Zhanrong  Guo

College of Oceanography & Environmental Science, Xiamen University, Xiamen, Fujian, 361005

Abstract: As early as 1964, Dalian City in China suffered sea water intrusion due to excessive extraction of groundwater. At the present time, 10-plus coastal cities are experiencing sea water intrusion to varying degrees. Sea water intrusion has not only damaged the ecological environment of seaside areas, but also hampered the economic development of coastal regions. Since the 1980s when the size of China’s coastal areas subjected to sea water intrusion increased rapidly, sea water intrusion has been showing a slowly declining tendency, with certain areas no long undergoing expansion of sea water intrusion. In order to obtain an in-depth understanding of the mechanisms of sea water intrusion, since the late 1970s, scholars in this field has conducted a series of research, such as water-rock cation exchange experiments, to identify the geochemical action of sea water intrusion, and built a transitional belt numerical model of three-dimensional finite element variable density to simulate and forecast the process and development trends of sea water intrusion. In order to prevent and treat sea water intrusion, besides controlling the amount of groundwater extraction and extraction layout, coastal areas must adopt relevant preventive and treatment measures according to actual local circumstances; such measures may include artificial recharge, building underground impervious screen, implementing water-saving irrigation, and long-distance water diversion.

Keywords: Sea Water Intrusion; Groundwater Extraction; Numerical Modeling; Geochemical Action; Preventive and Treatment Measures

1 History and Current Situation of Sea Water Intrusion

China has a zigzagging coastline extending more than 18,000km. Sea water intrusion primarily occurs in coastal cities with heavy groundwater extraction. Following the first-time detection of sea water intrusion in Dalian City in 1984, the same problem was found again in Qingdao City in 1970; the majority of coastal cities with sea water intrusion saw the emergence of this problem in the late 1970s and the early 1980s (Table 1). By now, sea water intrusion has hit more than 10 cities in China’s coastal areas. From north to south along the coast, major cities suffering sea water intrusion include Dalian, Yingkou, Huludao, Qinhuangdao, Laizhou, Longkou, Penglai, Yantai, Weihai, Qingdao, Rizhao, Ningbo, Wenzhou, Zhanjiang and Beihai (Fig. 1).

Fig. 1 Major Coastal Cities in China with Sea Water Intrusion

The combined sea water intruded areas of these cities exceed 900km². Dalian, the first city to experience sea water intrusion, has incurred an intruded area of 223.5km² over a 34-year period, with the rate of sea water intrusion averaging 6.6km² per year (Table 1). Laizhou is the city with the most extensive sea water intrusion with an intruded area of 260.0km², averaging 10.4km² per year (Table 1). Sea water intrusion occurred at a slower rate in some smaller cities, such as Beihai, which has incurred an intruded area of merely 4km², averaging less than 0.3km² per year. This is mainly attributable to the city’s small groundwater extraction and slow increase in water consumption (Table 1).

Table 1 Sea Water Intrusion in Some Coastal Cities in China

During its initial stage, sea water intrusion occurred at a comparatively slow rate in China’s coastal cities. However, in the 1980s, sea water intrusion grew rapidly, which could be attributed to the significant increase in groundwater consumption during this period of time. In the 1990s, thanks to the government’s implementation of a series of effective measures aimed at curbing sea water intrusion, the rate of sea water intrusion remarkably slowed down, and in some cities, the area of sea water intrusion no longer expanded. Sea water intrusion in Laizhou and Longkou cities on the Jiaodong Peninsula has followed such a development pattern. Since the occurrence of sea water intrusion in 1976, Laizhou’s area of sea water intrusion has been growing on a yearly basis. The city experienced the rapidest growth of sea water

intrusion in the 1980s. Its cumulative area of sea water intrusion was merely 15.8km² in 1979, but expanded to 238.7km² by 1989 (Zhumei Liu, 2003), averaging an annual growth rate of 22.3km² over the 1979-1989 period, with the highest annual growth rate reaching 39.1km²/year (1987) (Table 2). Since 1990, Leizhou city government has implemented measures aimed at checking sea water intrusion; as a result, the rate of sea water intrusion has slowed down (Table 2). The city’s cumulative area of sea water intrusion totaled 212.0km² in 1991 and 260.0km² in 2001, averaging an annual growth rate of 4.8km² over this 10-year period, far lower than the sea water intrusion rate in the 1980s (22.3km²/year) (Table 2).  In 2001, the annual intrusion dropped to 3.7km²/year (Table 2).

