Evaluation of groundwater vulnerability in Nanchang city

(Jiangxi Geological Survey and Research Institute, Xiangtang 33020 1)

The groundwater vulnerability of Nanchang city is evaluated by DRpASTIC evaluation model, and it is concluded that the beaches along Ganjiang River and Fuhe River in Nanchang city have high vulnerability and poor anti-pollution ability. Honggutan New District, Li Antang and Ganjiang Branch of Bayi Bridge have high vulnerability and poor anti-pollution performance. The Hejian blocks in Chaoyang and Yangtze Islands have medium vulnerability and medium anti-pollution performance. Fenghuangzhou, Jiangxiang and Northwest Gully areas have low vulnerability and good antifouling performance. The eastern and northwestern gangways of Laofu River have low vulnerability and good antifouling performance. This paper expounds and analyzes the main factors that form the high-high fragile area of groundwater. According to the groundwater vulnerability zoning and pollution source characteristics in Nanchang city, some suggestions and countermeasures for groundwater pollution prevention in Nanchang city are put forward.

Keywords: groundwater; Vulnerability assessment; Nanchang city

Groundwater has the characteristics of good water quality, stable water temperature and not easy to be polluted. Therefore, it is becoming more and more important as one of the urban water supply sources, especially when the city is faced with water shortage and seasonal water shortage. However, due to the massive discharge of industrial wastes and domestic garbage and the extensive use of chemical fertilizers and pesticides in agricultural areas, groundwater in some areas has been polluted to varying degrees. Relevant data show that in the old city and densely populated areas of Nanchang, the pollution sources are mainly domestic wastewater, domestic garbage and fecal pollutants, and the pollutants are mainly "three nitrogen" organic pollution and various viruses and bacteria. The pore water of Quaternary loose rocks in Nanchang Dairy Factory, Nanchang pesticide plant, Chemical Raw Materials Factory and Waizhou Hydrological Station has been polluted to varying degrees. The main pollution factors are pH value, Zn2++, Fe and Mn2++. Red bed groundwater pollution areas are mainly distributed in Luojiaji and peripheral hydrological stations. The pollution factors are Fe, Mn2++, Cl-, pH, total hardness and dissolved solids, and the pollution degree is moderate to mild. Therefore, it is of great practical significance to evaluate the vulnerability of urban groundwater in Nanchang and identify the potential danger of groundwater pollution on the basis of analyzing the evaluation results, which is of great significance to scientifically formulate the prevention and control plan of groundwater pollution and effectively realize the sustainable utilization of groundwater resources.

1 Urban physical geography and environmental geological background

1. 1 physical geography and research area

Nanchang, the capital of Jiangxi Province, is located in the alluvial plain in the lower reaches of Jiangxi (Jiang) and adjacent to Poyang Lake, the largest freshwater lake in China in the north. Generally speaking, the terrain is high in the northwest and low in the southeast, and low hills, hills and plains develop in turn, showing layered landform characteristics. With the Ganjiang River as the boundary, the northwest of Ganjiang River is low mountain and hilly, with structural erosion, and the east of Ganjiang River is a river erosion accumulation plain with dense rivers, lakes and harbors, with braided water system. The study area is Nanchang urban planning area [1]: Maqiu Town in the east and Meiling in the west; It starts from Changbei Airport in the north and reaches the Silver Triangle in the south (3 kilometers south of Nanchang County). East longitude:11546' 27 "~11606' 04"; North latitude: 28 30 ′ 48 ″ ~ 28 51′ 54 ″. The total area is1248.0km2.

Rock structural characteristics of 1.2 layer

The strata in the study area are pre-Sinian, Upper Cretaceous, Paleogene and Quaternary. Magmatic rocks are exposed in Jinning and Himalayan periods. Nanchang is located on the north side of the junction zone between Yangtze block and South China block and on the south edge of Yangtze block, with complex geological structure and developed faults and rift basins. Fold structure: bedrock in the area is exposed only to the west of Ganjiang River. Except for a series of NEE-trending secondary tight linear syncline folds composed of pre-Sinian phyllite, Pliocene-Paleogene folds are wide and gentle, showing nearly east-west trend. Faults in this area are well developed, mainly in NE, NE, NW and NW groups, followed by NE and EW faults.

