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Soil contamination is the aftereffects of man-made chemicals or other alteration in the natural soil environment. It typically arises from the rupture of underground storage tanks, application of pesticides, and percolation of contaminated surface water to subsurface strata etc.
The most common chemicals involved are solventspetroleum hydrocarbons, pesticides, lead and other heavy metals. This occurrence of this phenomenon is mutually associted with the grade of industrializations and intensities of chemical usage.
The concern over soil contamination originates primarily from health risks, from direct contact with the contaminated soil, vapours from the contaminants, and from secondary contamination of water supplies within and underlying the soil.
Following are the objectives:
- To understand the action of soil contamination.
- To study the cause, effects and control measures of soil contamination.
- To study various remediation technology available for soil contamination.
- To test soil specimen for shear strength using Direct Shear Test
- Depending upon the contaminant of the soil specimen, to provide a remedial solution for the soil contamination.
2. SOIL CONTAMINATION: CAUSES AND EFFECTS
2.1 Soil Contamination
Soil contamination is the occurrence of pollutants in soil beyond a certain level causing a declination of one or more soil functions. Thus a reliable baseline and background concentrations of contaminants have to be determined.
2.2Classification of hazardous substances
Various toxic and hazardous substances are responsible for the soil contamination and degradation. Indication of these hazardous substances is of paramount importance so as to find the remedial measures. There are 3 classes specified as follows:
2.3 Causes of Soil contamination
Soil can be contaminated by natural conditions or human activities. Contamination of soils is, generally, a result of careless use of oil and chemicals.Mostly human activities are responsible for widespread contamination. Few of the causes of the soil contamination as a result of human activities as follows.2
- Improper and irresponsible waste disposal&Uncontrolled waste water disposal
- Incorrect storage of products and/or waste in industrial activities
- Road accidents in goods transport
- Leaks from tanks or faulty operations
- Unlimited and improper use of pesticides
- Uncontrolled air pollution
2.4 Effects of Soil Contamination
Soil contamination can cause severe health problems for human beings, other animals as well as environment. Some of the effects of the soil contamination on environment as well as human beings are listed below.
- Reduced soil fertility & nitrogen fixation.
- Deposition of silt in tanks and reservoirs
- Reduced crop yield and increased erosion.
- Ecological imbalance & reduced vegetation
- Release of pollutant gases and radioactive rays causing health risks.
- Entrance of dangerous chemicals underground water.
- Clogging of drains and Inundation of areas.
- Release of gases & Foul smell.
- Waste management problems.
2.5 Health Risks
The major issue is that there are lots of sensitive land uses where people are in direct contact with soils like parks, residences, schools and playgrounds. Other contact mechanisms include inhalation of soil contaminants which have been vaporized or contamination of drinking water. There is a very large set of health risks from vulnerability to soil contamination depending on type ofpollutant, pathway of attack and vulnerability of the exposed population. Following are the risks:
- Contaminated soil can be discharged to municipal waterways. Thus, it accumulates in water used by human being for various purposes.
- Hazardous substances which are eluted from contaminated soil may get ingested through groundwater.
- There can be direct ingestion of contaminated soil including soil particulate.
- Hazardous substances which are emitted by contaminated soil to atmosphere may get inhaled and cause serious health issues.
- Hazardous substances on crops & the livestock on contaminated land can get accumulated and ingested by human beings.
2.6 Control Measures for Soil Contamination
Due to increased soil pollution and rising adverse effects on the environment and risk in safety, control measures are required for checking this problem. Following are the few:
- Recycling of plastic waste & purification of wastewater.
- Waste landfill control & ban on deforestation.
- Industries provided with adequate prevention and protection measures.
- Newer sewer networks as far as possible.
- Liquid manure storage basin provided as far as possible.
3. REMEDIAL SOULTIONS
3.1 Soil Remediation
Soilremediationis the collective term for various strategies that are used to purify and regeneratesoil. It is part of a wider effort i.e. environmental remediation, which also includes efforts to purify the air and the damage done to the ecological balance of the planet.
3.2 Soil Remediation Technologies
There are two distinct classes of soil remediation: in-situ, or on-site, and ex-situ, or off- site. On-site cleanup measures are generally preferred as they are cheaper. Ex-situ remediation has the added incentive of taking the mass of contaminants off-site before they can spread further.More caustic chemicals can be used & acid leaching can be done ex-situ without any harm to the environment.