Table 2   History of Sea Water Intrusion in Laizhou City

Sea water intrusion also occurred in Longkou City in 1976; the city’s cumulative area of sea water intrusion totaled 45.8km² in 1984 and expanded to 78.7km² by 1990, averaging an annual growth rate of 7.2km²over the 1984-1990 period, with the highest intrusion rate reaching 19.4km² per year (1989) (Table 3). In the 1990s, Longkou city government also implemented measures for checking sea water intrusion; as a result, the rate of sea water

intrusionslowed down. The city’s cumulative sea water intrusion totaled 89.0km² in 1991 and 105.0km² in 1998 (Chunhai Yao, 2000), averaging an annual growth rate of 2.3km² over this seven-year period, much lower than the intrusion rate in the 1980s (7.2km²/year) (Table 3). In 1998, the annual intrusion rate dropped to 1.7km²/year, with the highest intrusion distance reaching 5.9km.

Table 3   History of Sea Water Intrusion in Longkou City

2 Major Causes of Sea Water Intrusion

China started research on sea water in 1975. Over the years, a number of investigations of sea

water intrusions into cities have been conducted, suggesting that excessive groundwater extraction in seaside areas is the primary cause of sea water intrusion, and that precipitation also has an effect on sea water intrusion. In dry years, reduced groundwater recharge accelerates the rate of sea water intrusion; conversely, in wet years, increased groundwater recharge slows down the rate of sea water intrusion.

In the 1970s and 80s, due to a lack of unified planning and effective management of groundwater resources in Laizhou, a rising number of wells were dug in the seaside areas of the city and many newly dug wells had greater depth; soon, the density of wells for water extraction reached 10 holes per square kilometer. Over the 12-year period from 1978 to 1990, over-extraction of groundwater in seaside areas totaled 650 million cubic meters. As a result of the severe over-extraction of groundwater, the level of groundwater continued to fall, seriously disrupting the equilibrium between the original saline and fresh water interfaces and resulting in the occurrence and development of sea water intrusion. By 1989, the area of groundwater in Laizhou City whose water level was lower than sea level had reached 262.1km², and the water level in the central extraction areas had dropped to -16.7m (Dongyan Liu, 1999) (Table 3).             Increased precipitation can reduce the rate of sea water intrusion; for example, in 1990, Laizhou received remarkably increased precipitation, which reached 816mm; as groundwater recharge increased and extraction decreased, the area of groundwater whose level was below sea level was reduced by 55.1km² compared with 1989, and the cumulative area of sea water intrusion dropped by 52.2km² (Table 3). However, in 1991, precipitation decreased remarkably to a mere 461mm; consequently, the area of groundwater whose level was lower than sea level rose again, reaching289.8 km², and the groundwater level in central extraction areas continued to show a falling tendency, reaching -19.9m by 1992 (Table 3).

