Washington State, like many others, has begun a program to dispose of highway runoff in which a priority has been given to the use of infiltration technologies (e.g., infiltration basins, dry wells, etc.). Heavy metals are of particular interest, because they are the most prevalent priority pollutant in highway runoff. Though traditionally these heavy metals are thought to possess relatively large retardation factors within the subsurface, previous studies have indicated a possibility of their transport being facilitated through complexation with natural organic matter (NOM). If ignored, this form of transport may pose serious errors in the prediction of metal breakthrough times and mass loadings to underlying groundwaters. In addressing this issue, large-scale soil columns are tested under loading conditions similar to actual infiltration best management practices to assess the performance of these facilities in removing heavy metals (Cd, Cu, Pb, Zn) from infiltrating water. Experiments were performed investigating the effects of high and low concentrations of NOM on metals attenuation by field soils. Results show that high concentrations of NOM actually improved soil column attenuation of the metals.
An evaluation was conducted of the performance of three Washington State soils in attenuating trace heavy metals (Cd, Cu, Pb, and Zn) in contaminated water. Although previous investigations of this kind have used relatively small-scale soil columns as indicators of actual performance, large-scale soil columns (0.3 m in diameter and 1 m deep) were tested under loading conditions similar to actual field infiltration basins. The soil columns were loaded periodically to produce quasi-field conditions, with the corresponding metal transport being observed as a function of depth. The data indicate that metals speciation and background metals present in the soil are important factors to consider, because they can affect expected metals removal rates of soils. The hydraulics of the system played a secondary role to the geochemistry. Results suggest that infiltration can be a viable alternative in disposing of runoff at low metals concentrations, if the soil-water system's assimilative capacity is not taken for granted.
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