Stormwater runoff, a significant environmental concern, refers to precipitation, such as rain or melting snow, that flows over land surfaces rather than soaking into the ground. In developed areas, natural landscapes are replaced by impervious surfaces like paved streets, parking lots, rooftops, and sidewalks.
These surfaces prevent water from infiltrating the soil, causing it to flow across land and into nearby waterways. As this water travels, it collects pollutants from urban, suburban, and agricultural areas, carrying them untreated into streams, rivers, and other water bodies.
Contamination of Waterways
Stormwater runoff transports contaminants across the landscape, depositing them into aquatic environments.
Sediment, soil particles dislodged by erosion, is a prevalent pollutant that reduces water clarity, known as turbidity. This sediment can originate from construction sites, agricultural fields, or eroding stream banks.
When suspended, it limits sunlight penetration, which harms aquatic plants, and when it settles, it can smother habitats for fish and other organisms.
Nutrients, primarily nitrogen and phosphorus, are carried by runoff. Sources include fertilizers, pet waste, and other organic materials.
Excess nutrients can trigger eutrophication, leading to rapid overgrowths of algae, known as algal blooms. As these large algal masses decompose, they consume dissolved oxygen, creating low-oxygen zones often called “dead zones” that are detrimental to aquatic life.
Chemical pollutants are another group of hazardous substances in stormwater. These include oil, grease, and heavy metals (zinc, copper, lead) from vehicle leaks, industrial activities, and roof shingles.
Pesticides and herbicides, used in agriculture and residential landscaping, also become entrained in runoff. Such chemicals can poison aquatic organisms, disrupt their reproductive cycles, and accumulate within the food chain, posing risks to ecosystems.
Pathogens, including bacteria and viruses, are also transported by stormwater. These microorganisms frequently originate from pet waste, wildlife droppings, failing septic systems, or sewage overflows.
Elevated levels of fecal coliform bacteria often exceed safe recreational standards. Contact with water contaminated by these pathogens can lead to human illnesses like gastrointestinal problems, skin rashes, and ear infections, sometimes necessitating beach closures.
Thermal pollution also contributes to water quality degradation. Rainwater flowing over hot impervious surfaces, such as asphalt roads, parking lots, and rooftops, absorbs heat before entering waterways.
This warmed runoff can raise the temperature of receiving water bodies, even by several degrees Celsius. Increased water temperatures reduce dissolved oxygen, which is essential for aquatic life, and can also affect the metabolism and reproductive cycles of sensitive species.
Impacts on Aquatic Ecosystems
The influx of pollutants via stormwater runoff has widespread consequences for organisms within aquatic ecosystems.
Fish and invertebrates are directly affected by the toxicity of chemical pollutants, which can impair their health, growth, and ability to reproduce. Reduced oxygen levels, a common result of nutrient pollution and decomposition, can suffocate marine wildlife and lead to fish kills.
Sediment can clog fish gills, reduce their resistance to disease, and smother fish eggs and spawning grounds, disrupting their breeding patterns.
Aquatic plants also experience significant impacts. Excessive nutrients foster the overgrowth of algae, which forms dense mats that block sunlight from reaching other submerged vegetation.
This reduction in light hinders photosynthesis for bottom-dwelling plants, which provide food and oxygen for aquatic organisms. The overall species composition can shift as sensitive plant species are replaced by more tolerant, often invasive, varieties.
Habitat degradation is a direct outcome of these pollution pressures. Sediment deposition creates a “blanket” on riverbeds, smothering benthic (bottom-dwelling) organisms and destroying essential habitats like fish spawning areas.
Chemical pollutants can accumulate in the food chain, leading to higher concentrations in predators and broader ecological disruptions. The physical alteration of waterways, including the loss of important large organic debris, further diminishes suitable refuge areas for aquatic life.
Chronic pollution and habitat disruption contribute to a reduction in species richness, commonly referred to as biodiversity loss. As ecosystems degrade, sensitive aquatic species decline, and communities shift towards more pollution-tolerant organisms, altering the natural balance of the environment.
Physical Changes to Land and Water Bodies
Beyond chemical and biological contamination, the sheer volume and force of stormwater runoff also induce physical alterations to land and water bodies.
Increased runoff volume and velocity contribute to streambank erosion. This process involves the removal of soil and rock from stream edges, leading to land loss and an increased load of sediment in waterways.
Over time, this erosion can widen and alter the natural course of stream channels.
Impervious surfaces prevent precipitation from soaking into the ground, leading to increased flooding. Instead of natural infiltration, higher volumes of water rapidly flow over these surfaces, overwhelming drainage systems.
This results in more frequent and severe localized flooding in urban and suburban areas, causing property damage and safety hazards.
Natural water flow patterns are disrupted, leading to altered hydrology characterized by “flashy” flows. These rapid, high-volume surges in streams and rivers can scour channel beds, removing vegetation and dislodging substrate.
Such scouring destroys aquatic habitats and can impact the stability of the stream ecosystem.
Finally, the physical impacts of stormwater runoff can lead to infrastructure damage. Erosion and flooding can undermine and damage roads, bridges, and other human-made structures.
Additionally, sediment carried by runoff can clog stormwater drains and pipes, further exacerbating flooding issues and requiring costly maintenance and repairs.