Sea (salty) water intrusion on the south bank of Laizhou Bay

The sea (salty) water intrusion in the south bank of Laizhou Bay began in the late 1970s, when it was only scattered in some areas and the area was small. With the rapid development of regional economic construction, especially in the early 1980s, Han Wei, Changyi, Shouguang and other centralized water supply sources have been put into use one after another, and the amount of groundwater exploitation has increased year by year, and the exploitation intensity has been increasing. At the same time, a large number of surface reservoirs and ponds have been built in the middle and upper reaches of the main rivers flowing through the south bank of Laizhou Bay, which has improved the utilization rate of surface water storage, and the surface runoff entering the alluvial plain in the middle and lower reaches of the river has been decreasing, and the main rivers have been cut off for a long time, and the groundwater recharge has dropped sharply. The direct consequence of the long-term imbalance of groundwater resources in the south bank of Laizhou Bay is that the groundwater level continues to drop, and the natural balance established between groundwater fresh water and seawater (salty water) in natural state is broken. Groundwater flows in the opposite direction, the interface between salt and fresh water moves to the land, and sea (salt) water invades from the starting point to the sheet until a continuous intrusion zone runs through the east and west, which has caused great harm to the local ecological environment quality, industrial and agricultural production and people's life.

The coastal area on the south bank of Laizhou Bay is rich in underground brine resources, which is the largest sea salt production base in China. At present, the total area of Yantian is 32,800 hm2, and there are nearly 7,000 brine wells in coastal areas, the depth of which is generally between 40-70m, and about 90 million m3/a of underground brine is mined. Due to long-term continuous mining, the brine water level is in a state of continuous decline, and the brine concentration is also decreasing year by year. Brine mining can alleviate the intrusion of bittern to some extent.

I. Geological background conditions

Geomorphologically, the south bank of Laizhou Bay is adjacent to Taiyi Mountain in the south, Jiaodong Hilly Area in the east and Yellow River Delta Plain in the west. The landform is slightly inclined from south to north, with an average slope of 0.47‰. The landform types have changed from the piedmont alluvial plain in the south to the alluvial marine plain in the middle and the coastal marine plain in the north, and the coastal types are silty and muddy. The alluvial-diluvial plain in this area is composed of several alluvial-diluvial fan groups with overlapping fan edges. The front of alluvial-diluvial fan formation is buried by marine sediments or alluvial deposits of the Yellow River, and the aquifer has coarse particles, large thickness and multi-layer structure. In the vertical direction, the aquifer particles tend to be thinner from bottom to top, while in the horizontal direction, they have the hydrogeological characteristics of alluvial-diluvial fans. The buried depth of the main aquifer roof is 5 ~ 30m, and the lithology is all kinds of sand and gravel in the lot, and silty sand, silty sand and clayey silty sand mixed with ginger stone in the lot and the edge zone.

There is a weakly permeable layer of Middle Pleistocene sandy clay on the south bank of Laizhou Bay, with a buried depth of 40 ~ 70m and a thickness of about 20m. There are three transgression layers on it (Figure 7-5), corresponding to three transgression events since the Late Pleistocene in geological history. During the three transgressions, the ancient seawater (salty-bittern) enriched in the stratum was preserved and horizontally connected with the fresh water aquifer, which was the source of seawater (salty) invasion in this area. Seawater (salt) intrusion in the south bank of Laizhou Bay mainly occurred in the late Pleistocene and Holocene strata.

Figure 7-5 Cross-sectional view of marine sediments on the south bank of Laizhou Bay

Second, the characteristics of groundwater chemical environment

The characteristics of groundwater chemical environment in the south bank of Laizhou Bay are restricted by Quaternary paleogeographic environment, hydrogeological conditions and groundwater exploitation. In the horizontal direction, the hydrochemical field has obvious zonation, showing the changing law of fresh water-brackish water-salt water-brine from inland to coastal. The chemical types of groundwater are HCO3, HCO3-Cl, Cl-SO4 and Cl from south to north, and the salinity increases from 0.2g/L to 220g. The underground brine is distributed in the middle of the salty water area, with an east-west zonal distribution, and the trend is nearly parallel to the coastline (Figure 7-6), and the salinity of the brine is 50 ~ 220 g/L.

