John E. Tobiason
Chloride exports from the widespread application of road salt serve as a primary contribution to water body salinity in cold climate regions. The detrimental effects of road salt pollution on water quality include an apparent relationship between chloride and water corrosivity. This study utilizes regression based chloride load estimation to support mass balances and analyze transport flow-paths to the Wachusett Reservoir from its predominantly rural watershed. Results from hydrograph and load separation techniques show that average loading and discharge from baseflow contributed approximately 74% and 65% of the total loading and discharge, respectively. The baseflow dominated tributary chloride loading is not only a result of predominantly baseflow sustained discharge, but is also attributed to enhanced groundwater chloride concentrations that result from road salt transport favoring sub-surface infiltration. However, a long-term trend analysis suggests a slight shift towards an increased fraction for chloride loading via overland flow, perhaps as a result of late 20th century land development. Load estimation with LOADEST was generally insensitive to a variety of hydrograph separation methods, yet yearly estimates from LOADEST offered similar accuracy to those from simple linear interpolation. A chloride mass balance performed for 2000-2019 revealed that reservoir and watershed export only recently reached a level similar to inputs from road salt over the 20-year period of study. The decadal response of the reservoir system reflects the slow-moving nature of the baseflow dominated chloride loading and implies that decreased road salt application will not likely be reflected in near-term outflowing reservoir water quality. It is recommended that current and future corrosion control practices for drinking water treatment should consider chloride persistence in the reservoir.