The Menemen Basin, located western part of the Gediz River basin in western Turkey, is a critically important agricultural and urban groundwater system under increasing anthropogenic pressure. This study assesses the combined effects of groundwater abstraction, agricultural intensification and industrial activities on water quality, focusing on the risk of salinisation and future trends.
Data covering the period 1980–2026 reveal significant spatial heterogeneity and temporal degradation. At several points (Hatundere Çeşme, Hatundere, Yaylaköy, Yeniköy, Güzelhisar 2, Bucak and Ilıpınar), TDS, HCO₃⁻ and hardness values have increased between these two years. Historical data (1980–2018) show that TDS, Cl⁻ and Na⁺ values are already high at industrial and agricultural sites, confirming the long-term anthropogenic impact. High NO₃⁻, NH₄⁺ and PO₄³⁻ values indicate agricultural runoff, whilst local enrichment in As, B and Fe raises further concerns regarding bioavailability.
In the most affected areas, the hydrochemical facies is shifting from a Ca-HCO₃ composition towards Na-Cl/Na-SO₄ compositions, consistent with concentration via evaporation and the input of dissolved substances. The increase in EC and Cl⁻ values in 2025–2026 indicates that salinisation is accelerating.
Furthermore, the measured oxygen-18 (δ¹⁸O) and deuterium (δ²H) values are very close to the region’s Local Meteoric Water Line. This indicates that the primary source of the water is precipitation.
An enrichment in δ²H and δ¹⁸O values is observed from 1980 to 2026. This suggests that, following intensive groundwater abstraction, irrigation return flows and/or contributions from ponds and surface water have entered the system. Intensive agricultural use leads to partial evaporation of the water at the surface, followed by re-infiltration into the aquifer. Furthermore, the Menmen Basin’s proximity to the Aegean Sea means that increased groundwater abstraction poses a risk of seawater infiltrating the groundwater via fracture systems.
Consequently, if the current situation persists, it is projected that the median TDS and Cl⁻ values in sensitive areas will exceed 2,000 mg L⁻¹ and 400 mg L⁻¹, respectively, within 10–15 years. However, with moderate intervention, the TDS value must be maintained at a level of 800–1,200 mg L⁻¹.
The Menemen Basin is currently experiencing salinisation at measurable levels. Without coordinated management, the decline in water quality will threaten agricultural productivity and the long-term sustainability of water resources.