Fig. 7-6 Profile of groundwater salinity change in Guti-Yangzi area on the south bank of Laizhou Bay

In the vertical direction, groundwater exploitation in this area is mainly concentrated within 200 meters. According to the different hydrochemical characteristics within 200 meters, the groundwater chemical environment in the south bank of Laizhou Bay can be divided into five types: total fresh water area, brackish fresh water area, brackish water area, brackish fresh water area and total salt water area (Figures 7-7 and 7-8).

All fresh water area is located in the south of the study area, that is, the north of the piedmont alluvial plain. The aquifer is mainly composed of alluvial gravel and sand from Hezi River, Mihe River, Bailang River, Yu He River and Weihe River, and has a multi-layer structure. The salinity of groundwater is less than 1.5g/L, and the hydrochemical type is HCO3-Cl or HCO3.

Light brackish fresh water area, slight brackish fresh water area and brackish fresh water area are located in the middle of the study area, which are the junction of alluvial-diluvial plain and marine plain. The quality of groundwater is complex, and the salt content of groundwater is as high as 50g/L; Among them, the brackish water area is located in the middle of Changyi City and the east of Shouguang City, and the thickness of fresh water in the upper part is generally 10 ~ 100 m (Figure 7-9).

The saline water area is located in the northern part of the study area and is a marine plain. There are high-concentration brine and many salt fields in the 80m depth in the middle of this area.

Thirdly, the relationship between salt water intrusion and groundwater dynamic field changes.

1. Changes of groundwater dynamic field

Since 1970s, with the development and utilization of surface water and groundwater in the south bank of Laizhou Bay, the groundwater dynamic field has changed greatly. At the end of 1970s, the buried depth of groundwater level was generally 1 ~ 6m, and its flow direction was basically consistent with the topographic slope and surface water system, that is, it drained from south to north to Laizhou Bay and Xiaoqing River. In recent 20 years, the groundwater level in the piedmont alluvial plain has dropped sharply. The buried depth of the groundwater level is generally 8 ~ 30m, and the maximum water level drop is more than 20m. In addition, there are negative water level funnels with a total area of about 1493km2 in fresh water area and negative brine extraction funnels with an area of about 1047km2 in salt water area (Figure 7- 10). The appearance of the funnel of groundwater level decline changed the diameter of groundwater in the whole area.

Figure 7-7 Zoning Diagram of Groundwater Hydrochemical Structure Types

Figure 7-9 Profile of groundwater salinity change in Liutuan-Qing Xiang.

Figure 7- Isogram of Groundwater Level Decline from 10 to 2002

2. Relationship between salt water intrusion and groundwater dynamic field change.

Under the condition that there is no separated water body in the south bank of Laizhou Bay, because the density ρb of salt-brine is greater than the density ρf of fresh water, it is inevitable that salt water will invade the fresh water body. The static model of salt water intrusion is as follows:

Let the water levels of fresh water and salt water be hf and hb, respectively, and the depth of the interface between fresh water and salt water below sea level is Z (Figure 7- 1 1), then the following formula is obtained according to the Haiben-Herzberg model:

Study on Geological Environment Problems in Shandong Province

Figure 7- Hydrostatic model of salt water intrusion +0 1

According to the hydrochemical data of brine in this area, the density of brine is 1.04g/cm3, and that of fresh water is 1g/cm3, so the above formula can be approximately expressed as

Study on Geological Environment Problems in Shandong Province

It can be seen that the depth of salt-fresh water interface under sea level is proportional to the water level difference between fresh water and salt-brine on both sides of the interface, and the depth of salt-fresh water interface under sea level is about 25 times that of salt-fresh water interface. Due to the exploitation of underground fresh water, the decline of its water level will lead to a sharp increase in the interface between salt and fresh water. Every time the fresh water level drops 1m, the salt-fresh water interface will increase by 25m.

