The cost of municipal water treatment due to diminished water quality represents an important component of the societal costs of water pollution. Here the chemical costs of municipal water treatment are expressed as a function of raw surface water quality. Data are used for a 3-year period for 12 water treatment plants in Texas. Results show that when regional raw water contamination is present, the chemical cost of water treatment is increased by $95 per million gallons (per 3785 m3) from a base of $75. A 1% increase in turbidity is shown to increase chemical costs by 0.25%. ¿ 1998 American Geophysical Union