Cixian county and its adjacent areas

1. Deep earth structure

Figure 6-4 Lithospheric Thickness Map of North China (unit: km) (quoted from Niu, 1996)

As can be seen from Figure 6-4, Cixian area is just at the edge of the lithospheric thickness thinning belt. The eastern part is the North China Plain, and the lithosphere is 60 ~ 80 km thick. In Taihang Mountain area in the west, the lithosphere thickens to 120 ~ 160 km. Deng et al. (1994) and Niu et al. (1996) call this phenomenon of lithospheric thinning mantle plume rising and thermal thinning. Cixian area is just on the mantle ridge or mantle terrace at the edge of mantle plume uplift, which forms the crust thickness abrupt change zone, gravity cascade zone, geothermal isotherm cascade zone and earthquake-prone zone to the west.

2. Structure and fracture system

From Mesozoic to Cenozoic, the mantle uplift formed the famous northeast North China Graben in North China, while the Taihang Mountain fault zone (split shoulder structure) with block uplift was formed in Taihang Mountain area. Cixian area is in the transitional zone of this tectonic system.

The eastern boundary of the southern segment of Taihang Mountain fault zone is in Handan-Linzhang-Gongxian area, and the western boundary is in Pingding-Licheng-Gaoping-Lushi line (Figure 6-5). A series of NNE high-angle thrust faults are formed in the fault zone (Figure 4-2).

Fig. 6-5 Schematic Diagram of Distribution of Fault Zone and Esophageal Cancer in Southern Taihang Mountain

1-the boundary of the high incidence area of esophageal cancer; 2- fault zone boundary

3. Rock system

Exposed to the west of Cixian County. Except for a few Archean metamorphic rocks and Sinian sandstone and sandy shale, the rocks in this area are basically Cambrian-Ordovician and Carboniferous-Permian. These rocks are considered to be stable caprocks deposited offshore the North China Platform. Cambrian is a set of limestone, marl, sandstone and dolomite. Ordovician is mainly a set of dolomite and limestone deposits with flint belt. Carboniferous-Permian is black shale with coal seam, and sedimentary sandstone, shale and limestone are mainly distributed in the mountainous and hilly areas of Cixian County. These sedimentary rocks obviously have nothing to do with mantle plume activity.

Magmatic rocks are distributed in the south and north of this area (Figure 4-2), belonging to Mesozoic diorite, which is considered to be the product of tectonic evolution and heavy lava magma, and may be related to mantle activity. Skarn-type iron deposits are near the contact zone with limestone.

There is a lack of data on selenium content in rocks in this area. According to the adjacent data cited in Chapter 4, the selenium contents of argillaceous rocks, carbonate rocks and clastic rocks are 0.37, 0.27 and 0.28μg/g respectively. It can be inferred that the selenium content of black or carbonaceous shale and coal seam may be higher. Therefore, the background value of selenium in rocks in this area is generally higher than that in Zhangjiakou area.

According to the above data, the eastern part of Cixian area is a graben area, which is covered by Quaternary proluvial. Far away from the mantle plume in the west, the differential uplift of Taihang Mountain fault does not expose metamorphic rocks in the deep crust in a large area, but only distributes some sedimentary rocks in the platform cover. The activation of mantle plume magma only shows the emplacement of neutral diorite and hydrothermal solution near the contact zone of rock mass, which has little effect on this area. Therefore, the lack of mantle thermal fluid action, deep regenerative thermal fluid circulation and volcanic magmatism in the deep part of the earth in this area have an impact on the surface. The most important selenium geochemistry is the partial enrichment of selenium in limestone, shale and carbonaceous shale sandwiched with coal seams produced by offshore deposits such as platform delta facies and tidal flat. These strata rocks are controlled by Taihang Mountain fault.

(2) Supergene system of the earth

1. Topographic landscape

Affected by the uplift of Taihang Mountain fault block and the subsidence of graben in North China Plain, the landform in this area is mountain-hill-alluvial fan platform-alluvial plain from west to east. The altitude drops gradually from1000 ~1500m to < < 50m. Shexian and Cixian in the west are mountainous, Cixian in the middle is hilly, Cixian in the east and Linzhang in the west are alluvial fan platforms, and Linzhang and wei county in the east are plain areas.

2. Weather and climate characteristics

The climate in this area is restricted by topography, showing obvious differences between mountainous areas and plains. The plain area has a warm temperate climate. The annual average temperature in mountainous areas is about 2 ~ 3℃ lower than that in the plain, and it is cold in winter. In summer, due to blocking the warm and humid airflow in the western Pacific, the rainfall increases greatly, so the mountainous areas often form eroded landforms and steep peaks.

