Bacteria are microscopic, single-celled organisms that are ubiquitous in natural and built environments, including soil, water, air, and living organisms. In aquatic systems, bacteria play essential ecological roles by breaking down organic matter, recycling nutrients, and supporting food webs. However, in the context of stormwater management, bacteria are often of concern because certain types are associated with fecal contamination and can pose risks to human health.
Stormwater professionals typically focus on indicator bacteria, such as Escherichia coli (E. coli) and fecal coliforms, which are used to signal the possible presence of pathogenic microorganisms. While most bacteria are harmless or beneficial, elevated concentrations of these indicator organisms in stormwater runoff suggest that disease-causing organisms may also be present, particularly in receiving waters used for recreation or as drinking water sources.
In stormwater systems, bacteria are introduced through a variety of sources across the watershed. Common sources include waste from pets and wildlife deposited on impervious surfaces, agricultural runoff containing manure, leaking or failing sanitary sewer infrastructure, illicit sanitary connections to storm drains, and failing septic systems. Urban areas also contribute bacteria through combined sewer overflows in older systems, while rural and suburban areas may see contributions from livestock operations and improperly managed waste.
Bacteria can accumulate on land surfaces during dry weather and are then washed into stormwater systems during rainfall or snowmelt events, a process often referred to as washoff. Once mobilized, bacteria are transported through storm drains, ditches, and channels into receiving waters such as streams, lakes, and estuaries. In some cases, stormwater management practices like detention basins can temporarily retain bacteria, but they may also provide conditions that allow bacteria to persist or even regrow under favorable conditions.
Environmental factors such as temperature, sunlight exposure, sediment attachment, and nutrient availability influence bacterial survival and transport. Bacteria often attach to fine sediment particles, which can protect them from ultraviolet radiation and enhance their persistence in the environment.
Managing bacteria in stormwater systems typically involves source control measures, such as public education on pet waste disposal, maintenance of sanitary infrastructure, agricultural best management practices, and the elimination of illicit discharges. Structural practices, including filtration, infiltration, and disinfection in some cases, may also be used to reduce bacterial concentrations before runoff reaches receiving waters.
Overall, bacteria in stormwater are both a natural component of the environment and a key water quality concern, particularly where human exposure is possible, making their monitoring and management an important aspect of modern stormwater programs.