Chapter two Literature review Petroleum products

Chapter two
Literature review
Petroleum products
Composition of Petroleum Products
Petroleum is a naturally occurring, yellow-to-black liquid found in geological formations beneath the Earth’s surface. It is commonly refined into various types of fuels. Components of petroleum are separated using a technique called fractional distillation i.e. separation of a liquid mixture into fractions differing in boiling point by means of distillation, typically using a fractionating column.
It consists of hydrocarbons of various molecular weights and other organic compounds (EIA, 2018). The name petroleum covers both naturally occurring unprocessed crude oil and petroleum products that are made up of refined crude oil. A fossil fuel, petroleum is formed when large quantities of dead organisms, usually zooplankton and algae, are buried underneath sedimentary rock and subjected to both intense heat and pressure.
Petroleum has mostly been recovered by oil drilling (natural petroleum springs are rare). Drilling is carried out after studies of structural geology (at the reservoir scale), sedimentary basin analysis, and reservoir characterization (mainly in terms of the porosity and permeability of geologic reservoir structures) have been completed (Guerriero V, et al. 2011 and 2012). It is refined and separated, most easily by distillation, into a large number of consumer products, from gasoline (petrol) and kerosene to asphalt and chemical reagents used to make plastics and pharmaceuticals. Petroleum is used in manufacturing a wide variety of materials, and it is estimated that the world consumes about 95 million barrels each day.

Fig. 2.1 Separation of crude oil
Properties of petroleum products that affect their transport
There are several properties that determine the transport of contaminants in the environment. These properties include density, viscosity, solubility, vapour pressure, volatility, interfacial tension and wettability.
Density the volumetric mass density, of a substance is its mass per unit volume. The density, or more precisely, the volumetric mass density, of a substance is its mass per unit volume. It is the ratio of its mass to its volume and they vary in their molecular weight. Interaction and structure (Lyman et al., 1990). The symbol most often used for density is ? (the lower case Greek letter rho), although the Latin letter D can also be used.
Equ.. 2.1
When considering the transport of contaminant in the environment, the density of such contaminant is used to ascertain the pattern at which it migrates. Density is related in terms of specific gravity, which means that the density of a substance to compared to the density of a standard, in the case of liquid solid, water is usually used while air serve as the standard for gas.
The viscosity of a fluid is the measure of its resistance to gradual deformation by shear stress or tensile stress. For liquids, it corresponds to the informal concept of “thickness”: for example, honey has a higher viscosity than water (Symon, Keith 1971). Viscosity is the property of a fluid which opposes the relative motion between two surfaces of the fluid that are moving at different velocities. In simple terms, viscosity means friction between the molecules of fluid. When the fluid is forced through a tube, the particles which compose the fluid generally move more quickly near the tube’s axis and more slowly near its walls; therefore some stress (such as a pressure difference between the two ends of the tube) is needed to overcome the friction between particle layers to keep the fluid moving. For a given velocity pattern, the stress required is proportional to the fluid’s viscosity. Values of viscosity for organic liquids generally range from 0.3 to 20Nsm2 at ambient temperatures, while water has a viscosity of 1Ns/m2 at 20oC (Lyman et al., 1990). The lower the viscosity, the more readily a fluid will penetrate a porous medium (Huling and Weaver, 1991).
It is measured in terms of the maximum amount of solute dissolved in a solvent at equilibrium. The resulting solution is called a saturated solution. Solubility is defined as the maximum quantity of a substance that may be dissolved in another. It is the maximum amount of solute that may be dissolved in a solvent at equilibrium, which produces a saturated solution. When certain conditions are met, additional solute may be dissolved beyond the equilibrium solubility point, which produces a supersaturated solution.
Factors that affect the solubility of compounds include temperature, salinity, cosolvents, dissolved organic matter and pH. Most organic compounds become more soluble as the temperature increases, but some behave in the opposite way. The aqueous solubility of compounds decreases with increasing molecules weight and structural complexity (Huling and Weaver, 1991). For a contaminant, such as gasoline that comprises a mixture of range of compounds, solubility will lead to the rapid loss of the more soluble compounds, leaving behind the less soluble compounds.
