NPTEL – Civil – Geoenvironmental Engineering Joint initiative of IITs and IISc – Funded by MHRD Page 1 of 14 Module 1 FUNDAMENTALS OF GEOENVIRONMENTAL ENGINEERING A) Scope of geoenvironmental engineering Any project that deals with the interrelationship among environment, ground surface and subsurface (soil, rock and groundwater) falls under the purview of geoenvironmental engineering (Fang and Daniels 2006). The scope is vast and requires the knowledge of different branches of engineering and science put together to solve the multi-disciplinary problems. A geoenvironmental engineer should work in an open domain of knowledge and should be willing to use any concepts of engineering and science to effectively solve the problem at hand. The most challenging aspect is to identify the unconventional nature of the problem, which may have its bearing on multiple factors. For example, an underground pipe leakage may not be due to the faulty construction of the pipe but caused due to the highly corrosive soil surrounding it. The reason for high corrosiveness may be attributed to single or multiple manmade factors, which need to be clearly identified for the holistic solution of the problem. The conventional approach of assessing the material strength of the pipe alone will not solve the problem at hand. A lot of emphasis has been laid for achieving a “green environment”. Despite a lot of effort, it is very difficult to cut off the harmful effects of pollutants disposed off into the geoenvironment. The damage has already been done to the subsurface and ground water resources, which is precious. An effective waste containment system is one of the solutions to this problem. However, such a project has different socio-economic and technical perspectives. The realization of such projects require the contribution of environmentalist, remote sensing experts, decision makers, common public during its planning stage, hydrologists, geotechnical engineers for its execution stage and several experts for management and monitoring of the project. The totality of the problem can be
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NPTEL – Civil – Geoenvironmental Engineering
Joint initiative of IITs and IISc – Funded by MHRD Page 1 of 14
Module 1
FUNDAMENTALS OF GEOENVIRONMENTAL
ENGINEERING
A) Scope of geoenvironmental engineering
Any project that deals with the interrelationship among environment,
ground surface and subsurface (soil, rock and groundwater) falls under the
purview of geoenvironmental engineering (Fang and Daniels 2006). The scope is
vast and requires the knowledge of different branches of engineering and
science put together to solve the multi-disciplinary problems. A geoenvironmental
engineer should work in an open domain of knowledge and should be willing to
use any concepts of engineering and science to effectively solve the problem at
hand. The most challenging aspect is to identify the unconventional nature of the
problem, which may have its bearing on multiple factors. For example, an
underground pipe leakage may not be due to the faulty construction of the pipe
but caused due to the highly corrosive soil surrounding it. The reason for high
corrosiveness may be attributed to single or multiple manmade factors, which
need to be clearly identified for the holistic solution of the problem. The
conventional approach of assessing the material strength of the pipe alone will
not solve the problem at hand.
A lot of emphasis has been laid for achieving a “green environment”.
Despite a lot of effort, it is very difficult to cut off the harmful effects of pollutants
disposed off into the geoenvironment. The damage has already been done to the
subsurface and ground water resources, which is precious. An effective waste
containment system is one of the solutions to this problem. However, such a
project has different socio-economic and technical perspectives. The realization
of such projects require the contribution of environmentalist, remote sensing
experts, decision makers, common public during its planning stage, hydrologists,
geotechnical engineers for its execution stage and several experts for
management and monitoring of the project. The totality of the problem can be
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visualized under the umbrella of geoenvironmental engineering. Therefore, the
real challenge for a geoenvironmental engineer is how well he can integrate the
multi-disciplinary knowledge for achieving an efficient waste containment.
As mentioned earlier, in most parts of the world, damage has already
been done to the geoenvironment and groundwater reserves due to
indiscriminate disposal of industrial and other hazardous wastes. Owing to the
excessive demand, it becomes important to remediate and revive the already
polluted geoenvironment and groundwater. A geoenvironmental engineer has a
great role to play for deciding the scheme of such remediation practice. A lot of
concepts from soil physics, soil chemistry, soil biology, multi-phase flow, material
science and mathematical modelling, need to be taken for planning and
execution of an efficient remediation strategy. Therefore, it is essential for the
geoenvironmental engineer to think out of the box, to an extent that the
knowledge can help him visualize the problem better and suggest efficient
solution. Else, the solution to such problems becomes a trial and error process or
rather, learn from mistakes and rectify. Since such projects are cost intensive
one cannot afford to take too much of chances.
Another important issue is the reuse and recycling of waste materials,
which reduces the burden on our environment manifold. A very good example is
exploring the possibility of mass utilization of fly ash for geotechnical
applications. However, while using waste materials for meaningful applications
there are issues such as short term and long term impact, which is a governing
factor for deciding its selection as a viable material. Although, short term
behavior can be assessed using planned laboratory evaluations it often becomes
difficult and complex for understanding the long term behavior. The scope of
geoenvironmental engineering is to simplify the process of understanding the
behavior and resort to reliable predictions and estimations. This would require a
thorough knowledge on material science and chemistry and the reaction it
undergoes with time. This is indeed a tough task, but needless to say, such
challenges make this subject quite interesting.
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The frequent occurrence of landslides especially during rainy season has
drawn the attention of researchers and practicing engineers. The conventional
slope stability analysis is partially helpful in understanding the problem. A wider
perspective of the problem would be to include factors such as infiltration and
seepage of rain water through the slope. Such factors are going to add on to the
instability of slope. The scope and challenge for the geoenvironmental engineer
is to couple the geotechnical, geological and hydrologic concepts to explain
rainfall induced slope failure. Construction of flood protection works such as
embankments and levees also comes under the purview of geoenvironmental
engineering. Unless a thorough hydraulic study is conducted, any geotechnical
measures for flood protection would prove to be futile. This is specifically true for
large rivers and for meandering sections.
Geoenvironmental engineering is more research oriented and new
concepts and methodologies are still being developed. Therefore, this particular
course intends to introduce different avenues and overall scope of
geoenvironmental engineering to the reader. The course would highlight the
uncertainties and complexities involved and the wide research potential of the
subject. Special emphasis has been laid on the basics of soil-water interaction,
soil-water-contaminant interaction, which are essential for understanding the
impact of geoenvironmental contamination, its minimization and remediation.
B) Multiphase behavior of soil
Conventional or classical soil mechanics assumes soil media to be
completely water or air saturated. This is a typical example of a two phase media
consisting of soil solids and water/air. The assumption of two phases
considerably simplifies the mathematical quantification of the complex
phenomena that take place in porous media. Off late, geotechnical and
geoenvironmental engineering problems require the concept of three or
multiphase behaviour of soil for realistic solution of several field situations. For
example, a partially saturated soil is a three phase porous media consisting of
air, water and soil. The three phases result in transient and complex behaviour of
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unsaturated soil. Such cases are encountered while designing waste
containment facility where flow characteristics of unsaturated soil need to be
determined. When it comes to soil-water-contaminant interaction there are multi-
phase interactions involved. The migration of non-aqueous phase liquid (denoted
as NAPL) through porous media is a typical example. Fluidized bed, debris flow,
slurry flow, gas permeation through unsaturated soil media are some problems
where multiphase behaviour becomes important. Such studies are handy while
designing remediation scheme for contaminated soil and groundwater, which are
very important issues for the geoenvironmental engineer to solve. Understanding
the complex interaction of different phases is challenging and has paved way for
the study of multiphase behaviour of porous media. Such a realization has
generated a lot of interest in the research fraternity for developing experimental
and mathematical procedures for clearly delineating the phenomena in
multiphase porous media.
C) Role of soil in geoenvironmental applications
All civil engineering structures are ultimately founded on soil and hence its
stability depends on the geotechnical properties of soil. Conventional
geotechnology is more concerned about rendering soil as an efficient load
bearing stratum and designing foundations that can transfer load efficiently to
subsurface. Apart from this, soil is directly related to a number of environmental
problems, where the approach should be a bit different. Consider the case of
groundwater recharge as shown in Fig. 1.1. The infiltration and permeation
property of homogenous or layered soil mass above water table decides the rate
of recharge. In this case, a geotechnical engineer has to work closely with
hydrogeologists for deciding different schemes of artificial groundwater recharge.
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Fig. 1.1 Artificial groundwater recharge
Consider the case of waste dumped on ground surface. During
precipitation, water interacts with these wastes and flow out as leachate. When
the leachate flows down, soil act as buffer in retaining or delaying several harmful
contaminants from reaching groundwater. Such a buffering action obviously
depends on the texture and constituents of soil mass. While designing a waste
containment facility, the role of soil in such projects is enormous. A coarse
grained soil with filter property is required for leachate collection where as a fine
grained soil is required for minimizing flow of leachate. These are two entirely
different functions expected from soil in the same project. The cap provided for
waste dumps also necessitate the use of specific type of soils with the required
properties. The amount of water that infiltrates into the waste below is minimized
by soil used in such caps. Special type of high swelling soils is used as backfills
for storing high level radioactive waste in deep geological repositories. Another
important geoenvironmental problem, namely, carbon sequestration uses the
geological storage capacity for disposal of anthropogenic CO2 to mitigate the
global warming. Therefore, soil plays a very vital role in geoenvironmental
projects and the property by which it becomes important is problem-specific.
