**3. The driving forces of salinization**

#### **3.1. Conceptual model**

water, and washes minerals and nitrate away, which may cause low soil permeability and

There has been a history of salt and water movement under evapotranspiration conditions. Fritton et al. [7] examined the differences of water‐salt distributions under various evaporation intensities, and much research was subsequently focused on salt and water transport in soil [8]. A variety of formulas are widely used to calculate evaporation. However, studies on salt transport fell behind, relatively. Groundwater has significant effects on salt accumulation, although the drainage system is blocking it with flattening landform. The higher the salinity of groundwater, the more serious the salt accumulation. In the capillary rise zone, evaporation maintains a constant generally, but it decreases rapidly at the bottom of this zone and tends to zero, while the groundwater exceeds the impact depth. Throughout this process, the cumula‐ tive evaporation has a power function in relation to the diving depth [9]. It is easier to prevent salt upward and improve the leaching efficiency with a higher hydraulic gradient when the water table is deep. However, plants cannot grow up when the root zone lacks water. Zhang and Zhang [10] considered that the groundwater should be controlled at 0.7–0.8 m during the

In the high salinity environment, plants play an important role in salt regulation [11]. Plants may exacerbate salinization in sea reclamation areas during evapotranspiration, and the impact depth should be the sum of root depth and capillary rise height. The movements of salt and water become stable only when the groundwater is below the limited depth of phreatic

Sediment deposited on the seabed for years has high salinity of more than 10%. When the environment changes to reclamation, salt in sediment will be gradually released. Environ‐ mental factors have obvious effects on the release of salt. For example, wind and temperature can promote this process, while the initial mineralization of groundwater will hinder it. In an acidic environment, the chemical properties maintain relative balances. Once these ions are in alkaline conditions, Mg2+ and Ca2+ would flocculate to deposition, which also accelerate the

Moreover, the environment of land reclamation areas is more complex than that of the natural coast. Groundwater replaces part of the saline water with the movement of fresh‐salt water interface. Due to the restriction of upper soil, groundwater exchange is slow, and the accom‐

can migrate within groundwater freely, and of which the concentration is determined by the

2 −, which may cause hydrolysis and acid erosion on rock and generate more dissolved

is a conservative ion excluded by soil colloids, which

. Meanwhile, CO2 released by plant roots generates excess HCO3

is released from the sediment in exchanges

−

nutrient content.

evaporation.

*2.3.3. Salt releasing from sediment*

dispersion of magnesium and calcium.

panying removal of salt is too low. Cl−

for a continuous supply of H+

and CO3

salinity of groundwater. In an open system, Na+

*2.3.2. Salt accumulation*

growth period, and fell to 1.2–1.4 m without crops.

164 Soil Contamination - Current Consequences and Further Solutions

The formation mechanisms of salinization are complicated for the multiple factors of water and salt movements. The sediment under the reclamation soil releases a large amount of salt, which was originally deposited in the oceanic environment over time. In a humid environment, with the upstream groundwater and rainfall supplement, the alkaline water in the reclamation soil is replaced by low‐acid groundwater with higher dissolved oxygen. The fresh‐salt water interface is pushed forward to the sea. However, under semi‐humid climate where the evaporation rate is larger than the precipitation rate, with the effort of evapotranspiration, the salinity in reclamation soil increases, and thus, the fresh‐salt water interface moves backwards to the continent (**Figure 2**).

**Figure 2.** Concept map of water environment evolution in land reclamation regions.

### **3.2. Physical model**

Semi‐humid climate is an important incentive for coastal salinization. Taking Bohai Rim as an example, in recent years, global warming aggravates the evaporation process and salinization in sea reclamation region. Guan et al. [12] monitored the involvement of salinized coastal area around the Yellow River Estuary, and found that the saline land had expanded by 487.4 km2 over the last 15 years. To investigate the saline formation mechanism in compacted soil, we studied the salt transport processes under the joint action of phreatic water evaporation and lateral interflow.

#### *3.2.1. Experimental setup*

The experiment was carried out outdoor using the apparatus shown in **Figure 3**. Soil collected from the reclamation site in Dalian, China, was filled into a glass tank. The tank was 0.6 m high and was divided into three sections with two perforated plates which were the simulation of coastal constructions. The diameter of the holes was 1.0 cm, and the interval between each hole was 5 cm. In total, 25 monitor holes were reserved on the side of the tank as illustrated in **Figure 3**. Seawater collected from the reclamation site was poured initially into the tank at a depth of 10 cm. The reclamation soil was then filled into the middle of the tank in layers. Each layer of the filling soil was 5 cm. The soil tank was placed outside for 9 months with light‐tight cover to prevent water loss. Then, fresh groundwater was poured into the Marriotte's bottle until the water level in the left section reached to 50 cm. The purpose of this stage was to simulate the process of sideward flow in the soil. The pH value, electric conductivity and volumetric water content were measured during the experiment. Soil samples were also collected from the monitor hole at the end of each stage.

