Temporal Analysis of Groundwater Quality in the Harran Plain: Linking Land Use Change to Water Contamination (2005–2025) ^†

Abstract

This study evaluates groundwater quality dynamics in the Harran Plain (∼1500 km²), a key agricultural zone within Türkiye’s Southeastern Anatolia Project (GAP). Satellite images from Landsat 5 TM and Landsat 8 OLI/TIRS were used to assess land-use changes over the years 1990, 2000, 2010, and 2020, with the GIS employed for classification and analysis. In this study, groundwater samples collected from twenty different locations in 2005, 2015 and 2025 were analyzed. For each sample, pH, EC, and various ion concentrations (Na, K, Cl, SO₄, NO₃, Ca, Mg, HCO₃) were measured. All analyses were performed using standard hydrogeochemical methods. Data from 20 wells (2005–2015) revealed significant reductions in EC (8235 to 2510 µS/cm) and NO₃⁻ (720 to 327 mg/L), due to drainage systems, improved irrigation, and fertilizer management. Nonetheless, localized pollution persisted. Land-use shifts toward high-value crops improved water efficiency, while urban and industrial expansion introduced new pressures. Results emphasize integrated water–land policies for sustainable groundwater management in arid agroecosystems.

1. Introduction

On a global scale, industrialization, urbanization, and population growth are widely acknowledged as the predominant socioeconomic drivers of land use and land cover change [1]. In the case of the Harran Plain, however, the dynamics are more region-specific. This semi-arid region, long characterized by traditional dry farming practices, has experienced a major shift over the past three decades with the introduction of irrigated agriculture. This transition has not only transformed agricultural practices but has also given rise to a distinct set of socioeconomic driving forces that continue to shape the region’s development trajectory [2].

Concurrently, a major transformation in land use was observed, including a shift from traditional cotton and grain farming to high-value, low-water-demand crops. However, the expansion of residential and industrial zones in certain areas introduced new environmental pressures, with some wells recording increased NO₃⁻ levels. Uncontrolled land development and irregular irrigation practices were identified as contributing factors. Moreover, the conversion of pasture and fallow lands into cultivated areas appears to have altered the groundwater recharge regime, impacting overall water quality [3,4,5]. These findings highlight the critical role of integrated water–land policy approaches for sustainable groundwater management in arid agroecosystems.

Recent studies have demonstrated that LULC changes have a decisive impact on groundwater quality and quantity. Ali and Bilal [6] comprehensively examined the effects of LULC changes on groundwater recharge, storage and quality using ArcGis. They found that agricultural expansion and urbanization in particular led to increases in nitrate and salinity, while natural recharge rates decreased by 30–50%. Zhang et al. [7] examined the spatial and temporal effects of human-induced land use changes on water quality in China’s Songliao River Basin. The study evaluated 22 water quality parameters (nutrient elements, heavy metals, etc.) in three different rivers; GIS-based analyses and PCA and RDA methods were used to relate land use types to pollutant dynamics. The results showed that agricultural areas (especially rice fields) and urbanisation increased nitrogen, phosphorus and heavy metal pollution, while forests and wetlands had beneficial effects on water quality. Arsenic posed the most significant health risk, exceeding acceptable limits, particularly for children.

This study aims to reveal the impact of temporal and spatial land use changes on groundwater in the Harran Plain, considering factors such as vegetation and urbanization over the last 30 years.

2. Materials and Methods

2.1. Study Area

The Harran Plain, located in the southeastern part of Türkiye within the Sanliurfa province, represents the largest irrigated agricultural area in the region. Geographically, it lies between 36°42′ N–37°10′ N latitudes and 38°50′ E–39°10′ E longitudes, with an approximate surface area of 1700 km² when delineated by its geomorphological boundaries [4]. The plain is characterized by a semi-arid climate, with an average annual air temperature of 18 °C and a mean annual precipitation of 284 mm. Structurally, it constitutes the base of the Akçakale Graben, which extends in a north–south direction.

Topographically, the Harran Plain is bordered by the Tektek Plateau to the east, the Germuş Plateau to the north, and the Fatik Plateau to the west. These surrounding plateaus were shaped by fluvial processes during the neotectonics period. Their elevations gradually decrease, and their slopes become less steep from north to south. Consequently, while the northern part of the Harran Plain exhibits a distinct graben morphology, this feature becomes progressively less pronounced toward the south [5,8] (Figure 1).

Figure 1. Location of the study area.

