Complementarity of bioassays and microbial activity measurements for the evaluation of hydrocarbon-contaminated soils quality

In order to describe a soil polluted with hydrocarbons, the complementarity of bioassays and microbial activities measurements was studied. The sample s of soil were taken from a site which had received oil tank residues over 50 years. Five zones were sampled. Each sample was characterized by chemica l analyses, the measurement of dehydrogenase, phosphatase, hydrolysis of FD A and urease activities, soil respiration, and Microtox and Metplate bioass ays. The chemical analyses revealed different levels of total hydrocarbon c oncentrations (from 1.5 to 78.8 mg/kg of dry soil) but also relatively high quantities of nickel (from 14.5 to 841.6 mg/kg of dry soil) and lead (30.9 -355.4 mg/kg of dry soil) or cadmium (0-1.2 mg/kg of dry soil) in the diffe rent zones. Urease and dehydrogenase were sensitive to the presence of meta ls (31% inhibition of urease and 50% inhibition of dehydrogenase in the mos t contaminated soil). Measurements of Substrate Induced Respiration showed that the soil microflora were stressed in the presence of the pollutants. I n the zone containing the highest concentration of metals. the microbial ac tivities were low and the bioassays revealed a high potential toxicity (e.g . wIC50 for Microtox obtained with a 15% dilution of soil, 90% inhibition o f beta -galactosidase activity). In the other zones, the soil microbial act ivities were not depressed in comparison to the reference zone whereas the bioassays revealed the presence of toxic compounds extracted with the solve nt used. (C) 2001 Elsevier Science Ltd. All rights reserved.