Surfactants, or surface-active agents, are chemical compounds that reduce the surface tension between two substances, most commonly between water and air or between water and oils. In stormwater management, surfactants are considered a class of organic pollutants that alter the physical and chemical behavior of runoff, particularly by enhancing the mixing and transport of contaminants that would otherwise remain separate from water.
Surfactants have a unique molecular structure consisting of a hydrophilic, water-attracting head and a hydrophobic, water-repelling tail. This dual nature allows them to position themselves at interfaces, where they reduce surface tension and enable substances like oils, greases, and other hydrocarbons to disperse into water as emulsions. This property is why surfactants are widely used in detergents, soaps, industrial cleaners, and many commercial products.
In stormwater systems, surfactants are commonly introduced through residential, commercial, and industrial activities. Typical sources include vehicle washing, laundry discharge, car wash facilities, cleaning operations, and certain manufacturing processes. They may also enter runoff through illicit discharges or sanitary sewer overflows. Once present in stormwater, surfactants can facilitate the mobilization of other pollutants, particularly hydrophobic compounds such as oils, pesticides, and polycyclic aromatic hydrocarbons, increasing their transport through drainage systems.
From a water quality perspective, surfactants can have several impacts. They may contribute to visible foaming in streams and outfalls, which is often an indicator of contamination. They can also be toxic to aquatic organisms, particularly at higher concentrations, and may interfere with biological processes such as respiration in fish and invertebrates. Additionally, by increasing the solubility and mobility of other pollutants, surfactants can complicate treatment processes and reduce the effectiveness of sedimentation-based best management practices.
In stormwater treatment, surfactants are managed through a combination of source control and treatment practices. Bioretention systems, constructed wetlands, and soil-based filtration practices can promote biodegradation and adsorption of surfactants. However, their presence can also inhibit oil-water separation and reduce treatment efficiency in certain structural BMPs.
Surfactants are surface-active chemicals that reduce surface tension and promote the mixing of otherwise immiscible substances, playing a significant role in the transport, behavior, and environmental impact of pollutants within stormwater systems.