Table 4   Relationship between Sea Water Intrusion and Amount of Extraction and Precipitation in Laizhou City

Similar to the situation in Laizhou City, in the 1970s and 80s, Longkou City also suffered from extremely severe over-extraction of groundwater. In the 1986-1993 period, over-extraction of groundwater in the city’s seaside areas totaled 110 million cubic meters. During the period of time, dry weather lasting several consecutive years accelerated the development

of sea water intrusion. The 1981-1989 period was consecutive dry years, with annual precipitation

averaging a mere 47.18mm, accounting for just 80.0% of the mean annual precipitation. Rapid expansion of areas with sea water intrusion mainly occurred during this particular length of time, of which the 1986-1989 period was consecutive dry years, with an average annual precipitation of merely 381.9 mm, accounting for just 64.9% of the mean annual precipitation; consequently, sea water intrusion expanded most rapidly during this period of time. In 1989 alone, the area of sea water intrusion increased by 19.4km², and the area of groundwater whose level had dropped below sea level rose by 58.5km² (Chunmei Yao, 2000) (Table 3). In 1990, remarkably increased precipitation alleviated sea water intrusion; as a result, the area of groundwater whose level was below sea level dropped by 38.7km² compared with 1989, and the cumulative area of sea water intrusion fell by 5.0km² compared with 1989 (Table 3).

Table 5  Relationship between Sea Water Intrusion and Amount of Extraction and Precipitation in Longkou City

3 Geochemical Action of Seawater Intrusion

Prior to sea water intrusion, water-bearing media of terrestrial genesis have long been saturated by fresh water with relatively enriched Ca^2＋, and thus have absorbed an abundance of Ca^2＋ on their surface, whereas the mixture of seawater and freshwater contains a considerably high content of Na^＋ and Mg^2＋, which provides favorable conditions for the water-rock cation exchange.

In Longkou City, experiments have proved that in the process of sea water intruding into the freshwater aquifer, the water-rock cation exchange shows a very strong reaction (Jichun Wu, 1996; Yuchun Xue, 2000). The high-density Na^＋ and Mg^2＋ in the seawater and freshwater mixture replace Ca²⁺, which has been absorbed by the water-bearing media; thus, the content of Ca²⁺ in the groundwater in the transitional belt is higher than the content of Ca²⁺ in the local freshwater and seawater, forming Ca-Na and Ca-Mg mixture water on the Pipe trilinear chart; not a single cation or anion pair in the mixture water

exceeds 50%. The anion exchange reaction is primarily based on the Na⁺ and Ca²⁺exchange, complemented by the Mg²⁺ and Ca²⁺ exchange. Moreover, at the initial stage of sea water intrusion, the Mg²⁺ and Ca²⁺exchange is almost nonexistent. During the process of sea water intruding into the aquifer, the water-bearing media shows a relatively strong chemical absorption of K⁺ in the water mixture, with minimal exchange between K⁺ and Ca²⁺. The absorption of Cl^－ in the mixture water by the water-bearing media is almost nonexistent. The content Na^＋ will not increase dramatically and eventually give rise to C1-Na type groundwater with TDS of up to 3~11g/l until the percentage of the milligram equivalent of Cl^－ in the mixture water has exceeded 90%. Therefore, the mixture water in the transitional belt of sea water intrusion is not simple mixture of freshwater and seawater; rather, it is accompanied by physicochemical action during the mixing process.

4 Numerical Modeling of Sea Water Intrusion

Numerical modeling for sea water intrusion can be summarized into two models: the mutation interface model and the transitional belt model. Earlier on, many scholars used the mutation interface model in the plane or sectional two-dimensional modeling study of sea water intrusion. As a matter of fact, sea water and fresh water are completely mutually soluble; there is a transitional belt between them, and in most cases, the width of the transitional belt cannot be disregarded; only the use of a transitional belt model can provide an accurate simulation of the actual circumstances. Data obtained from the groundwater dynamic monitoring networks for China’s coastal cities over the years suggest that there exists no distinct mutation interface between the seawater zone and freshwater zone, but a wide transitional belt (mixed belt) in which sea water and fresh water are mixed with other. The width of the transitional belt reached 2.0~4.0km in Laizhou City and 1.5~2.5km in Longkou City.

