CHLORIDE STORAGE AND TRANSPORT IN A SMALL TRIBUTARY WATERSHED TO LAKE CHAMPLAIN

An unnamed tributary of Lake Champlain in the Town of Plattsburgh, NY offers an opportunity to evaluate the mechanisms of chloride storage and transport due to road salt application. The small (3 km²) watershed has no naturally-occurring sources of chloride and is a landscape with varying levels of development intensity (golf course, residential, commercial/industrial, airport, etc.) and few remaining forested areas (<16%). Several roadways and paved areas traverse the watershed including a major transportation route (U.S. Route 9) immediately upstream of a site that is monitored continuously as of Fall 2015 for temperature and electrical conductivity (EC) as a proxy for chloride. Grab sample analyses conducted for the previous three years indicate that chloride concentrations increase during winter months with peaks during spring snow melt events and decline somewhat throughout the summer and fall. However, preliminary data suggest that chlorides remain in storage along U.S. Route 9 and trigger short spikes in EC during storms. These spikes dissipate as runoff from upstream areas dilute the chloride contributed from roadside ditches. Additionally, limestones in the southern portion of the watershed can produce significant quantities of water from springs throughout the year. Many of these springs discharge into the roadside ditches and continue to mobilize chlorides during the summer and fall months. Limited data from grab sample analyses of the springs also suggest that groundwater has elevated chloride concentrations due to salt applications on roads and parking lots in areas topographically upgradient of the springs. Chlorides in groundwater and subsequent mobilization of chlorides from roadside ditches appear to sustain a minimum background concentration of approximately 60 mg/L during baseflow conditions in the summer and fall.