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Article

Sustainable Marginal Water Resource Management: A Case Study of Brackish Water Irrigation on the Southern Coast of Laizhou Bay

by
Wenquan Liu
1,*,
Fang Lu
2 and
Weitao Han
3,4
1
Observation and Research Station of Seawater Intrusion and Soil Salinization, Laizhou Bay, Ministry of Natural Resources, Qingdao 266061, China
2
College of Ocean Science and Engineering, Shandong University of Science and Technology, Qingdao 266590, China
3
Key Laboratory of Coastal Science and Integrated Management, Ministry of Natural Resources, Qingdao 266061, China
4
Weifang Marine Development Research Institute, Weifang 261035, China
*
Author to whom correspondence should be addressed.
Sustainability 2025, 17(5), 1956; https://doi.org/10.3390/su17051956
Submission received: 3 January 2025 / Revised: 17 February 2025 / Accepted: 18 February 2025 / Published: 25 February 2025
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Abstract

:
The secure and effective use of marginal water resources, such as brackish water, plays a crucial role in ensuring food security and promoting the sustainable development of agricultural land. This paper conducted indoor soil column experiments to simulate the infiltration of brackish water (0, 1, 3, and 5 g L−1) in order to study the effects of infiltration on the movement of soil water and salt, aiming to address the critical challenge of utilizing marginal water resources in coastal saline-alkali areas. The result showed that, as salt content increases, the movement speed of the moisture front and soil infiltration rate gradually decrease over the same period of time. The moisture front progress and infiltration volume showed a positive correlation. The moisture content of the soil profile gradually decreased, within the soil depth range of 0–40 cm, except for the 5 g L−1 saline water infiltration, and the Cl− content increased, while the other treatments showed a trend of first decreasing and then increasing. The higher salt content at the same depth, the higher the Na+ and Cl− contents. Under different irrigation water volume conditions, the soil profile conductivity shows a trend of first decreasing and then increasing. The research findings advance fundamental understanding of salinity-driven soil hydrological processes, offering theoretical support for the sustainable utilization of brackish water, balancing agricultural water demand and soil health in coastal areas.