**Figure 3.** Testing apparatus.

#### *3.2.2. Phreatic evaporation stage*

The volumetric water content was dynamic during the experimental process (**Figure 4**). It first increased during the first month and reached a peak. Then, it decreased gradually until getting back to its initial state. In spatial scale, the water content was larger in bottom layer and smaller in surface layer. It was relatively stable in surface and subsurface layer horizontally. While under the middle layer, it was larger along the right side. These facts indicate that capillarity is the main driving force of the increase in water content. The soil matrix potential gradually decreased with the increase in water content. When the water content reached 3%, the soil matrix potential meets the bottom and was stable ever since. The actual evaporation was little at first when the water content in the surface was low. With the increase in water content, the actual evaporation raised, which leads to the total loss of water content. The higher content along the right side was supplied by the seawater.

The total dissolved salt (TDS) in the soil increased during the evaporation process. It was larger in surface layer than that at the bottom and larger along the right side than the left. This indicates the salt accumulation with plenty supplement. The initial salt type of surface soil was CaSO4, and it gradually transited to CaCl2 and NaCl in the salt accumulation process. The salt type in the bottom layer was NaCl; Mg2+ presented tendency to dissolve in seawater and stayed in the right side of the tank. The pH value decreased on the left of the tank while increased on the right.

**Figure 4.** Variation in soil moisture and conductivity during the evaporation period.

#### *3.2.3. Seepage stage*

around the Yellow River Estuary, and found that the saline land had expanded by 487.4 km2 over the last 15 years. To investigate the saline formation mechanism in compacted soil, we studied the salt transport processes under the joint action of phreatic water evaporation and

The experiment was carried out outdoor using the apparatus shown in **Figure 3**. Soil collected from the reclamation site in Dalian, China, was filled into a glass tank. The tank was 0.6 m high and was divided into three sections with two perforated plates which were the simulation of coastal constructions. The diameter of the holes was 1.0 cm, and the interval between each hole was 5 cm. In total, 25 monitor holes were reserved on the side of the tank as illustrated in **Figure 3**. Seawater collected from the reclamation site was poured initially into the tank at a depth of 10 cm. The reclamation soil was then filled into the middle of the tank in layers. Each layer of the filling soil was 5 cm. The soil tank was placed outside for 9 months with light‐tight cover to prevent water loss. Then, fresh groundwater was poured into the Marriotte's bottle until the water level in the left section reached to 50 cm. The purpose of this stage was to simulate the process of sideward flow in the soil. The pH value, electric conductivity and volumetric water content were measured during the experiment. Soil samples were also

The volumetric water content was dynamic during the experimental process (**Figure 4**). It first increased during the first month and reached a peak. Then, it decreased gradually until getting back to its initial state. In spatial scale, the water content was larger in bottom layer and smaller in surface layer. It was relatively stable in surface and subsurface layer horizontally. While under the middle layer, it was larger along the right side. These facts indicate that capillarity is the main driving force of the increase in water content. The soil matrix potential gradually

lateral interflow.

*3.2.1. Experimental setup*

166 Soil Contamination - Current Consequences and Further Solutions

**Figure 3.** Testing apparatus.

*3.2.2. Phreatic evaporation stage*

collected from the monitor hole at the end of each stage.

Under the seepage, the groundwater supply rate decreased with the increase in soil water content until 22 h later when there was water leaking out from the end of the tank (**Fig‐ ure 5(a)**). The water content in the soil was stable since then. This dynamic was driven by the soil matrix potential. The potential‐driven change rate (**Figure 5(d)**) decreased with the rise in the water table until the active water absorption stopped and a relatively free horizontal flow started, which leads to a stable velocity. The TDS of the soil was first increased because the crystal structure of salt in the soil was dissolved in the infilled groundwater (**Figure 5(c)**). The high TDS in the soil was carried out with the horizontal flow, and the TDS in the leaking water was high at first (**Figure 5(b)**). The time that TDS became stable was later than the flow rate indicates that the change in the salt front is later than in the wetting front.

The stable wetting front moves to the offshore, while the water table rises. Surface area above the free water is in the salt accumulation state, and the salt is mainly CaCl2 and MgSO4. The area below the free water surface is in the state of desalination, and the salt type is mainly CaCl2 and NaCl. Water and salt movement will change the pH value of the soil environment; the pH value will increase with the increase in salt content first decreased slightly after the rise.

