Percolation and Contaminant Transport in Road Embankments with and without Technical Safeguards to Reduce Percolation – Lysimeter Experiments

Abstract

Alternative materials, like recycling materials, industrial by-products and soils already influenced by human activities (e.g. urban soils) are commonly accepted as building material in earthwork constructions. The risk of contaminant leaching from these materials can be limited by reducing the amount of percolating water. Technical safeguards are means to achieve this, but they must be adapted to the requirements of road construction. The research project aims to better knowledge of the percolation and leaching processes in road embankments constructed from alternative materials with different technical safeguards.The input and behavior of water and contaminants from re-used soils and road runoff is measured in eight field lysimeters at an urban road in an enterprise zone in Augsburg, southern Germany with about 9,700 vehicles per day. Each lysimeter contains the cut out of an embankment. The technical safeguards aim to decrease percolation and include a capillary barrier / drainage layer from geotextile and a second one from gravel, a bituminous film of 1kg/m2 and a modified mineral sealing of 10cm.A sandy gravel and a loamy soil were each used for construction of the embankment cut out in four lysimeters. The soils were spiked with a low contamination of Cd, Cu and CN- to test the efficiency of the technical safeguards. In the percolate water of the lysimeters as well as in the road runoff the pH value, electrical conductivity and the concentrations of sulphate, chloride, As, Pb, CN-, Cr, Cu, Hg, Ni, Mo, Sb, Tl, V and Zn have been analyzed in two-week composite samples since July of 2010. The amount of road runoff, lysimeter runoff and percolating water with its electrical conductivity are measured continuously. It has to be noted that measures were taken during a testing period in order to solve imperfections in the test set-up. These measures will have affected the observations presented.Results show that the unfiltered road runoff shows considerable concentrations of Zn and also elevated concentrations of Pb, as expected from former experience and literature. Up to now, all embankment types lead to a strong retention of these two substances. However, elevated Zn and Pb concentrations can be found in the percolating water during times of high chloride concentrations originating from deicing salt. Furthermore it was shown that the average contaminant leaching of each lysimeter was relatively small during the measurement period.Changes in concentrations of other parameters seem to be induced mainly by high chloride content of the infiltrating road runoff, but the time of occurrence of the changes in the effluent as well as the temporal variations depend also on the soil type, soil pH and the technical safeguards. Additionally, it seems that the soil type has a stronger influence on the percolation process compared to the different technical safeguards. Possible reasons will be discussed.