In-situ situations are limited as only the topside of the soil is accessible. The environmental limitations force in-situ remediation to fall into three categories: bioremediation, washing & venting.
Off-site facilities have the opulence of complete control over the cleaning chemical measures. Isolation of soil is easily possible in a controlled environment, without weather problems.
3.3 IMPORTANT REMEDIAL SOLUTIONS
3.3.1 Solidification and Stabilization
Stabilization/Solidificationis a remediation technology which relies on the reaction between a binder and soil to prevent the mobility of contaminants.
Stabilization - requires the addition of reagents to a contaminated material (e.g. soil or sludge) to obtain more chemically stable constituents.
Solidification - involves the addition of reagents to a contaminated material to give physical stability and to contain the contaminants in a solid product and abridge access by external agents (e.g. air, rainfall).
However, the uptake of S/S technologies has been relatively modest, and a number of barriers have been identified including:
- The relatively low cost and widespread use of disposal to landfill;
- Uncertainty over the durability and rate of contaminant release from S/S-treated material;
- Experiences of past poor practice in the application of cement stabilization processes used in waste disposal in the 1980s and 1990s.
- Residual liability associated with immobilized contaminants remaining on-site, rather than their removal or destruction.
3.3.2 Excavation or Dredging
Excavation processes can be very simple such as hauling the contaminated soil and maintain a regulated landfill. The process of aeration is applicable to Volatile Organic Compounds (VOCs). Dredging of bay mud or other silty clays containing contaminant can be carried out in case of contamination of a river soil or bay bottom.
Nowadays, because of the advance processes like "bio-augmentation" and "bio-stimulation" of the excavated material, it is possible to remediate semi-volatile compounds (SVOCs) onsite.
3.3.3 In-Situ Oxidation
This technology has become very popular and widely accepted due to its application for a wide range of soil and groundwater contaminants. Various types of oxidants such as hydrogen peroxide, potassium permanganate, ozone gas are injected in the process of chemical oxidation. Oxygen gas can also be used in the process. But also, the injected gases can cause the contamination to spread faster than normal.
3.3.4 Pump and treat
Pump and treat involves pumping out contaminated groundwater with the use of a submersible or vacuum pump, and allowing the extracted groundwater to be purified by slowly proceeding through a series of vessels that contain materials designed to adsorb the contaminants from the groundwater. For petroleum-contaminated sites this material is usually activated carbon in granular form.
Depending on geology and soil type, pump and treat may be a good method to quickly reduce high concentrations of pollutants. It is more difficult to reach sufficiently low concentrations to satisfy remediation standards, due to the equilibrium of absorption (chemistry)/desorption processes in the soil.
3.3.5 Soil Vapor Extraction
Soil vapor extraction (SVE) is an in situ unsaturated zone soil remediation technology in which a vacuum is applied to the soil to induce the controlled flow of air and remove volatile and some semi volatile contaminants from the soil. The gas leaving the soil may be treated to recover or destroy the contaminants, depending on local and state air discharge regulations
Granular activates carbon is used as a filter for water. This activated carbon is positively charged and thus removes the negative ions such as ozone, chlorine, fluorides. This carbon must be replaced periodically. It is not that much effective in removing heavy metals.
3.3.6 Soil Washing
Soil washing uses water to remove contaminants from soils. The principle of soil washing lies in separating the most polluted portion from the cleaner portion of the soil by scrubbing it. This scrubbing process reduces the amount that needs to be cleaned, and we get two fractions of soil; clean and polluted.
After understanding the different technologies applicable for the remediation, we have to identify the suitable technique for the soil. General selection criterions followed are:
The selection of a suitable remedial technology, thefollowing steps are necessary (US EPA 1991):
- Determination of remedial objectives and identification of response action: This step identifies existing site problems using preliminary remedial investigation data
- Technology screening: This step identifies and screens potential applicable remedial technologies. It focuses on eliminating those technologies which have severe limitations for a given set of waste-and site-specific conditions.
- Development and screening of remedial action alternatives: These alternatives represent workable number of options that appear to adequately address all site problems.
