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→Nutrient pollution
Excessive algal growth requires both phosphorus and nitrogen. While in some environments nitrogen is considered to be the limiting nutrient that controls such growth, phosphorus has been considered as the main limiting factor in most freshwaters (Howarth & Marino, 2006<ref>Howarth, R. W., & Marino, R. (2006). Nitrogen as the limiting nutrient for eutrophication in coastal marine ecosystems: Evolving views over three decades. Limnology and Oceanography, 51, 364-376. doi:10.4319/lo.2006.51.1_part_2.0364</ref>; Schindler et al., 2016<ref>Schindler, D. W., Carpenter, S. R., Chapra, S. C., Hecky, R. E., & Orihel, D. M. (2016). Reducing phosphorus to curb lake eutrophication is a success. Environmental Science & Technology, 50, 8923-8929. doi: 10.1021/acs.est.6b02204.</ref>). Therefore, controlling the load of phosphorous leaving a sub-watershed can reduce the chances of nutrient pollution in receiving surface waters. Formation of policies such as Lake Simcoe Phosphorous Offsetting Policy (LSPOP) are examples of such an approach. LSPOP requires a Zero Export Target where all new developments should control 100% of phosphorus from leaving the property (LSRCA, 2021<ref>LSRCA (Lake Simcoe Region Conservation Authority). (2021). Phosphorus Offsetting Policy. July). Available at: https://www.lsrca.on.ca/Shared%20Documents/Phosphorus_Offsetting_Policy.pdf.</ref>).
Excessive algal growth requires both phosphorus and nitrogen. While in some environments nitrogen is considered to be the limiting nutrient that controls such growth, phosphorus has been considered as the main limiting factor in most freshwaters (Howarth & Marino, 2006<ref>Howarth, R. W., & Marino, R. (2006). Nitrogen as the limiting nutrient for eutrophication in coastal marine ecosystems: Evolving views over three decades. Limnology and Oceanography, 51, 364-376. doi:10.4319/lo.2006.51.1_part_2.0364</ref>; Schindler et al., 2016<ref>Schindler, D. W., Carpenter, S. R., Chapra, S. C., Hecky, R. E., & Orihel, D. M. (2016). Reducing phosphorus to curb lake eutrophication is a success. Environmental Science & Technology, 50, 8923-8929. doi: 10.1021/acs.est.6b02204.</ref>). Therefore, controlling the load of phosphorous leaving a sub-watershed can reduce the chances of nutrient pollution in receiving surface waters. Formation of policies such as Lake Simcoe Phosphorous Offsetting Policy (LSPOP) are examples of such an approach. LSPOP requires a Zero Export Target where all new developments should control 100% of phosphorus from leaving the property (LSRCA, 2021<ref>LSRCA (Lake Simcoe Region Conservation Authority). (2021). Phosphorus Offsetting Policy. July). Available at: https://www.lsrca.on.ca/Shared%20Documents/Phosphorus_Offsetting_Policy.pdf.</ref>).
In addition to contaminating surface waters, abundance of nutrients can also contaminate the ground waters. Contrary to surface waters, nitrogen is the primary nutrient of concern regarding groundwater quality. Hobbie, et al. (2017<ref>) reported that only 22 % of phosphorous is retained within its watershed, while the same estimated for nitrogen is 80%. Therefore, most of the nitrogen is leached in the groundwater or transformed through denitrification. Phosphorus retained in the watershed is often in particle form. Therefore, it tends to be bound to soil particles and retained in the vadose zone (area between ground surface and the groundwater table). However, nitrogen is often available in dissolved form which is more mobile and bioavailable. Thus, it can travel through the vadose zone and contaminate the groundwater.
