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An evaluation methodology was developed to help transportation agencies make prudent decisions about the reuse of waste materials and by-products in highway construction. The primary objective of this evaluation process was to assess the potential impact on surface water and groundwater of constituents released from these materials. Test materials included coal fly ash, bottom ash, blast furnace slags, scrap tires, foundry sand, and recycled asphalt pavement. Test materials were screened for potential aquatic impact in their raw form. Then they were amended with either asphalt or aggregate for detailed assessment, including leaching (batch and flat plate or column tests) and environmental removal, reduction, and retardation (soil sorption, volatilization, photolysis, and biodegradation) tests. Short-term bioassays were used to measure directly the potential aquatic impact of derived leachates as a supplement to chemical analyses. For most materials, amendment with asphalt or aggregate largely reduced or eliminated any impact on the aquatic organisms. Among environmental factors, soil sorption appeared to be the most effective contaminant-removal mechanism. All waste-amended asphalts and aggregates exhibited no measurable impact on target organisms after soil sorption. However, for materials such as pressure-treated wood and deck sealer (used as is), environmental tests showed little or no effect in reducing their aquatic impact.

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The 6-year NCHRP Project 25-9, titled Environmental Impact of Construction and Repair Materials on Surface and Ground Waters, concluded in March 2000. The main purpose of the study was to develop and demonstrate a methodology for identification of the possible impact of highway construction and repair (C&R) materials on the quality of surface water and groundwater in the immediate highway environment. The evaluation methodology consisted of an initial screening of a raw (unamended) C&R material leachate for aquatic toxicity by using bioassay tests and of further detailed testing and evaluation of those materials identified as potentially toxic. In the detailed evaluation, leaching source term tests were conducted to determine leachate composition under varying environmental conditions. Environmental tests were then conducted on the leachate to determine the effects of removal, reduction, and retardation (RRR) processes in the highway environment. The environmental processes included photolysis, volatilization, biodegradation, and soil sorption. The leaching and RRR tests identified interactions between specific environmental processes and leachate contaminants and yielded empirically derived functions for each process. The functions were coupled with selected reference environments in a computer model to simulate fate and transport for surface and subsurface pathways. Model results were reported as concentrations, loads, or fluxes of contaminants as a function of time at the highway boundary. Complete evaluation methodology results are presented for a standard asphalt cement concrete developed to serve as a reference asphalt concrete for interlaboratory comparisons.

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