Air Quality at Mammoth Cave National Park
Most visitors expect clean air and clear views in parks. However, Mammoth Cave National Park (NP), Kentucky, home to the world’s longest recorded cave system and scenic river valleys, experiences relatively poor air quality. The park is downwind of many sources of air pollution, including power plants, urban areas, and industry in Kentucky and Tennessee. Pollutants emitted from these sources can harm the park’s natural and scenic resources such as upland surface waters, plants, fish, bats, and visibility. The National Park Service works to address air pollution effects at Mammoth Cave NP, and in parks across the U.S., through science, policy and planning, and by doing our part.
Nitrogen and Sulfur
Nitrogen (N) and sulfur (S) compounds deposited from the air may have harmful effects on ecosystem processes. Healthy ecosystems can naturally buffer a certain amount of pollution, but once a threshold is passed the ecosystem may respond negatively. This threshold is the critical load, or the amount of pollution above which harmful changes in sensitive ecosystems occur (Porter 2005). N and S deposition change ecosystems through eutrophication (N deposition) and acidification (N + S deposition). Eutrophication increases soil and water nutrients which causes some species to grow more quickly and changes community composition. Ecosystem sensitivity to nutrient N enrichment at Mammoth Cave National Park (MACA) relative to other national parks is very low (Sullivan et al. 2016); for a full list of N sensitive ecosystem components, see: NPS ARD 2019. Acidification leaches important cations from soils, lakes, ponds, and streams which decreases habitat quality. Ecosystem sensitivity to acidification at MACA relative to other national parks is high (Sullivan et al. 2016); to search for acid-sensitive plant species, see: NPSpecies.
From 2017-2019 total N deposition in MACA ranged from 9.5 to 10.3 kg-N ha-1 yr-1 and total S deposition ranged from 2.8 to 3.1 kg-S ha-1 yr-1 based on the TDep model (NADP, 2018). MACA has been monitoring atmospheric N and S deposition since 2002, see the conditions and trends website for park-specific information.
Seeps and surface waters that flow from limestone bedrock in MACA are well-buffered from acidification effects. Some soils, sandstone ridges, and dry upland areas may be more sensitive to acid deposition. There is a particular concern that during rainstorms, when there is little opportunity for rainwater to interact with deep soils, episodic acidification could occur.
Epiphytic macrolichen community responses
Epiphytic macrolichens grow on tree trunks, branches, and boles. Since these lichens grow above the ground, they obtain all their nutrients directly from precipitation and the air. Many epiphytic lichen species have narrow environmental niches and are extremely sensitive to changes in air pollution. Geiser et al. (2019) used a U.S. Forest Service national survey to develop critical loads of nitrogen (N) and critical loads of sulfur (S) to prevent more than a 20% decline in four lichen community metrics: total species richness, pollution sensitive species richness, forage lichen abundance, and cyanolichen abundance.
McCoy et al. (2021) used forested area from the National Land Cover Database to estimate the impact of air pollution on epiphytic lichen communities. Forested area makes up 201 km2 (96.2%) of the land area of Mammoth Cave National Park.
- N deposition exceeded the 3.1 kg-N ha-1 yr-1 critical load to protect N-sensitive lichen species richness in 100% of the forested area.
- S deposition exceeded the 2.7 kg-S ha-1 yr-1 critical load to protect S-sensitive lichen species richness in 100% of the forested area.
For exceedances of other lichen metrics and the predicted decline of lichen communities see Appendices A and B of McCoy et al. (2021).
Additional modeling was done on 459 lichen species to test the combined effects of air pollution and climate gradients (Geiser et al. 2021). A critical load indicative of initial shifts from pollution-sensitive toward pollution-tolerant species occurred at 1.5 kg-N ha-1 yr-1 and 2.7 kg-S ha-1 yr-1 even under changing climate regimes.
Plant species response
Plants vary in their tolerance of eutrophication and acidification, and some plant species respond to nitrogen (N) or sulfur (S) pollution with declines in growth, survival, or abundance on the landscape. Horn et al. (2018) used the U.S. Forest Service national forest survey to develop critical loads of N and critical loads of S to prevent declines in growth or survival of sensitive tree species. Clark et al. (2019) used a database of plant community surveys to develop critical loads of N and critical loads of S to prevent a decline in abundance of sensitive herbaceous plant species. According to NPSpecies, Mammoth Cave National Park contains:
- 31 N-sensitive tree species and 96 N-sensitive herbaceous species.
- 35 S-sensitive tree species and 78 S-sensitive herbaceous species.
Airborne mercury, and other toxic air contaminants, when deposited are known to harm birds, salamanders, fish and other wildlife, and cause human health concerns. These substances enter the food chain and accumulate in the tissue of organisms causing reduced reproductive success, impaired growth and development, and decreased survival.
Effects of mercury and toxics:
- Mercury deposition at Mammoth Cave NP is a concern given numerous nearby coal-burning power plants, which are significant sources of airborne mercury
- Mercury deposition at the park is high compared to other parks (NPS 2010)
- Kentucky has issued a statewide fish consumption advisory for mercury
- Elevated levels of mercury have been found in bats, fish, insects, water, and sediment samples in the park (NPS 2009)
Mammoth Cave NP has been monitoring mercury since 2006. Explore air monitoring »
Visitors come to Mammoth Cave NP not only to explore the vast and complex underground labyrinths, but also to enjoy vistas of the Green River valley and hill country of South Central Kentucky. Unfortunately, these scenic views are often affected by haze that reduces how well and how far people can see. Visibility reducing haze is caused by tiny particles in the air, and these particles can also affect human health. Many of the same pollutants that ultimately fall out as nitrogen and sulfur deposition contribute to this haze. Organic compounds, soot, and dust reduce visibility as well. Significant improvements in park visibility have been documented since the 1990’s. Still, visibility in the park is a long way from the Clean Air Act goal of no human caused impairment.