1.3 division of water-bearing rock groups

According to the lithologic characteristics, assemblage relationship, water storage space form and hydraulic connection of water-bearing rocks (layers), it is divided into four aquifers: (1) Quaternary loose rock pore aquifer: Quaternary pore aquifer is composed of sand and pebble layers alluvial by Holocene, Upper Pleistocene and Middle Pleistocene, which are closely hydraulically connected to form a unified aquifer with abundant water, and the recharge source is lateral recharge of river water and atmospheric precipitation. (2) Paleogene and Cretaceous "red beds" fractured aquifers: mainly replenished by atmospheric precipitation, with medium-weak water abundance. (3) Pre-Sinian fractured aquifer group of metamorphic rocks: groundwater exists in weathered fractures, and it is mainly replenished by atmospheric precipitation, with weak water abundance. (4) Magmatic fissure aquifer: Groundwater is mainly replenished by atmospheric precipitation, and its water abundance is weak.

2 Nanchang groundwater vulnerability assessment [2]

2. 1 Selection of evaluation methods and evaluation factors

According to the specific natural geographical conditions and environmental geological background conditions in Nanchang, the drought evaluation model is adopted. Seven parameters, including groundwater depth (D), rainfall infiltration recharge (Rp), aquifer medium (A), soil medium (S), terrain gradient (T), unsaturated zone medium (I) and aquifer permeability coefficient (C), were selected as the evaluation factors of groundwater vulnerability in Nanchang.

2.2 groundwater vulnerability assessment factor score

(1) Groundwater depth: Groundwater depth determines all kinds of hydrogeochemical processes that surface pollutants go through before reaching the aquifer, which provides the greatest opportunity for pollutants to contact with oxygen in the atmosphere and oxidize it. Generally speaking, the deeper the groundwater level, the longer it takes for surface pollutants to reach the aquifer, the greater the chance that pollutants will be diluted on the way, and the less likely that pollutants will enter the groundwater.

The less pollution the aquifer is. Therefore, according to the influence of groundwater depth on groundwater pollution, combined with groundwater depth, the range and score of groundwater depth are given (table 1).

Table 1 groundwater depth grade table

(2) Rainfall infiltration recharge: Pollutants can be transported vertically to the aquifer through rainfall infiltration recharge, and move and spread horizontally in the aquifer. Therefore, recharge is the main carrier of pollutant migration to groundwater. The greater the recharge, the greater the possibility of groundwater pollution and the greater the vulnerability of groundwater. See Table 2 for specific scoring criteria.

Table 2 Rainfall Infiltration Replenishment Scoring Table

(3) Aquifer medium: Aquifer medium here refers to the lithologic characteristics of aquifer. Aquifers with different lithology have different ability to degrade pollutants, and the degree of fracture development, rock consolidation, particle size and sorting degree of the same lithology have different ability to degrade pollutants. Generally speaking, the more cracks develop, the easier it is to pollute; The better the consolidation, the less likely it is to be polluted; The bigger the particles, the easier it is to be washed and the easier it is to be polluted; The lower the particle content, the better the sorting, and the easier the pollution. See Table 3 for specific scoring criteria.

Table 3 Classification Table of Aquifer Media Types

Note: Typical score values are used when detailed information is lacking.

(4) Soil medium: Soil medium is usually the lithology of the surface layer (rock weathering zone), with an average thickness of 2m or less. See Table 4 for specific scoring criteria.

Table 4 Soil Medium Scoring Table

(5) Topography: Topography controls pollutants being washed away or staying in a certain surface area for a long time or infiltrating underground, which not only affects the formation of soil, but also affects the dilution degree of pollutants. For the terrain that is conducive to the infiltration of pollutants, the vulnerability of groundwater in the corresponding section is high. When the slope is 0 ~ 2%, the possibility of pollutants infiltrating into the ground is the greatest. On the contrary, when the slope of the ground is greater than 18%, it is easy to form surface runoff, and the possibility of pollutants infiltrating into the ground is very small. See Table 5 for specific scoring criteria.

