Bioavalibility in Soils - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 732

16.4 Measurement of Bioavailability
The simplest and most straightforward method of determining the bioavailable
fraction of a contaminant in a soil is to expose the organism of interest, be it an earth-
worm or a geranium, and measure the uptake of the contaminant into that organism.
In the case of organic contaminants the bioavailable fraction could be assessed by
measuring the rate of biodegradation, i.e. perform a bioassay. This gives a direct
measure of what is bioavailable to a specific organism over a specific period of
time. Clearly this is not practical as there are too many contaminated sites, too many
potential receptors and too many contaminants for direct bioassays to be performed
as a matter of standard practise. For this reason, a variety of methods have been pro-
posed to determine bioavailability. These can be divided into models that quantify
scenarios on bioavailability and chemical extractions that mimic bioavailability and
subsequent uptake. Because of the differences in their behaviour (Table 16.1 ), mea-
surement procedures vary for inorganic and organic contaminants. Measurement
procedures which are the same for the two groups of contaminants are the physical
sampling of pore water (see MacDonald et al. ( 2008 ) for a review of methods) allied
to the assumption that contaminants in the pore water are bioavailable. Methods for
sampling pore water include simple centrifugation of the soil (e.g., Nahmani et al.
2007a ), vacuum sampling (e.g., Tipping et al. 2003 ) and passive sampling systems
based on a zero-tension principle (e.g. Haines et al. 1982 ). The most common vac-
uum sampler is the suction cell, where an imposed vacuum sucks pore water into
an in-ground porous cup. In simple systems, the water is stored in the suction cell
and is subsequently sucked or blown into a sample flask placed on the soil surface.
When employing these methods in the laboratory it is common to standardize the
soil in advance. The most common standardization treatment is fixing the water con-
tent in the soil and allowing the system to equilibrate for a specific period of time
(e.g. Tipping et al. 2003 ). Water concentrations are commonly expressed in terms
of percent of the maximum water holding capacity.
16.4.1 Extractions for Determining Bioavailability
Chemical extractions represent perhaps the most straightforward method for deter-
mining bioavailability of a contaminant. These methods typically involve shaking
contaminated soil material with a solution for a period of time and then analysing
the concentration of contaminants in the solution. Contaminants that are more read-
ily extracted are more bioavailable. For standard extractions the ratio of solid to
solution, period of shaking, nature of shaking, temperature, et cetera, are stip-
ulated in guidelines. Any chemical extraction of this nature should have been
validated against field based measurements, showing good correlations between
extractable chemicals and tissue concentrations measured in an organism of inter-
est. A problem is that the extractions are often applied outside the limits of their
validation, e.g., different soil texture, pH, contaminant concentration, organic matter
concentration.