Journal of Engineering Studies and Research Volume 17 (2011) No. 1 13 [619168]

Journal of Engineering Studies and Research – Volume 17 (2011) No. 1 13

THEORETICAL STUDIES CONCERNING THE INFLUENCE OF
PHYSICAL AND MECHANICAL PROPERTIES OF THE SOIL IN
THE PROCESS OF EPURATION AND SELF-EPURATION

CHIȚIMUȘ ALEXANDRA-DANA1*, NEDEFF VALENTIN1, LAZĂR GABRIEL1,
MĂCĂRESCU BOGDAN1, MOȘNEGUȚU EMILIAN1

1„Vasile Alecsandri” University of Bacau, 157 Calea Marasesti, RO-600115, Bacau,
Romania

Abstract: The paper describes theoretical aspects c oncerning the influence of physical and
mechanical properties of the soil in the process of epuration and self-epuration. Soil depollution is carried out by means of long-term procedures (in the case of auto-epuration) or
by way of costly procedures or methods which are, technically speaking, difficult to apply (in
the case of epuration), so in orde r to prevent the physical pollution of the soil, we need to study
the properties (physical and mechanical ones) and their infl uence on the soil considered as
filtering material.

Keywords: soil, distribution, pollutants, filtering material, saturated area, unsaturated area.

1. INTRODUCTION

As related to human life, the soil fulfills four essentia l functions: biological, nourishment, ecological filter and
usage as construction material [1-5]. The biological function results from the fact that soil represents the environment/location for numerous species
of animals and plants. The biological cycles pass through the soil, including it as a component part for numerous ecosystems [1, 2, 6].
The nourishment/feeding function is related to the fact that the so il contains all the chemical elements
necessary for living (calcium, magnesi um, potassium, phosphorus, etc.). It accumulates them and then conveys
them for usage to plants and animals, and it also provides the necessary water and air necessary to circulate them
[4].

The function of ecological filter (Figure 1) is mainly due to the fact that the soil is a porous medium. The water
first permeates the soil, which functions as a real biological filter, similar to a cleaning system, a neutralizer and
converter of the pollutants included in activities. Water, in the process of its infiltration in the soil, is thus
transformed so that its chemical and biological qualities will be influenced by the properties of the pedological
layer [1, 5, 7, 8].

* Corresponding author, email [anonimizat]
© 2011 Alma Mater Publishing House

Journal of Engineering Studies and Research – Volume 17 (2011) No. 1 14

The function of a raw material is given by its use in manufacturing (sand, calcareous crusts, clays etc.) and in
various semi-industrial sub domains. The soil is also a basis for constructions, roads, dams, canals [2, 7, 9].

Fig. 1. Soil as natural filter [7, 10, 11].

Soil pollution (Figure 2) consists of those anthropogenic activities which usually lead to the disturbance of its
normal functionality as a life sustainer and medium within various ecosystems [12-14].

SOIL POLLUTION
BIOLOGICAL CHEMICAL PHYSICS RADIOACTIVE

Fig. 2. Categories of soil pollution [5, 12, 14-19].

Soil is a much more complex system than water and air, so pollution affects its properties and, hence, its fertility.
Further more, pollutants can pass from the soil in plants, water or air, and depollution appears as a difficult
process, and even as an impossible one [8, 14, 20].

Journal of Engineering Studies and Research – Volume 17 (2011) No. 1 15

2. THE SOIL’S PROPERTIES AS FILTER ING MATERIAL AND TH EIR INFLUENCE ON
REDUCING POLLUTION

The most important properties of the soil which affect the process of self-epuration and epuration are physical
and mechanical properties, such as: texture, structure, granulometric composition, density, specific volume,
porosity, pores' number, consistency, permeability, temperature.
After permeating the geological medium, pollutants go through some phenomena related to: transport, transformation, transfer, accumulation and fixation (Figure 3) [10, 13]. These phenomena also contribute to their
spreading and distribution to the elements of the geological environment (soil, geological formations, and
subterraneous waters) and to the chemical alteration of the geological medium [4, 21].

All these phenomena: transpor t, transformation, transfer, accumulation and fixation depend on [4, 5, 15, 21]:
– the properties of the pollutant;
– the properties of the soil, of the geological formations, of the subterraneous waters;
– the properties of the media with wh ich the pollutant gets in contact (the transport medium, the receiver
medium).

From the surface of the soil towards its depth two areas can be distinguished:
– the unsaturated area, in which the pores are filled par tly with water and partly with air, and this fact
allows the coexistence, at this level, of solid, liquid and gaseous phases;
– the saturated area, in which the pores are totally filled with water, and the dominant component of
liquid flowing at this level coincides with th e direction of subterraneous water flowing.

The unsaturated area (for aeration) of the aquifers is the lo cation where intense phenomena of subterraneous
water-transfer take place, mainly vertically [infiltration (Ir) , evapotranspiration (Er)], sh own in Figure 4 [21-24].
The characteristics of this area determine the movi ng speed of pollutants from the surface sources
(concentrated/diffuse) towards aquifers where the moving speeds increase significantly [19, 21, 22, 24].

