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.
Monitoring Dust Palliative Use Along the Denali Park Road
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.
Monitoring for Long-Term Effects
While annual monitoring of CaCl2 applications and soil water tracks the annual input of chloride into the roadside ecosystem, it is also important to periodically investigate other components of the ecosystem that may be accumulating chloride such as soils and plant tissues.
In 2015, a roadside survey of plant damages revealed correlations between species exhibiting outward signs of chloride toxicity (Figure 5) and cumulative CaCl2 application rates. Additionally, chloride measured in select spruce needle tissues surpassed known levels of toxicity (3000 ppm). However, roadside plant health appeared good along most stretches of the Park Road. In 2016, botany staff initiated soil sampling on transects perpendicular to the road to assess whether 13 years of CaCl2 application to the road surface has altered soil chemical properties. Overall, a significant accumulation of chloride in soils has not yet occurred, but sites with high application loads did tend to have higher soil chloride.
Plans for 2017
The dust palliative monitoring dataset now includes over a decade of information about chloride accumulation in soil pore water and surface water, plus measures of chloride accumulated in soils and plant tissues after 13 years of palliative use. Botany staff are currently compiling a comprehensive report to summarize findings. Results gathered will assist Park management in determining what level of CaCl2 application is sustainable over the long-term.