late Pleistocene loess; The Paihe River and Fengle River in the west are metamorphic rocks of Dabie Group, and the near sources are volcanic rocks, clastic rocks and late Pleistocene loess. The Baishishan River and Zhaohe River in the south are mainly Jurassic volcanic rocks and late Pleistocene loess. Gaozhe River in the northeast is dominated by late Pleistocene loess, and locally exposed Sinian-Cambrian carbonate rocks. Generally speaking, the geological background of provenance is mainly late Pleistocene loess.

Figure 1 sediment isobath of Chaohu Lake

Fig. 2 Distribution map of soil-forming parent materials around Chaohu Lake

3 Geochemical characteristics of elements in sediments of Chaohu Lake

3. 1 element content characteristics

See table 1 for geochemical parameters of surface and deep sediments in Chaohu Lake. The coefficient of variation of As, Hg and CaO in surface sediments is between 0.5 and 0.76, which reflects the uneven distribution of these elements in surface sediments. The coefficient of variation of Cd, Cu, Mo, Ni, P, S, Sb, Zn, MgO and organic matter is between 0.25 and 0.50, and these elements are uniformly distributed, while the coefficient of variation of other elements is less than 0.25, showing uniform distribution.

Table 1 Statistical Table of Geochemical Parameters of Surface and Deep Sediments in Chaohu Lake

Note: Oxide, N and P are%, pH is dimensionless, and other elements are mg/kg; Surface layer/depth (average value of surface sediments/average value of deep sediments); Surface layer/soil (average value of surface soil in surface sediment/loess parent material distribution area).

The average content of elements in surface sediments of Chaohu Lake is higher than that in deep sediments (ratio coefficient ≥ 1.5), while As, Mo, S and Sr are higher than those in deep sediments (ratio coefficient 1. 1 ~ 1.5), and Ni and TFe2O3 are obviously lower (ratio coefficient 0.8 ~ 0).

Compared with the average value of the surface soil in the upstream loess parent material distribution area, it is rich in Cd, Sr, Na2O and CaO (the ratio coefficient is 1.2 ~ 1.5), and the contents of Cr, Cu, Mo, N, Ni, P, Pb, S, Sb, TFe2O3 and Al2O3 are lower than those in the surface soil (the ratio coefficient is 0.6)

Compared with the surface sediments of lakes along the Yangtze River (Table 2), Na2O, Zr, ce and Sr are obviously higher than those of lakes along the Yangtze River (ratio coefficient ≥ 1.2), while Ti, B, U and SiO2 _ 2 are similar to those of lakes along the Yangtze River (ratio coefficient 0.9 ~ 1.2), and other elements are lower than those of lakes along the Yangtze River (ratio coefficient <

Table 2 Average ratio coefficient of surface sediments in Chaohu Lake and lakes along the Yangtze River

3.2 Geochemical Distribution Characteristics of Elements

According to the distribution characteristics of multi-element geochemical fields such as Cu, Cd, Zr, ti and N in surface sediments, Chaohu Lake is divided into three geochemical regions (Figure 3 and Figure 5), which have a good correspondence with the three deposition (subsidence) centers of Chaohu Lake (Figure 1). It shows that the sedimentary environment of the lake directly affects the deposition and distribution of elements in the lake.

Fig. 3 Geochemical zoning map of Chaohu Lake sediments

Note: The following figure shows the geochemical map of Cu element.

Fig. 4 pH diagram of sediment and lake water in Chaohu Lake.

Note: The following figure shows the pH value of sediments, and the numbers indicated are the pH value of water.

Fig. 5 Geochemical map of zirconium, cadmium, nitrogen and titanium in surface sediments of Chaohu Lake.

The pH value of Chaohu Lake sediments gradually increased from west to east (Figure 4), with neutral in the central and western regions and alkaline in the east. However, the pH value of lake water in the west is less than 8, and that in the middle and east is greater than 8. The spatial distribution trend of pH values in sediments and water bodies is basically the same.

Zr, Ti, Hg, SiO2 _ 2 _ 2 and Pb in the east are low background areas (Figure 5), N is background area, and other elements are distributed in high background. Ti, Pb, N, Hg, Cd, K, Zn, As, Cu, P and organic matter in the west are mainly in high background, while CaO, Zr and SiO2 _ 2 are mainly in low background. The content of most elements in the middle is lower than that in the east and west, and other elements are mostly distributed in the low background except SiO2 and Zr in the high background.

Deposition law of element 4 in lakes.

Because the shore of the lake is straight, after the river enters Chaohu Lake, the elements have obvious sedimentary sequence due to the changes of hydrodynamic and hydrochemical conditions. Zr is generally deposited in estuaries and coastal areas (Figure 5), while elements such as Cd, P, Pb, Zn, As and S are only deposited near estuaries. After exchanging with lake water, Cu, Ni, Cr, N and other elements are generally deposited on both sides and front of lake water, while Ti and other elements migrate to the center of lake. See Table 3 for the sedimentary characteristics of elements in the estuaries of rivers entering Chaohu Lake.

Table 3 Statistical Table of Element Deposition Characteristics of Rivers Entering Chaohu Lake Mouth

Note:+deposited near the estuary,-generally not deposited in the estuary.

