Monongahela, colloquially "Mon City", is a City in Washington County, Pennsylvania, United States and is part of the Pittsburgh, Pennsylvania, Metropolitan Statistical Area, located approximately 17 miles south of the city proper. The population was 4,300 at the 2010 census.One of only two cities in Washington County, and the second smallest city in Pennsylvania, Monongahela sits at the intersection of Pennsylvania state routes 136, 88, and 837, all of which constitute the city's Main Street. The City is served by Ringgold School District.Monongahela is known for its Aquatorium, a 3,000 seat amphitheater style venue built on the Monongahela River. According to its website, the Aquatorium is the first of its kind venue in the United States.