Long-term analysis of road salt loading and transport in a rural drinking water reservoir watershed

Abstract

Chloride exports from the widespread application of road salt serve as a primary contribution to water body salinity in cold climate regions. Road salt pollution is a concern in drinking water supplies due to the relationship between chloride concentration and water corrosivity. In the Northeastern U.S., researchers and water quality managers are interested in better understanding how management practices can mitigate the negative effects of road salt pollution. This study uses regression-based chloride load estimates from LOADEST to produce mass balances and analyze transport flow-paths to the Wachusett Reservoir from a predominantly rural watershed. A chloride mass balance for 2000 through 2019 revealed that reservoir and watershed export only recently reached a level similar to estimated inputs from road salt (11,200 ton/yr). Results from hydrograph and load separation techniques show that average loading (4300 ± 1300 ton/yr) and discharge from baseflow (3.2 ± 0.9 m3/s) contributed approximately 74% and 65% of the total loading (5800 ± 1800 ton/yr) and discharge (4.93 ± 1.50 m3/s), respectively from water years 2001 through 2019. Approximately 40% of the total annual tributary load (30% baseflow; 10% runoff) occurred in the spring (March–May) and is attributed to elevated baseflow conditions from snowmelt. As indicated by a baseflow chloride index, baseflow concentrations are 14% greater than the total concentrations measured in the tributaries. Therefore, the baseflow-dominated tributary chloride loading is due to a combination of greater discharge and chloride concentration in baseflow as compared to direct surface runoff. However, a long-term trend analysis suggests a slightly increased fraction of chloride loading via overland flow in several reservoir tributaries, potentially indicative of late 20th century land development. This study offers a unique monitoring approach for management authorities to better assess road salt pollution in water systems. The decadal response of the reservoir system reflects the slow-moving nature of the baseflow-dominated chloride loading and suggests that measurable water quality improvements will only be realized with a sustained long-term decrease in the amount of road salt applied.