**Figure 5.** Variation in soil moisture, water supply and permeability.

#### *3.2.4. Salt migration in land reclamation regions*

In semi‐humid coastal region, the water content in surface layer and water table of ground‐ water are the main factors of salt accumulation due to low precipitation rate but high evapo‐ ration. During the dry seasons, low groundwater supply rate from the continent pushes the freshwater and seawater interface upwards to the continent. When the water table meets the phreatic evaporation depth, Cl− moves upwards and the CaSO4 in surface layer is replaced by CaCl2. With the consistent supply of Na+ and Mg2+, most of the salt in the soil is replaced by NaCl. In wet seasons, the groundwater water table rises and the wetting front moves down‐ wards to the sea with salt front following. In reclaimed regions where there are obstacles (clay or coastal constructions), the water table would be raised. Salt in groundwater would also be raised with the water table which would then accumulate in the surface layer. CaCl2 and MgSO4 accumulated in the surface layer near the obstacles and NaCl accumulated at the bottom. Therefore, in terrestrial groundwater recharge conditions, land reclamation area of salinization prevention and control work should focus on underground baffle layer or foundation of a building with a relatively dense region.

#### **3.3. Numerical model**

CaCl2 and NaCl. Water and salt movement will change the pH value of the soil environment; the pH value will increase with the increase in salt content first decreased slightly after the

In semi‐humid coastal region, the water content in surface layer and water table of ground‐ water are the main factors of salt accumulation due to low precipitation rate but high evapo‐ ration. During the dry seasons, low groundwater supply rate from the continent pushes the freshwater and seawater interface upwards to the continent. When the water table meets the

NaCl. In wet seasons, the groundwater water table rises and the wetting front moves down‐ wards to the sea with salt front following. In reclaimed regions where there are obstacles (clay or coastal constructions), the water table would be raised. Salt in groundwater would also be raised with the water table which would then accumulate in the surface layer. CaCl2 and MgSO4 accumulated in the surface layer near the obstacles and NaCl accumulated at the bottom. Therefore, in terrestrial groundwater recharge conditions, land reclamation area of

moves upwards and the CaSO4 in surface layer is replaced by

and Mg2+, most of the salt in the soil is replaced by

**Figure 5.** Variation in soil moisture, water supply and permeability.

*3.2.4. Salt migration in land reclamation regions*

168 Soil Contamination - Current Consequences and Further Solutions

phreatic evaporation depth, Cl−

CaCl2. With the consistent supply of Na+

rise.

Apart from the physical model, FEFLOW was employed to simulate the migration of water and salt within a unit of a typical land reclamation project. The combined effects of two driving factors, phreatic evaporation and rainfall infiltration, were selected to reveal the migration process in the simulation. Then, the effects of salinity suppression of different measures for rainwater utilization were analyzed.

#### *3.3.1. Model domain description*

Due to lack of geological survey data, long sequence groundwater level and solute concentra‐ tion monitoring data of practical projects, the model domain was an imaginary land reclama‐ tion project which was based on Lingshui Bay land reclamation project in Dalian city, China, and several engineering examples in North China. The theoretical model (**Figure 6**) was designed to explore the mechanism of soil water and salt transport in reclamation areas in the north region of China and provide trend analysis results for practical projects.

**Figure 6.** The schematic diagram of model domain.

The left border in **Figure 6** is the land frontier before the project implementation, and the right border is the new land frontier. Considering that the extended distance from landside to the sea was less than 1 km in general cases, the extended distance was set to be 1 km. The surface grade was 3‰, and the lateral cofferdam was 5.5 m in accordance with land reclamation engineering specifications. The structure of earth fill is designed according to Lingshui Bay land reclamation project.

#### *3.3.2. About Feflow*

FEFLOW is a finite element‐based groundwater simulation system. It is considered to be a comprehensive, well‐tested and reliable program for the simulation of flow, mass and heat transport processed in porous media. FEFLOW provides data interfaces for geographic information system and can generate spatial finite element grids automatically. The system is equipped with fast and accurate numeric algorithms to control and optimize the solution procedure, and advanced visual figures are embodied in output results. FEFLOW is used to compute groundwater flow dynamics in unconfined and confined aquifers and multiple free water surface(s); describe the spatial and temporal distribution of contaminants and/or temperature fields; estimate the duration and travel time of contaminants in groundwater; study saltwater intrusion and so on.

#### *3.3.3. Mass transport model building*

A two‐dimensional coupled groundwater flow and mass transport model in vertical section was established (**Figure 7**). The left border was generalized as the boundary of known flow, and the groundwater flow was determined by the measured value in Lingshui Bay land reclamation project. The right border was defined as the boundary of known water level. The model contained five layers in vertical direction according to the soil layer. The top layer was planting soil layer. The second, third and fourth layers were compacted fill layer. The bottom layer was natural sediments with weak permeability. The free water surface of the unconfined aquifer was the upper boundary, and the bottom of the aquifer was impervious boundary.

