John L. Leal | |
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| Born | May 5, 1858 |
| Died | March 13, 1914 (aged 55) |
| Resting place | Cedar Lawn Cemetery, Paterson, New Jersey |
| Education | Princeton College, (BA 1880); , Princeton College (MA, 1883); Columbia College of Physicians and Surgeons (MD, 1884) |
| Occupation(s) | Physician and sanitary adviser to drinking water companies |
| Known for | First use of chlorine for disinfection of a U.S. drinking water supply |
| Awards | Water Industry Hall of Fame, American Water Works Association, 1974. |
John Laing Leal (May 5, 1858 – March 13, 1914) was an American physician and water treatment expert who, in 1908, was responsible for conceiving and implementing the first disinfection of a U.S. drinking water supply using chlorine. He was one of the principal expert witnesses at two trials which examined the quality of the water supply in Jersey City, New Jersey, and which evaluated the safety and utility of chlorine for production of "pure and wholesome" drinking water. The second trial verdict approved the use of chlorine to disinfect drinking water which led to an explosion of its use in water supplies across the U.S.