Precipitation
Artificial recharge
Aquifer
Groundwater
Bed rock
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D) Importance of soil physics, soil chemistry,
hydrogeology and biological process
Soil physics is the study of the physical properties and physical processes
occurring in soil and its relation to agriculture, engineering and environment. It
deals with physical, physico-chemical and physico-biological relationship among
solid, liquid and gaseous phase of soil as they are affected by temperature,
pressure and other forms of energy. Hence, the knowledge of soil physics
becomes important for solving geoenvironmental problems. The concepts of soil
physics is used for determining the transport of water, solute and heat (matter
and energy) through porous media, which is important to solve the problems
related to subsurface hydrology, groundwater pollution, water retention
characteristics of soil, improving crop production, rainfall induced landslides etc.
Soil physics is mostly quantitative and mathematical in nature and requires the
knowledge of soil physical properties. The important soil physical properties
include soil texture which deals with the particle gradation; soil water which
include mechanisms such as retention, infiltration, run off, permeation,
evaporation, transpiration, irrigation scheduling etc; soil aeration to take into
account exchange of gases such as oxygen and carbondioxide by plant roots
and microorganisms present in the soil. While defining these physical properties
of soil, it is very important to consider representative elementary volume (REV)
which is required to describe or lump the physical properties at a geometrical
point (Scott 2000). REV therefore describes mean property of the volume under
consideration.
Soil chemistry is the study of chemical characteristics of the soil and is
one of the important information required for many of the geoenvironmental
problems. The emergence of discipline “soil chemistry” began when J. T. Way
(father of soil chemistry) realized that soil could retain cations such as NH4+, K+ in
exchange for equivalent amounts of Ca+2 (Thomas 1977). This means that soils
act as ion exchangers. This aspect is vital for using soil in waste management
application. The contaminants leaching out of the waste dumps find its way to
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groundwater flowing past the soil porous media. The concentration of
contaminant at a distance away from the source for a given time is fully governed
by the chemical interaction of contaminant and the soil. There are several simple
and complex chemical reactions that may take place in soil-water system
depending upon the prevailing favourable condition. An example is the
phenomenon of solubility and precipitation as governed by the pH of the soil-
water-contaminant system. The knowledge of soil chemistry is important to
understand interactions between soil solids, precipitates and pore water,
including ion exchange, adsorption, weathering, buffering, soil colloidal
behaviour, acidic and basic soils, salinity etc. There is an interesting story which
resulted in the effects of soil acidity and alkalinity. The investigation on poor crop
productivity in eastern United States in early 1800’s lead to the understanding of
high soil acidity, which was regulated by the addition of lime. This resulted in high
yield of crops. Similarly the deleterious condition of soil due to high alkalinity was
realized and investigated in detail. After 1920’s the understanding on structural
soil chemistry and soil organic chemistry improved a lot. The acidity and
complexation potential of organic matter was appraised. A lot of chemists
researched on the structure and reactivity of water on soil mineral surface. These
and many other findings lead to the development of soil chemistry and today it is
one of the important branches of science required to explain several phenomena
in geoenvironmental engineering.
Understanding subsurface for geoenvironmental problems requires
extensive knowledge of hydrogeology. Hydrogeologic parameters influence a lot
on how a waste containment facility performs over its design life. Therefore, while
deciding the location for such facility it is important that the subsurface
hydrogeology condition is fully explored and studied. Different in-situ
methodologies are used for remediation of a contaminated site. For effective
functioning of such methods one has to study the hydrogeological aspects of the
site. Hydrogeologists play a vital role in locating groundwater aquifer, its
management and optimal extraction. Efficient watershed management by
artificial recharge is possible only if the hydrogeology of a particular area is
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known. The knowledge of hydrogeology is also required for understanding the
direction of groundwater flow. This is often required for assessing the extent of
contamination occurring due to a particular source of pollution and for risk
assessment.
Off late a lot of emphasis is laid on biological processes occurring in soils.
Initially, agriculturists were more bothered about this subject. But the subject has
caught the attention of many researchers due to its potential in solving different
geoenvironmental problems. For example, some type of microorganisms such as
Pseudomonas aeruginosa is used for remediation of hydrocarbon contaminated
site. It is very essential to understand the rate of such reaction and the impact of
such remediation. A lot of researchers worldwide are working on this interesting
problem. Biological process in soils is dependent on temperature and climatic
condition of a place, which need to be studied in detail. The soil biological
process is found to influence the exchange of greenhouse gases between soil
and atmosphere and many other soil physical parameters such as water
retention characteristics.
E) Sources and type of ground contamination
Solid, liquid and gaseous waste forms contaminates subsurface and
groundwater due to indiscriminate disposal. Solid wastes come from municipal,
domestic and industrial sources. Municipal wastes amounts to around 50 percent
of the total wastes produced. Household, hospital, agricultural wastes forms part
of municipal wastes. Returning these wastes to soil is considered to be a low
cost option. Abandoned e-waste, batteries, vehicles, furniture, debris from
construction industry is considered as solid waste and is produced from both
urban and rural areas. Large scale industrial development produces huge
quantities of hazardous waste and the sources are iron and steel industries,
packaging factories, paints, dyes, chemicals, glass factories, fertilizer and
pesticide industries, mine excavation waste etc. Coal mining, radioactive fuel
mining, petroleum mining and thermal power plants generate hazardous solid
waste that requires effective management.
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The main source and type of hazardous liquid waste include industrial
waste water contained in surface impoundments, lagoons or pits. It is also
produced from municipal solid refuse and sludge that are disposed on land. If not
handled properly sewage becomes an important source of liquid waste that has
undesirable effect on environment. Petroleum exploration leaves waste brine
solution which needs to be managed to prevent groundwater pollution. Liquid
waste emerges due to mining operation which is hazardous. A typical example is
acid mine drainage from dumped mine wastes.
Some of the gaseous waste includes NOx, CO, SO2, volatile
hydrocarbons etc. Chemical reaction may take place in air producing secondary
pollutants. SO2 combines with oxygen to produce SO3, which in turn combines
with suspended water droplets to produce H2SO4 and fall on ground as acid rain.
Natural breakdown of uranium in the geoenvironment emits cancer causing
radon gas into atmosphere.
F) Impact of contamination on geoenvironment
In most of the cases, wastes are disposed off indiscriminately in low-lying
areas without taking adequate engineering measures to effectively contain it.
This results in a highly unhygienic and unhealthy environment leading to
breeding of pests, mosquitoes and several harmful microorganisms. Many of the
emerging diseases found these days are direct impact of geoenvironmental
contamination due to wastes. During precipitation, or groundwater coming in
contact with these wastes generates contaminated water called leachate that can
travel far field and pollute the surface and groundwater resources. Many of the
harmful heavy metals can also travel along with the leachate if it is not contained
properly. Some of the solid waste such as excavation and mining waste, fly ash
(wet and dry) from thermal power plants requires large area of land for its storage
as wastes. This in turn would interact with rain water and can cause
contamination. Several harmful heavy metals well above the contamination limit
can enter the life cycle of organisms living in close proximity with such disposal
sites.
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One of the complexities of contamination impact is its long term effects
without a chance for realization. Most of the impacts are realized much later from
rigorous studies, and by the time the damage would have been done. Hence,
remediation becomes a tedious and cost-intensive affair. This makes
geoenvironmental engineering a challenging and much needed subject. There is
a need to focus on research that would help to predict and minimize the long
term impact of indiscriminate and mismanaged waste contamination.
G) Case histories on geoenvironmental problems
Use of readily available local soil instead of expensive
commercial soil (like bentonite) for waste management
Engineered waste management scheme necessitates the construction of
highly impermeable barrier so that waste disposed on it does not find its way to
ground water resources. Mostly these barriers are made of high plastic clays
which are commercially available. This would considerably increase the cost of
such geoenvironmental projects. Exploring the possibility of using local soils for
such applications, therefore, becomes an important geoenvironmental problem.
Any success in this direction would add to the economy of the project. This in
turn would result in sustainable development of such very important project. The
following research paper is an excellent case history of finding solution to one of
the geoenvironmental problems.