Vapour pressure
The vapor pressure (P°) is the pressure of the vapor of a compound in equilibrium with its pure condensed phase (solid or liquid). Vapour pressures depend strongly on the temperature and vary widely with different compounds due to differences in molecule – molecule interactions. The normal boiling point of a liquid is defined as the temperature at which the vapor pressure of the liquid is 1 atmosphere (P°= 1 atm). The vapour pressure of a substance is an intrinsic physical property that plays a crucial role in determining its distribution to and from gaseous environmental phases (the atmosphere, marsh bubble gas). The vapour pressure is also crucial for the prediction of equilibrium distribution coefficients to and from natural waters, such as Henry’s Law constant (KH).
A method for measuring the vapor pressures of a wide range of materials using a conventional thermobalance and standard sample holders is described. The equipment is calibrated using pure reference materials of known vapor pressure and exploiting the relationship between volatilization rate and vapor pressure based on the Langmuir equation for free evaporation.

The vapour pressure of a compound determines how readily vapours volatilies from the pue liquid phase (Fetter, 1999; Lyman et al., 1990). It is generally reported as the pressure of the gas in equilibrium with the liquid at a given temperature (Hemond and Fechner-Levy, 2000; Munowitz, 2000; Schwarzenbach et al., 1993). Vapour pressure is a vital tool in predicting the behaviour and fate of chemicals that are introduced into the environment (Lyman et al., 1990; Schwarzenbach et al., 1993), since the persistence of chemicals that have been absorbed in the soil highly dependent on vapour pressure (Lyman et al., 1990). For example, when a chemical has been spilled, knowledge of the vapour pressure of the chemical is crucial in order to estimate its rate of evaporation or volatility.
The vapour pressure of contaminants affects their partitioning and volatilization rates and has been used to categorize contaminants into volatile, semivolatile and nonvolatile. Contaminants with vapour pressure greater than 10-2kpa are termed volatile while those with vapour pressure values between 10-5 and 10-2kpa and less than 10-5kpa are classified as semivolatile organic contaminants in the environment has revealed that vapour pressure is a key property that controls the transport rate of organic contaminants (Mackay and Wania, 1995).
Volatility is quantified by the tendency of a substance to vaporize. Volatility is directly related to a substance’s vapor pressure. At a given temperature, a substance with higher vapor pressure vaporizes more readily than a substance with a lower vapor pressure (James G. SpeightandKister, Henry Z. 2006).Knowledge of volatilization rates is necessary to determine the amount of a contaminant that enters the gas phase and the change of the contaminant concentrations in soils and water bodies. In case of spills or purposeful application of a chemical to the soil, the period of time the chemical persist in the soil is determined to a large extent by the rate of volatilization of the chemical. The rate at which a chemical volatilizes from soil is affected by its chemical properties, the soil properties and surrounding conditions. Some of the chemical properties involved during volatilization include vapour pressure, aqueous solubility, molecular weight and molecular structure. The soil and environmental properties that affect the volatilization rate of a contaminant are its concentration in the soil, the soil water content, the airflow rate over the surface, humidity, temperature, sorption and diffusion characteristics of the soil, bulk properties of the soil, such as organic matter content, porosity, density and clay content. All of these factors affect the distribution of a compound between the soil solid, soil water and soil air phases (Lyman, 1990).
Wettability is the tendency of one fluid to spread on, or adhere to, a solid surface in the presence of other immiscible fluids. Wettability refers to the interaction between fluid and solid phases. In a reservoir rock the liquid phase can be water or oil or gas, and the solid phase is the rock mineral assemblage. Wettability is defined by the contact angle of the fluid with the solid phase. A wetting fluid will tend to occupy the smaller pore spaces, while a non-wetting fluid will tend to be restricted to the largest interconnected pore spaces (Mercer and Cohen, 1990).
Transport of petroleum products
Infiltration is the movement of water into the soil from the surface. The water is driven into the porous soil by force of gravity and capillary attraction. First the water wets soil grains and then the extra water moves down due to resulting gravitational force. The rate at which a given soil can absorb water at given time is called infiltration rate and it depends on soil characteristics such as soil texture, hydraulic conductivity, soil structure, vegetation cover etc. the infiltration plays an important role in generation of runoff volume, if infiltration rate of given soil is less than intensity of rainfall then it results in either accumulation of water on soil surface or in runoff. The different soil conditions affect the soil infiltration rate. Compacted soils due to movement of agricultural machines have a low infiltration rate which is prone to runoff generation.
