Monitoring Dust Palliative Use Along the Denali Park Road

By Sarah Stehn (last updated August 2023)

Figure 1 - Dust clouds created by vehicular traffic may alter the visitor experience and allows transport of fine sediments into the native vegetation. NPS Photo / Rob Parenti

To reduce road dust created by vehicular traffic, Park maintenance crews apply an aqueous solution of calcium chloride (CaCl2) to the surface of the Park road. This application reduces visually displeasing dust clouds and the subsequent need to replace fine materials lost from the road as dust (Figure 1), but also has the potential to adversely affect ecosystems adjacent to the road. For example, the addition of CaCl2 in high quantity has the ability to alter soil chemistry and subsequently the naturally occurring balance of ions available for plants and microorganisms.

Figure 2 - Staff collecting soil water from a soil lysimeter buried along the park road. These white bottles are left out to collect sample material overnight 1-3 nights of the summer. During the rest of the year, no evidence of the soil lysimeters or monitoring is visible from the park road. NPS Photo / Tamás Szerényi

Botany program staff has thus developed a plan to assess and monitor the possible effects of CaCl2 applications on roadside soils, water, and vegetation. In 2005, staff installed 15 pairs of soil lysimeters (instruments designed to sample water from within the topsoil) – for each pair, one lysimeter was buried near the road and one 10 meters away. Water samples are collected annually from lysimeters (Figure 2) and nearby water bodies to assess whether potentially toxic levels of chloride in solution are being reached and determine how far from the road effects of CaCl2 application can be detected. Maintenance staff record the application rate and mileage of each CaCl2 application event, allowing for comparison of application rate and measured concentrations.

The need for CaCl2 application varies by year based on weather and road conditions, but overall application, both in terms of total mileage and concentration, has decreased over time, as maintenance personnel have learned the most effective times and concentrations at which to apply the compound (Figure 3). In general, water gathered from soil lysimeters at road mileages with high application rates returned higher chloride concentrations both 1m and 10m from the road (Figure 4), suggesting that chloride does indeed move away from the roadbed and into the native vegetation. Because an accumulation of CaCl2 over many years may be what affects the roadside biota the most, monitoring will continue as long as applications do.

Figure 3 - Application of CaCl2 across years. Bars show total miles affected by application and the proportion of those miles receiving different concentrations of CaCl2 (lbs./yd2). Line shows the average application rate per year across all miles in which CaCl2 was applied.

Figure 4 - Comparison of the cumulative CaCl2 applications (left axis: lbs. /yd2; 2003-2016) and average measured chloride concentrations (right axis: mg/L; 2005-2016) at 15 soil lysimeter stations.

Figure 5 – A roadside willow exhibiting signs of chloride toxicity.

Monitoring for Long-Term Effects

The dust palliative monitoring dataset now includes two decades of information about chloride accumulation in soil and surface waters, and periodic measures of chloride accumulated in soils and plant tissues. In general, the spatial and temporal patterns of chloride in soil solution reflect patterns of palliative application, with high concentrations occurring where applications of dust palliative are high.

Collection of this data helped define the unique background levels of 2 ppm chloride in soil pore water, 5 ppm in surface water, 40 ppm in soils, and 2000 ppm in spruce needle tissue in our subarctic study area. Importantly, monitoring data also indicates that there is a 2-4 year lag time between when applications occur, and when sampling detects it (Stehn and Roland 2018).

Observations of dust palliative effects on the roadside environment warrant a continued program of monitoring. Records of application events supplied by roads staff are critical to effective monitoring, such that Park managers can estimate when more application may cause segments of the roadside environment to exhibit symptoms of chloride toxicity.

For more information:

Stehn, S. E. and C. A. Roland. 2018. Effects of dust palliative use on roadside soils, vegetation, and water resources (2003–2016): Denali Park Road, Denali National Park, Alaska. Natural Resource Report NPS/DENA/NRR—2018/1580. National Park Service, Fort Collins, Colorado.

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