Environmental assessment of a road site built with MSWI residue

A 23-year-old road built with MSWI residue (mix of fly and bottom ashes) was investigated through the environmental assessment of the residue, soils (adjacent to and underlying the road) and seepage waters. The pH and ANC(4.3) of the material indicate a low carbonation and a high alkaline stock. The material leaches few trace elements. The underlying road soil shows significant effects with respect to K and Na, attributable to the MSWI residue. Effects regarding Cd, Cr, Cu and Ni are observed, yet pollution thresholds are not exceeded. Compared to groundwater thresholds, the MSWI residue percolate is of poor to very poor quality (French reference system - FWQAS) in terms of pH, K, Na, Cl(-), F(-), As and Cu. It lies above the intervention values of the Dutch Soil Protection Act (DSPA) for Cu, Ni and Pb. Downstream, water from the road shoulder (W(Sdr)) is of very poor quality with respect to Na and Cl(-), and fair to poor quality regarding K. All other parameters comply with the best quality thresholds (FWQAS and DSPA). The high Cl(-) concentration of W(Sdr) would not fit for direct discharge in any watercourse. Toward a weakly-mineralised water, Cd and Cu would also be penalizing. The geometry of the road site creates penalizing conditions increasing the washing of the MSWI residue. The road structure does not cause downstream effect as regards trace elements, but effects exist for Cl(-), Na and K. Opposite downstream effects are observed (Ca, Mg, Mn and SO(4)(2-)) due to farming treatments and the nature of the local soil.