ROAD SALT RESIDENCE TIME IN A LOCAL WATERSHED

Deicing of highways, roads, and parking lots can contribute large amounts of chloride to watersheds. The timing of road salt application relative to thaw events and spring rains is critical to understanding the potential effect of salt on aquatic ecology. Also important is determining whether the watershed flushes the chloride or accumulates it from year to year.

In this study, a wetland system in northeast Illinois was monitored for five years by collecting continuous (hourly) specific conductivity readings at key stations, supplemented by occasional hand measurements at many additional locations. The wetlands receive runoff from parking lots, road, forested areas, and a highway cloverleaf. In addition to the conductivity measurements, water samples were collected at various times and locations corresponding to a wide range of conductivity measurements and analyzed for chloride content. These were strongly correlated to the conductivity values (R²=0.99) and allowed the numerous conductivity readings to be used to represent chloride concentration with confidence.

The study demonstrated the value of numerous, quick, and inexpensive continuous and manual water conductivity measurements for assessing the road salt load of waters within and entering the wetland basin. The data demonstrated that the chloride concentrations at individual measurement stations fluctuate wildly throughout the year, with large increases during the salting season, and sudden, but brief, decreases resulting from winter, spring, or summer rain. The datalso show also show that winter salting has a prolonged effect on the chloride concentrations in the wetland, as chloride concentrations had an overall downward trend during the spring growth season and summer, yet remained above background levels until July. Such observations are significant in evaluating the effects of the salt on the wetland ecology, but they would not have been possible with a traditional water sampling program.