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Freshwater acidification

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Diagram depicting the sources and cycles of acid rain precipitation.

Freshwater acidification occurs when acidic inputs enter a body of fresh water through the weathering of rocks, invasion of acidifying gas (e.g. carbon dioxide), or by the reduction of acid anions, like sulfate and nitrate within a lake.[1] Freshwater acidification is primarily caused by sulfur oxides (SOx) and nitrogen oxides (NOx) entering the water from atmospheric depositions and soil leaching.[1] Carbonic acid and dissolved carbon dioxide can also enter freshwaters, in a similar manner associated with runoff, through carbon dioxide-rich soils.[1] Runoff that contains these compounds may incorporate acidifying hydrogen ions and inorganic aluminum, which can be toxic to marine organisms.[1] Acid rain also contributes to freshwater acidification.[2]

Causes

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Natural

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Atmospheric CO2 affects freshwater acidity.[3] Microbial activity breaks down of organic matter releases organic acids such as humic and fulvic acids. These acids accumulate in water bodies, especially those surrounded by forests and wetlands.[4] Peatlands and wetlands often produce acidic waters because of the high levels of organic matter decomposition.[5] This creates naturally acidic conditions, which are common in boreal and subarctic regions.

Volcanic activity can release sulfur dioxide (SO₂) and other acidic oxides into the atmosphere.[6] In air, sulfur dioxide converts to sulfuric acid:[7]

SO2 + 1/2 O2 + H2O → H2SO4

Anthropogenic causes

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Rio Tinto in Spain presents an acid drainage of both natural and artificial origin (mining)

Human activities can significantly accelerate freshwater acidification. In addition to carbon dioxide, the combustion of fossil fuels sulfur dioxide (SO₂) and nitrogen oxides (NOₓ). These gases react with water and air to form sulfuric acid (H₂SO₄) and nitric acid (HNO₃).[6][8][9] This process is particularly harmful in areas where the natural buffering capacity of the water is low, as these ecosystems are less able to neutralize the added acidity.

Mining can significantly contribute to freshwater acidification through the process of acid mine drainage. When sulfide minerals such as pyrite (FeS₂) are exposed to air and water during mining operations, they oxidize to form sulfuric acid.[10]

Buffering Capacity

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A map depicting Atlantic Canada.

The buffering capacity of ecosystems helps them resist changes in pH. When this is lacking, freshwater reservoirs become acidified. For example, the Atlantic region of Canada has the lowest acid deposition rates in Eastern North America, yet it has the most acidic waters on the continent due to the low buffering capacity of the regional bedrock and the addition of natural organic acids produced from close by wetlands. In most of the Atlantic region, granite and shale bedrock are found, which contain very little buffering material. Soil formed from low-buffering materials and the waters that drain from them are, therefore, susceptible to acidification, even under low acid deposition.[11]

Soil that undergoes acidification can negatively impact agriculture.[12] Some species are able to withstand low pH levels in their environment. For example, frogs and perches can withstand a pH level of 4.[13] This allows these species to be unaffected by the acid deposition in their aquatic environment, allowing them to survive in these conditions.[13] However, most aquatic species, such as clams and snails, are unable to withstand low pH levels which negatively impacts their growth and survival. The high acidic levels deteriorate their thick shells decreasing their protection from predators.[13]

Effects on aquatic ecosystems

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This pond shows an overabundance of Sphagnum.

Acidification of freshwater ecosystems can decrease native biodiversity and can alter ecosystem structure and function entirely.[7] Macro-invertebrates and large vertebrates exhibit higher mortality and lower reproductive rates under acidified conditions. Conversely, algae thrive in acidified environments, and may quickly dominate these habitats, outcompeting other species. In particular, it is common to see an increase in the abundance of the sphagnum. Sphagnum has a high capacity to exchange H+ for basic cations within freshwater. The thick layer of sphagnum restricts the exchange between surface water and sediment, further contributing to reduction in nutrient cycling in the ecosystem.[7] Aquatic biomonitoring can be used to examine the health of aquatic ecosystems.

