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Toxicity to terrestrial plants

Description of key information

This substance is a complex mixture of hydrocarbons with different biodegradation properties in soil. Testing of this substance is not technically feasible. Supporting information on literature for soils with similar weathered or spiked  fossil fuels was discussed. For the purpose of hazard assessment, and the exposure assessment and risk characterisation of indirect releases, Soil PNECs for representative hydrocarbon blocks were calculated with a modelling tool (PETRORISK, see CSR sections 9&10).

Additional information

This UVCB substance is a complex mixture of hydrocarbon compounds with variable physicochemical and ecotoxicological properties. Application of standard ecotoxicological tests to such UVCB substances includes a number of technical problems such as low water solubility and volatility of the substance. As supporting information the read-across studies on fossil fuels (fresh and weathered) contaminated soils are discussed.

In the study by Molina-Barahona et al. (2005) effectiveness of bioremediation on diesel contaminated soil and the toxicity of the remaining diesel by phyotoxicity test was examined. Test was conducted by growing Avena sativa, Triticum aestivum and Carthamus tinctoriu were for 7 days in freshly contaminated and bioremediated soil and their root and stem length, germination rate and biomass weight were measured. Test soil contained 49 % sand, 24 % silt, 27 % clay and 6.5 % total organic matter.

In general, diesel had a significant toxic effect on dry weight and also on root and stem elongation of the plant in freshly contaminated soil. This effect was decreased after the bioremediation process. On bioremediated soil, the germination rate decreased on all plants 64 %. Freshly contaminated soil caused greatest reduction in root elongation in safflower (49%) and stem elongation was most affected in wheat (35% reduction compared to control).

Low rate of germination even in bioremediated soil could be have been caused by impermeability of the weathered diesel in soil, because it could block plant's access to water and nutrients. Plant growth was observed to be smaller in bioremediated soil than dry weight in control samples. This was probably because the remaining presence of undegraded diesel was effecting plant growth.

In the second study by Dorn P.B. and Salanitro J.P. (2000) the toxicity of soils spiked with crude oil (Light-Gulf of Mexico) before and after of 9 and 11 months of bioremediation in Norrwood and Norrwood/Baccto soils were examined. Norrwood soil was silty loam obtained from cotton field near College Station, Texas, and contained 15 % clay, 60 % silt and 0.3 % organic carbon. The Baccto topsoil was commercially available sandy loam potting soil. Norrwood/Baccto soil mixture consisted 75 % from Norrwood and 25 % from Baccto soil (v/v) and contained 20 % clay, 56 % silt and 4.65 % organic content. Toxicity was examined by conducting 21-days plant growth and germination inhibition tests using corn (Zea mays), wheat (Tritium aestivum) and oat (Avena sativa). 

The crude oil used in this study had similar characteristics compared to renewable hydrocarbons of wood origin (diesel type fraction). The densities of the test crude oil and the renewable fuel are close to another (approx. 780 kg/m3 in Light-Gulf of Mexico and 804.4 -804.8 kg/m3 in the registering substance) and they contain about the same amount of aromatics (6.4 % in Light-Gulf of Mexico and ca. 5.5 % in renewable fuel). The test substance (Light-Gulf of Mexico) in the study by Dorn P. B. and Salanitro J. P. (2000) contains higher percentage of benzene than the renewable fuel (0.538 % in Light-Gulf of Mexico and < 0.1 % in the registering substance).

Results showed that in untreated soils, the plant seed germination was significantly decreased (50 -100 % inhibition) in all plants, but in bioremediated soils, seed germination was not significantly lower than in control soils. Plant growth was similarly reduced significantly in untreated soil, but although growth was enhanced in bioremediated soils, it was still 0-40 % lower compared to control soil.

In conclusion, diesel showed toxic effects on plants with concentrations of 45 g/kg, when oat, wheat and safflower growth decreased 20, 30 and 35 % respectively compared to control in untreated soil that had total organic matter of 6.5 % (Molina-Barahona et al. 2005). Plant growth was still affected with diesel concentration of 1 mg/kg in bioremediated soil with organic content 0.3 %, when corn, wheat and oat weight decreased 16, 32 and 50 % respectively compared to control (Salanitro J.P & Dorn P.B. 2000). Based on their result and findings from literature, Salanitro J.P. and Dorn P.B. (2000) suggested that hydrocarbon phototoxicity cannot be predicted and varies widely with oil and soil type, concentration and species tested.