Groundwater recharge in model area mainly included precipitation infiltration recharge and lateral recharge, and evaporation and runoff into the sea is the main way of groundwater discharge. Precipitation and evaporation data were referred to meteorological stations near Lingshui Bay. Spring and autumn were the evaporation periods. The precipitation was concentrated in summer, so summer was the leaching period. There was little rainfall from winter to early spring, and the evaporation was weak due to low temperature. Evaporation capacity was much higher than rainfall capacity, and the ratio was about 2.3, which meant that the driving effect by phreatic water evaporation was strong. Salt would accumulate in shallow ground in the process of migration, ultimately resulting in soil salinization. Boundary conditions including lateral runoff, sea level and chloride concentration, parameters such as groundwater chloride ion content, precipitation recharge coefficient, specific yield and porosity were assigned according to the Lingshui Bay land reclamation project. Initial groundwater level was 0.7 m, which was equal to local capillary height, and initial ground‐ water chloride content was equal to that in seawater. Initial dispersion coefficient values were referred to previous experience and revised repeatedly in the simulation process. The simu‐ lation period was from spring and lasted for 5 years (1825d).

**Figure 7.** Soil layers of the model.

#### *3.3.4. Simulation results and analysis*

#### *3.3.4.1. Groundwater level*

procedure, and advanced visual figures are embodied in output results. FEFLOW is used to compute groundwater flow dynamics in unconfined and confined aquifers and multiple free water surface(s); describe the spatial and temporal distribution of contaminants and/or temperature fields; estimate the duration and travel time of contaminants in groundwater;

A two‐dimensional coupled groundwater flow and mass transport model in vertical section was established (**Figure 7**). The left border was generalized as the boundary of known flow, and the groundwater flow was determined by the measured value in Lingshui Bay land reclamation project. The right border was defined as the boundary of known water level. The model contained five layers in vertical direction according to the soil layer. The top layer was planting soil layer. The second, third and fourth layers were compacted fill layer. The bottom layer was natural sediments with weak permeability. The free water surface of the unconfined aquifer was the upper boundary, and the bottom of the aquifer was impervious boundary.

Groundwater recharge in model area mainly included precipitation infiltration recharge and lateral recharge, and evaporation and runoff into the sea is the main way of groundwater discharge. Precipitation and evaporation data were referred to meteorological stations near Lingshui Bay. Spring and autumn were the evaporation periods. The precipitation was concentrated in summer, so summer was the leaching period. There was little rainfall from winter to early spring, and the evaporation was weak due to low temperature. Evaporation capacity was much higher than rainfall capacity, and the ratio was about 2.3, which meant that the driving effect by phreatic water evaporation was strong. Salt would accumulate in shallow ground in the process of migration, ultimately resulting in soil salinization. Boundary conditions including lateral runoff, sea level and chloride concentration, parameters such as groundwater chloride ion content, precipitation recharge coefficient, specific yield and porosity were assigned according to the Lingshui Bay land reclamation project. Initial groundwater level was 0.7 m, which was equal to local capillary height, and initial ground‐ water chloride content was equal to that in seawater. Initial dispersion coefficient values were referred to previous experience and revised repeatedly in the simulation process. The simu‐

lation period was from spring and lasted for 5 years (1825d).

**Figure 7.** Soil layers of the model.

study saltwater intrusion and so on.

170 Soil Contamination - Current Consequences and Further Solutions

*3.3.3. Mass transport model building*

After the model runs for 5 years (the fifth year after the completion of land reclamation project), the groundwater level is shown in **Figure 8**. Because of the construction of land reclamation project, the discharge outlet is cut off. Groundwater from the origin land frontier and the sea enters into the fill, causing a gradual increase in groundwater level in this area. Groundwater level in the model domain after model runs for 1, 3 and 5 years is analyzed. It indicates that groundwater level becomes stable over time. Groundwater table near the original land frontier is higher and gets closer to the limit‐evaporable depth of groundwater, in which condition salt can migrate to shallow ground. In spring and summer, the evaporation is intensive, while there is no adequate supply; so groundwater table is relatively low.

**Figure 8.** Groundwater level in the simulation area.

#### *3.3.4.2. Chloride concentration*

Groundwater chloride concentration after the model runs for 1, 3 and 5 years in model area is seen in **Figure 9**. As groundwater from the original land frontier enters into the reclamation area and precipitation infiltration recharge, groundwater salt within the capillary height is diluted. However, chloride concentration is still in high level. Groundwater chloride concen‐ tration near the sea is much higher. In spring and autumn, chloride concentration is over 14000 mg/L where groundwater is lower than 1 m due to insufficient lateral supply. On the whole, groundwater chloride concentration gets higher from land to the sea. Therefore, countermeasures such as recharge wells are suggested to promote salt water discharge into the sea and inhibit seawater intrusion.

**Figure 9.** Groundwater chloride concentration in model area.
