Bioavalibility in Soils - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 737

DDE, DDD and earthworm uptake (Tang et al. 1999 ), extractable PAH and earth-
worm uptake (Tang et al. 2002 ) and extractable PAH and PCB and earthworm uptake
(Krauss and Wilke 2001 ). Beads made from Tenax TA have also been suggested as
an extractant (Cornelissen et al. 1998 ). A slurry of soil, Tenax TA beads and salt
solution is shaken for a specified period of time and then the Tenax TA beads (which
float or stick to the vessel walls) are removed and extracted for organic contami-
nants. Morrison et al. ( 2000 ) report good correlations between Tenax TA extracted
DDT, DDE and DDD and earthworm tissue concentrations, but poor correlations for
dieldrin. Cuypers et al. ( 2002 ) report good correlations between Tenax extractable
PAH and biodegradation of PAHs. Ten Hulscher et al. ( 2003 ) also report good cor-
relations between extractable PAHs and uptake in earthworms, though the nature of
the relationship varied with exposure media. Results reported by De La Cal et al.
( 2008 ) suggest that 48 h of Tenax extraction is needed as proxy for the bioaccessible
fraction of highly hydrophobic organic contaminants like polybrominated diphenyl
ethers, DDT, and DDT metabolites. Finally, a proposed procedure for measuring
bioavailability using a solid phase extractant is “Solid-phase micro-extraction with
negligible depletion” (nd-SPME) which involves inserting a fibre thinly coated in
an organic compound such as poly(dimethysiloxane) (PDMS) and polyacrylate into
the soil, leaving it there for a period of time and then removing it and extracting
organic contaminants from the fibre. Van der Wal et al. ( 2004 ) report good corre-
lations between accumulation of HCB, telodrin, dieldrin and PCBs in earthworms
and extractions using SPME.
16.4.2 Modelling the Bioavailability of Contaminants
16.4.2.1 Metals and Metaloids
Models can be split into two varieties, mechanistic models (i.e. those with a theo-
retical basis) and empirical models (i.e. those which are correlations). An example
of a model based on theory is the biotic ligand model .
The biotic ligand model is used to predict the toxicity of contaminants to target
organisms. It was initially designed for aquatic systems (e.g., Di Toro et al. 2001 ),
but more recently has also been developed for soil systems (e.g., Allen et al. 2008 ;
Lock et al. 2007 ; Steenbergen et al. 2005 ; Thakali et al. 2006a , 2006b ; Van Gestel
and Koolhaas 2004 ). The majority of chemical extractions that are used as proxies
for bioavailability are correlated with the metal concentration in an organisms tis-
sue. In contrast, the biotic ligand model is used to predict a toxicological endpoint,
be that root elongation, earthworm or springtail reproduction, respiration et cetera.
Thus biotic ligand models have great potential for the assessment of ecological risks
related to contaminated sites, as they determine not just whether a contaminant is
bioavailable, but also whether that contaminant will have a toxic effect. Although
from a contaminated site Risk Assessment perspective the toxicity component of
biotic ligand models is their most valuable component and it is almost impossi-
ble to separate the bioavailability and toxicity components, the theory behind the