Table 5 Terrain Slope Classification Table

(6) Type of media in vadose zone: The selection of media in vadose zone follows the following principles: (1) Select the most vulnerable media; (2) When the medium has multiple layers, the relative thickness of each layer should be considered, and the layer with the largest thickness should be selected as the unsaturated zone medium; (3) The fragility of each layer of medium must be considered. For example, when the upper limestone is covered with a layer of clay and a larger gravel layer with the same thickness, clay is a significant control layer from the point of view of groundwater pollution, because the clay layer limits the migration of pollutants to the aquifer. At this time, clay layer is the most suitable choice for unsaturated media. Confined aquifer is assigned as1regardless of its overburden; For bedrock media, the development degree of cracks, bedding and karst pipes should be considered, and for limestone media with well-developed karst caves, 10 can be given; For limestone media with poor karst development or poor connectivity, the score should be lower, such as 9 or 8. See Table 6 for specific scoring criteria.

Table 6 Classification Table of Media Types in Unsaturated Zones

Note: Typical score values are used when detailed information is lacking.

(7) Permeability coefficient of aquifer: The permeability coefficient is determined by the size and connectivity of pores in aquifer. The greater the permeability coefficient, the easier the groundwater is to be polluted and the higher the vulnerability. See Table 7 for specific scoring criteria.

Table 7 Scoring Table of Aquifer Permeability Coefficient

2.3 Weight of groundwater vulnerability assessment factors

When applying DRpASTIC method to evaluate groundwater vulnerability, each evaluation factor is given a relative weight, and its value range is 1 ~ 5, which reflects the relative importance of each evaluation factor. The factor weight of the most significant influence on groundwater pollution is 5, and the factor weight of the least influence is 1.

For each evaluation factor in DRpASTIC model, the corresponding weight values are 5, 4, 3, 2, 1, 5, 3. See Table 8 for the weight of each evaluation factor.

Table 8 DrPastica Index System Evaluation Factor Weight Table

2.4 grid division of groundwater vulnerability assessment unit

According to the specific situation of Nanchang, the evaluation units are divided by grid method. The grid (evaluation unit) we use is 0.5km×0.5km, so the total * * * of the study area (except water, the same below) can be divided into 3900 grids. At the same time, combined with the existing graphic data, the units with sudden changes in the status of evaluation factors are manually adjusted to ensure the relative unity of the status of evaluation factors in a single evaluation unit.

2.5 groundwater vulnerability index calculation and vulnerability evaluation results

According to the weight of each evaluation factor of groundwater vulnerability, the vulnerability index of DRpASTIC groundwater system is determined and calculated according to the following formula: DRPASTIC = 5× D+4× RP+3× A+2× S+1× T+5× I+3× CA, where D, RP, A, S, T, I and CA.

According to the above groundwater vulnerability assessment method, firstly, the score map of each evaluation factor is generated by GIS software, and then the groundwater vulnerability index of each evaluation unit (that is, the weighted sum of each evaluation factor is between 22 and 200) is obtained through interlayer superposition analysis and statistical calculation according to the relative weight values of each evaluation factor (using DRpASTIC groundwater system vulnerability index calculation formula). The evaluation results of groundwater vulnerability unit grid in Nanchang City are shown in Table 9. Combined with the principle of DRpASTIC vulnerability division, the evaluation division is divided into five vulnerability grades (Table 10).

