The Use of Acute and Chronic Bioassays to Determine the Ecological Risk and Bioremediation Efficiency of Oil-polluted Soils

To compare the effectiveness of acute and chronic bioassays for the ecological risk assessment of polluted soils, soil samples from a site with an historical mineral oil contamination (< 50-3,300 mg oil/kg dry soil) at the Petroleum Harbour in Amsterdam, The Netherlands, were screened for ecological effects using acute and chronic bioassays. A two-step 0.001 M Ca(NO3)2 extraction at a final solution-to-soil ratio of 1:1 was used to prepare extracts for the acute bioassays. Acute bioassays (< or = 5 d) applied to the 0.001 M Ca(NO3)2 extracts from the polluted and reference soils included growth tests with bacteria (Bacillus sp.), algae (Raphidocelis subcapitata), and plants (Lactuca sativa), immobility tests with nematodes (Plectus acuminatus), springtails (Folsomia candida), and cladocerans (Daphnia magna), and the Microtox test (Vibrio fischeri). Chronic bioassays (four weeks) performed on the same soil samples included tests with L. sativa, F. candida, and earthworms (Eisenia fetida) and the bait-lamina test (substrate consumption). The acute bioassays on Microtox showed a response that corresponded with the level of oil pollution. All other acute bioassays did not show such a consistent response, probably because pollutant levels were too low to cause acute effects. All chronic bioassays showed sublethal responses according to the contaminant levels (oil and in some soils also metals). This shows that chronic bioassays on soil samples are more sensitive in assessing the toxicity of mineral oil contamination in soil than acute bioassays on soil extracts. A pilot scale bioremediation study on soils taken from the two most polluted sites and a control site showed a rapid decline of oil concentrations to reach a stable level within eight weeks. Acute bioassays applied to the soils, using Microtox, algae, and D. magna, and chronic bioassays, using plants, Collembola, earthworms, and bait-lamina consumption, in all cases showed a rapid reduction of toxicity, which could be attributed to the degradation of light oil fractions.