Steel slag, a by-product of steel industry, is known to have an alkaline nature, where pH of the virgin leachate approaches extreme values as high as 12.5–13.0. These high pH values harm aquatic life when mixed with nearby surface waters and may have long-term effects on human health along with animals and plants if it merges with groundwater. This study investigated the leaching behavior of two electric arc furnace slag (EAF-S) samples with different aging properties. In order to determine the pH of the pure EAF-S samples, water leach tests were performed in accordance with ASTM D 3987. In the next stage, the conventional water leach test was modified to evaluate the sequential leaching from slag samples through subgrade clay. A series of sequential water leach tests were performed using clays with different minerology and plasticity. The results have shown a significant decrease on the pH of the leachate when passing through the subgrade. Moreover, plasticity of the clay has a substantial effect on the leaching characteristics, emphasizing potential use of slag as embankment material in transportation geotechnics practice.