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

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 c rosses the Oak Ridges Moraine in southern Ontario. Total salt applicat ions over the winter ranged from 29 to 74 kg m(-1) of highway, and NaC l concentrations in snow banks adjacent to the roadway reached 9400 mg l(-1) during the later stages of snow cover development. This salt wa s released to the ground surface during snowmelt. Sodium chloride (NaC l) loadings to soil from snow cover during the final melt phase were r elatively uniform along the study section (3-5 kg NaCl m(-1) of highwa y). However, the snowpack at all transects retained <50% of applied Na Cl, and this shortfall probably reflected direct runoff and infiltrati on of saline meltwater from the road surface into the adjacent shoulde r and right-of-way. Cation exchange with Ca2+ in near-surface soils mo st likely resulted in preferential retention of Na+ relative to Cl-, a lthough total storage of NaCl in upper soil horizons by winter's end w as <15% of deicing salt applications. An environmental tracer (O-18) w as 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(-1), and ca. 280 mm of wa ter 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( -1) and 1000 mg l(-1), respectively. Approximately 75% of the net flux of NaCl below the upper soil was retained in the 0-2.8 m depth interv al at this site, and results from more permeable soils traversed by th e highway indicate an even greater penetration of the annual NaCl appl ication into the unsaturated zone along the moraine. This saline water likely recharges groundwater in this portion of the Oak Ridges Morain e.