Introduction to Contaminated Site Management - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 55

Anthropogenic background concentrations can relate to both man-made and nat-
ural contaminants. The reason for this is that human activities can be responsible for
the release of man-made contaminants over larger areas, for example by large scale
exhaust of man-made organic contaminants via the air, followed by atmospheric
deposition. But, human activities also can also lead to the spread of natural contam-
inants in soils, for example, by the soil surface rising, in the case of deep polder
areas with shallow groundwater tables, exposing soil material that has been natu-
rally contaminated. So the distinction is based on the type of activity that caused the
background concentrations and not on the origin of the contaminants.
Like every classification of background concentrations, this distinction is arti-
ficial, since there is a transition from one to the other, for example, human
manipulation of the flow characteristics of a river that will impact the deposition
of sediment with natural (or maybe also anthropogenic) contaminants. Moreover,
some contaminants may contribute to the background concentration as a result of
both natural processes and man-made activities, such as the combined presence
of naturally occurring arsenic and arsenic from pesticide applications or smelting
operations.
Naturally occurring contaminants are often metals. Most organic contaminants
are man-made, although many exceptions are known such as PAHs (polycyclic
aromatic hydrocarbons) that are formed through natural processes such as wood
fires. Cyanide-containing chemicals are produced by a wide range of organisms and
plants as part of their normal metabolism. Bacteria and fungi are known producers
of cyanide. A few species of centipedes, millipedes, insects, beetles, moths and but-
terflies secrete cyanide for defensive purposes in repelling predators such as toads
and birds (MERG 2001 ). Some of the common plants that contain cyanide are cas-
sava, sweet potatoes, corn, lima beans, almonds, radishes, cabbage, kale, brussels
sprouts, cauliflower, broccoli, turnips, lettuce, kidney beans, and it can be found in
the pits or seeds of cherries, plums, apricots, pears and apples (MERG 2001 ).
Many statistical procedures exist to determine the background concentrations. In
general, the type of contaminant (natural or anthropogenic) does not influence the
statistical or technical method used to characterize background concentrations (US
EPA 2002 ). In most countries, background concentrations have been established
on a national or regional level (e.g., Lavado et al. ( 2004 ), who determined metals
background concentrations in Pampas soils in Argentina).
Background concentrations can, at the least, be used for two different purposes.
First, in relation to Ecological Risk Assessment, it is often assumed that the back-
ground concentration does not pose any risk, or less risk, to the soil ecosystem,
since the organisms are adapted to the long-term situation ('Added Risk Approach').
Therefore, Risk assessors need to exclude or nuance the risks caused by the back-
ground concentration. Second, background concentrations can be politically used
for the definition of 'acceptable' (from a political perspective, not in term of risks)
soil quality, for example, as an end goal of remediation. The substantiation for this is
the fact that large scale remediation of areas of several square kilometres, for exam-
ple, is nearly impossible for practical and financial reasons. Besides, it is not always
defensible, from the standpoint of fairness, to upgrade the soil quality in one small