1. Introduction

Water scarcity has become a global issue that transcends national boundaries [1,2,3]. It has a profound impact on agricultural production, as water shortages hinder crop growth, reduce grain yields, and jeopardize food security, ultimately threatening human health and social stability [4,5]. In this context, China faces severe freshwater shortages, with its total freshwater resources accounting for only 6% of the world’s supply. As a result, regional and seasonal water shortages are a significant challenge. Despite being a major agricultural producer, China has a high demand for irrigation water, which accounted for 62.2% of total water consumption in 2023. At the same time, issues such as water wastage and low water utilization efficiency exacerbate the gap between supply and demand for freshwater [6,7,8]. Brackish water, a type of marginal water resource, has emerged as a promising solution to China’s water crisis. Its development and use have become critical strategies for many countries and regions to mitigate the shortage of freshwater resources [9,10,11].
Irrigation with brackish water presents a dual impact on soil and plant growth. On one hand, brackish water irrigation can lead to the accumulation of salts in the soil. Excessive concentrations of salt ions cause soil expansion, resulting in the disintegration of soil aggregates and the clogging of larger pores. This creates salt stress on vegetation and poses a potential risk of soil salinization. When the salt content exceeds a critical threshold, crop yields decline as the soil salinity increases [12,13]. On the other hand, brackish water irrigation can meet crop water requirements, improve soil moisture content, and enhance the stability of soil aggregates. It also reduces the concentration of soil solution, which can ultimately improve both the quantity and quality of crop yields, including increased grain protein content in certain crops [14,15]. The impact of salt in irrigation water on soil is primarily reflected in its influence on soil-exchangeable sodium and the conductivity of the soil solution [16]. An increase in soil salinity can promote the orderly aggregation of soil particles, which stabilizes the soil structure and enhances the permeability of larger pores. However, sodium salts are the primary contributors to soil degradation. Due to the relatively small ionic radius of sodium ions, their presence leads to the expansion and dispersion of soil particles, which alters the physical properties of the soil, particularly under alternating wet and dry conditions. Therefore, understanding the effects of brackish water infiltration on soil water and salt dynamics is crucial. In recent years, numerous studies have investigated the relationship between brackish water irrigation and the transport of soil water and salts. Selim et al., 2013 [17] investigated the distribution characteristics of water and salt in sandy loam soils under various drip irrigation systems using brackish water, employing both field experiments and numerical simulations. Their findings indicated that, under the biweekly irrigation system and subsurface drip irrigation treatment, both the maximum depth of water infiltration and the largest volume of infiltration were achieved. Chen et al., 2018 [18] examined the effects of mulched drip irrigation with brackish water on soil moisture content, soil salinity, and root length density over a two-year field study. The results revealed that saline water irrigation increased soil salinity, and the average root length density, aboveground dry weight, and cotton yield were lower compared to those under freshwater irrigation. Yang et al., 2020 [19] explored the relationship between soil salinity and cotton yield under drip irrigation with varying concentrations of brackish water. Their study found that increasing salinity led to a higher soil pH and a corresponding decrease in cotton yield. Yin et al., 2022 [20] compared the effects of brackish water and Yellow River water on salt transport in saline soils using column leaching loss simulations. Their results demonstrated that, for the 0–20 cm soil layer, both the desalination time and the required leaching volume for brackish water were lower than those for Yellow River water; however, for soil layers deeper than 20 cm, Yellow River water required less time and leaching volume than brackish water. Wang et al., 2023 [21] compared the effects of different water and salt conditions, along with various boundary conditions, on the infiltration process. Their findings showed that when the brackish water infiltration rate was 3 g L−1, the cumulative infiltration was maximized, the time to reach soil moisture saturation was minimized, and the wetting front reached the bottom of the soil column first. Compared to freshwater, the use of brackish water could help mitigate excessive groundwater extraction in coastal regions, thereby promoting the renewal and transformation of existing water resources [22].
According to statistics, China’s available brackish water resources are estimated to be approximately 2.0 × 1010 m3 a−1, of which 1.3 × 1010 m3 a−1 are exploitable resources. Most of these resources are found at depths ranging from 10 to 100 m below the surface [23]. The southern coast of Laizhou Bay is home to the most abundant underground brackish water resources in China, covering an area exceeding 2500 km2. However, soil salinization remains a serious environmental issue along this coastline. Large areas of brine salt fields have been reclaimed for farmland, and the extremely high soluble salt content has resulted in soils with high electrical conductivity, low nutrient content, poor physical structure, and limited fertilizer retention capacity. In recent years, rapid economic development has exacerbated the demand for freshwater resources to support saline-alkali land reclamation and agricultural production. Apart from a small portion of water supplied by local reservoirs, irrigation for agriculture is primarily dependent on rainfall, making it a typical rain-fed agricultural region [24,25]. The shortage of water resources has severely restricted local agricultural productivity, hindering sustainable development. Therefore, the extraction and utilization of brackish water resources offer a promising management strategy for addressing the freshwater scarcity on the southern coast of Laizhou Bay, where freshwater resources are relatively scarce compared to the abundance of brackish water resources.
To date, there have been few studies on the use of brackish water for irrigation and its effects on soil water and salt transport on the southern coast of Laizhou Bay. Brackish water has a limited dissolved salt content, and short-term irrigation does not significantly affect soil chemical properties, including salt content. However, long-term brackish water irrigation can lead to secondary soil salinization, ultimately impacting crop growth [26,27]. This study examines the dynamic interaction between brackish water salinity and soil water and salt movement over time, addressing a gap in previous research, which has mostly focused on short-term or steady-state conditions. It also explores the impact of irrigation volume on salt accumulation, an aspect largely overlooked in earlier studies. This is particularly relevant for regions like Laizhou Bay, where brackish water is plentiful but freshwater is scarce. Understanding how salinity, irrigation volume, and soil characteristics interact is crucial for developing sustainable irrigation strategies in water-scarce areas. Therefore, this study conducts sophisticated indoor soil column simulation experiments: (1) to investigate the effect of brackish water on soil water and salt transport under different salinity conditions and (2) to examine the effect of brackish water on soil water and salt transport under varying irrigation volumes.

