In an effort to remove contaminants from urban runoff, stormwater treatment systems are gradually being integrated into the built environment. These include detention basins, treatment wetlands, rain gardens, and commercial manufactured devices. They have been installed with an expectation that they will improve stormwater as it moves through them, and in some cases are part of an overarching, catchment-wide water quality improvement plan. Frequently, these stormwater treatment systems do not meet the upper contaminant removal efficiencies due to an incomplete understanding of the dynamic variables that affect their performance. Contaminant removal performance within various stormwater treatment systems shows a high variability between different systems, between systems of the same type at different locations, and even within an individual system between different storm events. Variables affecting performance range from human-derived factors such as lack of maintenance, improper installation, or poor design, to the physical environment, such as climatic conditions affecting rainfall intensity, frequency and duration as well as catchment impervious surface types and speciation of contaminants.
Sources of variation in stormwater treatment system efficiency can be expressed as three broad categories: (1) contaminant load variables, (2) treatment system variables, and (3) monitoring technique variables. Contaminant load variables are influenced by local climate and catchment land use and surface materials, treatment variables depend on a stormwater treatment system’s specific design, installation and maintenance history and operational mechanisms, while differing monitoring techniques likely contribute to variation in globally available data. This paper outlines and qualifies the key sources of performance variation within stormwater treatment systems. A better understanding of these variables will help improve choice of treatment system type, placement, and management.