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Added risk approach

Selenium is naturally present in all environmental compartments. The median ambient background concentrations in agricultural soil and grazing land are 0.35 and 0.40 mg Se/kg, respectively (Vercaigne et al., 2010). Background Se concentrations were only measured in a few soils used for the terrestrial ecotoxicity tests and vary between 0.23 and 1.5 mg Se/kg dw (Somogyi et al., 2007; Wilke, 1989; Cartes et al., 2005; Soltanpour and workman, 1980; Liu et al., 2016). Because these background concentrations are significant compared to the NOEC and EC10 values for effects of inorganic selenium substances (selenite or selenate) to terrestrial organisms, the added risk approach is employed as a pragmatic solution. All NOEC and EC10 values are therefore based on added selenium concentrations, without taking into account the natural background in the soil. In essence this added risk assessment approach assumes that species are fully adapted to the natural background concentration and therefore that only the anthropogenic added fraction should be regulated or controlled (Appendix R.7.13-2 of the REACH guidance on “Environmental risk assessment for metals and metal compounds”).

Summary toxicity data

The available ecotoxicity results for the effect of selenium on terrestrial organisms are all based on either Na2SeO3 or Na2SeO4. All data reported are based on nominal added or background corrected measured Se concentrations in soil. A clear difference in toxicity was observed between selenite and selenate, with selenate showing significantly higher toxicity to invertebrates (Somogyi et al. 2007 and 2012) and plants (Cartes et al., 2005; Carlson et al., 1991). This is consistent with the lower adsorption and resulting higher bioavailability of selenate in soil compared to selenite. Therefore, results for sodium selenite are used for the assessment of inorganic tetravalent Se substances and results for sodium selenate are selected for the assessment of inorganic hexavalent Se substances.

The data available do not allow conclusions on the effect of soil properties (pH, organic carbon content, etc.) on the toxicity of selenite or selenate to terrestrial organisms. Therefore, all reliable toxicity data, expressed on an added concentration basis, were grouped for either selenite or selenate. The table below presents an overview of the lowest reliable toxicity data selected for hazard assessment of selenite and selenate to terrestrial organisms. For both selenite and selenate, the lowest long-term NOEC or EC10 values were observed for the toxicity to plants.

 Trophic level Species Parameter  Endpoint  Value (mg Se/kg dw)  Reference 
 Selenite
 Invertebrates

Enchytraeus albidus

Reproduction EC10 2.6 Somogyi et al., 2007
 Plants Sorghum vulgare Shoot dry weight NOEC 1.0 Carlson et al., 1991
 Micro-organisms  native biomass N transformation NOEC 5.9 Wilke, 1989
           
 Selenate      
 Invertebrates  Eisenia fetida  Reproduction  NOEC  1.7 Checkai et al., 2004
 Plants  Medicago sativa  Root and shoot yield  NOEC  0.44

Soltanpour and Workman, 1980; Wan et al., 1988

 Micro-organisms

 native biomass

 N transformation

 NOEC

 5.9

Wilke, 1989

These ecotoxicological effects data are all based on laboratory studies and experimental conditions can differ significantly from those in the field. Results are also available for toxicity of selenium in a field study on a calcareous Chernozem soil amended with different selenium doses added as Na2SeO3 (Biacs et al., 1995; Kadar et al., 1994; Kadar, 1995; Nyarai-Horvath et al., 1997). The soil was amended with Na2SeO3 at 30, 90, 270 and 810 kg Se/ha (no real control), corresponding to an added dose of approximately 10, 30, 90 and 270 mg Se/kg in the plough layer (0-20 cm). Trace elements were applied in the spring of 1991 with twofold replication and each plot has a total area of 21 m2. Maize, carrot, potato and pea were grown in the first, second, third and fourth year, respectively. Measured total soil Se concentrations are only reported for samples taken in 1994: 7, 29, 81 and 224 mg Se/kg at the 0, 90, 270 and 810 kg Se/ha treatments, respectively. Crop growth ranged from 4 to 6 months. The reported NOEC values for crop yield range from 7 to 81 mg Se/kg dry weight, which is significantly higher than the lowest NOEC values obtained from laboratory or greenhouse experiments with selenite or selenate. This difference may be explained by decreased bioavailability of Se in the field compared to laboratory conditions due to slow equilibration reactions (ageing), and confirms the conservative nature of the NOEC values derived in laboratory experiments.

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