2. Materials and Methods

2.1. Study Area

The soil samples used in this study were obtained from the Changyi Marine Ecological Special Reserve (37°3′7″ N~37°7′12″ N, 119°20′19″ E~119°23′49″ E), located to the east of the Dihe River in northern Changyi City, Shandong Province, China. The reserve lies on the beach below the coastline and on the southern bank of Laizhou Bay, as shown in Figure 1. The region has a continental climate typical of the warm temperate semi-humid East Asian monsoon zone, characterized by four distinct seasons. Spring is dry and prone to drought, while summer is marked by high temperatures and frequent drought episodes, despite the rainy season. Autumn is mild and cool, with occasional droughts toward the end of the season. Winter is dry and cold, with limited rain and snow. The annual average temperature is 11.9 °C, with a yearly mean precipitation of 660.1 mm, most of which occurs in the summer, accounting for 64.5% of the total annual precipitation. The annual average evaporation rate is 1859.4 mm, with a daily average evaporation rate of approximately 3.0 mm [28]. The terrain slopes gently from south to north, with gradients ranging from 0.27‰ to 0.31‰. Geologically, the area is part of the Bohai Depression within the North China Platform, consisting of Quaternary alluvial deposits. The landform is a coastal accumulation plain, and due to the influence of tidal actions, soil moisture and salinity in the reserve’s tidal flats fluctuate, impacting plant growth and distribution. The reserve encompasses various natural wetland types, including shallow water, tidal flats, salt marshes, and Tamarix wetlands. The predominant soils are fluvo-aquic soils and solonchaks. The vegetation is characterized by shrubs, herbs, and lianas, with Tamarix species being the dominant shrub. Herbaceous plants include Suaeda salsa, Setaria viridis, Artemisia capillaries, Conyza canadensis, Atriplex patens, and Cynanchum chinense as the dominant liana. On the southern edge of the reserve, an area of abandoned salt fields has been reclaimed for agriculture, with cotton being the primary crop, along with smaller plots of corn and alfalfa.

2.2. Experiment Design

The column simulation experiment was conducted at the Observation and Research Station for Seawater Intrusion and Soil Salinization in Laizhou Bay, Ministry of Natural Resources. The objective of this experiment was to investigate the influence of varying salinities of brackish water infiltration on soil water and salt transport characteristics. Soil samples were collected on 12 October 2022, after the rainy season had ended, when the groundwater level was stable and the impact of climatic fluctuations on soil salinity was minimal. Samples were taken from different soil profiles at depths of 0–15 cm, 15–30 cm, 30–45 cm, 45–60 cm, and 60–75 cm. The collected samples were air-dried, ground, and passed through a 2 mm sieve. Table 1 presents the basic properties of the tested soil. Bulk density was determined using the ring knife method, with values ranging from 1.46 to 1.56 g cm−3. For the extraction of soil solutions, a 1:5 soil-to-water ratio was used, and electrical conductivity was measured using a YSI EC300 conductivity meter (YSI Company, Yellow Springs, OH, USA). Soil texture was determined using the hydrometer method. The concentrations of K+ and Na+ were quantified using flame photometry, while Ca2− and Mg2− concentrations were measured through EDTA complexometric titration. The concentration of Cl− was determined via AgNO3 titration, and SO42− concentration was estimated using an EDTA indirect titration method. The specific measurement procedures are detailed in the Soil Agricultural Chemical Analysis Method [29], and the relevant attribute indices are summarized in Table 2. As shown in Table 2, the salt concentration in the tested soil is relatively low, classifying it as non-saline soil.
In this experiment, a transparent plexiglass soil column was used for simulation. The experimental setup primarily consisted of a soil column and a cylindrical Mariotte’s bottle, as shown in Figure 2. The soil column was constructed from a 5 mm thick plexiglass, with a diameter of 20 cm and a height of 90 cm. A sampling hole, with a diameter of 2 cm, was drilled every 10 cm along the side of the column, resulting in a total of seven sampling holes, each sealed with rubber plugs to facilitate soil sample extraction.
Before loading the soil, a piece of filter paper, with a diameter equal to that of the soil column, was placed at the bottom. Over this, a 5 cm layer of quartz sand (2–3 mm in diameter) was added to serve as an inverted layer. Drainage holes were then incorporated at the base of the soil column to allow for the discharge of leachate. The soil column and the outer wall of the Mariotte’s bottle were both marked with scales to monitor the water level in the bottle and track the depth of the wetting front.
The soil was layered and compacted into the column in accordance with the properties outlined in Table 1 to ensure that the soil structure remained consistent with the original soil. During the loading process, the soil surface between layers was carefully leveled to prevent the formation of stratification. A total of 75 cm of soil was loaded into the column, which was then allowed to equilibrate overnight after the loading process was complete.
The irrigation water used in this experiment was prepared by mixing groundwater collected from Changyi with laboratory distilled water. Four irrigation water samples with different total dissolved solids (TDS) concentrations were prepared: 0 g L−1, 1.0 g L−1, 3.0 g L−1, and 5.0 g L−1. The quality indices of the collected groundwater are presented in Table 3.

