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Reliable chronic toxicity data are available for the long-term effect of arsenic on 19 terrestrial species or microbial endpoints covering the 3 trophic levels (13 terrestrial plants, 4 invertebrates and 2 microbial endpoints). A total of 101 reliable EC10 or NOEC values, ranging between 6.9 and 704 mg added As /kg dry weight (dw), were selected for derivation of a PNEC value. All results were derived for soluble pentavalent arsenic substances (Na2HAsO4 and Na3AsO4).

Arsenic is naturally present in all environmental compartments. The median ambient background concentrations in the topsoil of agricultural and grazing land are 5.5 and 5.6 mg As/kg dw, respectively (http://gemas.geolba.ac.at). Background As concentrations in the soils used for the terrestrial ecotoxicity tests ranged between 0.6 and 240 mg As/kg dw, with a median of 4.4 mg As/kg dw. These background concentrations are significant compared to the lowest reliable NOEC and EC10 values for effects of inorganic arsenic substances to terrestrial organisms. Therefore, an added approach was selected for the risk assessment of arsenic in soil. All NOEC and EC10values are based on added arsenic 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”).

The bioavailability and toxicity of arsenic to most soil organisms was significantly affected by the properties of the soils tested. Toxicity to terrestrial invertebrates and most plants decreased with higher clay content of the soil. Data for one plant (Cucumis sativus) showed decreasing toxicity with higher pH. No significant relationships between toxicity and soil properties were observed for As toxicity to microbial endpoints. The slope of a simple linear regression between log-transformed EC50values for the various organisms and the log-transformed soil property of the soils resulting in the largest adjusted R2 values were selected for normalization of the toxicity data to standard soil conditions, as shown in the table below.

 

Test organism

Endpoint

Regression equation

Adj. R2

N

P

Eisenia fetida

reproduction

log EC50 = 1.060 +0.989*log clay

0.59

6

0.045

Folsomia candida

reproduction

log EC50 = 0.847 +1.074*log clay

0.89

6

0.003

Avena sativa

shoot yield

log EC50 = 0.697 +0.981*log clay

0.83

6

0.007

Cucumis sativus

shoot yield

log EC50 = 3.587 –0.273*pH

0.80

7

0.004

Hordeum vulgare

root elongation

log EC50 = 0.711 +1.034*log clay

0.43

19

0.001

Solanum lycopersicon

shoot yield

log EC50 = 1.225 +0.712*log clay

0.68

5

0.054

 

Wherever possible (i.e., when data on the soil properties of the test soil and a proper normalization model were available), EC10 and NOEC values were normalized to reasonable worst-case soil properties (10% clay, pH 7). Species-specific geometric mean values were derived for the most sensitive endpoint per species in case multiple data were available for one species. These species-specific geometric mean values vary between 5.0 mg As/kg for reproduction of the invertebrate Enchytraeus albidus and 142.8 mg As/kg for root elongation of Triticum aestivum.

 

The table below presents an overview of the chronic toxicity data selected for the PNEC derivation for toxicity of inorganic arsenic to terrestrial organisms.

Test organism

Taxonomic group

Endpoint

Range (and amount)

of NOEC or EC10values
(not normalized)
m
g As/kg dw

Species mean EC10(normalized to pH 7 and 10% clay)
m
g As/kg dw

Eisenia andrei

Lumbricidae (annelida)

reproduction

15.0 – 300.0 (n=5)

31.5

Eisenia fetida

Lumbricidae (annelida)

reproduction

10.0 – 413.1 (n=7)

45.3

Enchytraeus albidus

Enchytraeidae (annelida)

reproduction

10.0 (n=1)

5.0

Folsomia candida

Isotomidae (arthropoda)

reproduction

20.0 – 320.0 (n=15)

36.1

Arthrobacter globiformis

Bacteria

dehydrogenase activity

30.0 – 112.6 (n=5)

53.3

Natural soil microbial community

Bacteria

microbial N transformation

58.9 (n=1)

58.9

Avena sativa

Poaceae (monocotyledon)

shoot yield

11.6 – 134.2 (n=6)

18.6

Cucumis sativus

Cucurbitaceae (eudicotyledon)

shoot yield

14.8 – 115.0 (n=5)

23.3

Helianthus annuus

Asteraceae (eudicotyledon)

yield

16.3 – 43.0 (n=2)

15.0

Hordeum vulgare

Poaceae (monocotyledon)

root elongation

16.3 – 704.0 (n=18)

33.2

Lactuca sativa

Asteraceae (eudicotyledon)

root elongation

40.0 – 156.4 (n=4)

77.4

Medicago sativa

Fabaceae (eudicotyledon)

shoot yield

25.0 (n=1)

25.0

Oryza sativa

Poaceae (monocotyledon)

grain yield

8.9 – 101.6 (n=10)

10.0

Phaseolus vulgaris

Fabaceae (eudicotyledon)

yield

8.2 – 15.5 (n=2)

5.3

Raphanus sativus

Brassicaceae (eudicotyledon)

yield

15.8 – 67.7 (n=2)

18.5

Solanum lycopersicon

Solanaceae (eudicotyledon)

shoot yield

6.9 – 227.6 (n=5)

21.5

Sorghum bicolor

Poaceae (monocotyledon)

yield

8.4 – 94.7 (n=3)

10.3

Triticum aestivum

Poaceae (monocotyledon)

root elongation

78.9 – 270.0 (n=6)

142.8

Zea mays

Poaceae (monocotyledon)

yield

18.3 – 68.2 (n=3)

20.0

 

In addition, several studies are available on the chronic effect of arsenic to mammals and birds, resulting in lowest NOECoral values of 30 and 41.6 mg As/kg diet for birds and mammals, respectively.

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