Air Quality at Great Sand Dunes National Park & Preserve
Most visitors expect clean air and clear views in parks. Great Sand Dunes National Park & Preserve (NP & Pres), Colorado, home to the tallest dunes in North America, is downwind of many pollution sources. Air pollutants blown into the park can harm natural and scenic resources such as soils, surface waters, plants, wildlife, and visibility. The National Park Service works to address air pollution effects at Great Sand Dunes NP & Pres, 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 Great Sand Dunes National Park and Preserve (GRSA) relative to other national parks is very high (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 GRSA relative to other national parks is very high (Sullivan et al. 2016); to search for acid-sensitive plant species, see: NPSpecies.
From 2017-2019 total N deposition in GRSA ranged from 3.0 to 4.9 kg-N ha-1 yr-1 and total S deposition ranged from 0.7 to 1.3 kg-S ha-1 yr-1 based on the TDep model (NADP, 2018). GRSA has been monitoring atmospheric N and S deposition since 1980, see the conditions and trends website for park-specific information.
Alpine ecosystem effects
Alpine environments are particularly vulnerable to large inputs of reactive nitrogen because of the sparse cover of vegetation, short growing seasons, large areas of exposed bedrock and talus, and snowmelt nutrient releases (Williams et al., 1996; Nanus et al., 2012). Approximately 5% of the land area in GRSA is alpine (~328 km2 above 1550 m). McClung et al. (2020) compared the 2015 estimated total N deposition (TDep; NADP, 2018) to the critical load of N for an increase in alpine sedge growth (alpine plant critical load = 3 kg-N ha-1yr-1) and the critical load of N for alpine soil nitrate leaching (alpine soil critical load = 10 kg-N ha-1yr-1; Bowman et al., 2012). They found that deposition exceeded the alpine plant critical load 100% of the park’s alpine area, but was below the alpine soil critical load throughout the park’s entire alpine area.
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 113 km2 (18.9%) of the land area of Great Sand Dunes National Park and Preserve.
- N deposition exceeded the 3.1 kg-N ha-1 yr-1 critical load to protect N-sensitive lichen species richness in 83.8% of the forested area.
- S deposition was below the 2.7 kg-S ha-1 yr-1 critical load to protect S-sensitive lichen species richness in every part 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, Great Sand Dunes National Park and Preserve contains:
- 3 N-sensitive tree species and 23 N-sensitive herbaceous species.
- 6 S-sensitive tree species and 20 S-sensitive herbaceous species.
Pollutants like mercury and pesticides are concerning because they are persistent and toxic in the environment. These contaminants can travel in the air thousands of miles away from the source of pollution, even depositing in protected places like national parks. In addition, while some of these harmful pollutants may be banned from use, historically contaminated sites continue to endure negative environmental consequences.
When deposited, airborne mercury and other toxic air contaminants are known to harm wildlife like birds and fish, and cause human health concerns. Many of these substances enter the food chain and accumulate in the tissue of organisms causing reduced reproductive success, impaired growth and development, and decreased survival.
- Mercury concentrations in fish sampled at Great Sand Dunes NP & Preserve did not exceed any known toxicity thresholds for fish, birds, or human consumption. Fish were analyzed for mercury from two sites at the park and compared to data across 21 western parks. The average fish mercury concentration (52.7 ng/g ww) was slightly lower than the study-wide mean (77.7 ng/g ww). However, the data may not reflect the risk at other unsampled locations in the park (Eagles-Smith et al. 2014). No fish sampled from the park were found to be intersex (Schreck and Kent 2013). Reproductive abnormalities such as intersex, the presence of both male and female reproductive structures in the same fish, can signify exposure to contaminants.
- Some dragonfly larvae sampled at Great Sand Dunes NP & Preserve had mercury concentrations at moderate or higher impairment levels. Dragonfly larvae have been sampled and analyzed for mercury from three sites in the park; 31% of the data fall into the moderate (100-300 ng/g dw) and 31% fall into the high (>300 ng/g dw) impairment categories for potential mercury risk. An index of moderate impairment or higher suggests some fish may exceed the US EPA benchmark for protection of human health (Eagles-Smith et al. 2020, Eagles-Smith et al. 2018).
- Pesticides found in park fish and water. Dieldrin (a historic-use pesticide) concentrations in 50% of cutthroat trout sampled from Sand Creek exceeded the subsistence consumption threshold established for human health. Concentrations of current-use pesticides in fish are particularly high for parks in the Rockies and Sierra Nevada, as compared to parks in Alaska and the Cascades (Flanagan Pritz et al. 2014). Pesticide concentrations in water samples from Sand Creek and Medano Creek did not exceed any known benchmarks for aquatic life (Keteles 2011).
- Airborne contaminants found in park air and vegetation samples. SOC (semi-volatile organic compound) concentrations found in Great Sand Dunes NP vegetation were generally higher than concentrations in other western parks (Landers et al. 2010; Landers et al. 2008). The dominant SOCs found in vegetation included PAHs (polycyclic aromatic hydrocarbons) and DDTs. SOC concentrations detected in air were moderate-to-high compared to other western parks.
- Mercury found in park snow samples. In 2021, the average mercury concentration found in snowpack at Music Pass was 6.72 ng/L (USGS 2021; Ingersoll et al. 2007).
Visitors come to Great Sand Dunes NP & Pres to enjoy views of impressive sand dunes against the backdrop of the Sangre de Cristo mountain range. Park vistas are sometimes obscured by haze, reducing 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 on clearest days have been documented since the 1990’s. However, no significant trends have occurred on haziest days and visibility in the park still needs improvement to reach the Clean Air Act goal of no human caused impairment.
- Reduction of the average natural visual range from about 170 miles (without the effects of pollution) to about 125 miles because of pollution at the park
- Reduction of the visual range to below 85 miles on high pollution days
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.
Over the course of a growing season, ozone can damage the leaves of plants, reducing their growth rate and making them less resistant to disease and insect infestations. Some plants are more sensitive to ozone than others. There are a number of ozone-sensitive plants in Great Sand Dunes NP & Pres including Rudbeckia laciniata (cut-leaf coneflower) and Populus tremuloides (quaking aspen). Along streams and seeps where conditions are wetter, plants may have higher ozone uptake and injury (Kohut et al. 2012). Search ozone-sensitive plant species found at Great Sand Dunes NP & Pres.
Visit the NPS air quality conditions and trends website for park-specific ozone information.
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