Soil properties of importance for assessing and reclaiming contaminated land
Peder Maribo, 12-2-2015
Composition and origin of Danish soils
The concept soil is often used for the mixture of fragmented rock, minerals, inorganic and organic
substances, microorganisms and plant residues etc. constituting the upper geological deposits of
land. Contamination of the soil can have a big significance for human activities and for the
environment. Contaminants can spread to crops, to the indoor climate of buildings or can be
transported down in the geology to aquifers and contaminate the groundwater.
I Denmark the upper soil layers are formed in the Quaternary period, comprising the last ice-age
and the subsequent post-glacial period. This period comprises 1 – 2 mio. years and the soil layers
formed in this period are called the Quaternary deposits. They are formed by glacial deposits
(moraine deposits), meltwater deposits, postglacial freshwater deposits, marine deposits and wind
deposits.
Moraine deposits, the result of minerals and ricks transported by the ice and deposited due to the
movement and finally melting of the ice, comprise a non-stratified and unsorted mixture of clay,
slit, sand, gravel and stones. Moraine deposits in Denmark has a magnitude (depth) ranging from of
a few to 200 meter, but most places the magnitude is less than 50 meter.
Meltwater (or alluvial) deposits are deposited by the glacial rivers running under the glacier and
forming the plains at the front of the glacier. They are characterised by being well graded and
horizontally stratified.
Postglacial freshwater deposits are formed in waterways and lakes after the ice melted away from
the landscape.
The last ice melted away from Denmark approximately 10.000 years ago, and big parts of what is
now northern Jutland was flooded by the sea. This was a consequence of the weight of the ice that
had compressed the underlying sediments. With the disappearance of the ice these sediments started
expanding gradually with up to 12 – 14 m above the present sea level. Quaternary sea deposits are
mainly found in northern Jutland and consist of sand and clay with shells of various kinds.
Wind deposits are mainly sand dunes formed along the coastal areas.
The soil
The top soil, soil at or just beneath the soil surface, constitute an ecosystem of it’s own. The soil is
constantly undergoing weathering and crumbling, deterioration processes of physical and chemical
nature, breaking down the solid rock minerals to smaller particles. The processes are due to wind,
water, frost, wear or influence from microorganisms, plants and animals in the top 2 – 3 meter of
the soil. Degradation of dead organic material lead to the formation of humic substances, large
complex organic molecules, that play a major role in the soil, in relation to facilitation of life and
exchange of ions and other organic substances in the soil environment. Clay minerals can have very
big surface area per volume (up to 800 m2/g) and this gives clay an ability to bind ions and water in
the soil.
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The soil is composed by a grain skeleton of particles of mineral or organic matter in varying sizes
and shapes. The space between the soil particles is referred to as voids and can be filled with air
and/or water. The ratio between the voids and total volume of the soil is called the porosity or the
soil void ratio n.
𝑛=
𝑉𝑣
(F.1)
𝑉𝑡
In which:
n: void ratio (or porosity)
Vv: Volume of the voids [l]
Vt: Volume of the soil (total volume) [l]
Soil water
The water content of a soil sample is normally given in relation to the wet weight of the soil. A
water content of 100g/kg expresses that 1 kg of soil contains 100 g of water and 900 g of dry matter
(DM).
The saturated zone refer to the level below with the voids of the soil are water-filled. The level in
which the pressure in the water-filled voids is equal to atmospheric pressure is called the ground
water table. This is the level at which we find water standing in a boring in equilibrium with the
surrounding soil water.
Capillary forces between the soil grains and water will cause water to fill the voids above the
ground water table. The large voids in a coarse sand will only cause the water to fill the voids a few
cm above the groundwater table, whereas silt or clay can hold water many meters above the
groundwater table. Moving up in the soil over the ground water table air pockets may start to appear
at some point. This level is called the capillary fringe. Above this the soil voids contains both water
and air (figure 1). The soil never becomes completely dry. Above the capillary fringe hygroscopic
forces attract water to the surface of the soil particles, and in the microscopic voids where the soil
grains meet. The zone above the capillary fringe is called the unsaturated zone.
Soil water
Particle
Unsaturated zone
Air
Capillary fringe
Saturated zone
Ground water table
Figure 1. The soil matrix with the grain skeleton, water and air.
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When precipitation is bigger than the evaporation from the soil infiltration of water through the soil
appears and infiltrating water contribute to the formation of groundwater, located in the saturated
zone.
Sub-soil with the capacity to store and transport water and called aquifers. Aquifers have a high
porosity (void ratio) and high permeability. Such soils can be uniform (well graded) sands, gravel or
fractured rock. Permeability k is a measure for the ability to allow a liquid to flow through voids in
the soil matrix, and has the unit m2. The permeability is only depending on the soil grain skeleton
and independent of the type of liquid. Hydraulic conductivity K is the ability of the soil to let water
flow though the grain skeleton. The relation between k and K is expressed in formula F.2:
𝐾=
𝑘∙𝜌∙𝑔
𝜇
(F.2)
In which:
K: hydraulic conductivity [m/s]
k: permeability [m2]
ρ: density of the liquid [kg/m3]
µ: the dynamic viscosity of the liquid [kg/(m·s)]
g: specific gravity (9,82 m/s2)
Fig 2. Values of soil parameters for typical fresh groundwater conditions — using standard values
of viscosity and specific gravity for water at 20°C and 1 atm. From: Wikipedia
http://en.wikipedia.org/wiki/Hydraulic_conductivity
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The following section is taken from Singer; M. and D. Munns: Soils – an introduction. MacMillan
Publ. 1987.
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Measured by a nest of sieves
Measured by hydrometer
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