EFFECTS OF ION-EXCHANGE ON STREAM SOLUTE FLUXES IN A BASIN RECEIVING HIGHWAY DEICING SALTS

At Fever Brook, a 1260-ha forested basin in central Massachusetts, hig hway deicing salt application increased the solute flux in streamflow by 120% above background flux (equivalent basis) during a 2-yr period. Attempts to isolate the nonsalt component of stream solute fluxes hav e commonly subtracted salt contributions based on the net Cl flux (Cl output in streamflow minus Cl input in precipitation). In these studie s, any net Na flux in excess of the amount needed to balance the net C l flux has been attributed to weathering. At Fever Brook, however, the net output of Na was less than the net output of Cl, suggesting a los s of Na within the basin. The Na sink was inferred to be cation exchan ge of Na for Ca and Mg in the soil. A method was developed to quantify the exchange based on a Na budget, which included an independent esti mate of the Na flux from weathering. The amount of exchange was apport ioned to Ca and Mg based on their relative concentrations in the strea m. The background fluxes of Ca and Mg (i.e., those that would occur in the absence of deicing salts) were calculated by subtracting the amou nts from ion exchange plus the much smaller direct contributions in de icing salts from the observed fluxes. Ion exchange and direct salt con tributions increased the net output fluxes of Ca and Mg, each by 44% a bove background. In basins that receive deicing salts, failure to acco unt for cation exchange thus may result in an underestimate of the flu x of Na from weathering and overestimates of the fluxes of Ca and Mg f rom weathering.