A Spatially Explicit Physically Based Modeling Framework for BOD Dynamics in Urbanizing River Basins: A Case Study of the Chao Phraya River—Tha Chin River

Biochemical Oxygen Demand (BOD) is a key indicator of organic pollution and a proxy indicator reflecting organic loading that can indirectly influence eutrophication processes in aquatic systems. This study presents a spatially explicit, physically based modeling framework for simulating BOD dynamics in the urbanizing Chao Phraya and Tha Chin Rivers Basin in central Thailand. The framework integrates the Personal Computer Storm Water Management Model (PCSWMM) with GIS-based datasets to represent pollutant sources, hydraulic flow, and land use. The model was calibrated and validated using data from 36 monitoring stations (2021–2022), achieving strong performance: an NSE of 0.72 and an MAE of 0.35 mg/L for the Chao Phraya River, and an NSE of 0.88 and an MAE of 0.12 mg/L for the Tha Chin River. Scenario simulations for 2032 projected BOD concentrations exceeded 4 mg/L in several downstream segments under the baseline (no-intervention) scenario, indicating elevated organic pollution and potential oxygen depletion that may indirectly exacerbate eutrophication risk in the Upper Gulf of Thailand, particularly in tidal zones with low dilution and nutrient accumulation. Model projections suggest that effective mitigation would require a 20–30% reduction in BOD loads, achievable through enhanced wastewater treatment and stricter pollution controls. Although BOD reduction alone cannot eliminate eutrophication, it supports broader nutrient management efforts by improving baseline water quality conditions. The proposed model offers a robust tool for identifying pollution hotspots, evaluating management strategies, and informing integrated river basin policies under continued urban growth.