Washington State has begun a program to dispose of highway runoff. A priority has been given to the use of infiltration type technologies (e.g. infiltration basins, dry wells, etc.) The purpose of this study was to examine the environmental performance of a typical infiltration basin under the framework of the possible presence of facilitated heavy metal transport due to natural organic matter runoff. The goal was to establish revised guidelines for the design of an environmentally compliant infiltration facility.
The results suggest that infiltration can be a viable alternative in the disposal of runoff at low metals concentrations. This statement is, however, qualified by the need for the new siting criteria. In particular, the presence of background metals in the soil should be identified. Moreover, the organic content of the soil is likely to a better indicator of potential metal retention, and as such, should be included as a siting condition along with the cation exchange capacity (CAC) and silt and clay content. Finally, serious consideration should be given to extending the minimum depth to groundwater form the existing three value of 3 feet to 10 feet (3 meters) or more, particularly in those areas in which background metals are present. Here, geochemical controls are thought to produce effluents beneath the infiltration basins which may lead to detectable quantities (particularly copper and zinc) within underlying groundwater, wherein creating a situation that may violate the anti-degradation laws for existing State groundwater resources.