2.3. Experiment Process

This experiment involved two factors: the total dissolved solids (TDS) of the irrigation water and the irrigation dose. A total of four experimental setups were calibrated, and four brackish water irrigation experiments with different TDS levels (0 g L−1, 1.0 g L−1, 3.0 g L−1, and 5.0 g L−1) were conducted simultaneously. The amount of irrigation was controlled using the valve of the Mariotte’s bottle for drip irrigation simulation, with the drip flow rate set at 1.0 L h−1. Irrigation was stopped once the wetting front reached the bottom of the soil column. During the experiment, the water levels in the Mariotte’s bottle and the depth of the wetting front in the soil column were recorded at regular intervals, initially at shorter intervals and then at longer ones. To minimize disturbances in water distribution and salt movement within the soil, samples were extracted from all sampling holes between the soil surface and the wetting front as soon as the wetting front reached a sampling hole. Soil conductivity, moisture content, Cl−, and Na+ concentrations were then measured. At the end of the experiment, the water consumption for each treatment (0 g L−1, 1.0 g L−1, 3.0 g L−1, and 5.0 g L−1) was 3013.0 mL, 3388.0 mL, 3189.0 mL, and 3516.0 mL, respectively. The infiltration times for these treatments were 470.0 min, 510.0 min, 490.0 min, and 570.0 min, in the same order.

2.4. Statistical Analysis

Soil sampling data were processed using WPS office. Origin 2021 software was employed for data visualization and plotting, while WPS Office software was used to create schematic diagrams of the experimental setups. Variance analysis of the soil test data for the various brackish water infiltration treatments was performed.

3. Results

3.1. Influence of Brackish Water Infiltration with Different Total Dissolved Solid Levels on Wetting Front

During brackish water infiltration under different total dissolved solid (TDS) conditions, the wetting front exhibited varying characteristics over time, as shown in Figure 3. As the TDS of the infiltration water increased, the wetting front advanced more rapidly, and the water infiltration rate increased. This effect can be attributed to the higher TDS of the infiltration water, which reduces the repulsive forces between soil particles, promoting aggregation and improving the soil’s water transmission capacity. Consequently, soils with higher TDS experienced faster infiltration and greater wetting front progression. As infiltration time increased, the total infiltration volume also increased. In Figure 3a, it can be seen that when the TDS reached 5 g L−1, the slope of the infiltration curve decreased, while the total infiltration volume continued to increase. This resulted in an increase in Na+ concentration in the soil, leading to soil particle expansion and pore shrinkage, which in turn caused a gradual reduction in the infiltration rate. Figure 3b illustrates the relationship between wetting front progression and infiltration volume under varying TDS conditions. The data demonstrate a positive correlation between infiltration volume and wetting front progression, with an overall decreasing trend in the wetting front rate as TDS increased. Figure 3c shows the temporal changes in the soil infiltration rate under different salinization conditions. As the infiltration time increased, the infiltration rate of all treatments gradually decreased, with rapid changes occurring initially, followed by stabilization after 100 min. At the end of the infiltration process, the soil infiltration rates for treatments with 0 g L−1, 1.0 g L−1, 3.0 g L−1, and 5.0 g L−1 TDS were 0.1, 0.07, 0.06, and 0.05 cm min−1, respectively. The infiltration rate decreased as the TDS level increased, indicating that higher brine concentrations slow down the rate of water infiltration.

