Environmental Engineering Reference
In-Depth Information
metals will do more harm to the soil ecosystem. On the other hand, since a higher
bioavailability usually (but not always) coincides with a higher potential for leach-
ing, the contaminants will disappear from the ecologically most important soil upper
layer in a relative short timeframe, after which the ecosystem can recover. Using the
same reasoning, immobile contaminants are less bioavailable, but will reside longer
in the ecologically most important soil upper layer. As long as bioavailability is low,
the effects may be limited, but when environmental conditions change, for example,
by changing the land use, they may become available again and show their effects
then. It is a political dilemma grounded in what is worse: a big adverse ecological
impact for a short period, a smaller adverse ecological impact for a longer period,
or an unknown ecological effect later on. This question is even more difficult to
answer when other protection targets (the groundwater that is impacted by leaching;
human health that is impacted by contaminants in vegetables) are also taken into
consideration. Given the diversity of soil ecosystems and the complex relationship
with the many other (abiotic) factors in soil, there is, of course, no uniform answer
to this question. The main message here is that the risk assessor and risk manager
must, for the sake of sustainability, generally focus on long-term risks.
The bioavailability of contaminants is closely related to the soil properties. The
absorption characteristics of the soil are, in addition to the bioavailability of harmful
contaminants, also important for nutrient supply and the bioavailability of essential
metals. Therefore, soil characteristics such as organic matter content and type, clay
content, the content of manganese, aluminium and iron (hydr)oxides are important
in buffering both useful chemicals as well as contaminants. They strongly influ-
ence sorption and, hence, bioavailability. The type of organic matter is important
for at least two reasons. Firstly, organic matter as part of the solid phase reduces the
groundwater concentration and, hence, bioavailability, while adsorption to dissolved
organic matter increases the total concentration in pore water and may increase
bioavailability. Second, different types of organic matter have different sorption
affinities. Cuypers ( 2001 ), for example, showed that the bioavailability was much
higher in the amorphous dissolved-organic-matter domain than in the condensed
dissolved-organic-matter domain.
With regard to ecological protection, pH has special status. The presence of a
high concentration of hydrogen atoms has huge impact on metals and metalloids
adsorbed in and on all sorbents. Therefore, pH also controls the bioavailability
of both useful chemicals and contaminants in soils. As a consequence, the acid
buffering capacity of soil is an important factor for a good soil quality.
As a consequence, the absolute concentration does not play an important role
in Ecological Risk Assessment. However, the 'bioavailable concentration', as a
measure of exposure, is much more important with regard to normalisation of
effect concentrations in soil (for example a Soil Quality Standard; a concentra-
tion that marks the difference between 'no risk' and 'possible risk'). Imagine, for
example, that an ecologically based Soil Quality Standard for a specific contam-
inant is 100 mg/kg dry weight , and that the actual bioavailability at a specific site
results in an actual exposure that is 50% lower than the prevailing bioavailability
related the experimental conditions under which the Soil Quality Standard is set
as 100 mg/kg dry weight . When a linear relationship between ecological impact and
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