Taha and Kabir (2005) have explored the possibility of using tropical
residual soil for waste containment, which is readily available over a considerable
part of peninsular Malaysia. Hydraulic conductivity is used as the criterion for
evaluation of soil suitability for the said application. The soil was compacted at
different water content and compaction effort and then permeated with de-aired
tap water. The results of hydraulic conductivity test indicates that the required
flow of less than 10-9 m/s can be achieved by using a broad range of water
content and compaction effort. The soil has minimum shrinkage potential and
adequate strength to support the load of waste overburden. These properties
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discussed would fall under the purview of geotechnical engineering. But the
evaluation of soil suitability is not complete without understanding its chemical
reactivity. In this study, cation exchange capacity (CEC) of soil is used as an
indicator of chemical reactivity. It is desirable that the pollutants released from
the waste disposal site should be effectively attenuated by the liners. This means
that the soil should have high chemical reactivity. A soil with high CEC indicates
high reactivity and hence high attenuation capacity of pollutants.
Bioremediation of oil spills:
The case history is discussed in U. S. Congress, Office of Technology
Assessment, Bioremediation for Marine Oil Spills report. It essentially deals with
a marine oil spill that has occurred on the beaches of Alaska, USA, in late 80s.
The reason was due to the grounding of a ship on the shores. Office of
Technology Assessment (OTA), USA, felt the need of technologies to fight such
calamities. A comprehensive review of the methods for oil spill clean up was
conducted to develop an environmental friendly solution. One of the effective
solutions that came up was bioremediation in which specific species of
microorganisms were used to degrade oil. This is a slow natural process and
hence the major focus was on accelerating and improving the efficiency of this
natural process. Even though, some research has been initiated, it was found
that there is a dearth of data and hence the advantage of bioremediation over
other methods of oil spill clean up is yet to be ascertained. It has been opined
that in case of emergency situation, mechanical process such as using
dispersants and in-situ burning may still be appropriate.
Protecting environment from harmful effects of mine waste
using cover system
O’Kane and Wels (2003) have discussed the performance based design
of covers for mine wastes dumped on ground. The objective of the cover system
is to control harmful contaminant release from the waste dumps, chemical
stabilization of acid forming mine waste, dust and erosion control and provide
growth medium for sustainable vegetation cover. The proposed methodology of
cover design links predicted performance of cover system to the groundwater
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and surface water impacts. This method is impact oriented performance criteria.
In this method, a conceptual cover is selected first based on the type of waste,
size and geometry of the waste disposal, climate etc. A detailed cover design
analysis is performed that correlates cover design parameters (for example cover
thickness) to cover performance (net percolation). Third step links cover design
parameters to environmental impact assessment (groundwater quality). Fourth
step is to assess the risk based on the result from third step and the regulatory
law. If unacceptable, then cover design is modified. If acceptable then field trial
with performance monitoring is suggested. The feedback loop between impact
assessment and cover design is crucial for developing efficient cover system
without being overly conservative.
Value addition of waste products: Geopolymers from fly ash
Andini et al. (2008) have discussed about the value addition of fly ash by
converting it to a product called geopolymers. Davidovits first introduced the term
geopolymers for a new class of three dimensional alumino-silicate materials
(Davidovits 1989). Geopolymers are alkali-activated alumino-silicate binders and
its synthesis takes place by polycondensation from a variety of raw materials
such as metakaolin, coal fly ash etc. Polycondenstation reaction was carried out
by mixing fly ash with alkali metal silicate solution and then curing at different
temperature and time. Amorphous geopolymers are obtained at condensation
temperature ranging from 20 to 90 °C. The geopolymers has excellent
mechanical properties, thermal stability, acid resistance and are durable. It has
got a wide application in ceramics, cements, hazardous waste stabilization, fire
resistant materials etc. Environmentally sound recycling of fly ash into
geopolymers by hydro-thermal treatment is an excellent example of value
addition to the waste material.
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References
1. Andini, S., Cioffi, R., Colangelo, F., Grieco, T., Montangnaro, F. and Santoro,
L. (2008) “Coal fly ash as raw material for the manufacture of geopolymer-
based products”, Waste management, Vol. 28, pp. 416-423.
2. Davidovits, J. (1989) “Geopolymers and geopolymeric materials”, Journal of
Thermal Analysis, Vol. 35, pp. 429-441.
3. Fang, H-Y. and Daniels, J. L. (2006) “Introductory geotechnical engineering-
An environmental perspective”, Taylor and Francis, London.
4. O’Kane, M. and Wels, C. (2003) “Mine waste cover system design - linking
predicted performance to groundwater and surface water impacts”, Sixth
International Conference, Acid, Rock, Drainage, Cairns, Queensland, Carlton
South: AUSIMM.
5. Scott, H. D. (2000) "Soil physics: agricultural and environmental applications”,
Iowa State /university Press, USA.
6. Taha M. R. and M. H. Kabir (2005) “Tropical residual soil as compacted soil
liners”, Environmental Geology, Vol. 47, pp. 375-381.
7. Thomas, G. W. (1977) “Historical developments in soil chemistry: Ion
exchange”, Soil Science Society of America Journal, Vol. 41, pp. 230-238.
8. U. S. Congress, Office of Technology Assessment, Bioremediation for Marine
Oil Spills-Background Paper, OTA-BP-O-70 (Washington, DC: U.S.
Government Printing Office, May 1991).
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Model Questions
1) Explain the importance and scope of geoenvironmental engineering. 2) With examples, discuss the multiphase behavior of soil. 3) Why soil becomes important in geoenvironmental engineering? 4) Discuss the multidisciplinary nature of geoenvironmental engineering.
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Module 2
SOIL-WATER-CONTAMINANT INTERACTION
Knowledge of soil-water interaction and soil-water-contaminant interaction is very
important for solving several problems encountered in geoenvironmental
engineering projects. The following section introduces soil mineralogy and
various mechanisms governing soil-water-contaminant interaction.
2.1 Soil mineralogy characterization and its significance
in determining soil behaviour
Soil is formed by the process of weathering of rocks which has great variability in
its chemical composition. Therefore, it is expected that soil properties are also
bound to the chemical variability of its constituents. Soil contains almost all type
of elements, the most important being oxygen, silicon, hydrogen, aluminium,
calcium, sodium, potassium, magnesium and carbon (99 percent of solid mass of
soil). Atoms of these elements form different crystalline arrangement to yield the
common minerals with which soil is made up of. Soil in general is made up of
minerals (solids), liquid (water containing dissolved solids and gases), organic
compounds (soluble and immiscible), and gases (air or other gases). This section
deals with the formation of soil minerals, its characterization and its significance
in determining soil behaviour.
2.1.1 Formation of soil minerals
Based on their origin, minerals are classified into two classes: primary and
secondary minerals (Berkowitz et al. 2008). Primary minerals are those which are
not altered chemically since the time of formation and deposition. This group
includes quartz (SiO2), feldspar ((Na,K)AlSi3O8 alumino silicates containing
varying amounts of sodium, potassium), micas (muscovite, chlorite), amphibole
(horneblende: magnesium iron silicates) etc. Secondary minerals are formed by
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the decomposition and chemical alteration of primary minerals. Some of these
minerals include kaolinite, smectite, vermiculite, gibbsite, calcite, gypsum etc.
These secondary minerals are mostly layered alumino-silicates, which are made
up of silicon/oxygen tetrahedral sheets and aluminium/oxygen octahedral sheets.
Primary minerals are non-clay minerals with low surface area (silica minerals)
and with low reactivity (Berkowitz et al. 2008). These minerals mainly affect the
physical transport of liquid and vapours (Berkowitz et al. 2008). Secondary
minerals are clay minerals with high surface area and high reactivity that affect
the chemical transport of liquid and vapours (Low 1961).
Silica minerals are classified as tectosilicates formed by SiO4 units in frame like
structure. Quartz, which is one of the most abundant minerals comprises up to
95percent of sand fraction and consists of silica minerals. The amount of silica
mineral is dependent upon parent material and degree of weathering. Quartz is
rounded or angular due to physical attrition. The dense packing of crystal
structure and high activation energy required to alter Si-O-Si bond induce very
high stability of quartz. Therefore, the uncertainty associated with these materials
is minimal. In the subsurface, quartz is present in chemically precipitated forms
associated with carbonates or carbonate-cemented sandstones.
Clay minerals, which can be visualized as natural nanomaterials are of great
importance to geotechnical and geoenvironmental engineers due to the more
complex behaviour it exhibits. Therefore, this chapter emphasise more on
understanding clay mineral formation and its important characteristics. Basic
units of clay minerals include silica tetrahedral unit and octahedral unit depicted
in Fig. 2.1.
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Fig. 2.1 Basic units of clay minerals (modified from Mitchell and Soga 2005)
It can be noted from the figure that metallic positive ion is surrounded by non-
metallic outer ions. Fig. 2.2 shows the formation of basic layer from basic units
indicated in Fig. 2.1. There are 3 layers formed such as (a) silicate layer, (b)
gibbsite layer and (c) brucite layer.
