Concentration and fractionation of heavy metals in roadside soils receiving de-icing salts

Soil was sampled along two lines of a highway, 0.5 m and 2.5 m from the asphalt surface, and in an infiltration pond for highway runoff. The study area was located in the infiltration area of the reserve water supply for a community. The concentrations of Cd, Pb and Zn in soil samples from the highway (0.5 m) and/or in the infiltration pond exceeded guideline values for less sensitive land-use with groundwater protection. The highest Pb concentration measured (542 mg kg−1) was 34 times the average Pb concentration in soils in Sweden, and exceeded the Swedish guideline value by a factor of almost two. Cadmium in the infiltration pond exceeded the guideline value almost three times. An increased concentration with soil depth for Cd, Pb, Cu, Zn and PAHs in the infiltration pond showed that downward transport had occurred. This was supported by a Pb concentration exceeding the limit for drinking water quality in the groundwater 4.5 m below the soil surface in the infiltration pond. The ESP (exchangeable sodium percentage) in some samples was high enough (10–27%) for dispersion of soil colloids to occur. The Tessierʹs sequential extraction scheme showed that Pb and Zn occurred mostly in association with the oxide bound fraction whereas Cu was mainly associated with the organic fraction, e.g. colloids. Another important fraction for Pb was the carbonate fraction. The study showed that a large part of the Pb, Cu and Zn in roadside soils is vulnerable to leaching when exposed to a high NaCl concentration, reducing conditions or to a lowering in pH. Regression analyses showed that a high concentration of Na predominately displaces Ca of the base cations from the exchange sites in the soil. The highly significant relationships observed between soil properties and chemical fractions of the metals make the result reliable for the fractions that predominate.