Groundwater Evaluation and Management in the Surat Thani Basin, Southern Thailand, Using Stable Isotope and Numerical Modeling

Groundwater is an important resource for domestic, agricultural, and ecological use in the Surat Thani basin, southern Thailand, where increasing demand from agriculture, urbanization, and tourism is placing pressure on aquifer systems. This study investigates groundwater recharge and basin-scale flow dynamics using integrated numerical groundwater modeling, stable isotope analysis, and hydrochemical interpretation. A three-layer MODFLOW model representing floodplain (Qfd), terrace (Qt), and semi-consolidated aquifers was calibrated for 2018–2024 using PEST with pilot-point parameterization. Despite there being only four observation wells, the model achieved excellent agreement between simulated and observed heads (weighted RMSE = 0.0707 m). Simulated groundwater generally flows from western uplands toward the central floodplain and eastern coastal plain, with the Tapee River acting mainly as a gaining stream. Recharge rates range from 1.2 to 77.3 mm/yr (mean 23.7 mm/yr), representing only 1–4% of annual precipitation, while evapotranspiration is the dominant form of water loss. Stable isotope signatures (δ2H, δ18O) indicate recharge from direct monsoonal rainfall with minimal evaporation. Hydrochemical facies evolve from Ca–HCO3 recharge waters to mixed facies along downgradient flow paths, reflecting increasing residence time and water–rock interaction. The results identify western and northern upland areas as key recharge zones requiring long-term protection and sustainable groundwater management.