Aluminium, Iron or
Magnesium
Oxygen
Oxygen
Oxygen
Silicon
Oxygen
Silica
tetrahedron
Hydrox
yl
Oxygen
Aluminium
octahedron
(Si4O10) -4
(a) Silicate layer
S
Symbol
Al4(OH)12
G
Symbol
(b) Gibbsite layer
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Fig. 2.2 Basic layer of mineral formation (modified from Mitchell and Soga 2005)
Gibbsite layer is otherwise termed as dioctahedral structure in which two-third of
central portion is occupied by Al+3. Similarly, brucite layer is termed as
trioctahedral structure in which entire central portion is occupied by Mg+2. These
basic layers stack together to form basic clay mineral structure. Accordingly,
there is two and three layer configuration as indicated in Fig. 2.3. More than
hundreds these fundamental layers join together to form a single clay mineral.
Fig. 2.3 Fundamental layers of clay minerals (modified from Mitchell and Soga 2005)
Description on common clay minerals
Some of the important and common clay minerals are described below in
Table 2.1.
Mg6(OH)12
B
Symbol
(c) Brucite layer
G
S
B
S
G
S
S
B
S
S
Two layer Three layer
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Table 2.1 Summary of important clay minerals
Mineral Origin Symbol
Bond Shape Remark
Kaolinite Orthoclase Feldspar (Granitic rocks)
Strong hydrogen
bond
Flaky and platy
Approximately 100 layers in a
regular structure d =7.2A0
Halloysite (Kaolinite
group)
Feldspar Tropical
soil
Less strong bond
Tubular or rod
like structur
e
At 600C it looses water and alter soil
properties
Illite Degradation of mica
under marine
condition Feldspar
K+ provides bond
between adjacent
layers
Thin and
small flaky
material
Bond is weaker than
kaolinite d =10A0
High stability
Montmorillonite
(Smectite group)
Weathering of
plagioclase
H2O molecules
pushes apart mineral
structure causing swelling
Presence of
cations
Very small
platy or flaky
particle
Exhibits high shrinkage and
swelling Weak bond
d >10A0
Vermiculite
Weathering of biotite
and chlorite
Presence of H2O and
Mg+2 predominantl
y Mg+2
Platy or flaky
particle
Shrinkage and swelling less
than montmorillonit
e
G S
G S
G S
G S
H2O
G S
S
G S
S
K+ K
+
d
G S
S
G S
S
H2O H2O
B S
S
B S
S
H2O Mg+2
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Kaolinite formation is favoured when there is abundance of alumina and silica is
scarce. The favourable condition for kaolinite formation is low electrolyte content,
low pH and removal of ions that flocculate silica (such as Mg, Ca and Fe by
leaching). Therefore, there is higher probability of kaolinite formation is those
regions with heavy rainfall that facilitate leaching of above cations. Similarly
halloysite is formed by the leaching of feldspar by H2SO4 produce by the
oxidation of pyrite. Halloysite formations are favoured in high-rain volcanic areas.
Smectite group of mineral formation are favoured by high silica availability, high
pH, high electrolyte content, presence of more Mg+2 and Ca+2 than Na+ and K+.
The formation is supported by less rainfall and leaching and where evaporation is
high (such as in arid regions). For illite formation, potassium is essential in
addition to the favourable conditions of smectite.
2.1.2 Important properties of clay minerals
Some of the important properties that influence the behaviour of clay minerals
are presented below:
Specific surface area
Specific surface area (SSA) is defined as the surface area of soil particles per
unit mass (or volume) of dry soil. Its unit is in m2/g or m2/m3. Clay minerals are
characterized by high specific surface area (SSA) as listed in Table 2.2. High
specific surface area is associated with high soil-water-contaminant interaction,
which indicates high reactivity. The reactivity increases in the order Kaolinite <
Illite < Montmorillonite. For the purpose of comparison, SSA of silt and sand has
also been added in the table. There is a broad range of SSA values of soils, the
maximum being for montmorillonite and minimum for sand. As particle size
increases SSA decreases.
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Table 2.2 Typical values of SSA for soils (modified from Mitchell and Soga 2005)
Soil SSA (m2/g)
Kaolinite 10-30
Illite 50-100
Montmorillonite 200-800
Vermiculite 20-400
Silt 0.04-1
Sand 0.001-0.04
For smectite type minerals such as montmorillonite, the primary external surface
area amounts to 50 to 120 m2/g. SSA inclusive of both primary and secondary
surface area, (interlayer surface area exposed due to expanding lattice), and
termed as total surface area would be close to 800 m2/g. For kaolinite type
minerals there is possibility of external surface area where in the interlayer
surface area does not contribute much. There are different methods available for
determination of external or total specific surface area of soils (Cerato and
Lutenegger 2002, Arnepalli et al. 2008).
Plasticity and cohesion
Clay attracts dipolar water towards its surface by adsorption. This induces
plasticity in clay. Therefore, plasticity increases with SSA. Water in clays exhibits
negative pressure due to which two particles are held close to each other. Due to
this, apparent cohesion is developed in clays.
Surface charge and adsorption
Clay surface is charged due to following reasons:
Isomorphous substitution (Mitchell and Soga 2005): During the formation of
mineral, the normally found cation is replaced by another due to its abundant
availability. For example, when Al+3 replace Si+4 there is a shortage of one
positive charge, which appears as negative charge on clay surface. Such
substitution is therefore the major reason for net negative charge on clay particle
surface.
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O-2 and OH- functional groups at edges and basal surface also induce negative
charge.
Dissociation of hydroxyl ions or broken bonds at the edges is also responsible for
unsatisfied negative or positive charge. Positive charge can occur on the edges
of kaolinite plates due to acceptance of H+ in the acid pH range (Berkowitz et al.
2008). It can be negatively charged under high pH environment.
Absence of cations from the crystal lattice also contributes to charge formation.
In general, clay particle surface are negatively charged and its edges are
positively charged.
Due to the surface charge, it would adsorb or attract cations (+ve charged) and
dipolar molecules like water towards it. As a result, a layer of adsorbed water
exists adjacent to clay surface, the details of which are presented in section
2.2.1.
Exchangeable cations and cation exchange capacity
Due to negative charge, clay surface attracts cations towards it to make
the charge neutral. These cations can be replaced by easily available ions
present in the pore solution, and are termed as exchangeable ions. The total
quantity of exchangeable cations is termed as cation exchange capacity,
expressed in milliequivalents per 100 g of dry clay. Cation exchange capacity
(CEC) is defined as the unbalanced negative charge existing on the clay surface.
Kaolinite exhibits very low cation exchange capacity (CEC) as compared to
montmorillonite. Determination of CEC is done after removing all excess soluble
salts from the soil. The adsorbed cations are then replaced by a known cation
species and the quantity of known cation required to saturate the exchange sites
is determined analytically.
Flocculation and dispersion
When two clay particles come closer to each other it experiences (a)
interparticle attraction due to weak van-der-Waal‟s force (b) repulsion due to –ve
+ +
Typical charged clay
surface
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charge. When particles are sufficiently close, attraction becomes dominant active
force and hence there is an edge to face configuration for clay particles as shown
in Fig. 2.4(a). Such a configuration is termed as flocculant structure. When the
separation between clay particles increase, repulsion becomes predominant and
hence the clay particles follows face to face configuration called dispersed
structure (Fig. 2.4b).
Fig. 2.4 Different arrangement of clay particle
A lot of micro and macro level behaviour of clays are associated with these
arrangement of clay particles (Mitchell and Soga 2005).
Swelling and shrinkage
Some clay minerals when exposed to moisture are subjected to excessive
swelling and during drying undergo excessive shrinkage. A lot of engineering
properties of soil is affected by this behaviour and the stability of structures
founded on such soils become detrimental. The swelling of clay minerals
decreases in the order montmorillonite > illite > kaolinite.
2.1.3 Minerals other than silica and clay
Other than silica and clay, subsurface contains a variety of minerals such
as oxides and carbonates that governs the reactivity of soil and its interaction
with the environment. Some of the abundant metal oxide minerals present are
iron oxides (hematite, magnetite, goethite etc.) and aluminium oxides (gibbsite,
boehmite). Other oxide minerals (such as manganese oxides, titanium oxides)
are far less than Fe and Al oxides, but because of small size and large surface
area, they would affect very significantly the geochemical properties of
subsurface. These oxides are mostly present in residual soils of tropical regions.
Other major components include soluble calcium carbonate and calcium
+ + +
+
+
+ (a) Flocculant
+ + + + + + + + +
+ + +
(b) Dispersed
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sulphate, which has relatively high surface area. In most soils, quartz is the most
abundant mineral, with small amount of feldspar and mica present. Carbonate
minerals such as calcite and dolomite are found in some soils in the form of bulky
particles, precipitates etc. Sulphate minerals mainly gypsum are found in
semiarid and arid regions.
