The impact of road de-icing salts (NaCl) on colloid dispersion and base cation pools in roadside soils

The maintenance of safe-driving conditions in snow and ice-affected areas in the wintertime includes the use of sodium chloride (NaCl) as de-icing salts. In this study, the impact of NaCl on soil-colloid mobilisation and exchangeable base-cation leaching has been evaluated. The chemistry of groundwater samples below an infiltration trench for highway runoff and leachate from column studies suggested that soil-colloid mobilisation had occurred, as the exchangeable sodium (Na) concentration and the electrical conductivity (EC) in the groundwater/column leachate reached the threshold values for colloid dispersion. Generally, samples with no dispersion problems had high Na and calcium (Ca) concentrations, suggesting that the initial effect of the de-icing salt was to stabilise the colloids. In the column study there was a good agreement between the degree of colloid dipersion problems and lead (Pb) concentration when the pH value was above 7.0. Significant negative correlations between Na/CEC (cation exchange capacity) and Ca/CEC in roadside soils from three sites indicated that Na preferentially displaces Ca from the exchange sites. However, the groundwater data indicated that Na ions also displace potassium (K) and magnesium (Mg). A positive effect of NaCl seen at one site was an increase in the K concentration, which is highly likely an effect of Na ions displacing fixed K between the layers of 2:1 type clay minerals. In soils lacking these types of clay minerals, severe K shortage may result from a high plant demand combined with the low K concentration in the readily available fractions in the original soil and a high susceptibility to leaching. The most significant impact on soil exchange processes was found to occur within 6 m from the road.