Fate and transport of deicing materials in an unconfined roadside aquifer
Inclement weather during winter months requires the roadway application of deicing materials to maintain safe driving conditions. Concerns regarding the proven, negative impacts of NaCl on the environment, motor vehicles, and roadway infrastructure have led to the development of alternative, environmental-friendly deicing agents, such as calcium magnesium acetate (CMA). This project investigated the subsurface fate and transport of CMA and NaCl deicing materials and their respective impact on groundwater quality in an unconfined, homogeneous, roadside aquifer. The influences of different highway drainage practices on deicing ion fate and transport were also investigated.
A variety of multi-disciplinary techniques were employed to characterize the aquifer hydraulic properties, and a detailed matrix of wells was installed to monitor the groundwater water quality and horizontal progression of the contaminant plumes. In situ vertical profiles of groundwater samples were obtained to monitor temporal variations in the vertical contaminant plume morphology. The groundwater flow code MODFLOW was coupled to the solute transport code MT3D to simulate the groundwater fate and transport of the various deicing ions from July 1997 through September 1998.
Hydraulic model results indicated that focused recharge from closed highway drainage systems creates a larger vertical interval of contaminated groundwater than results from roadside drainage onto highway shoulder areas. The subsequent contaminant plume beneath the focused recharge area is also more concentrated than the plume beneath the highway shoulders. Both drainage systems resulted in sodium concentrations exceeding the drinking water MCL, while only the focused drainage system resulted in chloride concentrations exceeding the secondary drinking water guidelines. Transport model results and field data indicated that deicing cation transport was conservative through the field site, and that aerobic acetate degradation was limited by groundwater dissolved oxygen (DO) content. Available groundwater DO beneath the focused recharge area was rapidly consumed, resulting in anoxic groundwater conditions and cessation of aerobic biodegradation. Levels of iron(II) were observed to increase in response to the anoxic conditions, producing groundwater total iron concentrations that exceeded the drinking water MCL. This phenomenon was only observed where CMA was applied, suggesting that CMA, as well as NaCl, may negatively impair groundwater quality.
0543: Civil engineering