Introduction to Contaminated Site Management - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 54

1.7.6 Essential Metals

Several contaminants that are found in soil, mainly metals, are essential for
the functioning of human beings or organisms. Twenty-five naturally occurring
elements are believed to have an essential function in plants and animals. As a
consequence, humans and/or organisms need to take these contaminants up, for
example in their diet. Among these essential metals are zinc, copper and selenium.
It is interesting to note that about 2 billion people worldwide, mainly children and
women, suffer from zinc deficiency, partly due to low zinc concentrations in soils in
Africa, Latin America and in some parts of Asia (Prasad 1998 ; Ramakrishnan 2002 ).
Most organisms have a narrow range between optimal concentrations (toxicity) and
deficiency.
From the perspective of essentiality, it seems a paradox to name essential
metals 'contaminants'. However, analogous to the definition used in this topic in
Section 1.3.1 , that is, that the toxicity of chemicals depends on the dose at which
humans or organisms are exposed, the period that this exposure takes place and the
frequency of exposure and the form (speciation) in which the chemical substance
is available, these essential contaminants are not beneficial by definition. Generally
speaking, however, essential contaminants are beneficial at specific low doses.
There has been a lot of debate about Risk Assessments in which essential metals
are involved. However, at high concentrations and, hence, toxic levels, risk decisions
must not be influenced by the fact that the same contaminants would have been
useful at lower concentrations. Of course, in the definition of an end goal of Risk
Management (e.g., remediation objectives), the essentiality of contaminants must
play an important role.

1.7.7 Background Concentrations

Many definitions are used for background concentrations (aka: baseline values or
Reference Values), often in combination with the adjective 'natural' (natural back-
ground concentrations). Generally speaking, background concentration refers to the
concentration in soil or aquifer over a larger area (that is, on a larger scale than
the site that is under investigation). These background concentrations can either be
related to natural processes or to anthropogenic activities (that is, anthropogenic
activities, which took place during years, decades, or even centuries, other than the
activities that caused the contamination under investigation). In the context of this
topic, the distinction is followed as described by the US Environmental Protection
Agency (US EPA 2002 ):

• Natural background concentrations: contaminants present in the environment in

forms that have not been influenced by human activity.

• Anthropogenic background concentrations: human-made or natural contami-

nants, present in the environment as a result of human activities (but not
specifically related to the site in question).