Evaluation of decontaminated soil. IV. Application of bioassays with earthworms and plants on reference soils

Within the framework of the project "Evaluation of decontaminated soil", the applicability of bioassays with earthworms and plants for the quality assessment of decontaminated soil is investigated. In order to establish a reference system for "normal" levels of metals in earthworms, radish and lettuce, bioassays with these organisms were applied to relatively unpolluted soils from nature reserve areas in the Netherlands. Such reference levels can be used to evaluate the results bioassays on decontaminated soils. Nineteen elements were analysed: As, Ba, Ca, Cd, Co, Cr, Cu, Hg, K, Mg, Mo, Na, Ni, Pb, Sb, Se, Sn, V, Zn. The concentrations of these elements (except for As, Cu, Hg, Ni en Pb) and the content of clay and the pH of the soils were lower compared to a study of Edelman (1983) on the same soils. The content of organic carbon was, however, higher. The soil concentrations of all elements appeared to be significantly correlated with the pH and the contents of clay and organic carbon of the soils. Also the availability, expressed as the concentrations in pore water sampled after the bioassays with earthworms, appeared to be correlated with a combination of these three soil characteristics. For most elements, bioavailability determined as the concentrations in earthworms, radish roots, radish tops and lettuce was also significantly correlated with a combination of these soil characteristics. The results of the Microtox-test, which was applied to the soil pore water, could not be explained by the pore water characteristics (pH, conductivity). These results and data gathered from the literature were used to formulate a proposal for a reference system of "normal" concentrations of elements in earthworms, radish roots and radish tops. Growth of lettuce was severely hampered, especially on acid soils; therefore lettuce was considered to be less suitable as a test plant. It is concluded, that the soil type correction presently used in the Dutch soil protection policy needs to be expanded with soil pH in addition to the contents of clay and organic matter presently used.Speerpuntprogramma Bodemonderzoe