2.1.4 Soil mineralogy characterization
One of the very well established methods for mineralogy characterization
of fine-grained soils is by using X-ray diffraction (XRD) analysis. Majority of the
soil minerals are crystalline in nature and their structure is defined by a unique
geometry. XRD identifies minerals based on this unique crystal structure. In
XRD, characteristic X-rays of particular wave length are passed through a
crystallographic specimen. When X-ray interacts with crystalline specimen it
gives a particular diffraction pattern, which is unique for a mineral with a
particular crystal structure. The diffraction pattern of the soil specimen (according
to its crystal structure), which is based on powder diffraction or polycrystalline
diffraction, is then analyzed for the qualitative and quantitative (not always)
assessment of minerals. Sample preparation method for XRD should be done
with great care as the XRD reaches only a small layer (nearly 50 µm) from the
surface of the sample. Hence, homogeneity is very important. Soil sample is
initially dried and sieved through 2 mm sieve. Sieved sample is homogenized in a
tumbler mixer for 30 min. A control mix of 30 g was taken and ground in lots of 15
g in a gyratory pulverizer. 15 percent by weight of KIO4 (internal standard) was
added to 5 g of specimen and again homogenized in a mixer. The prepared
specimen is then subjected to analysis. .
X-ray wave of monochromatic radiation (Kα) is commonly obtained from copper
radiation, which is commonly known as Cu- Kα. A typical XRD output is
represented by Fig. 2.5. It can be noted from the figure that ordinate represent
relative intensity of X-ray diffraction and abscissa represents twice of angle at
which a striking X-ray beam of wave length λ makes with parallel atomic planes.
Based on this diffraction pattern, the minerals can be identified by matching the
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peak with the data provided by International Centre Diffraction Data (ICDD)
formerly known as Joint Committee on Powder Diffraction Standards (JCPDS).
The solution for Eq. 3.22 corresponding to case (ii) can be represented as
follows (Carslaw and Jaeger 1959):
Ct/C0
Source Receiver
Lc
0 +z -z
Direction of diffusion
(a) Infinite cell
Source Receiver
Lc
z0 z 0
Direction of diffusion (b) Finite cell
Ct/C0
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ccm
cde
c
t
L
mz
L
mz
m
LRtmD
L
z
C
C 0
1
222
0
0
sincos.)/exp(2
(3.24)
where z0 is the interface between source and receiver.
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References
1. ASTM D 4646 (2004) “Standard test method for 24-h batch-type
measurement of contaminant sorption by soils and sediments”, Annual Book
of ASTM Standards, Vol. 04.11, ASTM International, West Conshohocken,
PA, USA.
2. Benson, C. H., Daniel, D. E. and Boutwell, G. P. (1999) “Field performance of
compacted clay liners”, Journal of Geotechnical and Geoenvironmental
Engineering, ASCE, Vol. 125, No. 5, pp. 390-403.
3. Carslaw, H. S. and Jaeger, J. C. (1959) “Conduction of heat in solids”, Oxford
University Press, New York.
4. Chang, N. Parvathinathanb, G. and Breedenc, J. (2008) “Combining GIS with
fuzzy multicriteria decision making for landfill siting in a fast-growing urban
region”, Journal of Environmental Management, Vol. 87, No. 1, pp. 139-153.
5. Crank, J. (1975) “The mathematics of diffusion”, Oxford University Press,
New York.
6. Daniel, D. E. and Benson, C. H. (1990) “Influence of clods on hydraulic
conductivity of compacted clay” Journal of Geotechnical Engineering, ASCE,
Vol. 116, No. 8, pp. 1231-1248.
7. Fetter, C. W. (1992) “Contaminant hydrogeology”, Macmillan publishing
Company, New York.
8. Hatzichristos, T. and Giaoutzi, M. (2006) “Landfill siting using GIS, fuzzy logic
and the Delphi method”, Journal of Environmental Technology and
Management, Vol. 6, No.2, pp.218–231.
9. Kmet, P., Quinn, K. J. and Slavic, C. (1981) “Analysis of design parameters
affecting the collection efficiency of clay-lined landfills”, Proc., Fourth Annual
Madison Conf. of Appl. Res. and Practice on Municipal and Industrial Waste,
Univ. of Wisconsin, Madison, Wis., Sept., 250–265.
10. Munier, N. (2004) “Multicriteria environmental assessment”, Kluwer Academic
Publishers, Netherlands.
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11. Rowe, R. K., Caers, C. J. and Barone, F. (1988) “Laboratory determination of
diffusion and distribution coefficients of contaminants using undisturbed
clayey soil”, Canadian Geotechnical Journal, Vol. 25, pp. 108-118.
12. Shackelford, C. D. (1991) “Laboratory diffusion testing for waste disposal- a
review”, Journal of Contaminant Hydrology, Vol. 7, pp. 177-217.
13. Sreedeep, S. (2006) “Modeling Contaminant Transport in Unsaturated Soils”,
Ph. D. Thesis submitted to the Department of Civil Engineering, Indian
Institute of Technology Bombay, India.
14. Younus, M. M. and Sreedeep, S. (2012a) “Evaluation of bentonite-fly ash mix
for its application in landfill liners”, Journal of Testing and Evaluation, ASTM,
in print.
15. Younus, M. M. and Sreedeep, S. (2012b) “Re-evaluation and modification of
plasticity based criterion for assessing the suitability of material as compacted
landfill liners”, Journal of Materials, ASCE, in print.
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Model Questions
1. Explain the concept of 3Rs and waste management hierarchy? 2. What is the aim of integrated solid waste management program? 3. Bring out the difference between a natural attenuation landfill and an
engineered landfill. 4. Explain the important details required for deciding landfill site. 5. Discuss in detail the multicriteria method for landfill site selection. 6. What is the importance of waste characterization? 7. What are the factors influencing leachate quality and quantity? 8. How to estimate leachate and gas generation rate? 9. With a neat figure, explain a conceptual liner and cover in landfill. 10. What is the major role of soil in a waste containment facility? 11. What are the requirements of compacted liner? 12. Explain in steps the design philosophy of waste containment liner system. 13. Starting from the basics, derive the differential equation for defining
contaminant transport for reactive contaminant. Every phenomena governing differential equation need to be discussed in detail.
14. With neat figures, explain laboratory method for establishing a) hydrodynamic dispersion coefficient, b) retardation coefficient, c) diffusion coefficient of unsaturated soil with low water content d) partition coefficient.
15. What are the major differences between physisorption and chemisorption? 16. Explain the batch method for establishing sorption characteristics of the
soil-contaminant system. 17. Explain the physical significance of sorption characteristics and its
importance in contaminant transport modeling. 18. What are the different isotherms used for establishing sorption
characteristics? 19. What are the different contaminant transport phenomena? 20. What is diffusion and when it is expected to dominate contaminant
transport phenomena? 21. What is retardation coefficient and how it is helpful in determining ionic
velocity? 22. A column test was conducted to determine dispersion coefficient. The soil
used was a silty clay with specific gravity 2.7. The diameter and height of the saturated soil column is 5 cm and 7cm, respectively with a water content of 35%. Calculate the pore volume of the soil column. An advective flux equal to 0.003 kg/day/m2 of 1000 mg/l SrCl2 has flown through the soil column for 5 hrs. Determine the total pore volume and number of pore volume for 5 hrs. The longitudinal hydrodynamic dispersion coefficient is 1.267 x 10-9 m2/s with a tortuosity coefficient of 0.7. The molecular diffusion coefficient of Sr+2 is 7.9 x 10-10 m2/s. Determine the longitudinal dispersivity for the soil-contaminant system.
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23. A batch test was conducted for 3 soil samples A, B, C with an initial concentration of 100 mg/l of SrCl2. 5 g of each of the soil sample is mixed with 50 ml, 100 ml, and 250 ml of SrCl2 and the values of Ce for A are 10, 8 and 6 mg/l, for B it is 12, 10 and 8 mg/l and for C it is 4, 3, 2 mg/l respectively. Compare the reactivity of the soil-contaminant system of the three soils and comment on the role of liquid to solid ratio on the sorption capacity of the three soil. Make any suitable assumptions.
24. Specific discharge in the field is given as 1.68x10-8 m/s. Bulk density of fully saturated porous medium is 1.6 g/cc with volumetric water content of 0.4. Partition coefficient of lead obtained by linear isotherm is 10 ml/g. Determine average velocity of lead. What will be the velocity of lead if it is assumed as non-reactive with porous medium?
