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Environmental fate & pathways

Bioaccumulation: terrestrial

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BAF (terrestrial) Endpoint Summary:

For the terrestrial food chains in which birds are the predator, it was assumed that the diet of birds consisted solely of worms. Two food chains were evaluated for worm eating mammals: a 100% earthworm diet and a mixed diet of earthworms and other invertebrates (30% and 70%, respectively). Estimating the Ni concentrations to which a worm-eating bird or mammal may be exposed requires representative Ni concentrations in soil and a bioaccumulation factor (BAF) to estimate the Ni concentration in the earthworm (or other invertebrate) from the soil concentration. The choice of earthworms as the prey for terrestrial predators follows TGD guidance. The TGD uses earthworms because they ingest soil. An assumption is made that when predators ingest earthworms, they also ingest the full gut content of the worms, which is comprised of soil. As such, the earthworm would represent the highest dose of soil among potential prey organisms, and hence the largest dose of Ni as well. In nature, the diet of birds and mammals does not consist 100% of earthworms. For example, the European starling, which consumes earthworms, also feeds on other invertebrates and fruits and grains. The common shrew may also consume substantial quantities of insects, spiders, isopods, centipedes, snails, and slugs (Churchfield 1990). Accordingly, assuming a 100% earthworm exposure for these species may overestimates Ni exposure if earthworms have higher Ni concentrations than other prey species. This appears to be the case. For example, based on Ni data reported in Torres and Johnson (2001), the mean Ni BAF in isopods was approximately 0.066 on a dry wt. basis,which is l ower than the geometric mean earthworm BAF of 0.30 that was calculated as part of this analysis. Therefore, the assumption that the diet of the birds and mammals addressed in this assessment are comprised 100% of earthworms is a cautious approach, as earthworms contribute a higher dose of soil to the consumer organism than other food sources, and because the earthworm shows a higher BAF than other soil invertebrates, e.g., isopods. As discussed above, therefore, a second food chain was assumed for the shrew that consists of earthworms and other invertebrates, as represented by isopods. The isopod was selected to represent another terrestrial invertebrate in the shrew diet because a Ni BAF is available and isopods were a relatively common prey item in a southern England shrew study (Churchfield 1990).

 

Earthworm BAFs were compiled for various soil types based on a review of the scientific literature and ranged from 0.05 to 1.86 on a dry weight basis. The maximum BAF of 1.86 is from a soil that likely has elevated Ni bioavailability due to its combined low cation exchange capacity (CEC) of 5.3 meq/g and low pH of 4.8. However, BAFs from other studies are similar in magnitude, suggesting that perhaps this maximum BAF is not unique. Although there is an overall trend of decreasing BAFs with increasing soil Ni concentrations (based on the Neuhauser et al. [1995] and Janssen et al. [1997a] studies), the two lowest BAFs (0.05 and 0.07 from Ma (1982) were derived using control soils. Accordingly, all BAFs were pooled and log normally distributed. The resulting BAF geometric mean from the cumulative frequency distribution is 0.30. 

 

Nickel bioaccumulation factors (BAFs) for earthworms. (Note: Soil content was voided from the earthworm gut, at least partially in each study). 

Species

Soil Type

pH

OM (%)

CEC (meq/

100 g)

 

Soil Ni

(mg/kg dry

wt.)

 

Earthworm Ni

(mg/kg dry

wt.)

 

BAF

(dry

wt.)

 

References

Allolobophora caligonosa

Marine clay loam

7.1

5.8

26.3

a

a

0.05

Ma 1982

Allolobophora caligonosa

Sandy loam

6.6

2.8

9.4

a

a

0.20

Ma 1982

Allolobophora caligonosa

Riverine clay loam

5.3

6.9

26.4

a

a

0.07

Ma 1982

Allolobophora caligonosa

Peaty sand

4.7

12.4

20.5

a

a

0.34

Ma 1982

Allolobophora caligonosa

Sandy podzolized soil

5.4

6.4

13.5

a

a

0.21

Ma 1982

Allolobophora caligonosa

Plaggen soil

4.8

2.8

5.3

a

a

1.86

Ma 1982

Allolobophora tuberculata

Silt loam (fine)

a

a

a

12.6

4.7

0.37

Neuhauser et al. 1995

Allolobophora tuberculata

Silt loam (fine)

a

a

a

12.6

4.5

0.36

Neuhauser et al. 1995

Allolobophora tuberculata

Silt loam (coarse)

a

a

a

64.5

14

0.22

Neuhauser et al. 1995

Allolobophora tuberculata

Silt loam (coarse)

a

a

a

95.5

20

0.21

Neuhauser et al. 1995

Eisenia andrei

Dutch soils

3.42

b

b

7.1

4.5

0.63

Janssen et al. 1997a

Eisenia andrei

Dutch soils

5.67

b

b

71.5

7.9

0.11

Janssen et al. 1997a

Eisenia andrei

Dutch soils

5.35

b

b

74.9

9.0

0.12

Janssen et al. 1997a

Eisenia andrei

Dutch soils

7.17

b

b

18.7

9.0

0.48

Janssen et al. 1997a

Eisenia andrei

Dutch soils

6.53

b

b

51.1

5.6

0.11

Janssen et al. 1997a

Eisenia andrei

Dutch soils

6.51

b

b

48.2

3.4

0.07

Janssen et al. 1997a

Eisenia andrei

Dutch soils

6.8

b

b

99.9

9.0

0.09

Janssen et al. 1997a

Eisenia andrei

Dutch soils

4.98

b

b

27.4

11.2

0.41

Janssen et al. 1997a

Eisenia andrei

Dutch soils

3.77

b

b

1.9

2.2

1.2

Janssen et al. 1997a

Eisenia andrei

Dutch soils

3.85

b

b

12.2

4.5

0.37

Janssen et al. 1997a

Eisenia andrei

Dutch soils

5.26

b

b

15.2

9.0

0.59

Janssen et al. 1997a

Eisenia andrei

Dutch soils

6.75

b

b

45.6

16.9

0.37

Janssen et al. 1997a

Eisenia andrei

Dutch soils

6.69

b

b

50.9

21.4

0.42

Janssen et al. 1997a

Eisenia andrei

Dutch soils

6.89

b

b

82.8

15.7

0.19

Janssen et al. 1997a

Eisenia andrei

Dutch soils

7.02

b

b

56.2

14.6

0.26

Janssen et al. 1997a

Eisenia andrei

Dutch soils

6.95

b

b

37.5

5.6

0.15

Janssen et al. 1997a

Not specified

Clay

a

a

a

859

66.7

0.08

Jacques Whitford Limited 2005a

Mixture

Mixed types

6.9

3.3

a

13.6

25.2

1.85

Gish and Christensen 1973

Mixture

Lansdale loam

5.9-6.3

a

a

14

16

1.14

Beyer et al. 1982

Mixture

Hagerstown silt loam

5.4-6.4

3.0

9

12

15

1.25

Beyer et al. 1982

Mixture

Lansdale loam

4.9-6.4

2.5

8

13

11

0.85

Beyer et al. 1982

Mixture

Readington silt loam

5.3-6.1

2.6

11

16

12

0.75

Beyer et al. 1982

Geometric mean BAF (all data): 0.30

 

aNot reported

bNot reported for individual samples, but the minimum, median, and maximum OM contents were 2.0, 5.5, and 21.8%, respectively, and

the minimum, median, and maximum CECs were 1.7, 10.6, and 41.8 meq/100 g (Janssen et al. 1997b).