Ecotoxicological effects of short-term exposure to elongated tunnel particles from two road construction sites on juvenile Atlantic salmon (Salmo salar)

Tunnelling through rock to facilitate transportation and logistics is common worldwide. However, the negative impacts of discharge emissions from tunnel construction on aquatic environments are not well understood. This study aimed to increase knowledge about the ecotoxicological effects of particle discharge from two tunnel construction sites (Rafoss and Verket) in Norway on juvenile Atlantic salmon (Salmo salar). Fish were exposed to environmentally relevant (100 mg/L) and high concentrations (Rafoss: 2800 mg/L, Verket: 5000 mg/L) for 192 h. Particles from both sites (47-51 % < 10 μm in diameter), primarily granite-gneiss bedrock and slightly more elongated than naturally eroded particles, created distinct water environments due to geological and chemical differences. This is the first peer-reviewed study to utilize μCT for quantification of particle retention in gills and it allowed identifying concentration-dependent retention, with higher retention for Verket particles, likely due to water chemistry differences. Despite high retention, no histological changes were observed. Mortality occurred at ≥3500 mg/L Verket particles after 48 and 24 h, plausibly due to suffocation from gill clogging. The LC₅₀ for Verket particles was 3137 mg/L. Suffocation likely caused a drop in plasma Cl levels due to elevated pCO₂. Elements with high particulate fractions accumulated in gills and stomach contents in a concentration-dependent manner for both particle types, and in livers at ≥3500 mg/L. No acute or sublethal effects, including metal toxicity, were observed at concentrations up to 2000 mg/L Verket and 2800 mg/L Rafoss particles for 192 h. These results suggest that common benchmarks of 50-400 mg/L in emission permits do not cause acute or sublethal effects in juvenile Atlantic salmon during short-term exposures.