technical guidance documents - Institute for Health and Consumer ...

EFFECTS ASSESSMENT
The weight of the gut contents can be rewritten using the fraction of gut contents in the total worm:
where:
Explanation of symbols
132
W = W ⋅ F ⋅ CONV
(82a)
gut
CONV
earthworm
gut
RHO
soil
soil
soil = (82b)
Fsolid
⋅ RHOsolid
CONVsoil conversion factor for soil concentration wet-dry weight soil [kgwwt . kgdwt -1 ]
Fsolid volume fraction of solids in soil [m 3. m -3 ] Table 5
Fgut fraction of gut loading in worm kgdwt . kgwwt -1 0.1
RHOsoil bulk density of wet soil [kgwwt . m -3 ] eq. (18)
RHOsolid density of solid phase [kgdwt . m -3 ] Table 5
Using this equation, the concentration in a full worm can be written as:
C
earthworm
=
BCF
earthworm
⋅C
porewater
1+
F
gut
+ Csoil
⋅ F
⋅CONV
soil
gut
⋅CONV
soil
(82c)
When measured data on bioconcentration in worms is available the BCF factors can be inserted
in the above equation. For most substances, however, these data will not be present and BCF will
have to be estimated. For organic chemicals, the main route of uptake into earthworms will be
via the interstitial water. Bioconcentration can be described as a hydrophobic partitioning
between the pore water and the phases inside the organism and can be modelled according to the
following equation as described by Jager (1998):
earthworm
( 0 . 84 + 0.
K ow ) RHOearthworm
BCF = 012
(82d)
where for RHOearthworm by default a value of 1 (kgwwt .L -1 ) can be assumed.
Jager (1998) has demonstrated that this approach performed very well in describing uptake in
experiment with earthworms kept in water. For soil exposure, the scatter is larger and the
experimental BCFs are generally somewhat lower than the predictions by the model. The reasons
for this discrepancy are unclear but may include experimental difficulties (a lack of equilibrium
or purging method) or an underestimated sorption. 4
4 According to certain studies some soil ingesting organisms may accumulate chemical substances not only from
the soil pore water but also directly (possibly by extraction in the digestive tract) from the fraction of the
substance adsorbed onto soil particles. This may become important for strongly adsorbing chemicals, e.g. those
with a logKow > 3. For these compounds the total uptake may be underestimated. In other studies however it
has been shown that soil digesters virtually only bioaccumulate the substance via the pore water, i.e.
bioconcentrate chemical substances from the soil pore water. At present the latter process can be modelled by
use of the equilibrium partitioning theory (cf. also Section 3.5).