Ecotoxicological assessment of road runoff residues for aquatic surface ecosystems in a scenario of reuse

Purpose Road runoff residues are often reused in road and civil works building. Although the physicochemical and ecotoxicological characteristics of these materials are known, no extensive ecotoxicological studies have been carried out on the leachates obtained from these residues once they have been spread and exposed to rainfall. This study was aimed at assessing the ecotoxicological risks for lentic aquatic ecosystems receiving the leachates of road runoff residues, either raw or treated, through granulometric sieving. Materials and methods Thirteen raw materials were collected in sites located in various parts of France, a majority of urban retention ponds with high traffic roads. Three of them were granulometrically sieved (fractions, 2-30 mm, 60 mu m to 2 mm, and <60 mu m). The materials were characterized on a physicochemical level and submitted to a leaching test reflecting the transfer of pollutants from the solid phase to the aqueous phase when the reused materials are exposed to rainfall. The contents in trace metallic elements of leachates were determined. Ecotoxicological single-species tests and microcosm assays were then carried out on the leachates to predict the potential effects of the leachates reaching a close aquatic lentic ecosystem. Results and discussion Twelve samples out of 22 displayed at least one metal content higher than ecotoxic thresholds. For the samples that were submitted to granulometric sieving, the finest fraction (<60 mu m) was also the most contaminated. The metal concentrations of leachates, compared with that of sediments, showed that only a low fraction (most often <0.1%) of sediment metals was mobilised by leaching. However, all leachates showed at least one metal concentration > predicted no effect concentration (PNEC). No acute toxicity was found at concentration <20% leachate (v: v). Among the 15 nonfiltered leachates, only two were toxic to some organisms of microcosms; the others were not toxic at the tested concentration of 10%. Conclusions The results obtained on 13 raw materials and 2 valorizable granulometric fractions suggest absence of acute ecotoxicity and absence of chronic ecotoxicity (21-day exposure) at dilution factor >10. The analysis of metallic contents and bioassay results lead concordantly to propose a no observed effect concentration close to 1% (v:v). On an ecotoxicological point of view, the granulometric fractioning does not seem to produce valorizable fractions, which would be systematically innocuous.