Bioavalibility in Soils - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 723

16.2 What is Bioavailability?
In the last five years a variety of international working parties and governmental
organisations have produced reports on bioavailability. There has also been a healthy
discussion in the scientific literature regarding the term. The emerging consensus
appears to be that, given the variability present in the environment, in terms of soil
properties and soil organisms a definition of bioavailability that is precise, universal
and useful is unlikely to be attained. However, there is an emerging consensus on
what the term bioavailability encompasses, how it should be used and what infor-
mation should be provided on a case by case basis to make the term meaningful and
useful.
In 2002 the United States of America National Research Council published
a report on bioavailability in soils (National Research Council 2002 ) which was
usefully summarised by Ehlers and Luthy ( 2003 ). The report notes that the term
bioavailability has been defined by various disciplines and goes on to state that this
has led to some confusion over the term and that therefore further definitions will be
avoided. Instead the report focuses on bioavailability processes , which are the bio-
logical, chemical and physical processes that result in an organism being exposed
to a contaminant present in the soil. These processes are: release of the contaminant
from the solid phase, transport of the contaminant to and across a biological mem-
brane and, incorporation into a living organism. The National Research Council
report goes on to document methods for measuring these processes. Partially in
response to the National Research Council report, but also noting the legal and reg-
ulatory implications of the concept of bioavailability within the Risk Assessment
process, Semple et al. ( 2004 ) proposed the introduction of the term “bioaccessibil-
ity” to complement bioavailability. According to Semple et al. bioavailable material
is “freely available to cross an organism's cellular membrane from the medium the
organism inhabits at a given time”, whilst bioaccessible material is “that which is
available to cross an organism's cellular membrane from the environment, if the
organism has access to the contaminant”. The key advance offered by Semple et al's
definitions is the inclusion of a time element. Within their paper Semple et al. note
that many classical chemical extraction methods for measuring “bioavailability”
(see Section 16.4.1 ) actually measure or predict bioaccessibility and that, for Risk
Assessment and remediation, it is often bioaccessibility rather than bioavailability
that is of concern. Semple et al. also make the points that bioavailability and bioac-
cessibility will vary between organisms and that the “membrane” in question can
vary with the organism, for example being just the cellular membrane for a bac-
terium, but including both the epidermis and gastrointestinal tract of an earthworm.
Following Semple et al. ( 2004 ) the term “bioaccessibility” is gaining usage in the
literature. Reichenberg and Mayer ( 2006 ) made a further contribution to the debate
over what bioavailability is when they proposed that bioavailability comprised both
“accessibility” and “chemical activity”. Their “accessibility” term is essentially the
same as the Semple et al. ( 2004 ) bioaccessibility term. Chemical activity is a clas-
sically defined term related to fugacity and freely dissolved concentrations, which
quantifies the partial molar free energy of a substance (Faure 1992 ). A variety of