Environmental Engineering Reference
In-Depth Information
influenced by ore deposits have been ignored. The arrows represent maximum lim-
its for clean fill set by the EPAV.
3
Figure
2.6
clearly shows that “background”
concentrations vary enormously, because the data represent a very large range of
soils of different derivations without necessarily the ecology dependent on these
soils showing any toxicity symptoms. Therefore total concentrations
per se
have no
informative content with regard to ecological toxicity. Hodson et al. (
Chapter 16
of
this topic) give a detailed analyses of bioavailability.
In layered and faulted geological materials it is common to find that preferred
pathways for groundwater seepage have created zones of high accumulation of
iron and associated trace metals and metalloids rendering the concept of natural
background concentrations unworkable. These fracture zones often develop high
concentrations of ferruginous material. Similarly, the process of vertical leaching of
rainwater through a soil profile, especially if it contains layers of contrasting perme-
ability, will distribute trace elements in a non-uniform manner through the profile.
Moreover, in Australia, the sharp boundary between a lighter-textured topsoil and
a clay subsoil, in soil profiles that are commonly called Duplex profiles (Northcote
1960
), often becomes a localised periodic low redox/high redox zone where fer-
ruginous gravels form by accretion of iron oxides/hydroxides on existing nuclei.
These become sites of preference for the accumulation of arsenic, vanadium, nickel,
chromium, cobalt and other metals if present in the material. For other kinds of
soil profiles one often finds that native inorganic contaminant concentrations form
a depth function similar to a chromatogram, in much the same way that calcium
carbonate and gypsum accumulate at a preferred depth in the soil profile.
Clearly, for these regolith bodies, which can have such complicated contaminant
distributions, it is not logical to maintain individual background concentrations for
trace elements to be used in soil related policy. However, so long as environmental
regulators in Australia continue to rely on single background concentration stan-
dards to determine where further investigations and assessments are required, then
the risk assessors must be free to apply the guidelines intelligently and the regulators
must be open to scientific argument in individual cases.
2.3 Contaminant Behaviour in Soils
2.3.1 Chemical Affinities and Solubilities
Contaminants occur in many soils as a result of having been added to the soil by
mankind or due to inherited geological processes. An understanding of the origin
and mobility of contaminants is essential in making judgements on their human
health and ecological impacts. In this section the characteristics of contaminant
groups is described in terms of affinities and solubilities.
3
Environment Protection Authority, Victoria, Australia.
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