- Detailed evaluation of remedial action alternatives, and remedial alternative selection: The alternative technologies are then subjected to detailed evaluation. For each remedial alternative, the following factors are analysed:
- Technical considerations
- Environmental concerns
- Public health concerns.
- Institutional concerns.
- Remedial alternative selection: After detailed evaluation of action alternatives, each of the alternatives is summarized relative to each of the criteria described above.
These criteria should limit improper and irresponsible use of the solutions.
4. MODEL ASSESSMENT USING DIRECT SHEAR TEST
The shear strength is one of the most important engineering properties of a soil, because it is required whenever a structure is dependent on the soil's shearing resistance. The shear strength is needed for engineering situations such as determining the stability of slopes or cuts, finding the bearing capacity for foundations etc.
4.2 Direct Shear Test
The direct shear test is one of the oldest strength tests for soils. In this laboratory, a direct shear device will be used to determine the shear strength of cohesion less soil. From the plot of the shear stress versus the horizontal displacement, the maximum shear stress is obtained for a specific vertical confining stress. After the experiment is run several times for various vertical-confining stresses, a plot of the maximum shear stresses versus the vertical (normal) confining stresses for each of the tests is produced. From the plot, a straight-line approximation of the Mohr-Coulomb failure envelope curve can be drawn, f may be determined, and, for cohesion less soils (c = 0), the shear strength can be computed from the following equation:
s = stanf
Equipments:Direct shear device, Load and deformation dial gauges, Balance.
The procedure which is followed in performing direct shear test is as follows:
- Measure inner side or diameter of shear box and find the area.
- Make sure top and bottom halves of shear box are in contact and fixed together.
- Weigh out 150 g of sand.
- Place the soil in three layers in the mold using the funnel. Compact the soil with 20 blows per layer.
- Place cover on top of sand.
- Place shear box in machine.
- Apply normal force.
Two soil specimens were chosen for the test.
Sample one: Clean and not contaminated with weight of 16 kg
Sample two: Contaminated with fuel ash with weight of 8 kg
- Determine the dry unit weight.
- Calculate the void ratio, e
- Calculate the Normal stress and Shear stress
As explained in the procedure above, two soil samples were tested. After the test, following results were obtained:
- Load at failure for sample one = 57.6 N
- Load at failure for sample two = 32 N
The contaminated sample will be used for the design project to asses it is commercial viability.
4.5 Remedial Solution
In order to get the remedial solution to the given problem with fuel ash as a contaminant, some more information is required concerning the physical and chemical properties, the distribution and migration behavior of the contaminants as well as concerning the geological and hydro-geological conditions of the contaminated site. So, the steps given below should get us to the remedial solution.
- State of aggregation and properties like stability, water solubility of the pollutant fuel ash.
- Location of the fuel ash i.e. in the soil-air, solved in the soil-water, adsorbed at soil particles or located in all phases.
- Description of soil payers
- Hydro-chemical characterization of the soil water
So, now the next step is to carry out an assessment of the resulting environmental risk depending upon the above mentioned steps. For acute high risks, in-situ procedures can be excluded as they are time consuming. So, final remedial solution should be selected considering the aspects such as costs and requirement of local authority.
- The above report comprises of the discussion of Soil contamination with its causes and effects.
- Various remediation technologies have been developed and used extensively. Appropriate technology is chosen according to the type of contaminant and environmental requirement.
- For the example assessment, a contaminated soil sample with fuel ash is chosen which is tested for its shear strength using Direct Shear Test.
- Results of the direct shear test are presented in terms of the load at failure for two different soil specimens. From results, it is inferred that the load at the failure for the contaminated soil specimen is quite less than that of clean soil specimen.
- As observed due to the various unwanted components the shear strength of the contaminated soil is reduced, thereby hampering its utility and value.
- Using the results of test and environmental conditions, the appropriate solution can be selected in accordance with the selection criteria.
- Soil is an essential component of natural ecosystems as environmental sustainability relies largely on soil ecosystem. Therefore, the contamination of soil is a very big social problem because of its importance for environment protection and human health.
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- Engineering Properties of Soils Based on Laboratory Testing ,Prof. Krishna Reddy, UIC