2.2. Satellite Data and Software

The land use status of Harran Plain for different years was determined from Landsat 5 TM and Landsat 8 OLI/TIRS satellite images [9] for the years 1990, 2000, 2010, and 2020. Data processing was performed using ArcGIS 10.8 software with the supervised classification method and the maximum likelihood algorithm [10,11]. Based on visual interpretation and spectral reflection profiles, five main LULC (Land Use/Land Cover Change) classes were defined: Irrigated agriculture, built-up area, dry agriculture, forest area, and steppe. Post-classification analysis can effectively identify the location, presence, and rate of change.

2.3. Groundwater Samples

In this study, groundwater samples collected from twenty different locations in 2005, 2015 and 2025 were analyzed. For each sample, pH, EC, and various ion concentrations (Na, K, Cl, SO₄, NO₃, Ca, Mg, HCO₃) were measured. All analyses were performed using standard hydrogeochemical methods [3]. Sampling points were selected to represent the hydrogeological characteristics of the region. The diversity in different geological formations, land use, and anthropogenic impact levels across the locations was taken into consideration. The sampling strategy aimed to comprehensively document the spatial and temporal variations in groundwater quality in the region. Analysis results were evaluated by referencing WHO [3]. Statistical analyses and comparisons were used to determine the significance of changes in parameters. The data used in this study for the year 2005 were sourced from Yeşilnacar et al. [12], while the 2015 data were taken from Yetiş et al. [13]. Additionally, the 2025 data was gathered through field surveys conducted by the authors, where samples were systematically collected for analysis

3. Results and Discussion

3.1. Land Use/Cover Change Classification

In 1990, the dominant land cover classes in Harran Plain were rangelands with 37.91% and dry farming areas with 37.06% of the total area, respectively. These were followed by irrigated agricultural areas with 21.01%, forest areas with 3.44%, and lastly, built-up areas with 0.58%. By 2020, irrigated agricultural areas constituted the largest share, covering more than half of the total area with 54.45%. Rangelands, dry farming areas, forest areas, and built-up areas occupied 25.58%, 7.13%, 6.78%, and 6.06% of the area, respectively. According to the LULC in Harran Plain between 1990 and 2020, there was a significant decrease in dry farming areas, while irrigated agricultural areas experienced a substantial increase. A reduction was observed in rangelands, whereas forest areas and built-up areas showed increases. It is observed that the land use/cover changes in the Harran Plain, and the increase in irrigated lands and urban area expansion, are driven by a combination of socioeconomic, environmental, and geographical factors. Although population growth is the primary reason for rapid urbanization, the inclusion of other factors such as economic growth and physical reasons should also be considered. This change in land use characteristics in the plain, in favor of irrigated agriculture, affects all groundwater in the plain. Between 1990 and 2020, an increase of 33.44% was observed in irrigated agricultural areas. This increase is associated with the commencement of irrigation under the GAP project. During the same period, a decrease of 29.93% occurred in dry agricultural areas. This reduction is a direct result of the transition to irrigated agriculture. A decrease of 12.33% was observed in steppe areas. This reduction is associated with the expansion of built-up areas and agricultural lands.

Agricultural Production Change: In the Harran Plain, one of Türkiye’s most important agricultural production areas, the most striking situation in terms of land use is the clear determination of the change in groundwater levels to meet the irrigation water demand during the transition from dry farming to irrigated farming.

Groundwater Level Change: In the plain, where dry farming was practiced for years, irrigation has been provided since 1995 within the scope of the GAP. Before the GAP project, the irrigation water needed for irrigated farming in the Harran Plain was supplied from groundwater. According to these records, poor drainage conditions affect approximately five thousand hectares of the plain and lead to increases in groundwater levels [5,8,14].

3.2. Location-Based Variation Analysis in Water Quality

The hydrochemical evaluation of the northern region (S1–S5; Çamlıdere, Karabayır, İkiağız, Vergili, Yardımcı) reveals that water resources are generally influenced by the carbonate-rich lithology of the area. Higher carbonate content, coupled with relatively low levels of anthropogenic pressure, results in lower pollution loads compared to other parts of the plain. Between 2005 and 2025, EC values exhibited only a slight decline (5–15%), while NO₃⁻ levels showed variable trends, including a 65% reduction at station S4. Concentrations of Ca²⁺ and Mg²⁺ remained relatively stable throughout the monitoring period. The predominance of the Ca–HCO₃ water type indicates the strong control exerted by the limestone and dolomite geology, which appears to be the principal factor governing groundwater chemistry in this region.

In contrast, the central region (S6–S13; Mutluca, Günbalı, Kısas, Konuklu, Hancağız, Uğurlu, Ozanlar, Keçikıran) reflects the influence of mixed land use patterns, particularly intensive agricultural practices. Over the study period, EC values showed a marked decline, especially at stations S8 and S11, accompanied by substantial reductions in NO₃⁻ concentrations, with up to an 85% decrease at S8. A decreasing trend was also observed in Cl⁻ values. Moreover, water types underwent a notable shift from Ca–NO₃ dominance toward Ca–HCO₃ types, suggesting a gradual reduction in anthropogenic nitrate loading. These improvements in water quality may be attributed to modifications in agricultural practices, such as reduced fertilizer application or enhanced irrigation management strategies, highlighting the potential effectiveness of sustainable land use interventions in mitigating groundwater pollution.