In addition to contaminating surface waters, abundance of nutrients can also contaminate the ground waters. Contrary to surface waters, nitrogen is the primary nutrient of concern regarding groundwater quality. Hobbie, et al. (2017<ref name="example2">Hobbie, S.E., Finlay, J.C., Janke, B.D., Nidzgorski, D.A., Millet, D.B., Baker, L.A., 2017. Contrasting nitrogen and phosphorus budgets in urban watersheds and implications for managing urban water pollution. Proc. Natl. Acad. Sci. U. S. A. 114 (16), 4177–4182.</ref>) reported that only 22 % of phosphorous is retained within its watershed, while the same estimated for nitrogen is 80%. Therefore, most of the nitrogen is leached in the groundwater or transformed through denitrification. Phosphorus retained in the watershed is often in particle form. Therefore, it tends to be bound to soil particles and retained in the vadose zone (area between ground surface and the groundwater table). However, nitrogen is often available in dissolved form which is more mobile and bioavailable. Thus, it can travel through the vadose zone and contaminate the groundwater.
==Nutrient Management==
Due to their differing stoichiometry, managing nitrogen pollution is different from that of phosphorous. To reduce the nitrogen pollution, the watershed inputs should be reduced in general, while reducing phosphorus pollution would require reducing the movement of phosphorus from the contributing areas in the watershed (Hobbie et al., 2017<ref name="example2" />).
The key method for nutrient management is source control, or prevention of pollutants from entering stormwater, at the source. Low Impact Development (LID) features are source control measures in this way. Source control policies are cost effective tools for nutrient management (Marsalek and Viklander, 2011<ref>Marsalek, J., Viklander, M., 2011. Controlling contaminants in urban stormwater: Linking environmental science and policy. In: Lundqvist, J. (Ed.), On the Water Front: Selections from the 2010 World Water Week in Stockholm. vol. 101. Stockholm International Water Institute (SIWI), Stockholm.</ref>) that support and encourage the implementation of LIDs. Examples of such policies are adapted by Lake Simcoe Region Conservation Authority, South Nation Conservation (SNC), Nottawasaga Valley Conservation Authority (NVCA), and Halton Region in Ontario, as well as Chesapeake Bay and Mississippi River Basin (Region of Waterloo, 2017<ref>Region of Waterloo. (2017). Phosphorus Offsetting: Review of Existing Ontario Programs and Opportunities. Available at: https://www.regionofwaterloo.ca/en/living-here/resources/Documents/water/projects/wastewater/plan/WS2018V5-Tech_Memo_9A_WWTMP-Phosphorus_Offsetting_2017.PDF</ref>), municipalities as nutrient management by-laws, provinces like Ontario Nutrient Management Act and the Great Lakes Protection Act, and countries like Great Lakes Agreement between Canada and the US (CWN, 2017<ref name="example1" />). Additionally, design guidelines such as this one, provide tools for design and implementation of LIDs. While historically these guidelines indicated percent removal rates, the recent approaches are guiding to meet specific numeric objectives such as concentration (Clark and Pitt, 2012<ref>Clark, S. E., and Pitt, R. (2012). Targeting treatment technologies to address specific stormwater pollutants and numeric discharge limits. Water Res., 46(20), 6715–6730.</ref>).
Selection of the LID type and its implementation for proper nutrient management, requires a good understanding of site limitations, sources of nutrients, their forms (particulate/soluble), and nutrient removal mechanisms associated with each LID type. The sources of nutrients and removal mechanisms are reviewed in this page, for additional details on [[phosphorus]] and [[nitrogen]], refer to their relative pages.
==Nutrient sources==
The extreme amounts of nutrients are both due to the general increase of their availability and the decreased removal capacity of natural systems. Following the important technological advancements in early 20th century (Haber-Bosch process), nitrogen in the form of ammonia has increased significantly due to the ongoing production of fertilizers. While the increase in agricultural production has been a beneficial effect of this finding, it has also led to excessive amounts of nutrient source loading. The decrease in nutrient retention capacity is due to the modification of natural systems such as the increased impervious surfaces along with reduction of [[vegetation]], stream channelization and modification and degradation of riparian zones (Collins et al, 2010<ref>Collins, K.A., Lawrence, T.J., Stander, E.K., Jontos, R.J., Kaushal, S.S., Newcomer, T.A., Grimm, N.B. and Ekberg, M.L.C., 2010. Opportunities and challenges for managing nitrogen in urban stormwater: A review and synthesis. Ecological Engineering, 36(11), pp.1507-1519.</ref>).
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" />).
==References==
==References==