A Novel Secondary Treatment Step for Tunnel Wash Water: Impact on Retention and Toxicity of Tire and Road Wear Particles, Tire Leachates, and Metals

Summary

Abstract Urban expansion and increasing vehicular traffic have significantly intensified road related pollution in recent decades. Among the emerging contributors to environmental degradation are tire and road wear particles (TRWPs) and tire-derived chemicals (TDCs), which represent a growing source of microplastic and chemical contamination. These pollutants enter ecosystems through air deposition and surface runoff. A particularly concentrated source is tunnel wash water (TWW), generated during the maintenance of road tunnels. In Norway, this water is frequently discharged into adjacent rivers and fjords without adequate treatment. This study aimed to investigate the impacts a novel secondary treatment step for TWW may have on retention and toxicity of TRWPs, TDCs and metals taking into account the contaminant fluxes and seasonal variations by using the Ekeberg Tunnel as a case. A two-step treatment system, consisting of a 21-day sedimentation phase followed by filtration with Leca Filtralite HMR, was evaluated by testing the effectiveness of sedimentation alone and in combination with filtration for reducing key contaminants. Fieldwork was carried out during spring and autumn of 2024 to capture seasonal differences, with samples collected before and after each treatment stage. Metal samples were filtered through a 0.45 µm membrane to distinguish between particle bound and dissolved fractions. All samples were subsequently analyzed for metals, TRWPs, and TDCs using Agilent 8800 ICP-MS QQQ, Py-GC/MS, and UPLC-TOF MS. A 14 day leaching test also evaluated metal release from particles retained before treatment. Acute toxicity was assessed using Daphnia magna at each TWW treatment stage, including detergent only exposures. Additional measurements included dissolved organic carbon, anions, and other water quality parameters. Results revealed that TWW contains a complex, seasonally variable contaminant mix. Higher concentrations of TRWP, TDCs and metals were observed in spring, due to winter accumulation. The treatment system effectively reduced particle bound metals (Al, Fe, Cu, Pb), TRWP and some TDCs like 6PPD and DPPD. However, dissolved metals (Zn, Mo, As) and water soluble TDCs such as HMMM, TMQ, and MTBT were not effectively removed, with some increasing post treatment. Biological assays showed that D. magna ingested particles in all treatments, but acute toxicity was observed only in untreated autumn samples at a 50% sample concentration. Microscopic analysis revealed that particles adhered to ii appendages and exoskeletons, causing immobilization through physical obstruction. The leaching test confirmed that retained TWW particles also release metals such as iron (30.0 µg/L), zinc (9.32 µg/L), and copper (2.00 µg/L) over time. This study confirms that TWW from the Ekeberg Tunnel poses considerable environmental risks, particularly during the spring season. While the two step treatment system effectively reduced coarse, particle bound pollutants, it was less capable of removing some dissolved metals and water soluble TDCs. The combination of high concentrations of untreated TRWPs, TDCs, and metals along with evidence of particle ingestion, mechanical interference, and gradual metal leaching, raises concerns for aquatic life and water quality. These findings highlight the importance of effective treatment in reducing environmental risks, while also showing the need to improve the removal of finer particles and certain dissolved pollutants. Ongoing monitoring is essential to protect sensitive ecosystems like the Oslofjord.