Fig. 3. Scheme of pollutants’ activity in the soil [7, 10, 11, 13]. Concentration of
liquid pollutants in
the soil Concentration of
pollutants in the
soil Concentration of
volatile pollutants
in the soil Distribution
of polutants
in the soil
Transfer of volatile
pollutants to the soil
surface
Dilution of volatile
pollutants at the soil
surface

Pollutants’
settling in
plants

Pollutants’
settling in
the soil
Transfer of
pollutants to
the vegetal
layer Transfer of
liquid pollutants
to the soil
surface Transfer of liquid
pollutants to the
subterraneous
water
Settling of
volatile
pollutants in
the air
contained by
the soil Pollutants’
storage on the
vegetal layer Transfer of liquid
pollutants in the
drinking water Infiltration in
in the
drinking
wate rPollutants’
transfer and
transformation
in the soil
Pollutants’
accumulation
and settling in
the soil Settling of liquid
pollutants in the
water contained by
the soil

Journal of Engineering Studies and Research – Volume 17 (2011) No. 1 16

The saturated area is a strong filter for the access of pollutants in the subterraneous area and that is why
knowledge of its characteristics is necessary [8, 19, 21, 24]:
– thickness;
– mineral matrix:
• granulosity;
• porosity;
• permeability;
– fluids present in the unsaturated area:
• gases; • miscible fluids;
• immiscible fluids: lighter than water or heavier than water;

– interaction between fluids – aqueous land:
• humidity/saturation degree;
• interfacial tension; • capillary pressure.

X0 – precipitations; S – leakage surface;
Np max/min – percolation level
maximum/minimum; Er – evapotranspiration;
I – infiltration ; Qs – leakage subterraneous.

Fig. 4. Components of water balance for the unsatura ted area (for aeration) and phreatic aquifer [11, 21,
23-25, 28].

The thickness of the unsaturated area, if it is sufficiently big and made up of lands with fine-grain structure and low permeability, with significant humidity scarcity, can en sure the reduction of vulnerability to pollution in case
of aquifers [11, 19, 21]. The characteristics of the mineral matrix are synthesized in the value of the permeability coefficient, and for its
evaluation it is necessary to paramete rize the characteristics of the porous space, features determined by the land
granulosity [25].

Journal of Engineering Studies and Research – Volume 17 (2011) No. 1 17

2.1. Major properties influencing the epuration and auto-epuration process
Permeability is an intrinsic characteristic of geological formations depending on the size and shape of the
cavities in which the fluids can move. The bigger the pores’ diameter is, the lower the environment’s resistance
to the fluids’ flow is, and the permeability of geological formations is higher [5, 10, 11, 23]. Permeability is quantified by means of the permeability coefficient (Figure 5) [7, 19, 26].

The value of the permeability coefficient (K) depends on the pores’ width. The finer the sediments’ granulosity is and, as a consequence, the pores’ width gets lowe r, the contact surface between the fluids and the solid
particles increases, as well as the resistance to fl owing, while permeability decreases [7, 19, 26-28].

Fig. 5. The characteristics of soil and drainage systems (K tot is determined by the soil layer which is the least
permeable) [7, 19, 26, 27].

Apparent density is one of the main indicators of structure of the soil and, at the same time, one of the main
determining factors of the soil’s physical properties (Figure 6). High apparent densities signify the reduction of
water-retaining capacity, of permeability and aeration [3, 16, 19].

Fig. 6. Frequent values of apparent density [3, 19].

Journal of Engineering Studies and Research – Volume 17 (2011) No. 1 18

Total porosity offers important information related to numerous characteristics of the soil. High values of
porosity indicate a high water-retaining capacity, high permeability and satisfying aeration, while sometimes
indicating low values of bearing. As in the case of appa rent density, the interpretation of the total porosity value
is only performed only in correlation with soil texture (Fig ure 7). At the same absolute value of total porosity, a
sandy soil can have quite favorable physi cal features, whereas in case of a clay ey soil these characteristics can be
less favorable [11, 16, 19, 26]. Draining porosity constitutes an important factor so as to assess the draining
capacity of soil with excess of humidity [3, 7, 11].

Fig. 7. Frequent values of drained porosity [3, 11].

Contraction is the soil’s property to diminish its volume due to reducing the water quantity in the soil. In Figure
8 a group of curves is represented, showing both the influence of the clay contents and that of the apparent density on the contraction index. We observe higher values of the contraction index, at the same texture, for
higher apparent densities [3, 16, 19].

Fig. 8. The variation of the contraction index depending on the clay contents and apparent density [3, 9, 19].

Journal of Engineering Studies and Research – Volume 17 (2011) No. 1 19

3. CONCLUSIONS

For certain pollutants, soil represents a transport medium, and for other pollutants it represents a storing tank.

The distribution of the pollutant in the soil depends on th e properties of the pollutant, the soil’s properties, of the
geological formations, of the subterraneous waters, and on the properties of the media it comes in contact with. The saturated area of the soil is a powerful filter for th e access of the pollutants in the subterraneous areas and
that is why it is necessary to acquire knowledge about its characteristics.
Soil depollution is made by means of long-term procedures (in the case of auto-epuration) or by means of costly
methods which are, technically speaking, difficult to apply (in the case of epuration), therefore in order to
prevent the physical pollution of the soil, we need to study the properties (physical and mechanical ones) and
their impact on the soil regarded as filtering material.

ACKNOWLEDGMENTS

The present research was performed in the frame of the BRAIN project: “Doctoral scholarships, an investment in
intelligence”, financed by th e European Social Found and Romanian Government.

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