Levels of phosphates in fresh waterways can be much higher than normal because of contamination from municipal and domestic wastewater, whether you're hooked up to a sewer or have a septic system, says Helen Suh MacIntosh, an associate professor in environmental health at Harvard University.
Like many cities, Cincinnati has a combined sewer system that is easily overwhelmed in wet weather, when untreated stormwater mixes with wastewater and overflows into water bodies or backs up into homes.