RESUMO
Road salt (NaCl) applications to highways have increased stream sodium and chloride concentrations due to retention within watersheds. 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 adjacent to a highway. Despite high concentrations of Ca(2+) and Mg(2+), elevated salt concentrations enable Na(+) to occupy up to 15% of the cation exchange capacity of shallow peat. Calculations of selectivity coefficients show that Na(+) preferentially exchanges with Mg(2+), and Na(+) can be desorbed 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(-) at the onset of melting, with maximum fluxes coinciding with peak discharge. From 7 March through 4 April 2005, the flux of dissolved salt exiting the wetland amounts to 13% (Na) and 17% (Cl) of annual rock salt applied to the highway. For all of 2005, the total salt mass leaving the wetland via Kampoosa Brook is similar to the amount of road salt applied; 50% of the annual salt efflux occurred during the snowmelt season of March through May. In general, exported Na(+) and Cl(-) correlate with the number of lane miles of highway crossing the watershed. Large rain events outside of winter months are more effective than snowmelt with reducing 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 salt contamination to levels less toxic to vegetation and aquatic organisms.