Introduction to Contaminated Site Management - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 27

1.3.4 Occurrence in Soils and Groundwater

Contaminants coming from various natural and anthropogenic sources may be
found in soils. Contaminants enter the soil via emissions onto the soil surface,
usually unintentionally (e.g., through atmospheric deposition, spills, etc.), some-
times intentionally (e.g., the use of metal-containing fertilizers, illegal dumping). In
most cases, the contaminants migrate downwards. The velocity of migration varies
greatly, depending on the type of contaminant, soil type, soil properties and climatic
conditions. Ultimately, contaminants leach into the groundwater, where migration
continues, both in vertical and in horizontal directions. As a consequence, contam-
inants are found in the entire depth range of a soil. However, since most soils have
an upper layer with high organic matter content, which provides a high potential
for adsorption of both inorganic and organic contaminants, generally the total soil
concentration is higher 'in the first few decimetres' of the soil.
Since contaminant characteristics differ widely, the contamination profile vis-
à-vis depth also differs. The shape of this profile is determined by the sorption,
desorption and degradation potential of contaminants and of the physical charac-
teristics (water flow transport) and physico-chemical characteristics (sorption and
desorption) of the soil. Generally speaking, immobile contaminants have a higher
ratio between the concentration in the solid phase of the soil and the concentration
in the pore water or the groundwater than do mobile contaminants. However, all
contaminants have higher concentrations in the solid phase of the soil and, hence,
higher total soil concentrations in the upper soil, rich in organic matter and in clay
horizons (mainly metals).
For practical reasons the concentration of contaminants in soil is usually
expressed by weight of contaminants per unit weight of dry soil (kilogram), while
the concentration of contaminants in groundwater commonly is expressed by weight
of contaminants per unit of volume (l). Since this proved to result in the most con-
venient figures, the contaminant weight is usually expressed in milligram (mg) for
soil and in microgram (

g) for groundwater. In summary, the concentrations are
expressed as mg contaminant /kg soil, dry weight for soil and in

μ

μ

g contaminant /l groundwater
for groundwater, most often shortened to mg/kg dry weight (or mg/kg), and

μ

g/l,

respectively.

1.3.5 Mixtures of Contaminants

In the great majority of contaminated sites, more than one contaminant is found in
soil or groundwater. There are two reasons for this. First, most materials from which
contaminants originate contain more than one contaminant. Metal ores, for example,
often contain several metals which may be simultaneously released frommetallurgic
industrial processes. Similarly, in most activities or processes, where contami-
nants are released into the environment, several contaminants are involved. One
example of this is a dry cleaning facility, where several chlorinated hydrocarbons
are simultaneously used. From both these examples it can be concluded that the