3.2. Influence of Brackish Water Infiltration with Different Total Dissolved Solid on Moisture Content

Figure 4 illustrates the impact of brackish water infiltration on soil profile moisture content under varying total dissolved solid (TDS) conditions. As shown in Figure 4, soil moisture content in the profile exhibited significant changes after brackish water infiltration, particularly in the upper and lower soil layers. The moisture content in the surface soil (0–10 cm) showed minimal differences between the treatments, with values close to the soil’s saturation moisture content. However, the variation range in this layer was more pronounced. As brackish water infiltration continued, soil moisture content decreased rapidly. Between the 10 and 30 cm soil depth, moisture content remained relatively stable. At depths of 30–70 cm, soil moisture content changed significantly, with a marked decrease. However, the moisture content change at this depth was smaller than at the surface (0–10 cm). At the same soil depth, as the TDS of the infiltration water increased, soil moisture content also increased. A proportional relationship was observed between moisture content and the TDS of the infiltrating water.

3.3. Influence of Brackish Water Infiltration with Different Total Dissolved Solid on Cl− and Na+

The influence of brackish water infiltration with varying total dissolved solid (TDS) concentrations on Cl− and Na+ differed significantly. As shown in Figure 5a, in the 0–40 cm depth range, the Cl− content under the 5 g L−1 TDS brackish water treatment initially decreased and then increased, while the content in the other treatments exhibited a continuous increase. In the 40–70 cm depth range, the Cl− content steadily increased with the infiltration of brackish water at different TDS levels. Higher TDS levels induced a marked increase in Cl− content. As illustrated in Figure 5b, Na+ and Cl− showed distinct variations, particularly in the upper to middle soil profiles, where Na+ content changes were more complex. In the 0–40 cm depth range, the variations of Na+ and Cl− in the 0 g L−1 and 1 g L−1 TDS treatments were largely consistent, while in the 3 g L−1 and 5 g L−1 TDS treatments, the changes followed a more erratic pattern. In the 40–70 cm depth range, Na+ and Cl− content variations became consistent, showing a trend of gradual increases. This increase in ion content reflected the soil desalinization process, although the overall soil salinity did not continuously rise. The 40 cm depth appeared to serve as a boundary, which may be closely associated with factors such as soil layer depth, initial soil salinity, moisture content, and the TDS of the infiltrating water.

3.4. Influence of Different Infiltration Rates with the Same Total Dissolved Solid on Electrical Conductivity and Moisture Content

Figure 6 illustrates the variation in electrical conductivity and moisture content at different soil depths under a 3 g L−1 infiltration treatment. As shown in Figure 6a, in the vertical direction, electrical conductivity increased with the depth of the wetting front. The overall conductivity of the soil profile first decreased and then increased, with an inflection point observed at 30 cm. In the 0–30 cm depth range, electrical conductivity rapidly decreased as the wetting front depth increased. In the 30–70 cm depth range, conductivity gradually increased with further wetting front advancement, indicating the continued infiltration of brackish water. This pattern suggests that the upper-middle soil layers underwent desalination, while the lower-middle soil layers experienced salt accumulation. In the horizontal direction, at the same depth, conductivity increased with continued infiltration, indicating that ongoing brackish water irrigation led to a rise in soil salinity content. As shown in Figure 6b, soil moisture content gradually decreased as infiltration progressed in the vertical direction. The deeper the wetting front, the greater the moisture content in the soil profile at that depth in the horizontal direction.