25. A drainage pipe became blocked during a storm event by a plug of sand and silty clay as shown in figure Q3.1. When the storm ceased, water level above ground is 1 m. Permeability of sand is 2 times that of silty clay. a) Obtain variation of head components and total head for the length of drainage pipe b) Calculate pore water pressure at centre of sand and silty clay c) Find average hydraulic hydraulic gradient in both soil plugs.
26. Determine the quantity of flow and seepage velocity for constant head set ups given below (Fig. Q3.2) in SI units.
Total height of air tube is 10 cm
in which 2 cm is below water
20 cm
10 cm
5 cm
5 cm
10 cm
ksat= 3*10-5
cm/s
saturated volumetric water content = 0.5
Total height of air tube is 10 cm
in which 2 cm is below water
20 cm
10 cm
5 cm
5 cm
10 cm
ksat= 3*10-5
cm/s
Specific gravity = 2.65; w=25 %
Datum and water exit
A C B
3.3 m
1.5 m 0.5 m
Sand Silty
clay
Water level
Fig. Q3.1
Fig. Q3.2
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Module 4
CONTAMINATED SITE REMEDIATION
Soil contamination by organic or inorganic pollutants is caused by a number of
industries such as chemical, pharmaceuticals, plastics, automobile, nuclear
industries, biomedical wastes, mining industries, municipal solid waste. At times
it becomes essential to decontaminate soil. Broadly the soil decontamination is
done in two ways: (a) pump and treat in which the pollutant is pumped out using
external energy source, treated using methods such as incineration, radiation,
oxidation etc (b) removal of contaminated soil, treat it and then returning back to
its original place. This module is meant to briefly introduce various soil/ water
decontamination processes. The scientific basis and the reactions involved in
these processes are acid-base chemistry, solubility-precipitation, ion exchange,
redox, complexation, sorption, etc. which are discussed in module 2.
4.1 Contaminated site characterization/ assessment
Broadly, site characterization or contaminated site assessment (CSA) is
important for:
a) Determining concentration and spatial distribution of harmful pollutants under
consideration.
b) Determining the extent of site remediation (zonation) based on which the
suitable remediation technique is selected.
c) For assessing environmental and human health risk due to contamination.
More specifically, CSA is required to answer the following questions:
a) What is the source of contaminants?
b) What is the type and physical form of contaminants?
c) Spatial and depth wise extent of contamination
d) Whether the contaminants are stationery or movable?
e) If they are movable, then identify the significant pathways.
f) Identify the potential receptors of contaminants.
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4.2 Selection and planning of remediation methods
Fig. 4.1 (USEPA 1991) presents a flowchart on various processes
involved in the planning of site remediation.
Fig. 4.1 Processes involved in deciding contaminated site remediation
It can be noted from Fig. 4.1 that the most important step for making a decision
on site remediation is collection of data. Table 4.1 summarizes the essential data
to be collected as part of site reconnaissance and site characterization.
Table 4.1 Summary of data required for planning contaminated site remediation
Data Details Method of acquisition
1) Site history and land use pattern
a) Population density within 3 km from the contaminated site b) Proximity to important geographical features like airport, railways, river etc. c) Ownership of the land d) Extent of contamination
Field
2) Geologic and hydrologic
a) Topography b) Soil profile up to bed rock c) Information on aquifer d) Groundwater depth and flow direction
Field
3) Geotechnical a) Soil sampling and classification b) Permeability of soil c) Chemical characteristics of soil
Field Field Lab
Site reconnaissance: Assessment of distribution, reaction and migration potential
Site characterization and sampling
Mass balance analysis using predictive mathematical modeling
Sufficient information to decide remediation Select, evaluate and apply remediation
Lab/ field studies to understand distribution, reaction and migration
Sensitivity analysis to understand the effect of various design
parameters on remediation performance
Field verification of remediation effectiveness
Yes
Information sufficient to demonstrate
remediation optimization
No
No
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d) Soil strength Lab
4) Waste a) Water quality b) Identifying the type of contamination c) Concentration of contaminants d) Spatial extent of contamination e) Depth of contamination f) Contaminant retention characteristics g) Contaminant transport characteristics h) Hazard assessment and zonation
Field/ Lab Field/ Lab
Lab Field/ Lab Field/ Lab
Lab Lab Lab
4.3 Risk assessment of contaminated site
Risk assessment or hazard assessment is required to decide the extent of
contaminant remediation required for a particular site. The factors influencing risk
assessment are:
Toxicity
A material is deemed toxic when it produces detrimental effects on
biological tissues or associated process when organisms are exposed to
concentration above some prescribed level. Acute toxicity is the effect that
occurs immediately after exposure where as chronic toxicity deals with long term
effects. It is expressed as mass unit of toxicant dose per unit mass of receiving
organism. It must be noted that concentration is an important factor while
deciding toxicity. Only when a contaminant crosses a particular concentration, it
becomes toxic. If the concentration is within the prescribed limit then no
remediation need to be performed. Only those site which have toxic level of
contaminant concentration needs remediation. For example, toxic contamination
level leading to cancer becomes the basis for some of the site clean-up
programs.
Test protocols such as toxicity characteristics leaching procedure (TCLP)
(Method 1311, EPA) have been developed for extraction of chemicals from
wastes to verify whether the concentration is within the prescribed toxicity limit.
TCLP is designed to determine the mobility of both organic and inorganic
analytes present in liquid, solid, and multiphase wastes. Several regulatory
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agencies such as central pollution control board (CPCB), India, United States
Environmental Protection Agency (USEPA) have prescribed toxic concentration
levels for various chemicals that get leached from the waste samples by
conducting TCLP. In some cases, multiple extractions from the wastes become
necessary. For performing TCLP appropriate extraction fluid need to be used.
Glacial acetic acid mixed with water is used as the extraction fluid. In some cases
sodium hydroxide is also added. For detailed procedure, readers are advised to
refer to Method 1311, EPA.
Reactivity
It is the tendency to interact chemically with other substances. These
interactions become hazardous when it results in explosive reaction with water
and/or other substances and generate toxic gases.
Corrosivity
Corrosive contaminants degrade materials such as cells and tissues and
remove matter. It is defined as the ability of contaminant to deteriorate the
biological matter. Strong acids, bases, oxidants, dehydrating agents are
corrosive. pH < 2 or pH > 12.5 is considered as highly corrosive. Substances that
corrode steel at a rate of 6.35 mm/year is also considered hazardous.
Ignitability
It is the ease with which substance can burn. The temperature at which
the mixture of chemicals, vapour and air ignite is called the flash point of
chemical substances. Contaminants are classified as hazardous if it is easily
ingnitable or its flash point is low.
Based on the above four factors the risk associated with a particular site is
determined by specifying maximum acceptable risk using risk estimation
equations (Reddi and Inyang 2000). Risk assessment provides a numerical
quantification of the probability of harm from hazardous or toxic contamination.
Risk management uses this input of risk assessment in deciding how much
regulation and corrective measure need to be taken. The corrective action is
mostly the practice of remediation of the contaminated site. The maximum
possible concentration that could lead to the maximum acceptable risk is back
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calculated. If the level of concentration at a particular site is greater than the
maximum possible concentration, then it requires remediation. This approach
would clearly indicate the extent of remediation required for the contaminated
site. Appropriate remediation scheme is then selected to bring the concentration
level much less than the maximum possible concentration. Since risk
assessment and risk management is a very broad topic, it is difficult to discuss
the mathematical formulation in this course. Interested readers are requested to
go through additional literature (USEPA 1989; Asante-Duah 1996; Mohamed and
Antia 1998).
4.4 Remediation methods for soil and groundwater
Based on the toxic level of contaminants and the risk it pose to the
environment, a suitable remediation method is selected. It must be noted that the
remediation does not aim for entire decontamination. The major focus is to bring
the contamination level well below the regulatory toxic limit. This is done by
removing the toxic contaminants and/or immobilizing the contaminant that
prevents its movement through subsurface geoenvironment. The remediation
methods are broadly classified as physico-chemical, biological, electrical, thermal
and combination of these methods.
4.4.1 Physico-chemical methods
Removal and treatment of contaminated soil
One of the simplest physical methods for remediation is by removing the
contaminated soil and replacing it with clean soil. Essentially it is a dig, dump
and replace procedure. Such a method is practically possible only if the spatial
extent and depth of the contaminated region is small. The dug out contaminated
soil can be either disposed off in an engineered landfill or subjected to simple
washing as shown in Fig. 4.2.
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Fig. 4.2 Soil washing for granular soils contaminated with inorganic pollutant
However, washing procedure is mostly suitable for granular soils with less clay
content and contaminated with inorganic pollutants. For clay dominated soils, a
chemical dispersion agent need to be added to deflocculate and then chemical
washing is employed to break the retention of contaminants with the clay surface.
Incineration is suggested for soils contaminated with organic pollutants. In case,
it is necessary to remove organic pollutants then certain solvents or surfactants
are used as washing agents.
