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Sources of nutrients are urban surfaces, agricultural activities, as well as the atmosphere itself. Among different urban surfaces, turfs, lawns, and gardens have a high contribution to nutrients in stormwater (Muller et al., 2020<ref name="example3">Müller, A., Österlund, H., Marsalek, J., & Viklander, M. (2020). The pollution conveyed by urban runoff: A review of sources. Science of the Total Environment, 709, 136125.</ref>). The grass clippings and application of fertilizer and pesticide, renders lawns and turfs as one of the main contributors of total and dissolved phosphorus in stormwater (Muller et al, 2020). Additionally, fallen vegetation foliage is another contributor of a watershed’s nutrient output and specially phosphorus. Selbig, 2016 has reported foliage, contributing to more than 50% of annual phosphorus loads, excluding winter season. The salt used for de-icing in in cold climate areas may contain impurities that carries nitrogen and phosphorus (Muller et al, 2020<ref name="example3" />). Other activities in urban areas such as construction, fuel deposition by vehicles, and leaking of sewer pipes or septic tanks can also contribute to overall nutrient pollution. Similar to urban landscapes, agricultural activity contributes to the nutrient loads by fertilizer and pesticide applications, as well as manure. The nutrient loads from agricultural activities are more significant than urban landscapes. Atmospheric deposition is another source of nutrients, where the atmosphere is rather a carrier than a source of aerosol nutrients created by industrial, or transportation activities (Muller et al, 2020<ref name="example3" />).
Sources of nutrients are urban surfaces, agricultural activities, as well as the atmosphere itself. Among different urban surfaces, turfs, lawns, and gardens have a high contribution to nutrients in stormwater (Muller et al., 2020<ref name="example3">Müller, A., Österlund, H., Marsalek, J., & Viklander, M. (2020). The pollution conveyed by urban runoff: A review of sources. Science of the Total Environment, 709, 136125.</ref>). The grass clippings and application of fertilizer and pesticide, renders lawns and turfs as one of the main contributors of total and dissolved phosphorus in stormwater (Muller et al, 2020). Additionally, fallen vegetation foliage is another contributor of a watershed’s nutrient output and specially phosphorus. Selbig, 2016 has reported foliage, contributing to more than 50% of annual phosphorus loads, excluding winter season. The salt used for de-icing in in cold climate areas may contain impurities that carries nitrogen and phosphorus (Muller et al, 2020<ref name="example3" />). Other activities in urban areas such as construction, fuel deposition by vehicles, and leaking of sewer pipes or septic tanks can also contribute to overall nutrient pollution. Similar to urban landscapes, agricultural activity contributes to the nutrient loads by fertilizer and pesticide applications, as well as manure. The nutrient loads from agricultural activities are more significant than urban landscapes. Atmospheric deposition is another source of nutrients, where the atmosphere is rather a carrier than a source of aerosol nutrients created by industrial, or transportation activities (Muller et al, 2020<ref name="example3" />).
==Nutrient removal mechanisms==
Nutrients within stormwater can be in particulate or dissolved form. While phosphorus and nitrogen are carried in both forms in stormwater, the majority of phosphorus is particulate bound (approximately 55% [Erickson et al., 2012]<ref>Erickson, A. J., Gulliver, J. S., and Weiss, P. T. (2012). Capturing dissolved phosphorus with iron enhanced sand filtration. Water Res., 46(9), 3032–3042.</ref>) and majority of nitrogen is dissolved (Taylor et al. 2005<ref>Taylor, G. D., Fletcher, T. D., Wong, T. H. F., Breen, P. F., and Duncan, H. P. (2005). Nitrogen composition in urban runoff—Implications for stormwater management. Water Res., 39(10), 1982–1989.</ref>). The stormwater pollution is often attributed to particles. However, the effect of dissolved pollutants on the loads is being realized, as they are more mobile and bioavailable, and therefore can have a quicker effect on the receiving waters (LeFevre et al, 2015<ref>LeFevre, G. H., Paus, K. H., Natarajan, P., Gulliver, J. S., Novak, P. J., & Hozalski, R. M. (2015). Review of dissolved pollutants in urban storm water and their removal and fate in bioretention cells. Journal of Environmental Engineering, 141(1), 04014050.</ref>).
The removal mechanisms depend on the nutrient’s form and can be explained in three categories of ''physical'', ''chemical'', and ''biological processes''. Physical mechanisms are sedimentation and filtration. Chemical mechanisms include adsorption and precipitation. The chemical processes often transform or attach dissolved forms into insoluble solids, which can then be removed by physical processes. Biological processes are those vital for livelihood of organisms such as plant uptake and nitrification/denitrification. A detailed description of each removal mechanism is provided below:
* '''Sedimentation''' – As water slows down within a stormwater feature, the suspended particles settle at the bottom, leaving a rather clear water on top. Sedimentation often removes larger particles and particle-bound pollutants.
* '''Filtration''' – As water passes through [[vegetation]] or [[filter media]] of an LID, some pollutants are caught within their structure. Filtration is the mechanism that removes finer particles and pollutants bound to those.
* '''Adsorption''' – Through adsorption, the pollutants in the dissolved form are attached to the surfaces of solids due to electrical attraction. This results in the creation of a molecular layer on the surface of the adsorbent solid. The constituents of the solid may also dissolve and return to soluble form.
* '''Precipitation''' – Precipitation refers to the chemical reactions that convert dissolved substances into solids. Such solids can then be removed from stormwater by physical processes of sedimentation and filtration.
* '''Plant uptake''' – Plant uptake or assimilation is the removal of nutrients from stormwater by the plant roots for growth purposes. While during the growth season, plants consume nutrients and remove them from stormwater, this mechanism is inactive during their dormant seasons. Additionally, when plants die off or their clippings are left in the feature, the nutrients can return to the system. Therefore, it is important to consider removal of the plant litter from the feature, to avoid any additional contribution to the outflow pollution.
* '''Nitrification/denitrification''' – This is a microbial process where ammonia is converted to nitrite and then to nitrate by nitrifying bacteria. Through the denitrification process, the nitrate is further converted into gaseous nitrogen. This process is carried by denitrifying bacteria and requires anaerobic conditions. Anaerobic condition, can occur in lower depths of an LID, given the saturated conditions last long enough to minimize oxygen concentrations. Both processes require presence of organic matter as a source of energy.
Each LID practice has the potential to offer one or all the mentioned removal mechanisms. In practice, the chemical and biological removal mechanisms each require favorable environments for activation. These environmental factors include oxygen availability, percentage of available organic matter, potential hydrogen (pH), salinity, and temperature. Please refer to the [[phosphorus]] and [[nitrogen]] pages for further details. Additionally, proper maintenance of the LID practice in question is the key to maintain the removal capacity of the feature and ensure that it does not become an exporter of nutrients itself.
==References==
==References==