The process of sea water intrusion with the characteristics of transitional belt must be described with two partial differential equations. The first equation is intended to describe the flow of liquid (freshwater and seawater mixture) with a changing density, and the second equation is used to describe the transfer of dissolved saline matter in the mixture water. In Longkou City, in response to sea water intrusion into the phreatic aquifer, a three-dimensional characteristic finite element transitional belt model was built (Yuqun Xue, 1995). On this model, not only the impact of precipitation on solute transfer was considered, the impact of the phreatic fluctuation on the process of sea water intrusion was also taken into account, and the means of boundary conditions was adopted to resolve these two impacts; thus, the sea water intrusion model under the conditions of the phreatic aquifer was developed. At the same time, a three-dimensional finite element solute transfer transitional belt model was built in Longkou City as well. This model, on the basis of the preceding model, provided a new function of depicting the transfer behavior of the water-rock exchange cations Na^＋, Mg^2＋ and Ca^2＋ in the sea water intrusion process (Jichun Wu, 1996). In Yantai City, a three-dimensional characteristic infinite element transitional belt model was built (Jianmei Cheng, 2001), which, on the basis of the actual local hydro-geological conditions, not only considered the change of the density of mixture water on the transitional belt, but also ascertained the sea-bottom extension boundary of the artesian aquifer, providing a solution to the problem with the simulation of the sea water intrusion process in multilayered artesian aquifers (five layers). In Beihai City, a quasi-three-dimensional finite element transitional belt model (X. Zhou, 2000) was built to consider the change of the density of mixture water on the transitional belt and to conduct a numerical simulation of the seawater intrusion into the first artesian aquifer. The building of the above models has not only deepened the understanding of the patterns of sea water intrusion, but also greatly facilitated the prediction and forecasting of the development trends of sea water intrusion and helped the government to adopt relevant measures to prevent and treat sea water intrusion.

5 Environmental and Social Problems Arising from Sea Water Intrusion

5.1 Water salinization in the aquifer

Sea water intrusion has led to fresh water salinization, which has rendered numerous extraction wells useless, resulting in a severe impact on residents’ life, agricultural irrigation and industrial production. As a consequence of sea water intrusion, more than 2,600 extraction wells in Laizhou City became useless, with 150,000 local residents experiencing a shortage of drinking water and 17,000hm² farmland suffering reduced irrigation to varying degrees (Zhumei Liu, 2003). In Longkou City, over 1,000 extraction wells in the sea water intruded area were made useless, with 30,000 local residents experiencing a shortage of drinking water and 6,700hm² farmland suffering reduced irrigation to varying degrees. Moreover, sea water intrusion has caused severe rusting on industrial equipment and undermined product quality, forcing some enterprises to suspend production or switch to a different industry, which has in turn led to a rising unemployment rate. Lost industrial output value in Laizhou City due to sea water intrusion was estimated at about RMB 150 million.

5.2 Soil salinization

Sea water intrusion has increased the salinity of groundwater; with extended use of high-salinity groundwater for irrigation, salt has continuously accumulated on the upper soil layer, resulting in soil salinization, which in turn caused a decrease in soil fertility and eventually led to a reduction of grain yield. By 1995, the area of soil salinization had reached 4,600hm² in Laizhou City and 3,300hm² in Longkou City. Due to the impact of soil salinization, the yield of farmland mostly dropped by 20%-40% and fell by 50-60% in some serious cases, with zero harvest in some extreme cases. In 1989, in sea water intruded areas in Longkou City, 168 hm² grain crops were affected by soil salinization, with yield felling by 12,347 tons; 49 hm² oil crops were affected, with yield sliding by 1,120 tons; 48 hm² orchards were affected, with yield dropping 3,730 tons; the combined value of economic losses incurred by yield reduction amounted to RMB 15.92 million (Chunmei Yao, 2000). Due to the impact of the deterioration of soil salinization, Laizhou City’s grain output dropped from 520 million kilograms in 1979 to 302 million kilograms in 1989. By 1991, the area of soil salinization in Huludao City caused by sea water intrusion totaled 467 hm², with grain output dropped by 2,250kg/hm².

