STORAGE AND RELEASE OF ROAD-SALT CONTAMINATION FROM A CALCAREOUS LAKE-BASIN FEN, STOCKBRIDGE, MASSACHUSETTS, USA

Road salt applications to highways have caused steady increases in sodium and chloride concentrations of streams due to retention of this pollution within watersheds. However the mechanisms for retention and export of Na and Cl from different environments are not fully understood. This field study examines the hydrologic and cation exchange processes that store and release Na and Cl from a calcareous fen, whose chemistry has been significantly altered by road salt runoff from a highway. Average groundwater concentrations of dissolved salts are greatest in the upper 2m of peat in the fen and extend more than 600m from the highway, suggesting that groundwater transport is a near-surface process. Despite alkaline geochemistry, high road salt concentrations enable Na to occupy up to 15% of the cation exchange capacity of shallow peat in the more contaminated regions of the open fen. Calculations of selectivity coefficients show that Na preferentially exchanges with Mg, and Na can be desorbed and released under more dilute conditions caused by precipitation and snowmelt.

Detailed sampling of surface and ground waters during three snowmelt events illustrate early releases of Na and Cl from shallow groundwater at the onset of melting, with maximum fluxes of ions coinciding with peak discharge. The flux of dissolved salts exiting the wetland during one snowmelt event accounts for 13% (Na) and 17% (Cl) of rock salt applied to the highway during that year. The mass of exported Na and Cl is proportional to the number of lane miles crossing the watershed. During 2005, the annual flux estimates of Na and Cl are on par with the amount of road salt applied, with more than half of the annual flux occurring during the months of March, April and May. Subsequent large rain events disrupted prior apparent equilibrium conditions, causing Na to desorb from cation exchange surfaces and increase net export of Na and Cl from the fen. Large rain events outside of winter months are more effective than snowmelt with reduction of dissolved salts because snowmelt also introduces contamination. For this and other wetlands having alkaline geochemistry and high flushing rates, management strategies that reduce rock salt amounts to roadways will assist with reducing contamination to levels that may be less toxic to vegetation and aquatic organisms.