USING NA/CL MOLAR RATIO FOR ESTIMATING SODIUM RETENTION IN ARTIFICIALLY SALTED WATERSHEDS

Based on the relationship between Na/Cl molar ratio and sodium retention determined by soil column chromatographic experiments, PHREEQC computer simulations, and field data analysis, an alternate method for estimating sodium retention in a watershed is presented. The underlying concept on which this alternate method is based is the observation that sodium tends to be adsorbed more than chloride by soil colloids due to its greater cation exchange potential. Therefore, the Na/Cl molar ratio in river water from an artificially salted watershed, where sodium and chloride are adsorbed unequally, is typically less than 1.0. The more sodium is adsorbed preferentially, the lower the Na/Cl molar ratio. This ratio tends to be low during the initial salting period (January to March in eastern US) and increases after the bulk of the salt is flushed out of the watershed during the summer, fall, and early winter. Watershed soil cation exchange capacity is the major factor that determines the Na/Cl molar ratio. The annual sodium retention rate over the last two decades, based on this new method, is estimated to be between 30% and 40% in both the Mohawk River Watershed, NY and the Delaware River Watershed in NY, PA, and NJ. These values are comparable to the results from previous sodium retention studies, which used the traditional budget analysis method. The relatively high rates of sodium retention determined in this study speak to the difficulty of successfully instituting any environmental remediation policy that intends to quickly reduce sodium concentrations in a watershed.