EFFECT OF WINTER DEICING SALT MIXTURES OF CaCl2 AND NaCl ON WATERSHED SODIUM RETENTION

Soil column chromatographic experiments and PHREEQC computer simulations of the injection of 0.125 to 1.0 mole solutions of CaCl₂ and NaCl, at weight ratio of 6:4, into approximately 400 grams of soil were conducted in order to investigate their effect on watershed sodium retention at this mixing ratio mandated by the State of Rhode Island for winter deicing salt applications. Our study results show that the amount of soil sodium retention depends on the total concentration of the mixed salt injection. For high concentrations of injected mixed salt, there are only insignificant amounts of sodium retention due to the higher affinity of calcium for cation exchange with soil colloids. However, for low concentrations of injected mixed salt, such as 0.125 mole solutions, there will be significant sodium retention because calcium cannot saturate all soil cation exchange sites. This effect is more apparent if the soil has a high cation exchange capacity, such as found in organic-rich soils. These results help explain why there has not been a significant reduction in sodium concentrations for Rhode Island river water, even after 20 years of application of mixed salt with a state-madated CaCl₂ and NaCl ratio of 6:4. Other than the possibility of areas immediately adjacent to a salted road, watershed soil colloid cation exchange sites are rarely saturated by the amount of salt typically applied. Therefore, sodium will be retained during the salting period and released into river water at a later time in response to changes in relative cation concentrations, similar to the sodium behavior observed in a NaCl salt-only application.