Minimizing acidification

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Agricultural runoff is a major source of nitrogen and phosphorus, which contribute to freshwater acidification. Implementing best management practices (BMPs) in agriculture, such as reducing the use of chemical fertilizers, improving manure management, and adopting precision agriculture techniques, can significantly reduce nutrient runoff into water bodies.[14] Establishing riparian buffer zones—strips of vegetation planted along water bodies—can also help to filter pollutants from agricultural fields before they reach freshwater systems.[15] These measures not only reduce acidification but also mitigate eutrophication and improve overall water quality.

Wetlands and peatlands serve as buffers for freshwater systems by absorbing pollutants regulating water flow.[16]. Wetland restoration projects have been shown to increase the resilience of freshwater systems to acidification and other environmental stressors[17].

Liming, where calcium carbonate (CaCO3) is added to these system, increase pH levels.[18]

Regulations

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Regulation of anthropogenic emissions, specifically SOx and NOx, can lead to large decreases of acid rain and acidic bodies of water.[19] For example, the Canada-United States Air Quality Agreement has greatly minimized acid rain and ozone levels by 78% in Canada and 92% in the United States, as of 2020.[20] Moreover, investing in scientists to monitor and collect data is essential to create a model used to establish successful policies.[21] For instance, a protocol can be implemented to mitigate the issue.[21] Also, governments could invest funds to subsidize companies to decrease their pollution and incentivize them to use innovative methods of production, to lower both greenhouse gas emissions and the amount of acidic substances created. Furthermore, government institutions across the globe can connect on the issue of acidification and work together to find a feasible solution through international agreements.[12] Some successful government implementations include the Acid Rain Program[22] established in the United States in 1995, and the most recent Gothenburg Protocol, established by the United Nations Economic Commission for Europe (UNECE) to reduce acidification.[23]

Further reading

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  • "Measurements and observations : OCB-OA". Whoi.edu. Retrieved 2019-03-24.