Infiltration is a key process that helps in determining the quantity of contaminant spill that enters the soil, whereas the rate of infiltration depends on pore size distribution, state of dryness or wetness of the soil and hydraulic conductivity of the soil (Sumner, 2000). The Environmental Protection Agency (EPA) defines natural attenuation as “a variety of physical, chemical, or biological processes that, under favorable conditions, act without human intervention to reduce the mass, toxicity, mobility, volume, or concentration of contaminants in soil or groundwater. These in situ processes include biodegradation; dispersion; dilution; sorption; volatilization; radioactive decay; and chemical or biological stabilization, transformation, or destruction of contaminants” EPA, 1999 (Figure 1.2).

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Fig. 2.2 Conceptual illustration of the important natural attenuation processes that affect the fate of petroleum products in aquifers

As illustrated by Powers et al., (2001b), the infiltration of gasoline through the vadose zone is one of the transport processes that affects its transport in the subsurface. When released to the soil, either from leaks or spills, gasoline infiltrates down through the unsaturated zone due to gravity. This is accompanied to some extent by lateral spreading due to the effect of capillary forces and medium spatial variability, with a fraction of it being retained in the pore spaces due to interfacial forces.
The transport of gasoline in the subsurface is affected by the following factors;
i. Properties of the gasoline
ii. Volume of the gasoline
iii. Subsurface flow conditions
iv. Time and duration of release
v. Area of infiltration
Ugwoha et al., (2016) reported that the transport of kerosene compounds to groundwater after accidental release will be greater with coarse textured soils than fine textured soil and the transport of kerosene compounds in different soil types will be in the order of sand;silt;clay.
Various studies have shown that contaminants, such as hydrocarbons, can be transported from the saturated zone to the groundwater via diffusion (Pasteris et al., 2002; Dakhel et al., 2003; Powers and Heermann, 1999; Hemond and Fechner-Levy, 2000; Lahvis, 2003).According to Bhandari (2007), the diffusion of contaminants is influence by the tortuosity of diffusion path. Other factors affecting diffusion rate are the properties of both the contaminants and soil. According to Yu (1995), diffusion is a major driving force behind subsurface contaminants transport. Diffusion is readily observed in the subsurface environment and the larger the amount of contaminant the lager and farther the effects of diffusion can be (Miller and Hogan, 1997).
Environment concerns of petroleum products
Petroleum and its by-products are used to fuel various forms of transportation. Over the years there has been increased concerns over the environmental effects of the petroleum industry. The environmental impacts of petroleum are mainly negative. This is due to the toxicity of petroleum which contributes to air pollution, acid rain, and various illnesses in humans. Petroleum also fuels climate change, due to the increased greenhouse gas emissions in its extraction, refinement, transport and consumption phases. The toxicity of oils can be understood using the toxic potential or the toxicity of each individual component of oil at the water solubility of that component. (Di Toro, Dominic M.; McGrath, Joy A.; Stubblefield, William A. (2007). Oil exploration, production and processing represent prime sources of exposure to petroleum products. But there are other possible sources, which includes vehicle and generator emissions, burning of vegetable and trash, food processing and use of cooking fuels.
Impact on the environment
In general, spilled oil can affect animals and plants in two ways: dir??t from the oil and from the response or cleanup process. (Bautista H. and Rahman K. M. M. 2016) (Sarbatly R.; Kamin, Z. ; Krishnaiah D. 2016). There is no clear relationship between the amount of oil in the aquatic environment and the likely impact on biodiversity. A smaller spill at the wrong time/wrong season and in a sensitive environment may prove much more harmful than a larger spill at another time of the year in another or even the same environment (Bautista, H.; Rahman, K. M. M. 2016). Oil penetrates into the structure of the plumage of birds and the fur of mammals, reducing their insulating ability, and making them more vulnerable to temperature fluctuations and much less buoyant in the water.
