Characteristics of Natural and Urban Soils - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 98

serpentine rock, the vegetation has evolved to cope with the higher availability of
nickel. Reimann and de Caritat ( 1998 ) warn that locally elevated concentrations of
a specified element in a rock type will have an influence on all media that interact
with that rock, such as soils, groundwater, vegetation and wind blown dust. Likewise
their readers are told to treat so-called “average” data on concentrations for specific
media with care as the sampling and analytical methods are often unknown.
Metals and metalloids and other trace elements vary greatly in natural abundance
in different geological materials (Krauskopf 1967 ; Reimann and De Caritat 1998 ),
as shown in Table 2.2 .
Basalt-derived soils in low rainfall climates that have not undergone much leach-
ing invariably are high in calcium (Ca) and magnesium (Mg) as well as in several
metals, such as cobalt, chromium, copper, iron, manganese, nickel, and vanadium.
Other rock types such as granites or shales have substantially lower metal concen-
trations, but in the soils that formed on these rocks these metals often have been
translocated to the subsoil. High arsenic is common in soils on the edges of certain
mineralised ore bodies. Another example of naturally elevated metal concentrations
is the high zinc soils in parts of the Netherlands and Belgium. Their abundance in
soils derived from these lithologies is determined by the abundance in the parent
material, but strongly affected by subsequent weathering and geochemical pro-
cesses, as well as by the mixing that occurs when unconsolidated earth materials
undergo transportation by wind, water or ice sheets.
Weathering processes operating on fresh rocks as well as previously weathered
geological materials will transform and release many or most of these elements,
with secondary minerals forming and capturing particular trace elements in their
lattices, according to their geochemical affinities. In high rainfall areas, well weath-
ered basalt soils have undergone high losses of silica, calcium and magnesium, but
the heavy metal concentrations have remained high due to their transformation to
low-solubility oxides/hydroxides.
Large areas of the world are covered in geologically recent (Holocene) alluvial
or windblown sediments or glacial morainic materials. Loess and morainic deposits
are widespread in Northern North America, Northern Europe and Northern Asia.
These consist of transported soils, having often a mixed geological origin and having
undergone relatively little weathering, as they tend to be of Post-Glacial age and,
hence, are younger than 12,000-16,000 years. All the great mountain belts and hilly
areas of the world are affected by continuing erosion exposing freshly weathered
soil material on slopes and colluvial mantles.
Elsewhere, large areas of the world, particularly in the continents of Africa, South
America and Australia, consist of old Tertiary-age land surfaces covered by a deep
regolith that is largely in situ. Regolith 1 is a general term for the layer of unconsoli-
dated (non-cemented), weathered material, including rock fragments, mineral grains
and all other superficial deposits, that rests on unaltered, solid bedrock. Its lower
limit is the weathering front. Soil is regolith that often contains organic material
1 Oxford Dictionary of Earth Sciences, Oxford University Press, 1999.