Water quality in raw water sources is an important factor that affects treated water quality and is necessary information to be considered in developing water quality management plan of MWA. The results of water quality investigation in the Chao Phraya river at Samlae raw water pumping station, PathumThani province, and in the Mae Klong river at ThaMuang raw water intake, Kanchanaburi province, in the fiscal year 2010 show that water quality of the Mae Klong river is generally better than water quality of the Chao Phraya river.

The amount of dissolved oxygen (DO) in the Mae Klong river is in the range of 4.7 – 6.9 mg/L whereas the DO level in the Chao Phraya river is in the range of 1.6 – 4.6 mg/L. Water turbidity in the Chao Phraya river is in the range of 15 – 178 NTU, and during September – October which is the flood period the turbidity of river water increases about 10 times of the values in other months. For the Mae Klongriver, water turbidity varies in the range of 5 – 48 NTU, and the highest value is found in October. Electrical conductivity of Chao Phraya river water is about 1.5 – 2 times of Mae Klong river water, showing that the amount of dissolved solids in the Chao Phraya river water is higher. The pH values of water in both rivers are close to each other which are slightly alkaline.