A Preliminary Study on The Prevention and Control of Seawater Intrusion in Northern Pingtan Island with Freshwater Recharge Curtain

： Seawater intrusion caused by groundwater over-exploitation is a kind of geological disaster caused by human economic activities and natural environment changes. Using freshwater recharge curtain technology is an effective means to prevent seawater intrusion. Based on the analysis of the characteristics of seawater intrusion in Pingtan island, Fujian Province, and in view of the possible seawater intrusion problems in the construction of Luyangpu underground reservoir in Pingtan Island, this paper expounds the basic principle of seawater intrusion prevention with freshwater recharge curtain , the technical measures of installing a row of recharge wells in the northern coastal zone of Luyangpu plain in Pingtan Island and using freshwater recharge curtain are put forward to prevent seawater intrusion. Through groundwater numerical simulation,the dynamic distribution of groundwater level in the north of Pingtan Island in the next 20 years is analyzed and predicted. The results show that the seawater intrusion can be effectively prevented by installing freshwater curtain recharge wells in the northern coast of Luyangpu.


Introduction
In recent years, seawater intrusion in coastal zone can not be ignored. As a geological disaster, it is seriously affecting the lives and economic development of local residents.
Seawater intrusion is caused by over-exploitation of groundwater, and is affected by geology, lithologic and atmospheric precipitation and other factors [1] .
At home and abroad, the technical measures to properly retaining groundwater runoff to reduce the flux of underground freshwater into the sea, and constructing groundwater hydraulic curtain can intercept groundwater runoff and prevent seawater intrusion [1] . In the technical research of preventing seawater intrusion by hydraulic curtain, foreign countries started earlier, and the earliest research can be traced back to the 19th century. J.Du Commun (1828), Bado n-Ghyben (1889) and Herzberg (1901), gave the expression of the depth below sea level at any point on the interface between salt water and freshwater when seawater invaded [2] . In 1953, the United States carried out the artificial recharge project along the coast of Los Angeles, the water in the reservoir in the northern mountain area was used to recharge the groundwater and raise the groundwater level, which formed a freshwater barrier to effectively prevent seawater intrusion [3] . In 1972, the Teboulba aquifer in Tunisia was recharged by rainwater collected from the dam, which made the sag disappear before recharging, improved the storage capacity of aquifer and effectively prevented seawater intrusion [4] . Guangrao County, Shandong Province [5] .A seawater intrusion prevention and control system invented by Wang Xiaojun and Chen Feng in 2018 solves the problem that the current technology can only prevent or desalinate seawater intrusion, realizes the combination of prevention and utilization, and improves the application efficiency of the system [6] . In the study of measures for recharging freshwater, Niu Sixiang changed the recharge rate of freshwater, which significantly shortened the distance of seawater intrusion. The larger the recharge amount of freshwater, the smaller the intrusion distance of seawater. The larger the recharge amount, the more effective to prevent seawater intrusion [7] .
In view of the possible seawater intrusion in Pingtan Island, this paper studies the technical measures to prevent seawater intrusion by using freshwater recharge curtain, and verifies the feasibility of the measures by numerical simulation analysis.

Geological conditions
Pingtan Island is located in the eastern part of Fujian Province, with flat terrain, mainly marine plain, and Junshan is the highest peak of the whole island. There  Atmospheric precipitation is the main recharge of aquifer, followed by lateral recharge of bedrock fissure water. Pingtan Island is flat, with slow underground runoff and weak hydrological cycle. Groundwater is mainly evaporated by capillarity or discharged into the sea by an undercurrent. Bedrock fissure water is recharged by atmospheric precipitation, and is mostly discharged by surface runoff. Hydrological cycle is fast, and it is generally recharged and discharged locally.
Groundwater is usually collected in the form of springs, supplied plain areas or flowed into the sea.
Groundwater dynamic changes obviously with climate in Pingtan Island. During the rainy season, the water level rises sharply; During the dry season, the water level drops slowly. The period of high groundwater level is from mid-February to September, and the period of low groundwater level in dry season is from October to February of the following year.

Characteristics of seawater intrusion
In the natural state, the salt water and freshwater in the coastal underground aquifer is in a state of dynamic equilibrium.
Because of over-exploitation of groundwater, the water level of freshwater drops, the equilibrium interface between salt water and fresh water is destroyed, and the seawater intrudes into the freshwater aquifer, resulting in salty water quality [8] .
Seawater intrusion is serious in some areas of Pingtan

Basic principle of preventing seawater intrusion with freshwater curtain
The following illustrates the basic principle of seawater intrusion prevention with freshwater recharge curtain through single well recharge curtain, and the schematic diagram is shown in Figure 2. When a recharge well is arranged on the onshore freshwater side of the coastline, a recharge mound is formed between freshwater and seawater on the section perpendicular to the coastline, which can block seawater from invading freshwater.

Prevention and control scheme of seawater intrusion in Luyangpu, Pingtan
Luyangpu underground reservoir is located in the north of Pingtan Island, and the northern boundary of the reservoir area is close to the coastal zone of Changjiang'ao. During the operation of the underground reservoir, when the groundwater level in the reservoir area is lower than the seawater level, seawater will often be poured into the underground aquifer in the reservoir area.
In order to avoid seawater from intruding into    is the first boundary of seepage area, 2  is the second boundary of seepage area.

Source and sink items
Source and sink items is determined by local data.

Identification and Verification of Models
The It is verified that the number of observation wells with water level difference less than 0.5m accounts for 80% of the total observation wells, and the simulation fitting degree is high, which meets the requirements.

Result analysis
freshwater recharge curtain in the next 20 years is shown in Figure 5, and the prediction curve of groundwater level along the freshwater recharge curtain in the next 20 years is shown in Figure 6.  Finally, the purpose of preventing seawater from flowing into freshwater aquifer is achieved，which means that the freshwater recharge curtain well can effectively control seawater intrusion.

Conclusions
Through the above analysis, the following conclusions are drawn: Freshwater recharge is used to recharge the aquifer, forming a freshwater recharge curtain, which can effectively prevent seawater intrusion.