Contribution for the derivation of a soil screening value (SSV) for uranium, using a natural reference soil

Abstract

In order to regulate the management of contaminated land, many countries have been deriving soil screening values (SSV). However, the ecotoxicological data available for uranium is still insufficient and incapable to generate SSVs for European soils. In this sense, and so as to make up for this shortcoming, a battery of ecotoxicological assays focusing on soil functions and organisms, and a wide range of endpoints was carried out, using a natural soil artificially spiked with uranium. In terrestrial ecotoxicology, it is widely recognized that soils have different properties that can influence the bioavailability and the toxicity of chemicals. In this context, SSVs derived for artificial soils or for other types of natural soils, may lead to unfeasible environmental risk assessment. Hence, the use of natural regional representative soils is of great importance in the derivation of SSVs. A Portuguese natural reference soil PTRS1, from a granitic region, was thereby applied as test substrate. This study allowed the determination of NOEC, LOEC, EC20 and EC50 values for uranium. Dehydrogenase and urease enzymes displayed the lowest values (34.9 and <134.5 mg U Kg, respectively). Eisenia andrei and Enchytraeus crypticus revealed to be more sensitive to uranium than Folsomia candida. EC50 values of 631.00, 518.65 and 851.64 mg U Kg were recorded for the three species, respectively. Concerning plants, only Lactuca sativa was affected by U at concentrations up to 1000 mg U kg(1). The outcomes of the study may in part be constrained by physical and chemical characteristics of soils, hence contributing to the discrepancy between the toxicity data generated in this study and that available in the literature. Following the assessment factor method, a predicted no effect concentration (PNEC) value of 15.5 mg kg-1dw was obtained for U. This PNEC value is proposed as a SSV for soils similar to the PTRS1.

Figure 1. Soil enzyme activities, N mineralization and potential nitrification.

Response of the arylsulphatase, dehydrogenase, cellulase urease, activity, N mineralization and potential nitrification to soils spiked with a range of uranium concentrations. The error bars indicate the standard deviation. The asterisks point out significantly differences from the control (P<0.05).

Figure 2. Reproduction of invertebrates.

Results obtained exposing Eisena andrei, Enchytraeus crypticus and Folsomia candida, to natural PTRS1 soil, contaminated with different concentrations of U. The error bars indicate the standard deviation. The asterisks point out significantly differences from the control (P<0.05).

Figure 3. Seed germination of plants.

Average number of emerged seeds in monocotyledonous, Avena sativa and Zea mays and in dicotyledonous species, Lycopersicon esculentum and Lactuca sativa, grown in PTRS1 soil contaminated with U. The error bars indicate the standard deviation.

Figure 4. Growth of plants.

Average values of fresh mass and dry mass in monocotyledonous, Avena sativa and Zea mays and in dicotyledonous species, Lycopersicon esculentum and Lactuca sativa grown in PTRS1 soil, contaminated with U. The error bars indicate the standard deviation. The asterisks point out significantly differences from the control (P<0.05).