The Hydraulic and Geo-Environmental Behavior of the Sub-Pavement Environment Andrew Dawson 1 , Annette Hill 2 and Philip Boothroyd 1 Abstract The paper considers the environmental properties and possible impacts of a wide range of candidate non-geologic, i.e. “alternative”, materials for pavement construction. Previous studies had shown that, depending on the material, they were likely to perform adequately from a mechanical point-of-view when used as-is, lightly stabilized and self- cemented. Thus the materials were subjected, in these conditions, to leaching testing (using two laboratory-based methods) and to outdoor lysimeter tests over a 1 year period at a test site. Outline results of observed performance are reported and the results compared and interpreted. The data was then used in a geo-environmental groundwater flow study, together with information on pavement run-off and percolation, in order to predict the mobility of both the run-off and the leached species in the sub-pavement environment. For the studies made, it is shown: that concerns about large scale pollution from leachate are unfounded; that, where there is excessive leaching from a contaminated base or sub-base layer ‘source’, the immediate pavement environment rapidly attenuates contaminant levels to acceptable values, and that, for most species, run-off should be of much greater concern than leaching from the pavement construction materials. Introduction There are increasing pressures for engineers to employ unconventional materials, e.g. slag, ash, construction and demolition waste, scrap tire, etc. as replacements for conventional crushed stone base and sub-base materials. Such materials have an undeniable environmental benefit in reducing despoliation of the environment by reducing quarrying and landfill, but are increasingly giving rise to concerns that their use in pavements may provide a disperse contaminant source which could lead to widespread pollution of ground and surface waters. At the same time, the pollution deposited on the pavement surface and migrating into the pavement construction is becoming a subject about which engineers are becoming more and more aware. Therefore studies have been performed by laboratory and outdoor tests to evaluate likely leaching of contaminants from such materials and by analytical modeling to predict the pathway and destination of contaminant fluxes from these sources. Contaminants in the Highway Environment In order to predict the impact of alternative aggregates on groundwater it is necessary to understand the sources, transport and fate of contaminants in the pavement subsurface. 1 Nottingham Centre for Pavement Engineering, University of Nottingham, University Park, Nottingham, UK. First named is corresponding author: andrew.dawson@nottingham.ac.uk 2 Formerly University of Nottingham. Now Scott Wilson, Chesterfield, UK