Tunnel wash water in a cold climate: characteristics, ecotoxicological risk, and effect of sedimentation

Abstract

The characterization of tunnel wash water (TWW) from 12 Norwegian tunnels showed very high concentrations of total suspended solids (TSS), metals, and polycyclic aromatic hydrocarbons (PAHs). The particulate matter is dominated by iron, aluminium, manganese, and zinc, where iron and aluminium hydroxides account for about 40% of the TSS. But the concentrations of metals such as copper, zinc, and cadmium cause the TWWs studied to be classified as having poor water quality, using environmental quality standards. Metals in the particulate fraction are removed effectively by sedimentation. However, those responsible for the classification of bad water quality seem to be present to a greater extent in small particles or colloids and are removed to a lesser extent from the particulate fraction. No removal of metals from the dissolved fraction by sedimentation was found. However, PAHs were removed more significantly from the dissolved phase than from the particulate phase. It is evident from the results that PAHs with four to six aromatic rings preferentially adsorb to particles during the sedimentation process, while smaller PAHs, with fewer rings, stay in the dissolved phase. Finally, a closer evaluation of secondary treatment of TWW, regarding effectivity and economic efficiency, is recommended.