Table 9 Statistical Results of Groundwater Vulnerability (Evaluation Unit) Network in Nanchang City

From Table 9 and Table 10, it can be seen that the beaches along Ganjiang River and Fuhe River in Nanchang City have high vulnerability and poor antifouling performance, with an area of 36.0km2, accounting for 3.7% of the research area; Honggutan New District, Li Antang and Ganjiang Branch of Bayi Bridge have high vulnerability and poor antifouling performance, covering an area of 32.75km2, accounting for 3.4%. The Hejian block between Chaoyang and Yangzizhou has moderate vulnerability and anti-pollution performance, covering an area of 287.0km2, accounting for 29.4%. Fenghuangzhou, Jiangxiang and the gully region in the northwest have low vulnerability and good antifouling performance, with an area of 1 15.75km2, accounting for1/0/.9%; The inter-hillock zone east and northwest of Laofu River has low vulnerability and good antifouling performance, covering an area of 503.50km2, accounting for 565,438+0.6%. In addition, it can be seen from table 10 that the smaller the vulnerability index, the less likely the aquifer is to be polluted, that is, the lower the vulnerability; Conversely, the greater the vulnerability index, the easier it is to be polluted, that is, the higher the vulnerability;

Table 10 Nanchang Groundwater Vulnerability Assessment Table

3. Analyze the main factors that form the high-high fragile area of groundwater.

It can be seen from the table 10 that the areas with high groundwater vulnerability in Nanchang are mainly distributed on the beaches along the Ganjiang River and Fuhe River, and the areas with high groundwater vulnerability are mainly distributed in Honggutan New District of Li Antang and Ganjiang Branch of Bayi Bridge. On the premise of fully analyzing the geological background of these areas, the specific reasons for its formation are analyzed from seven aspects: (1) Groundwater depth: through investigation, it is found that the groundwater depth in these areas is shallow. The results of groundwater level survey in 2004 show that the groundwater depth of Honggutan New District (Huiren Pharmaceutical Factory) in Li Antang and Ganjiang Branch of Bayi Bridge (Taohua Township) is 4m respectively. All of them are in the range of 2 ~ 5m, so there is no doubt that the depth of groundwater is so shallow. It takes a short time for surface pollutants to reach the aquifer, and there is little chance for pollutants to be diluted on the way, so it is very likely to enter the groundwater, with a single factor score of 8. (2) Rainfall infiltration recharge: According to the available data, the rainfall infiltration recharge in these areas is generally in the range of 150 ~ 200 mm/a, and as the main carrier of pollutants migrating to groundwater, the recharge amount is large, and the groundwater is likely to be polluted, with a single factor score of 8. (3) Aquifer media: These areas are all located in the alluvial plain of the lower reaches of Ganjiang River, and their aquifer media (lithology) are all gravel layers formed by ancient rivers or modern rivers. They are all loose sediments with good inter-particle sorting and pore connectivity, which are easy to cause groundwater pollution. The single factor score is 4-9, and the typical score is 8. (4) Soil medium: most of these areas are exposed, such as the beaches along the Ganjiang River and Fuhe River, which are directly covered by loose gravel layers; Where there is a soil layer, the soil layer is also thin, and the lithology is generally uncompacted sandy loam. Therefore, as far as soil medium is concerned, these areas are all areas with poor antifouling performance. (5) Topography: These areas belong to the alluvial plain area in the lower reaches of Ganjiang River, with a topographic slope of 0 ~ 2. It is difficult for surface water to form surface runoff and quickly take away pollutants. Instead, pollutants are left in these areas, allowing them to continuously penetrate into the ground, eventually causing groundwater pollution. The single factor score is 10. (6) Type of media in vadose zone: The media in vadose zone in these areas are mainly sand, gravel or gravel containing silt and clay, and the gap between particles is well connected, so sewage can easily pass through it, reach the aquifer smoothly, and finally pollute groundwater, with a single factor score of 6-9. (7) Permeability coefficient of aquifer: As mentioned above, the aquifer media (lithology) of groundwater in these areas are all gravel layers formed by ancient rivers or modern rivers, and its permeability coefficient varies with the lithologic conditions, location and geological age of the aquifer. For example, the permeability coefficient of beaches along Ganjiang River and Fuhe River is 260 ~ 360 m/d, and the antifouling performance is poor, with a single factor score of 6 ~ 8; However, the permeability coefficient of the Ganjiang branch dikes of Honggutan New District, Li Antang and Bayi Bridge is 53 ~ 160.9 m/d, and their antifouling performance is better than the former, with a single factor score of 2 ~ 3.

