MASS BALANCE OF CHLORIDE FROM DEICING SALTS IN A SMALL HUDSON RIVER TRIBUTARY

Chloride derived from deicing salt is a persistent contaminant in streams of the northeastern United States and other northern localities. Elevated concentrations in summer baseflow indicate that road salt does not merely wash off of roads into storm sewers but also enters the groundwater system through splash off of impervious surfaces. We used a Yellowsprings Instruments sonde in conjunction with a Hobo water level logger to determine the flux of chloride over time in the Casper Creek, a small (18-km long, 31 km² drainage area) tributary of the Hudson River in Dutchess County, New York. Chloride concentrations ranged from 35 to 230 mg/L and stream discharge varied between 0.03 and 1.2 m³/s over a four month interval from August through November of 2007, yielding a projected annual chloride flux of 4.5*10⁵ kg in the absence of new winter deicing applications. Using GIS, we determined the total length of roads upstream of the monitoring site, which allowed us to estimate the amount of salt (8.9*10⁵ kg/yr as chloride) applied during the previous winter based on lane-mile application rates provided to us from local Department of Transportation agencies. The ratio of projected chloride from groundwater inputs to chloride applied to roadways measures 0.51. Continued monitoring through the winter will allow us to complete a mass balance to determine whether chloride concentrations appear to be increasing, decreasing, or staying the same in groundwater. Determining the trend of chloride concentrations is important for aquatic life, which can begin to show chronic impairment at concentrations as low as 250 mg/L, and may point to the need for changes in development practices toward clustered development and other factors that reduce impervious surfaces.