3. Groundwater and surface water systems

Rocks in mountainous areas have strong water permeability and few barriers. The water in the surface rocks leaks deep underground, the groundwater level is very low, and there is no spring outcrop on the surface. Local villagers make a living by digging deep wells and even drinking kiln water. The rocks in the hilly plain are mostly interbedded with shale and sandstone, which has a good barrier to surface water leakage, high groundwater level and developed surface water system. Residents drink more well water.

There is Fuyang River system in the north and Zhanghe River system in the south. The source of Zhanghe River system is in Taihang Mountain basin west of Shexian County. When crossing the mountainous area, there is basically no tributary supply, and sediment, cuttings and gravel fragments are piled up on the riverbed along the river. The source of Fuyang River system is in the mountainous area at the junction of Cixian County and Shexian County, and it mainly flows through the hilly plain area, forming many secondary and tertiary tributary water systems, and the riverbed is mostly alluvial gravel accumulation landscape. Two rivers form a huge alluvial-diluvial fan platform at the exit of Taihang Mountain. Alluvial sand is formed in ancient rivers in plain areas.

According to the analysis of drinking water samples from high incidence area-middle incidence area-low incidence area of esophageal cancer, the average value of selenium is 1.236→0.676→0.243ng/ml. Although the highest value of 2.43ng/ml appears in the low-incidence area, most samples larger than 1ng/ml appear in the high-incidence area. Even so, selenium in drinking water has no obvious effect on human health compared with selenium in food.

4. Soil system

Coarse-boned cinnamon soil is developed in valleys and basins, and is mostly formed by in-situ weathering and denudation of rocks. Zhanghe area is alluvial sand. Meadow soil and cinnamon soil are developed in mountainous and hilly areas of Fuyang River Basin. The original rocks are mostly limestone, sandstone, shale and coal seam, and alluvial red sand, clayey sand and cinnamon soil are formed in the ancient river bed. The alluvial fan platform forms sandy clay and sandy cinnamon soil. On the alluvial plain, sandy soil and clayey sandy soil are formed.

According to the analysis of soil samples from high incidence area-middle incidence area-low incidence area of esophageal cancer, the average value of selenium is 0.420 → 0.174 → 0.148 μ g/g in turn. Obviously, the soil selenium level in both high-incidence and low-incidence areas has exceeded the limit value of low-selenium soil (0. 125μg/g) proposed by Qin Jian 'an (1989), and reached the level of low-selenium edge-full selenium-high selenium. This may be related to the genetic type of soil. In the seriously ill area in the upper reaches of the river, the soil accumulated in situ reflects the characteristics of selenium in the original rock. There are more sulfides with high selenium content and dark minerals with high specific gravity piled up along the river, and the soil developed by them has high selenium content. With the transport and dissolution of the river, some soluble selenium will enter the river, and the selenium in the sediments (mainly feldspar and Yingshi) unloaded along the downstream mild disease area will decrease. Selenium in the soil they develop will also decrease.

5. Plants and food systems

From mountain-hilly-plain, with the change of climate and soil type, natural vegetation also changes regularly. Oak, pine and other trees are the main trees in mountainous areas; Vitex negundo and Zizyphus jujuba are dominant in hilly areas; Due to man-made reclamation, natural vegetation in the plain area is scarce, mainly including seabuckthorn, tribulus terrestris, poplar, Sophora japonica, willow and other afforestation and fruit forests. Due to man-made destruction, only a small number of secondary forests remain in mountainous and hilly areas at present.

The main crops in this area are wheat and corn, as well as cotton, potatoes and peanuts. Usually do it twice a year. A total of 96 grain samples (mainly corn and millet) were collected in this study. Because the difference between corn selenium and wheat selenium is not obvious, the average value of grain selenium is calculated by combining them. The sequence of high incidence area-middle incidence area-low incidence area is 0.0805→0.0494→0.0260μg/g, except that the grain selenium in low incidence area is close to the grain limit in low selenium area (< < 0.025μ g/g) (Qin Jian 'an, 1989), the grain selenium in other high and middle incidence areas is in the range of.

6. People's lifestyle, food structure and selenium nutrition level

There are obvious differences in lifestyle and food structure between mountain people and primitive people. Due to inconvenient transportation, backward economy and relatively closed environment, mountain residents mainly eat local wheat and corn and like pickles. The plain area has convenient transportation, developed economy and open environment. The residents are mainly wheat and rice, and the foreign food is gradually increasing, and the food structure is more complicated.

In this study, a total of * * 105 healthy human hair samples were collected, and the average high incidence area-middle incidence area-low incidence area was1.159 → 0.972 → 0.912μ g/g, although all of them far exceeded the hair selenium limit of low selenium belt (< Is this related to the complex food structure and the addition of exogenous selenium in the low-incidence area?