5.3 Spread of endemic diseases

Sea water intrusion has also resulted in declining population health and population health. Due to lack of fresh water, residents in sea water intruded areas have to use salty water from time to time or throughout the year, resulting in a spread of endemic diseases. Consequently, many residents suffer from goiter and dental fluorosis. Statistics from 19965 showed that the number of sufferers of dental fluorosis stood at 15,600 and798 in Laizhou City and Longkou City, respectively. As a result of the rising incidence of endemic diseases, the average death rate of the population in areas affected by sea water intrusion was 1‰ higher than in non-affected areas (Mei Han, 1997).

6 Measures for the Prevention and Treatment of Sea Water Intrusion

In order to prevent and treat sea water intrusion, many scholars around the world have presented a great variety of viewpoints and methods, such as limiting the amount of coastal underground fresh water extraction, arranging well rows on the coast for artificial recharge, arranging a row of pumping wells near the coast to create a pumping channel, injecting certain substance along the coast to crate a water-segregating screen, etc. Each of these methods has some contradictory problems that must be resolved, and their feasibility in terms of technology and economy has yet to be established according to actual local circumstances. In the 1990s, in response to the actuality of the Chinese cities affected by sea water intrusion, local governments implemented effective measures aimed at preventing and treating sea water intrusion, with significant results achieved. For example, Laizhou City and Longkou City strengthened groundwater management and implemented groundwater recharge projects, farmland water-saving irrigation projects and long-distance water diversion projects, which have remarkably slowed down sea water intrusion, with certain sections of the affected land no longer experiencing an expansion of the sea water intruded area.

6.1 Strengthening groundwater resource management

Prior to the 1990s, ground water extraction was in a disorderly state. In order to prevent the area of sea water intrusion from expanding, coastal cities strengthened the management of groundwater extraction in seaside areas, strictly implemented a licensing system for water extraction, imposed a strict ban on authorized well digging and on the digging of deep wells, reduced the amount of groundwater extraction through administrative methods, and managed to limit the amount of groundwater extraction to a permissible extent.

6.2 Groundwater artificial recharge

As there is a limit to the amount of groundwater that can be extracted, groundwater recharge must be increased if the extraction amount is to be raised. Groundwater recharge can be increased by catching and holding precipitation and surface runoff and other methods, such as building flood detention gates, leaching wells and leaching ditches. Laizhou City has a very strong seasonal change in stream flow, with flow concentrated in July and August every year, accounting for 70%～80% of the annual flow volume; moreover, the streams in the city have short sources and rapid flow, with sharply rising and rapidly receding floodwater alternating during the flood season, which is extremely unfavorable for groundwater recharge. In the 1990s, Laizhou City built 10 small-scaled barrages (dams) on the lower reaches of its streams, which had a combined storage capacity of 3.30 million cubic meters; the city also built conveyance canals to divert floodwater to recharge works such as seepage pits, leaching ditches and leaching wells. These groundwater recharge works cost a combined sum of funding in the amount of RMB 150 million. Leaching ditches and leaching wells built on the lower reaches of the Wang River in Laizhou City are typical groundwater recharge works.

Before 1990, Laizhou city government built 122 leaching ditches and 244 leaching wells on a 2.9km section of the riverbed on the lower reaches of the Wang River. The leaching ditches were 80m long, 2m wide and 2m deep each, and had a 50m interval. In each ditch, two makeshift leaching wells were dug, each with a diameter of 2m and a depth of 4-9m; the upper water-resisting clay stratum was penetrated with a depth of 0.5～1.0m into the sandy gravel stratum; the wells and ditches were filled with pebbles and gravels. During three precipitations in the flood season in 1990, 15.23 million cubic meters of floodwater in the Wang River basin were intercepted with flood detention dams, and then diverted to leaching ditches and leaching wells for groundwater recharge. Groundwater level monitoring statistics showed that over a 144-hour period, groundwater recharge from rainwater infiltration totaled 317,000 cubic meters and the groundwater level in the recharged areas rose an average of 3.17m, indicating the recharging results were quite significant (Zhumei Liu, 2003).