The exploitation of underground brine on the north side of the salt-fresh water interface will reduce the water level of salt-fresh water and hb, increase the water level difference between the two sides of the salt-fresh water interface, and make the salt-fresh water interface move down, thus slowing down the intrusion of salt-fresh water.

Four. Spatial distribution of salt water intrusion

Before 1980s, due to the small scale of seawater (salty water) intrusion and the lack of groundwater analysis data in the whole region, the water quality analysis data of 1980 and 2002 were used to define the seawater-brine intrusion range. The analysis shows that the change characteristics of groundwater salinity in the invaded area are as follows: from less than 1g/L to 1 ~ 2g/L, the shallow groundwater salinity isoline moves inland, with an area of 594.6km2, and the 2g/L isoline moves inland with an area of 127. 1km2. The study of water quality analysis data shows that the concentration of Cl- increases with the increase of salinity (Figure 7- 12), and the relationship between them is approximately expressed as follows

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Where: d is salinity (mg/l); L is the cl- concentration value (mg/L).

Because Cl- is the most important stable constant element in underground brine, it is the most sensitive to the intrusion of brine, and has obvious harm to fresh water and agricultural ecology, and the experimental data is rich, so Cl- is used as an index to judge the intrusion of brine, and the standard value is 250 mg/L.

Figure 7- 12 Relationship between groundwater salinity and Cl- concentration in alluvial-diluvial plain area on the south bank of Laizhou Bay

On the basis of drawing 1980 and 2002 isograms of Cl- content in groundwater, the range of salt water intrusion is delineated. Saline intrusion is distributed in the middle of the study area in a strip shape, with an intrusion distance of 1.5 ~ 12.0 km and a total intrusion area of 696.8km2 (Figure 7-65438+).

Fig. 7- 13 distribution map of salt water intrusion in the south bank of Laizhou Bay

Based on a comprehensive analysis of the stratum structure, spatial distribution of salt water, groundwater exploitation status and groundwater quality monitoring data in the south bank of Laizhou Bay, it is found that the depth of salt water intrusion is mainly 80 ~ 100 m, which is the main channel for salt water intrusion because of the large alluvial sand layers, coarse particles, wide distribution and good continuity in the study area. And most of the sand layers are separated by clay and sandy clay with weak water permeability, so the intrusion type is mainly horizontal bedding intrusion (Figure 7- 14), and overflow intrusion can occur to some extent in some strong groundwater mining areas.

Fig. 7- 14 Profile of Salt Water Intrusion in the Lower Weihe River

Fig. 7- 15 salt water intrusion process model diagram

Verb (abbreviation of verb) the development process of salt water intrusion

1. evolutionary model

After the formation of salt-fresh water interface in geological history, a relatively stable position can only be reached because of the balanced relationship between groundwater inflow into the sea and salt diffusion to the land. When the virtual velocity v of groundwater flow moving the interface to the sea is equal to the virtual velocity u of salt dispersion moving the interface of salt and fresh water to the land, the interface is in a relatively stable state.

The exploitation of underground fresh water leads to the formation and development of funnel. When the edge of the funnel extends to the interface of salt and fresh water, the hydraulic gradient I decreases and the hydraulic gradient V decreases (Figure 7- 15). However, because the transfer speed of salt ions is 2-3 orders of magnitude lower than the change of water pressure, U is almost unchanged. The interface between salt and fresh water moves to land, and the salt water intrusion begins, and the dispersion attenuation lags behind the seepage attenuation, which is the initial stage of salt water intrusion.

After the interface between salt and fresh water meets the watershed line B on the downstream side of the funnel, the seepage at the interface points to the center of the funnel, that is, it flows to the land, not to the sea. Land surface seepage and land surface dispersion make the saline-fresh water interface move to land, and the salt water intrusion intensifies. The local reversal of seepage is the main driving force of invasion in strengthening stage, and the weakening of dispersion lag is still one of the driving forces.