Animals who rely on scent to find their babies or mothers cannot due to the strong scent of the oil. This causes a baby to be rejected and abandoned, leaving the babies to starve and eventually die. Oil can impair a bird’s ability to fly, preventing it from foraging or escaping from predators. As they preen, birds may ingest the oil coating their feathers, irritating the digestive tract, altering liver function, and causing kidney damage. Together with their diminished foraging capacity, this can rapidly result in dehydration and metabolic imbalance. Some birds exposed to petroleum also experience changes in their hormonal balance, including changes in their luteinizing protein (C. Michael Hogan 2008). The majority of birds affected by oil spills die from complications without human intervention (Dunnet, G.; Crisp, D.; Conan, G.; Bourne, W. 1982)
Impact on water
Petroleum product can enter water through direct spills or from a spill originally occurring on land and subsequently reaching water bodies through the effects of rain, wind, surface or sub-surface flow. Not minding the means of entry, there will be adverse impacts though the nature and severity of such impacts is dependent on the specific chemical composition and physical characteristics of the product involved and the degree of concentration/dilution.
Toxic pollutants in water refer to a whole array of chemical which are leached into ground water or which are discharged directly into rivers. Contamination of aquatic environment by crude oil and petroleum products constitute an additional source of stress to aquatic organisms (Omoregie et al., 1997) and is of importance to wetland environment. Water pollutants can also include excessive amounts of heavy metals, radioactive isotopes, faecal coliform bacteria, phosphorus, nitrogen, sodium and other useful (even necessary) elements as well as certain pathogenic bacteria and viruses (Botkin and Keller, 1998). The water environment experiences many dynamic changes induced by various natural events such as the spillage of toxic chemicals that may have significant impact on aquatic life (Camougis, 1981).
Baker (1970) reported that oil pollution effects vary according to the type and amount of oil involved, the degree of weathering, the time of the year, the plant species concerned and the age of the plant. Cowell (1977) include the physical and chemical properties of the oil as well as the quantity of the water being polluted. The water soluble fraction of crude oils has been found to reduce the growth rate of biomas turnover of some marcophytes (Gunlack and Hayes, 1977). Kauss and Hutchinson (1975) found that aquatic macrophytes population was reduced in the presence of water-soluble petroleum components. The inhibitory effects of petroleum components are known to be dependent upon the concentration of the petroleum product as well as that of water soluble components (Shew, 1977).
Impact on soil
Hydrocarbon (petroleum products such as Kerosene, Fuel and gasoline) pollution of soil can occur in several ways, from natural seepage of hydrocarbons in areas where petroleum is found in shallow reservoirs, to accidental spillage of crude oil on the ground. Not minding the source of contamination, once hydrocarbons (kerosene, fuel or gasoline) come into contact with the soil, they alter its physical and chemical properties. The degree at which the soil is altered depends on the type, the quantity spilled and the specific composition of the hydrocarbon spilled. Contaminated soil can affect the health of organisms through direct contact or via ingestion or inhalation of soil contaminants which have been vaporized. Soil also acts as a reservoir of residual pollution.
Impact on Human
Petroleum products can enter into the human body when they breathe in air, eat fish, bathe, drink water or accidentally eat or touch soil or sediment that is contaminated with oil. Oil spills may occur all around us. Since oil products include a lot of common fuels, it is obvious that oil spills may happen at high rates and in many locations, including residential areas. Surface oil spills are easy to identify and will leave visible traces such as oil stains, as well as other characteristic signs such as odors due to the vapors emitted by the spilled oil. The underground oil spills are more difficult to catch and yet may be more problematic (oil may reach groundwater more easily and travel with it). Both surface and underground oil spills have the potential to contaminate soils, sediment, water (groundwater and surface water bodies), and air (due to many volatile compounds emitted by the spilled oil into the air).
Oil spills have negative impacts on the residents of the affected areas. These effects can be aggravated by severe weather conditions. An example in this sense is the Murphy oil spill (due to the failure of a storage tank at the Murphy Oil USA refinery) that had particularly affected residential areas in Louisiana. This happened because of the contamination of flood waters following the levee breaks during Hurricane Katrina in August 2005.