4. Discussion

4.1. Infiltration Dynamics and the Role of Brackish Water Salinity

The effective and sustainable application of brackish water irrigation has become a significant area of research. Proper brackish water irrigation can enhance the moisture content in the root zone, optimize water and heat distribution within the soil, promote vegetation growth, and increase biological yield [30]. However, as the salinity of irrigation water increases, salt is transported into the soil, raising soil salinity levels, which in turn impairs the root system’s ability to uptake water and negatively affects the physiological growth of vegetation [31,32]. Brackish water salinity plays a crucial role in controlling the dynamics of water infiltration, impacting both the speed of wetting front movement and the overall infiltration process. This study confirmed that higher TDS levels in infiltrating water accelerate the wetting front’s initial movement, a trend that was consistent with previous research [33,34]. This phenomenon can be explained by the reduced resistance to infiltration caused by the osmotic pressure exerted by saline water, which lowers the water retention capacity of soil particles. Initially, soil water movement is dominated by soil matrix potential, resulting in rapid infiltration, but as the infiltration process continues, the system approaches equilibrium, where the rate of infiltration stabilizes. The increased salinity levels, particularly in the higher TDS treatments, quickly reach a critical point beyond which the infiltration rate significantly declines. The findings align with the concept that the osmotic potential of saline water diminishes the soil’s permeability and accelerates the initial infiltration process, but over time, this effect wanes. As the TDS levels increase, the rate of infiltration decreases due to higher ion concentrations in the soil solution, which may hinder water movement. This suggests that, while brackish water could initially enhance infiltration, prolonged exposure to high salinity may result in long-term soil structural degradation, limiting the potential for sustained water infiltration and increasing the risk of salinization. The wetting front for each treatment increased proportionally with the level of infiltration, while the effect of salinity variation on cumulative infiltration was found to be statistically insignificant. The differences in cumulative infiltration among the various treatments were relatively small, which is consistent with the findings of Zhu et al., 2024 [35]. The soil infiltration rate under different brackish water contents exhibited a transition from a rapid decline to gradual stabilization over time. Compared to the 0 g L−1 treatment, the soil infiltration rates for the brackish water contents of 1.0 g L−1, 3.0 g L−1, and 5.0 g L−1 decreased by 5.26%, 7.89%, and 21.05%, respectively. This indicates that a higher brackish water content is associated with a lower soil infiltration rate.

4.2. Soil Moisture Distribution and Salt Accumulation Patterns

The vertical distribution of soil moisture and salt accumulation under different salinity treatments reveals critical insights into how brackish water affects soil profiles over time. In this study, the soil moisture content decreased gradually with increasing soil depth. Throughout the infiltration process, the surface soil was continuously permeated by slightly brackish water, which led to near-saturation moisture content at the surface. The moisture content was minimal at the moisture front, with the deeper soil layers retaining their initial moisture content. This vertical gradient in soil moisture distribution is expected, as the upper layers are exposed to the infiltrating water for a longer period, while the lower layers are less affected due to lower permeability and slower water movement. The significant role of TDS in soil moisture content was highlighted by the direct correlation between salinity and moisture retention at each soil depth. Higher TDS levels in the infiltrating water increased the soil moisture content at the surface and the depth of the wetting front, indicating that saline water facilitates the movement of moisture deeper into the soil profile compared to freshwater or low-salinity irrigation. Across all treatments at the same soil depth, higher brackish water content levels were consistently associated with increased soil moisture content. Zhang et al., 2021 [36] examined the effects of irrigation with water of varying salinity on water and salt transport, revealing a positive correlation between increasing salinity levels in irrigation water and successive increases in soil water content and salt concentration. Similarly, Liu et al., 2024 [37] observed increases in both soil moisture and salt content with elevated salinity levels in brackish water irrigation.
To meet the physiological and biochemical needs of vegetation while minimizing negative environmental impacts, it is essential to control chloride ion content in irrigation water based on the specific growth requirements of plants in different regions. Research shows that long-term use of 3 g L−1 brackish water for irrigation did not significantly alter rhizosphere microorganism diversity or community structure [38]. This study further investigates soil water salinity conditions under varying irrigation volumes using 3 g L−1 brackish water. In the vertical direction, as the wetting front depth increased, soil conductivity initially decreased before rising again throughout the soil profile. Horizontally, with increasing irrigation water volume at a constant depth, conductivity also increased, indicating that continuous brackish water irrigation elevates soil salinity. Niu et al., 2014 [39] found that the migration distance of the horizontal wetting front was largest when the total dissolved solids (TDS) of the infiltration water were 3 g L−1, which they considered a critical point. When the TDS of the infiltration water was either higher or lower than 3 g L−1, the progress of the wetting front slowed. In contrast, in this study, as the TDS of infiltration water increased, the wetting front progressed more rapidly. This can be attributed to the similarity in the TDS levels of infiltration water and the differences in ion component content, which inevitably affect the soil’s physicochemical properties. Additionally, variations in soil loading within the columns could contribute to different results.
Several studies have shown that brackish water infiltration can lead to salt accumulation in the soil, although the overall salt content of the entire soil profile does not increase rapidly. The rate of salt accumulation is significantly influenced by factors such as soil depth, initial soil salt content, moisture content distribution, and the total dissolved solids (TDS) of the infiltration water [36,40,41]. The salt content in the soil profile changes with infiltration water of varying TDS levels. Specifically, as the TDS of the infiltration water increases, the soil salt content also rises. This study demonstrated that, under continuous brackish water infiltration, the soil profile undergoes a cycle of desalination at the surface and salt accumulation in the deeper layers.

