**1. Introduction**

Coastal regions are identified as productive and sensitive ecosystems with abundant biodi‐ versity. They are water bodies connected with both the land and the sea, and within which seawater mixes with inland freshwater discharge. Most of the megacities in the world are located in coastal regions, and more than 3 billion people which cover almost half of the world's population live along the coastline. The overloaded population increases the pressure on land

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

resources. Along with the population growth, the utilization and development of coastal regions have increased in recent years and the changes in socioeconomic and environmental conditions are continuing. To cope with the expansion of urbanization, land reclamation was carried out.

Overloaded population and the needs for more agricultural land and for flood protection are the main reasons for the reclaiming. Land reclamation has expanded rapidly to adjust to economic development in coastal regions. It brings about more space, which alleviates the contradiction between supply and demand of land resources. Many coastal countries, includ‐ ing the developed ones such as the USA, Japan and Netherlands, have long histories of coastal reclamation. Netherlands, as an example, reclaimed about 7000 km2 from the sea and inland lakes since the 1300s, which covers up to 21% of the total land surface of the country [1]. Currently, these new formed lands have exceeded 140,000 km2 in the worldwide scale, and still increasing rapidly in some countries such as China [2].

However, reclamation disturbs the hydro‐environment near the coast. It disrupts the water‐ salt movement and causes engineering, environmental and ecological problems. The quality of groundwater was affected by saline intrusion in the Netherlands [1]. Mangrove forest in China has been reported to be reduced by 53% than that in 1950s [3]. These degradations of marine habitats indicate that coastal ecosystem and hydrodynamic conditions are disturbed. The high density of salt in reclaimed regions exerts pressure on the local plants. If the salt pressure is weak, the injury to the plant could be recovered. The salinity in reclamation soil is 1–4% in 1 m3 which is much larger than the largest salinity that the most plants could bare (0.3%). The mineralization ability in groundwater is more than 50 g/L. Only plant with shallow roots and high salt tolerance could survive in reclamation areas. Once the salt pressure exceeds the salt resistance of the plant, the life cycle of it will be destroyed and hence disturbs the whole ecosystem. Apart from this, social underground infrastructures are other victims of soil salinization. Seawater accelerates the corrosion rates of reinforced concretes and underground pipe networks which would threaten the security of coastal structures. Therefore, the under‐ standing and mitigation of soil salinization in reclamation regions are important for coastal environmental protection.

Soil salinization is a tough problem for coastal environment and has drawn attention on a worldwide scale. Efforts have been made to study the mechanism and mitigation measures of it. Armstrong et al. [4] studied the seasonal variation in water and salt distribution in fields with both grassland and arable saline‐sodic clay soils under temperate rain‐fed conditions. Chen and Jiao [5] analyzed the groundwater chemistry in coastal aquifer and found that groundwater pumping was the reason for seawater intrusion. Iost et al. [6] found that reclamation influenced the local pH and carbonate content by decreasing calcium, magnesium and potassium while studying the initial pedogenesis of reclaimed saline marsh soils.

The objective of this chapter is to explore the mechanism of soil salinization in reclaimed coastal regions, especially that under semi‐humid climate where evaporation is more than precipita‐ tion. In this chapter, the Bohai Rim, China, is selected as an example to study the water and salt migration in reclaimed soil. Physical model and numerical model are built for under‐ standing, quantifying and modelling the dynamic of seawater intrusion. Promising mitigation measures are also proposed.
