Introduction
Salinity accumulation in arid and semi-arid regions due to irrigation is a significant issue leading to depletion of food resources and environmental hazards like soil erosion and elevated air dust levels. Studying the key factors that influence the increase or decrease in soil salinity in these regions is a priority for agricultural advancement and risk mitigation. Salinity development can be categorized into natural processes and those expedited by human activities. Natural salinity arises from phenomena such as weathering, saltwater springs, and salt domes (Darwishzadeh, 2002). Human activities such as irrigation, mining, land drainage, runoff into water streams, fertilizer use, road salt application, and oil extraction contribute to increased salinity. This research aims to analyze how natural and human factors impact salinity development in a section of the southern Khuzestan plain.
Materials and methods
This study started in March 2017 in a southeastern region of Khuzestan. The research area covers a portion of the southeast of Ahvaz, situated in Khuzestan province, spanning approximately 20000 hectares and located 40 km from the Ahvaz-Abadan road. Sampling was conducted along 5 routes, with three running east-west (perpendicular to the Karun River) and two running north-south (parallel to the river). Sampling sites were 300 meters apart, and samples were collected from depths of 0-50, 50-100, 100-150, and 150-200 cm, totaling 600 soil samples from 150 points. The ECe and EC of groundwater were assessed using an EC meter, followed by determining the correlation between ECe and ground water EC.
Results and discussion
From the west, represented by the Karun River, to the east, characterized by the Shadgan marshland, there was a noticeable decrease in soil salinity (Figure 2). Additionally, the groundwater levels in the areas increased as they neared the marshland. This outcome can be attributed to the impact of the wetland as well as the influence of the river in draining the bordering lands. In 2002, a substantial drain channel with a capacity of approximately 50 m3/s was constructed to facilitate the drainage of water from the sugarcane-cultivated lands of the Salman Farsi and Daabl Khazai units. This drainage system has fed the water table of the adjacent lands and led to surface salinity due to evaporation.
There was a strong relationship observed between the salinity of the surface layer and groundwater across all paths (R2=0.83), suggesting that surface soil salinity originates from groundwater (Figure 3). While surface salinity variations in relation to groundwater salinity did not show a clear pattern from north to south, groundwater salinity changes followed a similar trend to soil salinity along the three east-west routes. In contrast to the prior east-west routes, significant fluctuations in salinity were noted along this specific route.
The average salinity across five routes was 51.2 dS/m (classified as C4). Salinity ranged from 4 to 152 dS/m, with the highest salinity observed in the E route, running north-south. Various factors such as drainage conditions, soil texture, and human activities like altering drainage pathways or constructing new channels play a significant role. Jafari (2021) linked a considerable rise in soil salinity in some lands near this area to human interventions like dam construction and diverting river water away from the plain.
An additional contributing factor to the elevated salinity along this transect is its location east of the region, parallel to the drainage water conveyance canal of the sugarcane agro industrial complexes. This configuration has led to a rise in the water table, thereby exacerbating soil salinization. In contrast, transect C (east–west direction) exhibited the lowest salinity among all transects, attributable to continuous cultivation and leaching practices.
A highly significant positive correlation was observed among soil salinity measurements across all four investigated soil depths. However, the correlation coefficient between groundwater salinity and soil salinity at these depths was consistently low and statistically non significant. These findings contrast with the depth specific salinity relationships when analyzed per transect individually. In general, soil salinity is influenced by groundwater salinity and water table depth, but this relationship is modulated by various factors including soil texture, vegetation cover, and drainage discharge conditions.
Salinity levels in the study area are strongly governed by drainage conditions. The obstruction of traditional agricultural drain outfalls to Shadegan Wetland caused by the construction of large scale drainage systems for sugarcane plantations represents the most significant anthropogenic factor driving increased soil salinity in the eastern part of the region. Substantial spatial heterogeneity in groundwater and soil salinity is evident from east to west and from north to south. Key determinants include land drainage status, soil textural properties, and human induced modifications such as altered water flow paths and the installation of various drainage networks. The Karun River serves as the primary natural agent improving drainage in these lands, effectively lowering the water table and thereby reducing groundwater and soil salinity in the western sector.
Ghobadian (12) reported that, 50 years ago, the water table along the Ahvaz–Mahshahr corridor (present day Persian Gulf Highway), from Ahvaz to the northern margin of Shadegan Wetland, was substantially higher than at the time of the present study. Based on Ghobadian (12), the average water table depth during 1968–1969 was less than 1 m, whereas in the current study it exceeded 1 m. Over this five decade interval, the water table has declined by an average of 60 cm, accompanied by an increase in groundwater salinity. Soil salinization both results from and contributes to the reduction in vegetation cover. As salinity surpasses the tolerance thresholds of even salt resistant species, the exposed surface soil becomes bare, leading to increased surface heating and elevated evaporative demand. This, in turn, promotes capillary rise, perpetuating a vertical salinity gradient from depth to surface.
Conclusion
The highest salinity levels were observed in the eastern sector, as well as in parts of the northern and southern portions of the study area. This spatial pattern underscores the significant role of drainage in mitigating soil salinization. Drainage and consequently salt export was either enhanced or restricted by both natural and anthropogenic factors. A strong relationship was observed between soil salinity and water table depth, although this relationship varied across different transects. The primary drivers of increased salinity is anthropogenic activity, particularly the construction of large scale drainage systems associated with sugarcane cultivation units. In contrast, the Karun River exerts a counteracting influence by improving local drainage flow, thereby reducing salinity in adjacent lands. The results indicate that land salinization in the region has been intensified by human interventions. The construction of embankments along the Karun River has prevented floodwaters from inundating adjacent agricultural lands; on the other hand, lateral seepage from drainage canals into surrounding areas has raised the water table. This vegetation loss has, in turn, intensified wind erosion and contributed to the occurrence of dust storms, particularly over the last two decades. As a result, the study area located in the southeastern part of Ahvaz has become one of the active dust source regions in the country. It is recommended that land salinization be recognized as one of the most critical environmental hazards threatening the region. Continuous monitoring and systematic management efforts are urgently needed both to prevent further land degradation and to mitigate associated threats such as dust emission through targeted salinity reduction measures.