This paper presents the results of a study to assess the effects of impervious and pervious (or porous) asphalt on the quality of runoff from highways in the Netherlands. Furthemlore, the effects of settling and filtration on the quality of runoff of both types of asphalt have been elaborated. This study has been performed to support decisionmaking on how to deal with polluted runoff from highways in the Netherlands. The results show that runoff from well-maintained pervious asphalt contains a relatively low concentration of pollutants such as heavy metals, mineral oil, polynuclear aromatic hydrocarbons, and suspended solids compared to runoff from impervious asphalt. In runoff from both types of asphalt, copper, lead, and zinc are the prevailing heavy metals. The impression exists that especially the hard shoulders along highways provided with pervious asphalt act as a sink for suspended solids, soil particles, and other pollutants. To maintain its permeability and filter action, the hard shoulders should be regularly cleaned.Water treatment tests in the laboratory revealed that treatment of runoff from pervious asphalt results in lower efficiencies in the removal of heavy metals, compared with the treatment of runoff from impervious asphalt. This is probably because of the lower concentration of suspended solids and the difference in partitioning of the metals in the soluble and particle-bound phase. However, despite encouraging findings, long-term research on the continued permeability of pervious asphalt is lacking. As a result, its long-term performance with and without aggressive maintenance cannot be predicted at this time.The results of this study show that the current switch from impervious to pervious asphalt on Dutch highways has certain environmental benefits. The combination of low concentrations of pollutants, poor treatment efficiencies, and high costs makes it questionable whether additional treatment of runoff from pervious asphalt by means of settling basins or sand filtration is meaningful. Despite other positive properties of pervious asphalt such as noise abatement and less spray and skid resistance during wet weather, the rather moderate behavior of pervious asphalt with respect to slipperiness in winter should always remain an important point of attention when choosing this type of pavement. Water Environ. Res .. 71, 183 (1999).

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Road traffic is a diffuse source of heavy metals and oil that leads to pollution of verges and surface water in areas immediately surrounding roads. The Commission for Integrated Water Management (CIW) has drawn up a policy document addressing methods for managing this type of pollution. The document is based on results from numerous studies in The Netherlands targeting pollution caused by traffic. The Commission concludes that measures at the source are the only way to realise sustainable solutions. For example, attention should be devoted to the issue of zinc emissions from car tyres and crash barriers. The concept of controlled infiltration is recommended for combating pollution caused by spray and runoff from roads. This includes periodic chemical inspection of verge pollution and, where necessary, replacement of the verge's top layer. The application of porous asphalt on highways in The Netherlands has also proven highly effective in limiting pollution caused by traffic, with far less pollution caused by spray from the highway and runoff as compared to traditional asphalt.

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