Degradation of deicing chemicals affects the natural redox system in airfield soils

Abstract

During winter operations at airports, large amounts of organic deicing chemicals (DIC) accumulate beside the runways and infiltrate into the soil during spring. To study the transport and degradation of DIC in the unsaturated zone, eight undisturbed soil cores were retrieved at Oslo airport, Norway, and installed as lysimeters at a nearby field site. Before snowmelt in 2010 and 2011, snow amended with a mix of the DICs propylene glycol (PG) and formate as well as bromide as conservative tracer was applied. Water samples were collected and analyzed until summer 2012. Water flow and solute transport varied considerably among the lysimeters but also temporally between 2010 and 2011. High infiltration rates during snowmelt resulted in the discharge of up to 51 and 82% PG in 2010 and 2011, respectively. The discharge of formate remained comparatively low, indicating its favored degradation even at freezing temperatures compared with PG. Manganese (Mn) and iron (Fe) were observed in the drainage in autumn owing to the anaerobic degradation of residual PG during summer. Our findings suggest that upper boundary conditions, i.e., snow cover and infiltration rate, and the extent of preferential flowpaths, control water flow and solute transport of bromide and PG during snowmelt. PG may therefore locally reach deeper soil regions where it may pose a risk for groundwater. In the long term, the use of DIC furthermore causes the depletion of potential electron acceptors and the transport of considerable amounts of Fe and Mn. To avoid an overload of the unsaturated zone with DIC and to maintain the natural redox system, the development of suitable remediation techniques is required.

Constraints of propylene glycol degradation at low temperatures and saturated flow conditions

“…PG containing surface runoff can endanger aquatic life in surface water due to the creation of anaerobic conditions (CCME 2007;Corsi et al 2006). Furthermore, large quantities of residual ADFs accumulate together with runway deicing fluids (containing formate, FO) in snow next to runways, where they pose a risk for the soil system and groundwater upon onset of snowmelt in spring (French et al 2001;Lissner et al 2013).…”

“…Although PG is readily degraded by common aerobic and anaerobic bacteria (Toscano et al 2012;Veltman et al 1998;Willetts 1979), research on the temperature dependency of PG degradation has shown that the metabolic activity of PG degraders is severely inhibited by low temperatures (Jaesche et al 2006;Klecka et al 1993;Revitt and Worrall 2003). By contrast, FO degradation occurs quickly, even at −2°C, and a relatively shallow unsaturated zone is sufficient for complete FO degradation and groundwater protection (Hellsten and Nysten 2003;Hellsten et al 2005;Lissner et al 2013). Thus, the rapid degradation of FO during melt water infiltration and high soil water saturation favors the development of anaerobic conditions, which might additionally decrease the degradation rate of PG .…”

“…Consequently, the rapid transport of PG, e.g., along preferential flow paths, can pose a risk for groundwater (Cancilla et al 1998;Lissner et al 2013;Schotanus et al 2012;Wehrer et al 2013). The faster degradation of FO compared to PG at freezing temperatures also implies potential differences between the microbial consortium developing during PG and FO degradation, respectively.…”

“…After spring, increasing soil temperatures accelerate the degradation of residual PG, and available oxygen (O 2 ) is locally depleted (French et al 2001;Lissner et al 2013). Consequently, Fe and Mn oxides are used as alternative electron acceptors, and reduced Mn (II) and Fe(II) species occur in the pore water (French et al 2001;Jaesche et al 2006;Lissner et al 2013;Schotanus et al 2013).…”

“…Consequently, Fe and Mn oxides are used as alternative electron acceptors, and reduced Mn (II) and Fe(II) species occur in the pore water (French et al 2001;Jaesche et al 2006;Lissner et al 2013;Schotanus et al 2013). The overall redox reaction for PG and FO degradation is shown in Table 1.…”