Mapping Groundwater Concentration in the District of Ula, Muğla, Türkiye Using Spatial Interpolation Methods and Geostatistics

In this, karst systems located in the Akyaka district, Ula county, Muğla city, SW Turkey were investigated  because the systems are unique geological formations created by the dissolution of soluble rocks through the action of surface water or groundwater. These formations, typically developed in carbonate rocks and gypsum over extended periods, are shaped by environmental and climatic factors.  For this study, 16 measurement points were determined and collected water samples from the points. The collected water samples were chemically analyzed for major ions. Analyzed of the data indicated that the groundwater quality in the studied area demonstrates low to moderate ionic and physicochemical content, making it suitable for diverse uses, including domestic and irrigation purposes. Electrical Conductivity (EC) values, ranging from 248 to 1054 µS/cm, with an average of 508.69 µS/cm, indicate moderate salinity. Classification per the U.S. Salinity Laboratory Staff (1954) places most samples (85.72%) in the medium-salinity category (C2), with limited samples in low (C1) and high salinity (C3) categories. Compared to prior studies, such as those by Altun et al. (2017), the EC values align closely, suggesting consistent regional groundwater quality. Calcium (Ca²⁺) and magnesium (Mg²⁺) concentrations range from 1.58–8.85 mg/L and 0.21–6.2 mg/L, respectively, with averages suggesting soft to moderately hard water. These levels ensure lower scaling potential in plumbing, supporting both residential and industrial applications. While the concentrations exceed the WHO’s recommended limits for drinking water, their moderate variability contributes to a balanced ionic composition. Sodium (Na⁺) and potassium (K⁺) are present at lower concentrations, with averages of 0.87 mg/L and 0.10 mg/L, respectively. Chloride (Cl⁻), sulfate (SO₄²⁻), bicarbonate (HCO₃⁻), and carbonate (CO₃²⁻) show significant variability, with HCO₃⁻ dominating among anions, indicative of carbonate mineral dissolution. Total Dissolved Solids (TDS) range between 172 and 739 mg/L, categorizing all samples as freshwater. The chronological order of major ions follows Ca²⁺ > Na⁺ > Mg²⁺ > K⁺ for cations and HCO₃⁻ > Cl⁻ > SO₄²⁻ for anions. Elevated bicarbonate and calcium levels highlight carbonate rock dissolution, supported by the geological context, which includes extensive carbonate formations like dolomites and limestones. The Piper diagram analysis underscores the dominance of Ca-Mg-HCO₃ groundwater type, revealing the influence of alkaline earth metals and weak acids over alkali metals and strong acids. Water salinity indices further reflect the suitability of groundwater for agricultural use. The majority of samples (85.714%) fall within the medium salinity hazard category (C2), appropriate for irrigation with moderate leaching. However, higher salinity samples, such as GK1 and KZ1, with EC values of 1054 µS/cm and 737 µS/cm, respectively, are limited to salt-tolerant crops and require careful management in soils with restricted drainage. Importantly, none of the samples fall into the high-salinity hazard category (C4), indicating overall positive irrigation potential. Geological formations, including Quaternary alluvium, Jurassic-Triassic dolomitic limestone, and Mesozoic peridotites, shape groundwater chemistry. For example, the low EC value at UB01 (248 µS/cm) reflects limited interaction between surface water and surrounding carbonate-rich formations.