Thermodynamic Reformulation of Water Quality Indices: A Gibbs Free Energy Approach for Philippine Coastal Waters

This research compares the DENR-WQI framework by the Philippine Department of Environment and Natural Resources (DENR) with a new thermodynamic-based Gibbs-WQI method using surface water data collected from Iloilo City, Philippines. The Gibbs-WQI was developed to investigate cases where coastal waters labeled as suitable by the DENR-WQI displayed fecal coliform levels that exceedingly surpassed legal limits. The Gibbs-WQI uses entropy-based calculations to evaluate deviations from ideal reference states which helps detect anomalies dynamically. The analysis of 287 surface water samples included key parameters such as fecal coliform, phosphate, and dissolved oxygen through computational methods implemented in Python. The Gibbs-WQI system tended to place samples initially classified as Class SB or SC by the DENR-WQI into poorer quality categories when fecal coliform concentrations surpassed 5,000 MPN/100 mL. The Chi-square statistical test ( χ = 134.43 and p = 0) demonstrated a highly significant difference between the two indices. The Cohen’s Kappa coefficient value of 0.321 showed that there was only fair agreement between the classification systems. While DENR-WQI meets regulatory standards the study reports that Gibbs-WQI offers improved real-time environmental responsiveness. The study recommends combining both indices to achieve optimal monitoring and management outcomes. Predictive modeling in water quality management requires future research to explore both temporal-spatial analysis and the incorporation of machine learning techniques.