6.3 Groundwater impervious screen

Part of the underground fresh water on the land flows into the sea as groundwater runoff; if this part of underground fresh water could be utilized, the amount of groundwater extraction could be increased. In 1995, Longkou City adopted the method of high-pressure directional jet injection to build underground cutoff walls on the lower reaches of the Balisha River and Huangshui River; the cutoff walls each was 6km long and 2~3m wide. Thus, two groundwater reservoirs were built, with the Shuangshui River underground reservoir having a total capacity of up to 52.39 million cubic meters and a maximum regulating capacity of 33.29 million cubic meters, allowing the groundwater level within a 180km² area to rise by an average 2.5m (Chunmei Yao, 2000). Following their completion, the groundwater reservoirs have not only inhibited the development of sea water intrusion, but also alleviated water shortage in some areas in Longkou City.

6.4 Saving irrigation water consumption amount

Agriculture is the sector with the largest potential for saving water, as agricultural irrigation methods are undeveloped and irrigation quotas for farmland remain high. Laizhou City has implemented a series of effective measures to reduce the irrigation quotas for farmland, including converting large stretches of farmland into smaller ones, making ditches impervious, and developing low-pressure conduit irrigation and micro irrigation. This agricultural irrigation water-saving project involved a total investment of RMB 180 million. Experience over the years shows that irrigation through semi-fixed low-pressure conduit irrigation can save water about 900m³/hm² per year, and that micro irrigation can save water around 3000m³/hm² per year. By now, Laizhou City has developed 28,000 hm² of farmland irrigated with low-pressure water conveyance conduits and 800 hm² of micro irrigated farmland, saving about 30 million cubic meters of water per year. At the same time, many coastal cities have practices ecological agriculture, implemented forestation, improved the environment, developed drying farming, and reduced groundwater extraction.

6.5 Long-distance water diversion

Due to salinization, groundwater in seaside areas in Laizhou City become undrinkable, causing 150,000 local residential to suffer from a shortage of drinking water; moreover, the digging of wells in other locations by residents from sea water intruded areas exacerbated the excessive inland water extraction. In response to this grave situation, Laizhou City has implemented a water diversion project to provide residents in seaside areas with drinking water; an annual amount of 4 million cubic meters of water is diverted from upstream reservoirs, providing all residents in seaside areas with access to tap water. This water diversion project, which involves an investment of RMB 100 million, has not only met household water needs of 200,000 residents in seaside areas and part of industrial water needs, but also reduced the amount of underground extraction in seaside areas and slowed down sea water intrusion.

7 Conclusion

Seawater intrusion is one of the most critical environmental issues confronting China’s coastal areas, causing tremendous economic losses and seriously hindering the development of the local social economy and the improvement of resident’s living standards. The primary cause of sea water intrusion is excessive groundwater extraction; the level of precipitation also has a certain impact on the speed of sea water intrusion. The interface between sea water and fresh water is not an abruptly changing interface; rather, it’s a transitional belt of a considerable width. In the transitional belt, there exists a water-rock cation exchange effect; cation exchange reaction is predominantly exchange between Na⁺ and Ca²⁺, complemented by exchange between Mg²⁺ and Ca²⁺. Numerical modeling of sea water intrusion must take into account factors such as the change of the density of mixture water in the transitional belt, the impact of precipitation and phreatic surface fluctuation on solute transfer, water-rock cation exchange, and the extension of the boundary of the sea bottom. Artificial groundwater recharge, underground cutoff walls, water-saving farmland irrigation and long-distance water diversion are effective measures for the prevention and treatment of sea water intrusion; integrated implementation of these measures will deliver significant results.

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