Many studies in the past have attempted to characterize the typical chemical concentrations of contaminants in highway runoff. Some have also attempted to develop statistical relationships between highway metrics, adjoining land use/land cover, hydrology, and resultant highway runoff quality, with inconclusive results. In general, highway runoff quality throughout the country appears to conform to similar trends:
- Highways in urban areas, which also tend to have the highest traffic densities, have significantly higher contaminant concentrations in their runoff than corresponding rural highways per unit impervious area. FHWA studies found significant statistical differences in highway runoff quality between highways with less than 30,000 ADT (mostly rural and arterials) and those greater than 30,000 (more urban). Limited highway runoff characterization studies in Washington exhibit the same trends.
- Highway runoff tends to contain heavy metals at varying concentrations. The dissolved forms of copper and zinc are toxic to fish, and are difficult to remove using most conventional, passive stormwater treatment systems.
- The relationship between pollutant loading and traffic characteristics appears to be highly nonlinear and in some situations may be highly affected by outside influences such as ambient air quality and adjacent land uses. Existing studies indicate that nutrients and metals may be significant atmospheric inputs to stormwater runoff quality.
- The long-term trend in pollutant concentrations (since the early 80’s)in highway runoff has been reduced event mean concentrations. The only exception to this trend appears to be for petroleum hydrocarbons. Recent sediment quality data indicates that the long-term trend in petroleum hydrocarbons may be correlated (on a watershed scale) with increases in automobile use, since long-established “built-out” urbanized areas are also exhibiting increases in petroleum hydrocarbons concentrations in sediments.