4.3. Implications for Sustainable Brackish Water Irrigation Management

The results of this study provide important insights for managing brackish water irrigation in areas facing freshwater scarcity but with access to brackish water sources. While brackish water can offer an effective means of improving soil moisture and supporting plant growth, it must be managed carefully to prevent long-term salinization. In this paper, the research findings indicate that while brackish water can initially enhance moisture distribution, the high salinity levels can lead to the gradual accumulation of salts in the soil, which may negatively affect soil health over time. Therefore, sustainable irrigation strategies must strike a balance between the benefits of water infiltration and the risks of increased salinity. The study highlights the importance of tailoring irrigation practices to local conditions. Factors such as soil type, water quality, and the specific needs of the crops being grown must be considered when developing management plans. For example, regions with clay-rich soils may face slower infiltration and greater salt accumulation compared to areas with sandy soils. These differences call for customized approaches to prevent salinization, such as periodic leaching or adopting irrigation methods that enable controlled drainage. These practices are particularly crucial for crops that are sensitive to high salinity, as excessive salt concentrations can hinder plant growth and reduce yields. Furthermore, optimizing irrigation efficiency—using techniques such as drip irrigation—can help minimize the volume of brackish water applied, reducing the likelihood of salt buildup. Drip irrigation, in particular, allows for a more targeted application of water, ensuring that crops receive adequate moisture while minimizing the risks of waterlogging and salinization. In conclusion, while brackish water can serve as a valuable resource in water-scarce regions, its sustainable use hinges on effective management. By integrating soil monitoring, salinity control measures, and efficient irrigation technologies, it is possible to harness the benefits of brackish water irrigation without compromising soil health or agricultural productivity over time.

5. Conclusions

This study shows that higher TDS in brackish water infiltration accelerates the progress of the wetting front, but slows the infiltration rate once a certain depth is reached. Despite this, infiltration continues, and the moisture profile follows a fast-slow-fast trend. Soil moisture is directly related to TDS at each depth, with desalination in the upper layers and salt accumulation in the deeper layers. For brackish water with 3 g L−1 TDS, electrical conductivity decreases and then increases with varying irrigation doses. Continuous infiltration leads to higher conductivity and soil salinization, while deeper wetting fronts maintain a higher moisture content. The findings provide valuable insights for regions facing similar irrigation challenges. However, the impact of brackish water on soil health depends on local factors such as soil type, water quality, and climate. Sustainable irrigation strategies should consider these factors, focusing on soil salinization control, efficient water use, and long-term agricultural sustainability.