- Reduced visibility due to human-caused haze
- Reduction of the average natural visual range from about 110 miles (without the effects of pollution) to about 50 miles because of pollution at the park
- Reduction of the visual range to below 25 miles on high pollution days
Visit the NPS air quality conditions and trends website for park-specific visibility information. Mammoth Cave NP has been monitoring visibility since 2000. Check out the live air quality webcam and explore air monitoring »
At ground level, ozone is harmful to human health and the environment. Ground-level ozone does not come directly from smokestacks or vehicles, but instead is formed when other pollutants, mainly nitrogen oxides and volatile organic compounds, react in the presence of sunlight.
Ozone levels in the park have come down significantly over the past 10 years but still occasionally exceed the National Ambient Air Quality Standard set by the U.S. Environmental Protection Agency to protect public health. Ozone is a respiratory irritant, causing coughing, sinus inflammation, chest pains, scratchy throat, lung damage, and reduced immune system functions. Children, the elderly, people with existing health problems, and active adults are most vulnerable. When ozone levels exceed, or are predicted to exceed, health standards, Mammoth Cave NP staff post health advisories cautioning visitors of the potential health risks associated with exposures to elevated levels.
Ground-level ozone also weakens plants making them less resistant to disease and insect infestations. There are several ozone-sensitive plants in Mammoth Cave NP including Asclepias syriaca (common milkweed), Liquidambar styraciflua (sweetgum), and Liriodendron tulipifera (tulip poplar). A risk assessment concluded that plants in at Mammoth Cave NP were at high risk for ozone damage (Kohut 2007; Kohut 2004). Assessments in the park have also documented visible ozone injury on the leaves of approximately 25% of Common milkweed plants surveyed (CUPN 2010). Sweetgum and Tulip poplar have also shown ozone damage on leaves. Search ozone-sensitive plant species found at Mammoth Cave NP.
Visit the NPS air quality conditions and trends website for park-specific ozone information. Mammoth Cave NP has been monitoring ozone since 1997. Check out the live ozone and meteorology data from Mammoth Cave, NP and explore air monitoring »
Clark, C.M., Simkin, S.M., Allen, E.B. et al. Potential vulnerability of 348 herbaceous species to atmospheric deposition of nitrogen and sulfur in the United States. Nat. Plants 5, 697–705 (2019). https://doi.org/10.1038/s41477-019-0442-8
[CUPN] Cumberland Piedmont Network. 2010. Summary of Results of 2009 Foliar Injury Surveys by Cumberland Piedmont Network. Presented at the National Park Service Air Quality Planning Meeting. January 5-7, 2010. Denver, CO.
Geiser, Linda & Nelson, Peter & Jovan, Sarah & Root, Heather & Clark, Christopher. (2019). Assessing Ecological Risks from Atmospheric Deposition of Nitrogen and Sulfur to US Forests Using Epiphytic Macrolichens. Diversity. 11. 87. 10.3390/d11060087.
Geiser, Linda & Root, Heather & Smith, Robert & Jovan, Sarah & Clair, Larry & Dillman, Karen. (2021). Lichen-based critical loads for deposition of nitrogen and sulfur in US forests. Environmental Pollution. 291. 118187. 10.1016/j.envpol.2021.118187.
Horn KJ, Thomas RQ, Clark CM, Pardo LH, Fenn ME, Lawrence GB, et al. (2018) Growth and survival relationships of 71 tree species with nitrogen and sulfur deposition across the conterminous U.S.. PLoS ONE 13(10): e0205296. https://doi.org/10.1371/journal.pone.0205296
Kohut R.J. 2007. Ozone Risk Assessment for Vital Signs Monitoring Networks, Appalachian National Scenic Trail, and Natchez Trace National Scenic Trail. NPS/NRPC/ARD/NRTR—2007/001. National Park Service. Fort Collins, Colorado. Available at https://www.nps.gov/articles/ozone-risk-assessment.htm
McCoy K., M. D. Bell, and E. Felker-Quinn. 2021. Risk to epiphytic lichen communities in NPS units from atmospheric nitrogen and sulfur pollution: Changes in critical load exceedances from 2001‒2016. Natural Resource Report NPS/NRSS/ARD/NRR—2021/2299. National Park Service, Fort Collins, Colorado. https://doi.org/10.36967/nrr-2287254.
[NADP] National Atmospheric Deposition Program. 2018. NTN Data. Accessed January 20, 2022. Available at http://nadp.slh.wisc.edu/NADP/
[NPS] National Park Service. 2009. Assessing the Impact of Mercury Bioaccumulation in Cumberland Piedmont Park Units. PMIS 110144. National Park Service Annual Report.
[NPS] National Park Service. 2010. Air Quality in National Parks: 2009 Annual Performance and Progress Report. Natural Resource Report NPS/NRPC/ARD/NRR–2010/266. National Park Service, Denver, Colorado. Available at https://irma.nps.gov/DataStore/Reference/Profile/2166247
Porter, E., Blett, T., Potter, D.U., Huber, C. 2005. Protecting resources on federal lands: Implications of critical loads for atmospheric deposition of nitrogen and sulfur. BioScience 55(7): 603–612. https://doi.org/10.1641/0006-3568(2005)055[0603:PROFLI]2.0.CO;2
Sullivan, T. J. 2016. Air quality related values (AQRVs) in national parks: Effects from ozone; visibility reducing particles; and atmospheric deposition of acids, nutrients and toxics. Natural Resource Report NPS/NRSS/ARD/NRR—2016/1196. National Park Service, Fort Collins, CO.