The method is directly applied in situ where solvent, surfactant solution or
water mixed with additives is used to wash the contaminants from the saturated
zone by injection and recovery system. The additives are used to enhance
contaminant release and mobility resulting in increased recovery and hence
decreased soil contamination.
Vacuum extraction
This method is one of the most widely used in situ treatment technologies.
The method is cost-effective but time consuming and ineffective in water
saturated soil. The technique, as depicted in Fig. 4.3, is useful for extracting
contaminated groundwater and soil vapour from a limited subsurface depth. The
contaminated water is then subjected to standard chemical and biological
treatment techniques. Vacuum technique is also useful when soil-water is
contaminated with volatile organic compound (VOC). The method is then termed
Contaminated soil Grinding/ dispersing
and slurry preparation
Filtering (liquid solid
separation)
Polluted water for
treatment
Clean solid for reuse
To disposal Reuse of clean water
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as “air sparging”. Sometimes biodegradation is clubbed with air sparging for
enhanced removal of VOC. Such a technique is then termed as biosparging.
Fig. 4.3 A schematic diagram for vacuum extraction procedure (Reddi and Inyang, 2000)
The vacuum extraction probe is always placed in the vadoze zone. The success
of the method depends on the volatilization of VOC from water into air present in
voids. An injecting medium is used to extract soil-water and/ or soil-air. When
oxygen is used instead of nitrogen as the injecting medium, it enhances aerobic
biodegradation.
Soil structure influences a lot on the passage of extracted water and
vapour and hence on the success of vacuum extraction technique. It is not only
important that the injecting medium is delivered efficiently but also the extracted
product reaches the exit with less hindrance. Granular soils provide better
passage where as the presence of clay and organic matter impedes the
transmission of both fluid and vapour. Organic matter provides high retention
leading to less volatilization. High density and water content also minimize
transmissivity. Apart from soil, the VOC properties such as solubility, sorption,
vapour pressure, concentration etc. also influence the extraction process.
Solidification and stabilization
This is the process of immobilizing toxic contaminants so that it does not
have any effect temporally and spatially. Stabilization-solidification (SS) is
Vadoze zone
Saturated zone
Nitrogen/
oxygen Nitrogen/
oxygen
Aeration Aeration
Contaminated
water
extraction
Water trap
Vapour
extraction Vacuum
Contaminated air for
treatment
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performed in single step or in two steps. In single step, the polluted soil is mixed
with a special binder so that polluted soil is fixed and rendered insoluble. In two
step process, the polluted soil is first made insoluble and non-reactive and in the
second step it is solidified. SS process is mostly justified for highly toxic
pollutants. In-situ SS process is mostly influenced by the transmissivity
characteristics of the soil, viscosity and setting time of the binder. Well
compacted soil, high clay and organic content do not favour in-situ SS.
In ex-situ methods, polluted soil is first grinded, dispersed, and then
mixed with binder material. The resultant SS material need to be disposed in a
well contained landfill. It is essential that the resultant SS product does not
undergo leaching. The common binders used in practice include cement, lime, fly
ash, clays, zeolites, pozzolonic products etc. Organic binders include bitumen,
polyethylene, epoxy and resins. These organic binders are used for soil
contaminated with organic pollutants.
Chemical decontamination
This method is mostly applicable for those soils which have high sorbed
concentration of inorganic heavy metals (IHM). The first process in this method is
to understand the nature of bonding between the pollutant and the soil surface. A
suitable extractant need to be selected for selective sequential extraction (SSE)
of IHM from the soil mass. The extractants include electrolytes, weak acids,
complexing agents, oxidizing and reducing agents, strong acids etc. The use of
these extractants in single or in combination will depend upon the concentration
of IHM and nature of the soil mass.
In-situ application (as depicted in Fig. 4.4) of extractants would remove
IHM from the soil surface and enter into the pore water. The pore water is
pumped and treated (pump and treat method) on the ground. While treating the
pumped water, both extractants and IHM are removed.
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Fig. 4.4 A schematic diagram for in-situ chemical decontamination
Another method is to allow the contaminated pore water to flow through a
permeable reactive barrier (PRB). Hence the placement of the barrier is
determined by the direction of flow of ground water. The material packed in the
barrier will retain IHM by exchange (sorption), complexation or precipitation
reaction. The transmission and the reaction time determine the thickness of the
reactive barrier to be provided. The material to be provided in the barrier is
influenced by the knowledge of IHM to be removed. This is mainly due to the fact
that the above mentioned reaction occurs differently when IHM is present as
single or as multiple species.
The successful use of PRB or treatment wall (TW) depends upon its
location such that majority of the contaminated groundwater flows through it. It is
essential to have a good knowledge on the hydrogeological conditions where
such barriers need to be placed. In some cases, sheet pile walls are used to
confine the flow towards the permeable barrier. Some of the materials used in
PRBs are exchange resins, activated carbon, zeolites, various biota, ferric
oxides, ferrous hydroxide etc. Hydraulic conductivity of the PRB should be
greater than or equal to the surrounding soil for proper permeation to occur. The
Ground surface
Ground water
flow direction
Row of injection wells
Extraction Permeable
reactive barrier
Contaminated
zone with
extractants and
IHM
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knowledge on reaction kinetics and permeability of the barrier would determine
the thickness of the wall to be provided such that enough residence time is
achieved for the removal reaction to occur.
4.4.2 Biological methods
Remediation by biological treatment is mostly applicable for soil
contaminated with organic pollutants and the process is termed as
bioremediation. In this method, certain soil microorganisms are used to
metabolize organic chemical compounds. In the process these microorganisms
degrade the contaminant. If naturally occurring microorganisms such as bacteria,
virus or fungi is not capable of producing enzymes required for bioremediation,
then genetically engineered microorganisms would be required. At the same
time, it should be ensured that such microorganisms do not produce any
undesirable effect on the geoenvironment (such as toxins). The process of
bioremediation is dependent on reactions such as microbial degradation,
hydrolysis, aerobic and anaerobic transformation, redox reaction, volatalization
etc. An example of bioremediation is discussed in the next section where in the
process is used for the remediation of oil spill land.
4.4.3 Electro-kinetic methods
Electro-kinetic methods are popular field method for decontaminating a
particular site by using electrical principles. The procedure is more effective for
granular type of soils. Two metal electrodes are inserted into the soil mass which
acts as anode and cathode. An electric field is established across these
electrodes that produces electronic conduction as well as charge transfer
between electrodes and solids in the soil-water system. This is achieved by
applying a low intensity direct current across electrode pairs which are positioned
on each side of the contaminated soil. The electric current results in
electrosmosis and ion migration resulting in the movement of contaminants from
one electrode to the other. Contaminants in the soil water or those which are
desorbed from the soil surface are transported to the electrodes depending upon
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their charges. Contaminants are then collected by a recovery system or
deposited at the electrodes. Sometimes, surfactants and complexing agents are
used to facilitate the process of contaminant movement. This method is
commercially used for the removal of heavy metals such as uranium, mercury etc
from the soil.
4.4.4 Thermal methods
Thermal methods include both high temperature (>5000C) and low
temperature (<5000C) methods and are mostly useful for contaminants with high
volatilization potential (Evangelou 1998). High temperature processes include
incineration, electric pyrolysis, and in-situ vitrification. Low temperature
treatments include low temperature incineration, thermal aeration, infrared
furnace treatment, thermal stripping. High temperature treatment involves
complete destruction of contaminants through oxidation. Low temperature
treatment increases the rate of phase transfer of contaminants from liquid to
gaseous phase there by causing contaminant separation from the soil. Radio
frequency (RF) heating is used for in situ thermal decontamination of soil having
volatile and semi-volatile organic contaminants. Steam stripping or thermal
stripping is another process useful for soils contaminated with volatile and semi-
volatile organic contaminants. It is an in situ process in which hot air, water or
steam is injected into the ground resulting in increased volatilization of
contaminants. Sometimes vacuum is applied to extract air or steam back to the
surface for further treatment. The effectiveness of this method is increased by the
use of chemical agents that are capable of increasing the volatility of the
contaminants. High cost and its ineffectiveness with some contaminants (with low
volatilization potential) make thermal method less attractive. Also, in some cases
incineration process produces more toxic gases.
4.5 Some examples of in-situ remediation
Harbottle et al. (2006) have compared the technical and environmental
impacts of taking no remedial action with those of two remediation technologies.
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The main objective of this study is to verify the sustainability of remediation
technologies. The two remediation technologies evaluated in this study are
solidification/ stabilization (S/S) and landfilling. In both these methods
contaminants are contained rather than destroyed. Therefore, it is extremely
important to analyze the long-term effect to avoid any potential problems in
future. In this study, sustainable remediation project is defined as the one that
satisfies the five criteria listed as follows:
Criterion 1: Future benefits outweigh the cost of remediation.
