Rain and snow generate stormwater runoff that flows over the surface of saturated soils and over non-porous or impervious surfaces like roof tops and parking lots. Many contaminants, like fine sediment, metals, oils, nutrients from fertilizers and bacteria, are located on the surface. They’re easily dislodged and carried by stormwater into storm sewers, watercourses and eventually into drinking water supply reservoirs. Studies have shown that under certain circumstances, runoff generated during the first hour of moderate-to-high intensity storm events, commonly referred to as the first-flush, can have a greater pollutant concentration than raw sewage. It’s this first-flush that New York City’s many stormwater management programs are designed to treat.