References

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  1. ^ a b c d Psenner, Roland (March 1994). "Environmental impacts on freshwaters: acidification as a global problem". Science of the Total Environment. 143 (1): 53–61. Bibcode:1994ScTEn.143...53P. doi:10.1016/0048-9697(94)90532-0. ISSN 0048-9697.
  2. ^ Irwin, J.G.; Williams, M.L. (1988). "Acid rain: Chemistry and transport". Environmental Pollution. 50 (1–2): 29–59. doi:10.1016/0269-7491(88)90184-4. ISSN 0269-7491. PMID 15092652.
  3. ^ Jean-Pierre Gattuso; Lina Hansson, eds. (2011). Ocean acidification. Oxford University Press. ISBN 9780199591084. OCLC 975179973.
  4. ^ Berner, Robert A.; Lasaga, Antonio C. (1989). "Modeling the Geochemical Carbon Cycle". Scientific American. 260 (3): 74–81. doi:10.1038/scientificamerican0389-74. ISSN 0036-8733. JSTOR 24987179.
  5. ^ Nordstrom, D. K. (2011). "Mine waters: Acidic to circumneutral". Elements. 7 (6): 393–398. doi:10.2113/gselements.7.6.393.
  6. ^ a b Schindler, D. W. (1988-01-08). "Effects of Acid Rain on Freshwater Ecosystems". Science. 239 (4836): 149–157. doi:10.1126/science.239.4836.149. ISSN 0036-8075. PMID 17732976.
  7. ^ a b c Henriksen, Arne; Kämäri, Juha; Posch, Maximilian; Wilander, Anders (1992). "Critical Loads of Acidity: Nordic Surface Waters". Ambio. 21 (5): 356–363. ISSN 0044-7447. JSTOR 4313961.
  8. ^ Likens, Gene E.; Bormann, F. Herbert (1974-06-14). "Acid Rain: A Serious Regional Environmental Problem". Science. 184 (4142): 1176–1179. doi:10.1126/science.184.4142.1176. ISSN 0036-8075. PMID 17756304.
  9. ^ Cardoso, A.C.; Free, G.; Nõges, P.; Kaste, Ø.; Poikane, S.; Solheim, A. Lyche (2009). "Lake Management, Criteria". Encyclopedia of Inland Waters. Elsevier. pp. 310–331. doi:10.1016/b978-012370626-3.00244-1. ISBN 9780123706263.
  10. ^ Nordstrom, D. K. (2011-12-01). "Mine Waters: Acidic to Circmneutral". Elements. 7 (6): 393–398. doi:10.2113/gselements.7.6.393. ISSN 1811-5209.
  11. ^ Clair, Thomas A.; Dennis, Ian F.; Scruton, David A.; Gilliss, Mallory (December 2007). "Freshwater acidification research in Atlantic Canada: a review of results and predictions for the future". Environmental Reviews. 15 (NA): 153–167. doi:10.1139/a07-004. ISSN 1181-8700.
  12. ^ a b Chen, Changan; Lin, Juntong; Liu, Yuhang; Ren, Xiangru (2022). "Effects of freshwater acidification and countermeasures". IOP Conference Series: Earth and Environmental Science. 1011 (1): 012035. Bibcode:2022E&ES.1011a2035C. doi:10.1088/1755-1315/1011/1/012035. S2CID 248122033.
  13. ^ a b c "Effects of Acid Rain - Surface Waters and Aquatic Animals" (PDF). Landuse.alberta.ca. Retrieved 19 April 2022.
  14. ^ Camargo, Julio A.; Alonso, Álvaro (August 2006). "Ecological and toxicological effects of inorganic nitrogen pollution in aquatic ecosystems: A global assessment". Environment International. 32 (6): 831–849. doi:10.1016/j.envint.2006.05.002. hdl:10261/294824. PMID 16781774.
  15. ^ Mayer, Paul M.; Jr. Reynolds, Steven K.; Canfield, Timothy J.; McCutchen, Marshall D. (2005). "Riparian Buffer Width, Vegetative Cover, and Nitrogen Removal Effectiveness: A Review of Current Science and Regulations" (PDF). Journal of the American Water Resources Association. 43 (2): 311–324.
  16. ^ Gorham, Eville (May 1991). "Northern Peatlands: Role in the Carbon Cycle and Probable Responses to Climatic Warming". Ecological Applications. 1 (2): 182–195. doi:10.2307/1941811. ISSN 1051-0761. JSTOR 1941811. PMID 27755660.
  17. ^ Mitsch, William J.; Gosselink, James G. (2015). Wetlands (Fifth ed.). Hoboken, NJ: John Wiley and Sons, Inc. ISBN 978-1-118-67682-0.
  18. ^ Mant, Rebecca C.; Jones, David L.; Reynolds, Brian; Ormerod, Steve J.; Pullin, Andrew S. (2013-08-01). "A systematic review of the effectiveness of liming to mitigate impacts of river acidification on fish and macro-invertebrates". Environmental Pollution. 179: 285–293. Bibcode:2013EPoll.179..285M. doi:10.1016/j.envpol.2013.04.019. ISSN 0269-7491. PMID 23707951.
  19. ^ Menz, Fredric C.; Seip, Hans M. (2004-08-01). "Acid rain in Europe and the United States: an update". Environmental Science & Policy. 7 (4): 253–265. Bibcode:2004ESPol...7..253M. doi:10.1016/j.envsci.2004.05.005. ISSN 1462-9011.
  20. ^ Canada, Environment and Climate Change (2005-01-25). "Canada-United States Air Quality Agreement: overview". www.canada.ca. Retrieved 2023-03-25.
  21. ^ a b Grennfelt, Peringe; Engleryd, Anna; Forsius, Martin; Hov, Øystein; Rodhe, Henning; Cowling, Ellis (2020-04-01). "Acid rain and air pollution: 50 years of progress in environmental science and policy". Ambio. 49 (4): 849–864. Bibcode:2020Ambio..49..849G. doi:10.1007/s13280-019-01244-4. ISSN 1654-7209. PMC 7028813. PMID 31542884.
  22. ^ US EPA, OAR (2014-08-21). "Acid Rain Program". www.epa.gov. Retrieved 2023-03-24.
  23. ^ "Protocol to Abate Acidification, Eutrophication and Ground-level Ozone | UNECE". unece.org. Retrieved 2023-03-25.
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