Composition, characterization, and management of street sweepings
Link to fact sheet on managing street sweepings - [1]
Street sweepings are materials such as sand, salt, leaves and debris removed from city streets, parking lots and sidewalks to prevent these materials from being washed into storm sewers and surface waters, and to improve the appearance of public roadways. For a discussion of potential benefits associated with street sweeping, see Overview, water quality benefits, and other co-benefits of street sweeping.
Contents
Sources of materials in street sweepings
The primary sources of materials in street sweepings include the following.
- Automobiles and other vehicles. These are important sources of hydrocarbons associated with fluids (e.g. oil, gasoline, antifreeze) and metals associated with vehicle parts (e.g. brake linings, tire wear). These materials, particularly metals, are typically released at higher rates as vehicle speed increases and as the extent of stopping increases.
- Vegetation. Trees and grass adjacent to impermeable surfaces are important sources of organic materials to impervious surfaces that are swept. These organic materials are important sources of phosphorus and nitrogen, may contribute to oxygen demand, and may act as source areas for bacteria.
- Atmospheric deposition. Deposition is an important source of inorganic solids. This includes deposition from near-source and distant-source areas. These solids may be important sources of pollutants attached to the solids, including metals, PAHs, and a range of emerging chemicals (e.g. pesticides, microplastics).
- Local sources. This includes a wide range of sources that may have significant impacts locally. Examples include active construction sites (sources of sediment), industry (sources of organics), and contaminated sites (e.g. salvage yards, vehicle operations facilities, waste management facilities).
- Trash and debris. With streets that are frequently swept, trash is typically not an important source of pollutants. However, in some areas where inputs of these are high, or where sweeping is infrequent, thus allowing the materials to break down, trash and debris can be important contributors of certain pollutants, such as microplastics.
- Exposed and poorly managed soils. Soil, when exposed, poorly managed, or on steep slopes, can be an important source of sediment. Unless the soil is associated with another source (e.g. metals in high transportation areas), eroded soil transported to impervious surfaces is not an important source of other pollutants.
- Irrigated lawns. Lawns that are over-watered can be important sources of bacteria, organic material, and phosphorus if the source water contains polyphosphates.
- De-icing materials and sand applied for traction. In cold climates, de-icing materials are important sources of chloride, sodium, and phosphorus. Sand applied for traction is an important source of sediment.
Composition and characteristics of street sweeping materials
The composition and characteristics of street sweepings depends on multiple factors, including but not limited to the following. The discussion describes typical conditions.
- Source area (land use). Concentrations of metals, PAHs, and other organics, including many emerging chemicals, are higher in medium and high intensity developed land uses, such as industrial, heavy commercial, and high transportation areas, compared to lower intensity developed areas, such as residential areas. Sources of nutrients associated with organic material are greatest in residential areas.
- Type of sweeper.
- Time of year.
- Buildup period/interval.
- Soil. An important factor affecting the occurrence and distribution of pollutants in sweeping material is particle size. Concentrations of metals and organics that bind with inorganic sediment increase as particle size decreases. Thus, soil can locally play an important role in the composition of material accumulating on impervious surfaces, particularly when soils are exposed (e.g. active construction sites) or poorly managed (e.g. compacted or exposed soils, or soils on steep slopes).