Due to the poor antifouling performance of the above seven aspects, it is difficult to prevent groundwater pollution in these areas. According to the sampling analysis results in 2004, the components exceeding the standard in Li Antang (near Huiren Pharmaceutical Factory) are as follows.

Compilation of geological survey results in East China: 1999~2005

In fact, the measured content is 25mg/L, which is 1.25 times of the national groundwater quality standard III. The over-standard component in Wang Jia area of Honggutan is NH4++, and the measured content is 0.7mg/L, which is 3.5 times of the national groundwater quality class III standard. In addition, the analysis results of groundwater organic pollution in Li Antang (Xiaolan Industrial Park) show that the δ HCH of organochlorine pesticides in groundwater is 23.87ng/L, and the P, P ′-DDT is108.3 ng/L; /L； The detection rate of polycyclic aromatic hydrocarbons was 56%, and the comprehensive pollution index was 0.9, which reached the warning line. Groundwater pollution has caused some harm to this area, and if it is not prevented in time, it will have further adverse consequences. For example, these polluted areas are distributed on both sides of the Ganjiang River and upstream of Nanchang City. Alluvial gravel layers of Upper-Middle Pleistocene or Holocene are distributed, which have good permeability and are closely related to Ganjiang River hydraulics. Generally, because the groundwater level is higher than the Ganjiang River, the groundwater is discharged into the Ganjiang River. At this time, the polluted groundwater continuously brings pollutants into the Ganjiang water body, which pollutes the Ganjiang water body. The polluted Ganjiang River flows to the middle and lower reaches of Nanchang City, polluting all water intakes (industrial and domestic water supply sources) and causing immeasurable losses. The above situation shows that the prevention and control of groundwater pollution has become very urgent and necessary, especially for those areas with high groundwater vulnerability, effective measures need to be taken to control the further development and spread of groundwater pollution. 4 Nanchang groundwater pollution prevention and control suggestions

As mentioned earlier, due to the poor antifouling performance in some areas of Nanchang, groundwater in many places has been seriously polluted, and may even further pollute the water source of Ganjiang River. In order to prevent the further development and spread of groundwater pollution, the prevention and control of groundwater pollution has become a top priority. According to the survey data, groundwater pollution sources in Nanchang can be roughly divided into four types: point source pollution caused by industrial waste discharge; Application of agricultural chemical fertilizers and pesticides, non-point source pollution of sewage irrigation; Non-point source pollution of domestic waste discharge; Lateral infiltration recharge of polluted surface water. According to the characteristics of groundwater vulnerability zoning and pollution sources in Nanchang, it is suggested to establish the following groundwater health protection zones to prevent aquifer pollution.

(1) First-class protected area: mainly distributed in the floodplain along the Ganjiang River and Fuhe River. This zone corresponds to a highly vulnerable area, with poor antifouling performance and strong permeability of water-bearing medium, which is closely related to the water power of Ganjiang River. Once its groundwater is polluted, it will further bring pollutants into the water source of Ganjiang River. Therefore, this area is off-limits, and all agricultural planting activities in this area and the discharge of domestic garbage and domestic sewage should be prohibited. In a word, all activities that may cause groundwater pollution should be banned.

(2) Secondary protected areas: distributed in Honggutan New District, Qingyunpu Scenic Resort Scenic Area and Li Antang, corresponding to high vulnerability areas, with poor anti-pollution performance and strong water-bearing medium permeability. Groundwater is generally discharged into Ganjiang River. In 2004, it was verified by water samples that groundwater in some areas had been polluted, which may further bring pollutants into the water source of Ganjiang River. There are some polluting enterprises in this area, such as Huiren Pharmaceutical Factory and some chemical plants in Xiaolan Industrial Park in Li Antang, which pollute the groundwater. These polluting enterprises should be treated, and those that cannot be treated should be shut down, merged, transferred and relocated. We should try our best to avoid building all kinds of polluting enterprises, actively develop technology and capital-intensive enterprises, and gradually change to the industrial structure dominated by the tertiary industry and high technology, and from resource-based to benefit-oriented In addition, residential areas can be built.