Because of the different forms of elements carried by different rivers, the chemical properties of river water are different, and the deposition order of elements is also different. The pollutants in Nanfeihe River and Shiwuli River are relatively high, and the element content in river water (about pH 7.7) is quite different from that in normal lake water (about pH 8.0). Cd, Hg, As, S, Pb, P and organic matter are deposited near the estuary, generally within 4 ~ 6 km from the estuary, while MgO, K2O, Al2O3 and Mn are generally far away from the estuary.

5 The influence of polluted rivers on the water quality of Chaohu Lake

The heavily polluted rivers flowing into Chaohu Lake are Nanfeihe River and Shili River, followed by Paihe River, all of which are distributed in the western half of the lake. Pollution indicators include heavy metal elements, nutrients and organic matter. The main sources of pollutants are industrial wastewater and domestic sewage, and Fengle River is an important source of P and K elements that cause eutrophication in the western half lake. A large number of pollutants from the above rivers bring in, which makes the water quality of the western half lake mostly V ~ worse than V, thus causing the water quality of the western half lake to be obviously worse than that of the eastern half lake.

Compared with the lakes along the river, the contents of Cd, Cr6+, Be, Mn, F, Se, Mo and cyanide in Chaohu Lake are obviously higher (Figure 6), and the highest concentration of Cd is 0.024mg/L, which seriously exceeds the standard.

Fig. 6 Comparison of element contents between Chaohu Lake water and lakes along the Yangtze River. Content unit: mg/L.

The influence mode and scope of river polluted water on the water quality of Chaohu Lake. After polluted water bodies such as Nanfeihe River flow into Chaohu Lake, elements such as Cd, Hg, Fe, Mn, Cr6++, Pb and Zn spread southward along the east side of the western half of Hubei Province, forming a continuous distribution and gradually decreasing pollution zone (Figure 7). The pollution range is mainly confined to the west of Zhongmiao-Laoshan Uplift, and it is also a cyanobacteria disaster-prone area.

The pH value of Chaohu Lake also has similar distribution characteristics (Figure 4). With Zhongmiao-Laoshan Uplift as the boundary, the pH value of most lakes in the west is less than 8.0, and that of sediments is less than 6.7. The pH value of the eastern lake is above 8.0, and the pH value of the sediments is above 6.7, among which the pH value of the surface sediments at the bottom of the eastern lake is above 8.0.

6 concluding remarks

The sediments in Chaohu Lake are rich in Na2O, Zr, Ce and Sr, but the contents of most nutrients and heavy metals are lower than those in surrounding soil and sediments along the river, and are basically close to those of rare earth, rare and chemically stable elements W, Ti and Zr.

The geochemical zoning of Chaohu Lake sediments is obvious, and the eastern, central and western regions are consistent with the three sedimentary centers of the lake. The geological bodies with high background are distributed in the provenance, and the content of heavy metals in the east is high and the distribution range is wide, which shows that the content and distribution characteristics of elements in lake sediments are mainly controlled by the geological background of provenance, followed by the influence of sedimentary environment, and the influence of rivers on geochemical field is limited.

The sedimentary sequence of elements imported into Chaohu Lake is: Zr-estuary and coastal areas-Cd, P, Pb, Zn, As and S-Ni, Cr, Cu and N-Ti in the center of the lake.

Fig. 7 Geochemical map of cadmium in Chaohu Lake. Cadmium is measured in milligrams per liter.

The influence of river pollutants on the water quality of Chaohu Lake is greater than that of sediments. The diffusion of polluted water is controlled by the topography of the lake bottom, and its pollution influence is often limited to the western half of the lake, which is also a frequent area of cyanobacteria disasters.

refer to

[1] Anhui Bureau of Geology and Mineral Resources. Regional geology of anhui province. Beijing: Geological Publishing House, 1987.

[2] Anhui Soil Census Office. Anhui soil. Beijing: Science Press, 1996.

[3] Anhui Provincial Bureau of Statistics. Anhui Statistical Yearbook (2002). Beijing: China Statistics Publishing House, 2003.

[4] Editorial Committee of Anhui Disaster Reduction Yearbook. Anhui disaster reduction yearbook. Hefei Anhui University Press 2003

[5] Anhui Provincial Water Resources Department, Anhui Provincial Environmental Protection Bureau. Water function zoning in Anhui Province. Beijing: China Water Resources and Hydropower Press, 2004.

[6] Hefei Environmental Protection Bureau. Environmental Quality Report of Hefei City, Anhui Province (200 1 ~ 2005)

Geochemical characteristics of sediments in Chaohu Lake

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(Anhui Geological Survey Institute, Hefei 23000 1)

Abstract: The contents of nutrients and heavy metals in Chaohu Lake sediments are low, but rich in cerium, strontium and Na2O. The distribution of elements leads to the formation of three geochemical zones, which are basically controlled by the geological background and sedimentary environment of lakes. The sedimentary sequence of elements carried by rivers is As follows: Cd, P, Pb, Zn, As and S are at the place where rivers enter the lake, Ni, Cr, Cu and N are at the sides and 3 ~ 8 km ahead of rivers, Zr is at the edge of the lake, and T is at the center of the lake. Zhongmiao-Sam Uplift has played a role in limiting the sewage from extending to the eastern half of the lake.

Keywords: Chaohu Lake; Sediments; Geochemical characteristics