After the interface between salt water and fresh water moves to the line E with the largest reverse hydraulic gradient on the downstream side of the funnel, the reverse hydraulic gradient gradually decreases as it moves to the center of the funnel. Although seepage and dispersion are still in the same direction, the role of seepage in intrusion dynamics is getting smaller and smaller, and salt water intrusion has entered a weakening stage. When the top moves to the upstream side of the funnel, the hydraulic gradient is just the first line f of the original hydraulic gradient I, and the balance between seaward seepage and landward seepage is re-established, and the salt water intrusion is terminated.

It can be seen that the process of salt water intrusion can be divided into three stages: initial stage, rapid development stage and slow-down stage, and the invasion rate of each stage is obviously different.

2. Development process analysis

The monitoring data of groundwater quality in the study area shows that the salt water intrusion in the south bank of Laizhou Bay has stages, and the analysis of each development stage is as follows:

(1) initial stage (1976 ~ 1979)

From 1973 to 1975, the study area suffered from drought for years, with less precipitation, increased industrial and agricultural water demand and decreased groundwater level, forming a negative funnel of groundwater level with a total area of less than 800km2. The water quality in some areas on the fresh water side of the saline-fresh water interface becomes salty seasonally, and a few corresponding wells record that the water quality becomes salty.

(2) rapid development stage (1980 ~ 1990)

From 198 1 ~ 1989, the study area entered a drought period lasting for 9 years, and several centralized water supply sources were built one after another, and the groundwater exploitation increased sharply, and the area of the negative funnel area of groundwater expanded rapidly, from 792km2 in 1980 to 3244km2 in 1990. The salt water intrusion boundary began to fall into the negative funnel of groundwater level, and the intrusion speed was obviously accelerated. During this period, the average intrusion speed of salt-brine is 292m/year.

(3) remission stage (199 1 ~ 2002)

From 199 1 to 2002, the precipitation increased, a certain amount of Yellow River water was pumped into this area, and the negative funnel area of groundwater level continued to decrease, from 1990 to 256 1km2 in 2002, with an average annual decrease of 56km2. The intrusion boundary of bittern gradually approaches the negative axis of funnel, and some areas cross the central axis, so the intrusion speed slows down, and the average intrusion speed of bittern drops to166 m/a.

Prediction of development trend of salt water intrusion with intransitive verb

The intrusion boundary of Guangrao-Shouguang funnel area in the study area has reached the central axis of negative value area, and the vicinity of Liulv Town in Shouguang City has crossed the central axis of funnel. Although the intrusion boundary between Changyi funnel area and the western end of Guangrao-Shouguang funnel is still far from the central axis, the intrusion line has also crossed the maximum point of the reverse hydraulic gradient on the downstream side of the funnel, reflecting that the salt water intrusion on the south bank of Laizhou Bay is in a slow stage at present, and the future intrusion rate will gradually weaken until it is terminated. Only when the layout of local groundwater exploitation changes, especially when the location of centralized groundwater exploitation area changes and the negative funnel center of groundwater level moves, the salt water intrusion line will move until it reaches a new equilibrium point.

According to the above analysis, the salt water intrusion termination line is located at the position where the hydraulic gradient upstream of the negative funnel is exactly the original hydraulic gradient. According to the monitoring data of groundwater, the original hydraulic gradient of groundwater in the negative funnel area is 1.00 ‰ ~ 1.50 ‰, with an average value of 1.20‰. On this basis, we can determine the location of the termination line of salt water intrusion (Figure 7-65438+). From this, we can draw the following conclusions: Under the condition of maintaining the present situation of groundwater exploitation layout, salt water intrusion in this area will further develop. If preventive measures are not taken in time, the salt water intrusion area will eventually increase by 854.0km2, and the total intrusion area will reach 1550.8km2, accounting for 26% of the total area of the study area (Table 7-3). In the future, the development of brine intrusion is mainly concentrated in Guangrao, western Shouguang and Changyi City, and will invade 3 ~ 15 km inland in the future. Other areas are close to the invasion termination line, and the expansion area will not be too large.

Fig. 7- 16 Prediction of Salt Water Invasion Trend in the South Bank of Laizhou Bay

Table 7-3 Prediction Results of Salt Water Intrusion in the South Bank of Laizhou Bay Table Unit: km2