Author Contributions

W.L.: Writing—original draft. F.L.: Writing—review and editing. W.H.: Investigation. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by a National Natural Science Foundation of China funded project (42276223) and an open fund project of the Key Laboratory of Coastal Science and Integrated Management, Ministry of Natural Resources (2023COSIMZ001).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Acknowledgments

We would like to thank Changyi National Marine Ecology Special Reserve for the convenience of on-site sampling. Finally, we sincerely thank the anonymous reviewers for their constructive comments.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Geographic location of soil sample collection.
Figure 1. Geographic location of soil sample collection.
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Figure 2. Schematic diagram of brackish water irrigation experiment.
Figure 2. Schematic diagram of brackish water irrigation experiment.
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Figure 3. (a) Relationship between wetting front and time under different salinity; (b) relationship between wetting front and water infiltration under different salinity; (c) relationship between infiltration rate and time under different salinity.
Figure 3. (a) Relationship between wetting front and time under different salinity; (b) relationship between wetting front and water infiltration under different salinity; (c) relationship between infiltration rate and time under different salinity.
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Figure 4. Changes of soil moisture content at different soil depths under different salinity.
Figure 4. Changes of soil moisture content at different soil depths under different salinity.
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Figure 5. (a) Contents of Cl− in different soil depths under different salinity; (b) contents of Na+ in different soil depths under different salinity.
Figure 5. (a) Contents of Cl− in different soil depths under different salinity; (b) contents of Na+ in different soil depths under different salinity.
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Figure 6. (a) Electrical conductivity of soil at different depths under different irrigation volumes; (b) moisture content of soil at different depths under different irrigation volumes.
Figure 6. (a) Electrical conductivity of soil at different depths under different irrigation volumes; (b) moisture content of soil at different depths under different irrigation volumes.
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Table 1. Basic properties of tested soil.
Table 1. Basic properties of tested soil.
Mechanical CompositionSoil Layer (cm)Average
0–1515–3030–4545–6060–75
Clay (0~0.002 mm)%17.2414.3310.049.8510.6712.43
Silt (0.002~0.02 mm)%72.1573.6174.2376.9777.8374.96
Sand (0.02~2 mm)%10.6112.0615.7313.1811.5012.62
Soil TextureSilty LoamSilty LoamSilty LoamSilty LoamSilty LoamSilty Loam
Bulk Density (g cm−3)1.541.551.461.461.561.51
Table 2. Determination data of physical and chemical properties of original soil.
Table 2. Determination data of physical and chemical properties of original soil.
Soil Layer (cm)EC (μS·cm−1)Ion Content (mg kg−1)
Na+K+Mg2+Ca2+Cl−SO42−
0–1588.38.169.562.096.4912.147.7
15–30162.037.321.00.403.6612.147.7
30–45190.049.329.10.712.4812.057.2
45–60187.062.640.50.662.5712.171.5
60–75202.054.033.10.552.5711.895.3
Table 3. Ion content of groundwater.
Table 3. Ion content of groundwater.
ItemEC (ms cm−1)Ion Content (mg L−1)TDS (g L−1)
Na+K+Ca2+Mg2+Cl−SO42−HCO3−
Groundwater32.55454.0451.0177.0630.014,740.01730.0433.026.43
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Liu, W.; Lu, F.; Han, W. Sustainable Marginal Water Resource Management: A Case Study of Brackish Water Irrigation on the Southern Coast of Laizhou Bay. Sustainability 2025, 17, 1956. https://doi.org/10.3390/su17051956

AMA Style

Liu W, Lu F, Han W. Sustainable Marginal Water Resource Management: A Case Study of Brackish Water Irrigation on the Southern Coast of Laizhou Bay. Sustainability. 2025; 17(5):1956. https://doi.org/10.3390/su17051956

Chicago/Turabian Style

Liu, Wenquan, Fang Lu, and Weitao Han. 2025. "Sustainable Marginal Water Resource Management: A Case Study of Brackish Water Irrigation on the Southern Coast of Laizhou Bay" Sustainability 17, no. 5: 1956. https://doi.org/10.3390/su17051956

APA Style

Liu, W., Lu, F., & Han, W. (2025). Sustainable Marginal Water Resource Management: A Case Study of Brackish Water Irrigation on the Southern Coast of Laizhou Bay. Sustainability, 17(5), 1956. https://doi.org/10.3390/su17051956

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