Criterion 2: Overall environmental impact of the remediation method is less than
the impact of leaving the land untreated.
Criterion 3: Environmental impact of remediation process is minimal and
measurable.
Criterion 4: The time-scale over which the environmental consequences occur is
part of the decision-making process.
Criterion 5: The decision making process
The site selected in this study was an industrial location polluted by BTEX
(benzene, toluene, ethylbenzene and xylene) and TPH (total petroleum
hydrocarbon). About 4400 m3 of contaminated soil has been remediated. The
stabilization mix used was cement:bentonite of 2.5:1 and water:dry grout of 3.8:1.
It was found that due to S/S, groundwater pollution reduced by 98 percent and
the leachate from S/S sample was well within the limit. S/S process resulted in
the increase in strength, reduction in permeability and increase in pH of the soil.
The same quantity of contaminated soil has been landfilled at a distance of 96
km from the source. In long term, S/S has been found to perform better than
landfill and no action taken for remediation. Other advantages of S/S are low
material usage, low off-site waste disposal, potential ground improvement for
immediate re-use, and lesser impact on the local community. However, the
contaminants remain on the site which increases the level of uncertainty in long
term. In the case of landfilling, long term impacts are less due to the fact that
contaminated soil is removed from the site. The resources that need to be
mobilized for landfill are more than S/S.
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Ludwig et al. (2011) have explained the use of permeable reactive barrier
(PRB) for the treatment of Cr6 in groundwater. PRB in the form of trench and fill
system, chemical redox curtain or organic carbon based biotic treatment zone
induce reduction condition for converting Cr6 to relatively immobile and non-toxic
Cr3. The most efficient trench and fill application is granular zero valent Iron (ZVI)
fillings, which rapidly converts Cr6 to Cr3. Alternatively, organic mulch and
compost has been used to initiate microbially active Cr6 reduction. However, the
use of organic matter as well as organic carbon does not have the longevity of
ZVI. The study quotes an example of ZVI based PRB installed at North Carolina
in 1996. This PRB is of 10 m depth, 0.6 m wide and 46 m long. This PRB is
found to treat groundwater containing Cr6 (approximately 15 mg/l concentration)
for more than 15 years. This study also quotes the use of chemical reducing
agent such as sodium dithionite at US department of energy, Hanford, site for
treating large Cr6 containing groundwater plume.
Asquith and Geary (2011) have compared bioremediation of petroleum
contaminated soil by three methods, namely, biostimulation, bioaugmentation
and surfactant addition. Bioremediation process depends on microbial activity for
biodegrading petroleum hydrocarbons. Since it is a natural process, it is a slow
reaction. The above mentioned three methods are used for increasing the rate of
bioremediation reaction. Biostimulation enhances the growth and activity of
microorganisms by the addition of nutrients and/or additives. Bioaugmentation is
the addition of hydrocarbon degrading microbial cultures. Surfactant addition
would enhance solubility, emulsify and disperse hydrophobic contaminants to
overcome the problem of low contaminant bioavailability. Sandy loam soil with
total petroleum hydrocarbon (TPH) > 30000mg/kg has been used to evaluate the
three methods. It was noted from this study that biostimulation with nutrients
enhanced bioremediation process. Organic amendments provided a better
bioremediation than inorganic amendments. Surfactant addition was found to
increase bioavailability of hydrocarbon and hence enhance bioremediation.
Ascenco (2009) has discussed about contaminated site characterization
and clean up based on two case studies. The first case study pertains to the
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excavation and washing of soil in an industrial estate site of 0.12 km2.
Preliminary investigation of the site revealed contamination upto a depth of 6m
with TPH, volatile aromatics such as toluene, ethylbenzene and xylene. Soil was
found to be free of heavy metals. A quantitative risk assessment indicated the
need for remediation. 40000 tonnes of soil was excavated from the affected site
and subjected to soil washing. Washing has been performed in a unit with a
capacity of 70 tonnes/ hour. Washed soil has been declared safe after adequate
laboratory testing and the clean soil reused in the site. The soil has been first
homogenized and sieved. The required surfactant and extracting agents were
mixed with water and used for soil washing. The waste water which comes out
after washing has been treated and reused. Contaminated sludge and fines after
waste water treatment and oversized soil mass rejected during sieving was
transferred to landfills.
The second case study is another industrial area of 3 km2 near Lisbon.
The industrial site comprised mainly of organic and inorganic chemistry industries
producing pesticides, acid, copper, lead, zinc, iron pyrites etc. The site consists
of 52000 tonnes of hazardous sludge from zinc metallurgy and iron pyrite ashes.
The site required investigation and remediation due to the placement of an
airport in the vicinity of this site. The groundwater exhibited high levels of arsenic,
lead, mercury, cadmium, copper, zinc, cobalt. In some areas the pH was as low
as 1, which increased metal mobility. The investigations were mainly focused on
developing a conceptual site model and environmental risk analysis for defining
remediation options. The efforts are still on for this particular site.
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References
1. Asante-Duah, D. K. (1996) “Management of Contaminated Site Problems”,
Lewis Publ., CRC Press Inc., Boca Raton, Florida.
2. Ascenco, C. (2009) “Contaminated site characterization and clean up-two
case studies”, NASA/c3p- 2009 International workshop on environment and
alternative energy: Global Collaboration in Environmental and Alternative
Energy Strategies” http://www.wspgroup.com/en/WSP-
Group/Sustainability/Case-Studies-3/Land- remediation (Website visited on 7-
11-2011)
3. Asquith, E. A. and Geary, P. (2011) “Comparative bioremediation of
petroleum hydrocarbon-contaminated soil by biostimulation, bioaugmentation
and surfactant addition”, 4th International Contaminated Site Remediation
Conference, Clean up 2011, Adelaide, South Australia, pp. 261-262.
4. Evangelou, V. P. (1998) “Environmental soil and water chemistry: principles
and applications”, Wiley-Inderscience, New York.
5. Harbottle, M. J., Al-Tabbaa, A. and Evans, C. W. (2006) “Assessing the true
technical/ environmental impacts of contaminated land remediation – a case
study of containment, disposal and no action”, Land, Contamination and
Reclamation, Vol. 14 (1), pp. 85-99.
6. Ludwig, R. D., Wilkin, R. T. and Su, C. (2011) “Treatment of Cr6 in
groundwater using prb systems”, 4th International Contaminated Site
Remediation Conference, Clean up 2011, Adelaide, South Australia, pp. 7-8.
http://www.cleanupconference.com/program.html (website visited on 7-11-
tension-infiltrometer/ (website visited on 4-1-2012).
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Model Questions
1. Prepare a review on different methods of soil contaminant analysis and clearly list its limitations.
2. The concentration of contaminant sorbed on the soil need to be determined. What are the different single and sequential procedures for extraction of contaminants from soil?
3. Based on the available information in literature, try to device a scheme for measuring electrical and thermal property of soil.
4. What are the uses of measuring electrical property of soil? 5. What is the difference between calibration and validation procedure? 6. Discuss about the dielectric and electrical properties of soil-water-
contaminant system and its important features. 7. Explain steady state and transient methods for measuring thermal properties
of soil. 8. What is application of thermal property of soil? 9. What are the factors influencing thermal and electrical property of soil? 10. What are the various methods used for measuring volumetric water content of
soil? 11. From the available literature, prepare the procedure for measuring
permeability using Guelph permeameter, tension and minidisk infiltrometer. 12. What are the different modeling approaches in geotechnical and
geoenvironmental engineering? Discuss the relative merits and demerits of each method.
13. What are the different geophysical methods for subsurface investigation/ 14. Explain the principle and working of ground penetrating radar for delineating
subsurface contamination. 15. Explain the philosophy of accelerated physical modeling and how the stress
similitude is achieved. 16. With respect to permeability of soil, demonstrate mathematically how
accelerated physical modeling is useful in studying any seepage induced phenomenon.
17. Suggest and justify a less time consuming procedure in the lab for obtaining advective-dispersive contaminant transport parameters for a compacted bentonite soil layer
18. A falling head permeability test is conducted in centrifuge. The details of falling head test is as follows: Area of stand pipe is 0.28 cm2. Area of soil column is 80 cm2. Length of soil column is 10 cm. There is a change in head from 90 cm to 84 cm for a time of 15 minutes. The centrifuge is rotated at 700 RPM. Effective radius is 50 cm. Determine prototype permeability, prototype length, model velocity and prototype velocity, prototype seepage velocity. (report all results in SI and time in seconds). Weight of wet soil sample is 1500 g and after oven drying the weight reduced to 1200 g. Specific gravity is 2.45. What will be the time taken in days if the same test is conducted at 1g.