Sea Water Intrusion

In coastal areas over-exploitation of groundwater leads to seawater intrusion (see, e.g., SWIM'96).13 Due to the density difference between fresh and saline water, a saltwater wedge forms naturally in coastal aquifers. If the freshwater flow is diminished by pumping, the wedge will proceed further inland, eventually leading to the contamination of wells. Wells which are screened in the freshwater layer may still draw salt by the phenomenon called upconing.14,15

At present, six out of ten people live within 60 km of a coast, and by the year 2000 more than two-thirds of the population of developing countries will live in the vicinity of the sea. The increasing concentration of human settlements in coastal areas gives rise to excessive pressure on groundwater resources, resulting in seawater intrusion and related deterioration of water quality.16 Seawater intrusion is rampant along the coasts of India, Israel, southern China, Spain and Portugal, to name only a few.

In China, seawater intrusion has become more and more serious since 1970 in the eastern coastal zones. The following figures illustrate the situation on the Shandong coast, including the cities Laizhou, Weifang, Qingdao, Yantai, and Pingdu among others. The facts presented are taken from Wang et al.17

In the coastal area of Qingdao, Yantai and Weifang, the current water supply shortage in an average year is 3.1 km3. Available water resources can only meet 60% of the total demand. In a dry year, the shortage is 5.1 km3 or approximately 52% of the total demand. The well density rose from the original 10 wells/km2 to 25 wells/km2, and in some places even up to 70 wells/km2. From 1976 to 1986, overabstraction caused a large groundwater level drawdown in the northern Weifang area. Up to now, the total area with groundwater levels lower than mean sea level is 2,400 km2 in the Laizhou area. The lowest elevation is 20 m below sea level.

From Yantai City to Laizhou City, the area affected by seawater intrusion is up to 400 km2 and growing fast. By the year 2000, the affected area will be 2000 km2. The intrusion, which accelerated from a frontal speed of 46 m/yr to 400 m/yr between 1976 and 1988, caused serious damage to this area, in which around 450,000 people were affected. With growing salinity the soil productivity drops or may even be lost completely. About 80% of the cultivated land of the coastal plain of Laizhou City is affected and the grain loss is up to 75,000 tons per year.

To solve the problem, water resources management organizations were set up in Yantai City and water prices were increased, taking into account seasonal variation of scarcity. In order to save irrigation water, two measures were taken. First, drip irrigation and low pressure pump irrigation were introduced, which can save up to 90% and 30% of water, respectively. Second, earth canals were lined with concrete and open drains were replaced by pipes to decrease losses by evaporation and seepage.

The consequences of salinization are not seen all of a sudden, but progress gradually. Agriculture can adapt to a certain degree by planting more salt resistant, but usually less valuable, products. This happens on the Tamil Nadu coast in India, which is one of the most prominent examples for severe seawater intrusion. Rice cultivation has now given way to some more salt tolerant crops, such as trees for firewood. This allows the farmers to still procure income, but it does not reverse the trend of further degradation. Exploitation of groundwater with the help of electric and diesel pumps is continuing uncontrolled.18 The main cause is the fact that electricity is free of charge for the villages and a change in this policy is virtually taboo. This shows the political and social dimensions of harnessing a problem which on the scientific side is well understood.

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