ROAD SALT ACCUMULATION IN HIGHWAY SNOW BANKS AND TRANSPORT THROUGH THE UNSATURATED ZONE OF THE OAK RIDGES MORAINE, SOUTHERN ONTARIO

摘要

Pathways and fate of road deicing salt (NaCl) applied during the 1994–1995 winter were studied for a 14-km section of a major highway that crosses the Oak Ridges Moraine in southern Ontario. Total salt applications over the winter ranged from 29 to 74 kg m of highway, and NaCl concentrations in snow banks adjacent to the roadway reached 9400 mg l during the later stages of snow cover development. This salt was released to the ground surface during snowmelt. Sodium chloride (NaCl) loadings to soil from snow cover during the final melt phase were relatively uniform along the study section (3–5 kg NaCl m of highway). However, the snowpack at all transects retained <50% of applied NaCl, and this shortfall probably reflected direct runoff and infiltration of saline meltwater from the road surface into the adjacent shoulder and right-of-way. Cation exchange with Ca in near-surface soils most likely resulted in preferential retention of Na relative to Cl, although total storage of NaCl in upper soil horizons by winter's end was <15% of deicing salt applications. An environmental tracer (O) was used to trace movement of saline meltwater through the unsaturated zone underlying the highway. Average meltwater particle velocities at a site underlain by loam soils were 0.02 m d, and 280 mm of water was displaced below a depth of 1.86 m over a 78-day period in the spring and summer of 1995. Sodium ion and chloride ion concentrations in water sampled in late summer 1995 at depths >2 m exceeded 500 mg l and 1000 mg l, respectively. Approximately 75% of the net flux of NaCl below the upper soil was retained in the 0–2.8 m depth interval at this site, and results from more permeable soils traversed by the highway indicate an even greater penetration of the annual NaCl application into the unsaturated zone along the moraine. This saline water likely recharges groundwater in this portion of the Oak Ridges Moraine.