(3) Third-class protected areas: mainly distributed in agricultural areas in Chaoyang and the Yangtze River valley, which are equivalent to moderately vulnerable areas and have average antifouling performance. It is suggested that residential areas, entertainment places and light industrial areas should be established in this area, instead of polluting enterprises, ecological agriculture should be developed, intensive management should be promoted, scientific use of chemical fertilizers and pesticides should be advocated, utilization efficiency should be improved, non-point source pollution should be reduced, and more organic fertilizers should be promoted, especially sewage irrigation should be put an end to. Eliminate pollution sources, treat pollutants scientifically, and ensure that all pollutants meet the discharge standards.

(4) Grade IV protected area: located in the vast area east of Laofu River, the gully between posts in the northwest, Fenghuang Island, the northern end of Bayi Bridge, and Jiangxiang Town in the south of Jiangxi Province. This area corresponds to low-low vulnerability area, so it is suggested to establish agricultural activity area, heavy industrial area, manufacturing and food industrial area in this area. The eastern part of this belt is the main distribution area of groundwater resources. Therefore, protection should be carried out to eliminate pollution sources, industrial waste residue and domestic garbage should not be dumped at will, and sewage irrigation is prohibited.

5 conclusion

(1) evaluated the groundwater vulnerability in Nanchang, and concluded that the beaches along Ganjiang River and Fuhe River in Nanchang have high vulnerability and poor anti-pollution performance; Honggutan New District, Li Antang and Ganjiang Branch of Bayi Bridge have high vulnerability and poor anti-pollution performance. The Hejian blocks in Chaoyang and Yangtze Islands have medium vulnerability and medium anti-pollution performance. Fenghuangzhou, Jiangxiang and Northwest Gully areas have low vulnerability and good antifouling performance. The eastern and northwestern gangways of Laofu River have low vulnerability and good antifouling performance. It provides a scientific basis for the prevention and control of groundwater pollution in Nanchang city.

(2) The main factors that form the high-high fragile area of groundwater are elaborated and analyzed in detail.

(3) According to the groundwater vulnerability zoning and pollution source characteristics in Nanchang, some suggestions and countermeasures for groundwater pollution prevention in Nanchang are put forward.

refer to

[1] Nanchang Urban Planning and Design Institute. Master Plan of Nanchang City from 2002 to 2005

[2] Selected by Institute of Hydrogeology and Engineering Geology, Ministry of Geology and Mineral Resources. Study on the theory and method of groundwater resources evaluation. Beijing: Geological Publishing House, 1982.

Evaluation of groundwater vulnerability in Nanchang city

, Ma Zhenxing, Yang, Tao, Lu, Qin Yan, Wang Daoying

(Jiangxi Geological Survey Institute, xiang tang 33020 1)

Abstract: A dynamic evaluation model of groundwater vulnerability in Nanchang City is proposed. The results show that the groundwater along Ganjiang River and Fuhe River in Nanchang City has high vulnerability and poor anti-pollution ability. Honggutan New District, Li Antang and Ganjiang Branch of Bayi Bridge have high vulnerability and poor anti-pollution ability; The Hejian plots of Chaoyang Island and Yangzi Island are moderately fragile and moderately pollution-resistant; Phoenix Island, Jiangxiang and gully areas in the northwest have low vulnerability and good anti-pollution ability; That is, the eastern region and the Qianfu River ridge region in the northwest have low vulnerability and good pollution resistance. This paper expounds and analyzes the main factors that form the high-high vulnerability area of groundwater. According to the characteristics of pollution sources and fragile groundwater areas in Nanchang, some suggestions and measures for groundwater pollution prevention and control are put forward.

Keywords: groundwater; Vulnerability assessment; Nanchang