ABSTRACTPeriodically, Alum Creek at the Westerville water treatment plant contains excessive concentrations of chloride, producing a salty taste. The chloride is not removed during the water treatment process. Uncontaminated surface water and ground water throughout Alum Creek basin contain less than 25 mg/l of chloride. Larger concentrations are related to man's activity in the basin, particularly oil production.The chloride content in samples of contaminated surface water ranged from 26 to nearly 28,000 mg/l while samples from oil‐field brine pits ranged between 3,000 and 57,000 mg/l.Even a brief examination of the data indicates that most of the chloride contamination in Alum Creek is due to (1) the discharge of oil‐field brines directly into the mainstem or its tributaries in the upper reaches of the basin or (2) to the discharge of contaminated ground water into streams. In many areas, the highly mineralized ground water that is now seeping into the streams may have been contaminated a decade