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RESEARCH ARTICLE (Open Access)

Derivation of ecological standards for risk assessment of molybdate in soil

Koen Oorts A G , Erik Smolders B , Steve P. McGrath C , Cornelis A.M. van Gestel D , Michael J. McLaughlin E and Sandra Carey F
+ Author Affiliations
- Author Affiliations

A ARCHE, Liefkensstraat 35D, BE-9032 Ghent (Wondelgem), Belgium.

B Department of Earth and Environmental Sciences, Division of Soil and Water Management, Katholieke Universiteit Leuven, Kasteelpark Arenberg 20, BE-3001 Leuven, Belgium.

C Rothamsted Research, Harpenden, Hertfordshire, AL5 2JQ, UK.

D Department of Ecological Science, Faculty of Earth and Life Science, VU University Amsterdam, De Boelelaan 1085, NL-1081 HV Amsterdam, Netherlands.

E CSIRO Land and Water, Contaminant Chemistry and Ecotoxicology Program, Waite Campus, Waite Road, Urrbrae, SA 5064, Australia.

F International Molybdenum Association, 4 Heathfield Terrace, London, W4 4JE, UK.

G Corresponding author. Email: koen.oorts@arche-consulting.be

Environmental Chemistry 13(1) 168-180 https://doi.org/10.1071/EN15086
Submitted: 24 April 2015  Accepted: 31 July 2015   Published: 4 November 2015

Journal Compilation © CSIRO Publishing 2016 Open Access CC BY-NC-ND

Environmental context. In order to assess the potential risks of elevated molybdenum concentrations in soil due to anthropogenic activities, toxicity thresholds must be known and environmental criteria defined. Setting such criteria for metals is not straightforward because of varying natural background concentrations and differences in toxicity between typical laboratory and field conditions and across soil types. Toxicity data and models were derived that account for these parameters so that soil quality criteria can be derived based on total molybdenum concentrations in soil.

Abstract. An extensive testing programme on the toxicity of sodium molybdate dihydrate in soil was initiated to comply with the European REACH Regulation. The molybdate toxicity was assayed with 11 different bioassays, 10 different soils, soil chemical studies on aging reactions, and toxicity tests before and after 1-year equilibration in field conditions. Differences in molybdate toxicity among soils were best explained by soil pH and clay content. A correction factor of 2.0 was selected to account for the difference in molybdate toxicity between laboratory and field conditions due to leaching and aging processes. Toxicity thresholds were determined as the HC5–50 (median hazardous concentration for 5 % of the species, i.e. median 95 % protection level) derived from the species sensitivity distribution of ecotoxicity data after bioavailability corrections. Uncertainty analysis illustrated that the HC5–50 provides a robust and ecologically relevant predicted no-effect concentration (PNEC) for risk characterisation. The 10th and 90th percentiles for site-specific PNEC values in European agricultural soil are 10.7 and 168 mg Mo kg–1 dry weight respectively based on a large survey of metal concentrations and soil properties in arable land soils. Total soil Mo concentrations in these soils are below corresponding PNEC values at most locations, suggesting no regional risks of molybdate to soil organisms at this scale. The information presented can be used in the EU risk-assessment framework as well as for national and international regulatory purposes for the setting of soil quality criteria based on total molybdenum concentrations, soil pH and clay content.

Additional keywords: bioavailability, soil ecotoxicity.


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