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Over two decades, climate variability has caused a significant change in human well-being, particularly in the availability of water resources. The water shortage and insufficient water supply especially affect the quality of life in many ways. The Pollution Control Department (PCD) of the Metropolitan Waterworks Authority (MWA) has selected thirteen parameters considered essential as indicators of raw water suitability. The water quality of the Chao Phraya River at 18 PCD stations along the 379-km stretch from Chainat to Samutprakarn has been recorded for analysis and considered for inclusion in a Water Quality Index (WQI) that includes color, turbidity, Biochemical Oxygen Demand (BOD), Dissolved oxygen (DO), pH, Nitrate-Nitrogen (NO3-N), Fe, Mn, Total Dissolved Solids (TDS), Total Phosphorous (TP), Electrical Conductivity (EC), salinity, and hardness. The WQI provides a single value for water quality suitability, is useful for the MWA in water supply production, and reflects the constituents that may be harmful to the consumer and the efficiency of production. For example, if the salinity in raw water is greater than 0.25 mg/l, the salt content of the water cannot be removed by the MWA system, resulting in a water supply with a salty flavor. Therefore, if the deteriorated water quality of raw water is predicted in advance, measured at a point upriver from the Samlae intake station, appropriate solutions can be put in place. Using PCD water quality records, the average WQI score of the Chao Phraya River in the rainy season is 74.63 and in the dry season is 78.37 (from a full score of 100). These values indicate the deterioration of raw water quality during the low flow period, a situation that the MWA should acknowledge and make periodical inspections to identify and control.
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