What is typically the main concern with urban runoff into surface water?

Urban runoff carries a variety of pollutants, and the main concern often revolves around bacteria and algae blooms. This type of runoff can include waste from pets, leaking sewage systems, and other sources of fecal contamination, all of which contribute to elevated levels of bacteria in local surface water bodies.

The presence of excess nutrients, particularly nitrogen and phosphorus, often found in fertilizers, can lead to significant ecological consequences. These nutrients promote excessive growth of algae, leading to blooms that can deplete oxygen levels in the water as they decay and can produce toxins harmful to aquatic life and even human health.

While excessive sedimentation, low oxygen levels, and increased turbidity are valid concerns associated with urban runoff, they are often outcomes of broader issues, including the overgrowth of algae due to nutrient enrichment. Therefore, the focus on bacteria and algae blooms highlights both a direct and important consequence of urban runoff impacting water quality and ecosystem health.