Approximately 60% of NYC is served by combined sewers that carry both sewage and stormwater in a single pipe. During dry weather, all flow goes to wastewater treatment plants. But during heavy rain, the combined volume can exceed the system capacity, causing combined sewer overflows (CSOs) that discharge a mix of stormwater and untreated sewage into local waterways. NYC has 460 CSO outfalls across the five boroughs. The city is investing billions in infrastructure upgrades, green infrastructure, and storage tanks to reduce CSO events. Property owners can help by reducing stormwater runoff from their properties.