Migration of contaminants associated with pavement construction
In order to achieve the goals of 'Sustainable Development', alternative (secondary) materials are being increasingly used as bulk-fill aggregate within pavement construction as substitutes for traditional aggregates. This finds an end-use for stockpiles of industrial by-products (and hence the allowing the land on which they stand to be reclaimed for other uses) and protects finite, natural resources which they replace from over-extraction. Previously, there has been very little research concerned with the leaching of contaminants from alternative materials in pavement construction and the subsequent risks to water bodies from pavement drainage. It is this topic which is addressed here. Two flow regimes within a pavement have been studied in order to predict contaminant movement: (1) vertical flow through the aggregate and pavement and then vertically through the natural subgrade to ground water below and (2) horizontal flow through the aggregate to be discharged through pavement sides drains. Using these analyses a generic user-friendly risk assessment guide by which contractors may assess an aggregate prior to use is presented. A case-study is provided to illustrate some of the issues of concern. Guidelines in the risk assessment guide recommend the suitability of different physical parameters of a potential aggregate at a proposed pavement construction site for both water flow directions. If the subgrade at the site does not allow sorption by the soil to enable any contaminants in vertical flow to be below Water Quality Standards (WQS), the use of a geotextile clay liner to further increase sorption is recommended. If the concentrations of contaminants in water discharged from side drains is not below WQS for horizontal flow, guidelines determine whether the site rainfall and surface runoff allow sufficient dilution. In most situations alternative materials appear to be acceptable for use if pavement construction is on clay subgrades, with an exception of sites where the subgrades are shallower than those recommended or where they are close to areas of higher sensitivity, such as those in close proximity to protected groundwater.