The southern region (S14–S20; Kızıldoruç, Yardımlı, Özlü, Olgunlar, Yaygılı, Bolatlar, Uğraklı) initially exhibited the highest pollution loads in 2005, reflecting both geological sensitivity and anthropogenic pressures. However, by 2025, water quality indicators demonstrated remarkable improvement. EC values, for instance, decreased dramatically by up to 88% at S15, while significant reductions were also recorded in SO₄²⁻ and Cl⁻ concentrations. NO₃⁻ levels also declined considerably across the region. These chemical changes were accompanied by a transformation in water types, shifting from more complex ion compositions to simpler Ca–HCO₃ or Ca–NO₃ facies. The substantial reduction in pollution levels may be linked to implemented management measures or land use changes over the last two decades, which collectively contributed to restoring groundwater quality in this part of the plain.

On the whole, the spatial analysis of the Harran Plain demonstrates that groundwater quality has been strongly influenced by both geological settings and socioeconomic activities. While the carbonate-rich northern region remains relatively resilient, the central and southern regions reveal the importance of land management practices in shaping long-term hydrochemical trends. These findings underscore the necessity of region-specific strategies that integrate geological constraints with sustainable agricultural practices to protect and improve groundwater resources. Regional differentiation of the study area and distribution of groundwater monitoring points. Hydrogeochemical changes observed in different regions were influenced by both natural processes and anthropogenic factors. It was determined that the greatest improvement over the 20-year period occurred in the southern region, which had the highest pollution load in 2005 (Figure 2).

Figure 2. Number of groundwater samples exceeding WHO standards.

Figure 2 shows which parameters and how often groundwater samples exceeded WHO standards in 2005, 2015 and 2025. The analysis revealed that NO₃⁻ and SO₄²⁻ concentrations frequently exceeded WHO permissible limits, with exceedances being particularly pronounced in 2025. In the same year, a noticeable increase was also observed in Na⁺, Cl⁻, and EC values, indicating a deterioration in groundwater quality linked to salinization processes. In contrast, K⁺ and Mg²⁺ levels displayed only limited exceedances, occurring sporadically across certain years rather than consistently. Importantly, pH values remained within the acceptable WHO range throughout the study period, suggesting that groundwater acidity–alkalinity balance was not significantly disrupted. Furthermore, while the exceedance rate was approximately 27% in 2005 and approximately 22% in 2015, it rose above 35% in 2025, reaching the highest value within the period studied.

3.3. Comparative Evaluation According to Drinking Water Standards

Evaluation of groundwater samples according to WHO drinking water standards shows a significant improvement in water quality between 2005 and 2015. While 70% of the samples in 2015 did not meet drinking water standards for at least one parameter, this rate decreased to 40% in 2025. The most critical parameter is NO₃⁻; in 2005, 55% of samples contained nitrate above the 50 mg/L limit set by WHO, whereas in 2015, this percentage dropped to 70%. For EC, 25% of samples exceeded standards in 2015, but no samples exceeded this limit in 2025. Similar improvements were observed for Na⁺, Cl⁻, and Ca²⁺ parameters. A more limited improvement was recorded for SO₄²⁻ concentrations. pH and Mg²⁺ parameters were found to be within standards in both periods. These results indicate a positive development in water quality management in the region. However, the fact that 40% of samples still did not meet drinking water standards in 2025 emphasizes the need for continued sustainable measures to monitor and protect groundwater quality.

4. Conclusions

The Harran Plain stands as one of Türkiye’s most significant agricultural production zones and was the first area where the GAP was implemented. A defining feature of land use dynamics in the plain is the pronounced shift in groundwater conditions, closely linked to the transition from traditional dry farming to irrigated agriculture to meet growing irrigation demands. With the introduction of large-scale irrigation in 1995, substantial LULC changes have taken place. Over the past four decades, dry farming areas have dramatically declined—from 37% in 1990 to just 7% in 2020—while irrigated farming areas have expanded considerably, rising from 21% in 1990 to 54% in 2020. These transformations have had profound implications for groundwater quality. The intensification of irrigated agriculture has led to notable nitrate contamination and salinization of groundwater resources, while also altering the soil structure of the plain. In arid and semi-arid regions such as Harran, salinization emerges as an unavoidable outcome of uncontrolled and excessive irrigation practices coupled with inadequate drainage infrastructure.