More than 1 million gallons of mixed crude oil were released from the Murphy refinery tank. The flooding enabled the spreading of spilled oil over larger areas, affecting about 1,700 homes in several residential neighborhoods.
Effects on the General Population
The effects of oil spills on humans may be direct and indirect, depending on the type of contact with the oil spill.
Direct exposure to oil spills – occurs close to where people live or work and where they may come in contact with oil spill components:
i. By breathing contaminated air – since oil and products (petroleum products) have many volatile compounds which are emitted as gases from spilled oil, the air becomes contaminated with those volatile oil products or vapors producing specific odors. Even when odors are not felt, a health risk may exist for some individual compounds if residents are exposed (breath the air) for a long time. Of course, when the smell is obvious the health risk increases. Once in the air, contamination may travel over long distances. Of course, that vapors will also become more diluted with the distance traveled. So, the original contamination levels at the source along with specific weather conditions may dictate the final spreading of oil contaminated air vapors.
ii. By direct contact with the skin – people may come in direct contact with oil and/or oil products while walking in a contaminated area (e.g., beach). An initial irritation will be obvious. Additionally, contaminants may be absorbed through the skin and enter the body
Indirect exposure to oil spills – even when people live in places far from where the actual oil spill took place:
i. By bathing in contaminated water – for example swimming in a contaminated water stream – even when an oil sheen may not be visible, dissolved oil contaminants may exist in the water if it was impacted by an oil spill
ii. By eating contaminated food – some oil compounds bioaccumulate in living organisms and may become more concentrated along the food chain. Humans may become exposed to concentrations of contaminants in the food that could be orders of magnitude higher than in the contaminated environment. This is especially problematic since residents could be exposed even if they live far away from an oil spill if they consume food coming from a spill affected area
The main oil spill effects include a variety of diseases, negative economic impact, pollution with crude oil or petroleum products (distillates such as: gasoline, diesel products, jet fuels, kerosene, fuel oil, as well as heavy distillates like hydraulic and lubricating oils) and the aesthetic issues that affect the residents of the affected areas in multiple ways.
The negative economic impact is a major effect of oil spill pollution. It can affect the community where the oil spill occurred in a number of ways, among which the following are the most important:
i. Long-term ceasing of activities such as fishing in the polluted waters that affects fishermen and fisheries if a very large amount of oil is spilled; for example, the BP oil spill in the Gulf of Mexico had already impacted many local fishermen’s and fisheries’ normal activity, and this looks like a long-term effect due to the very large amount of spilled oil. Effects on local fishermen are detailed below.
ii. Property value reduction depends on the magnitude of the oil spill and affects all the properties in a certain area exposed to oil spill pollution; this negative effect on property value applies not only to those properties directly affected by the oil spill, but to all the properties in a certain area exposed to oil spill pollution or at risk of becoming polluted at some point in time
iii. The reduction of tourism in the affected areas;
iv. The disturbance of land and sea traffic, which affects import-export activities;
The aesthetic and recreational impact is related to the visible effects of oil spill pollution (oil slick, sheens) appearing on coast waters, shoreline, and beaches, wetlands, etc. When more serious, the complete closure of such recreational areas may occur, at least temporary, until the spill is removed and the cleanup process ends.
Aguilera et al (2010) reviewed human health evaluations associated with oil spills around the world and found that most provided evidence of a relationship between exposure to spilled oils and acute physical and psychological effects, as well as possible genetoxic and endocrine effects. Effects of oil exposure on the developing foetus are also not well understood, although adverse effect have been observed in studies involving petroleum product ( P.M.S, Kerosene, Diesel etc.) (IOM, 2010; Edward et al 2010).
Soil Types
Soil can be seen as the organic and inorganic materials on the surface of the earth. It can also be defined as an assemblage of discrete particles in the form of a deposit, usually of mineral composition, but sometime of organic origin, which can be separated be separated by gentle mechanical means and that includes variable amount of water and air (BS 1377-1, 1990). Soil serve as an important component as a bio-physico-chemical reactor that decomposes dead biological materials and recycles them into nutrients. (Hillel, 1998).
Sandy Soils
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles. A Sandy soil contains 70% sand with particle ranging in diameter from 0.0625mm to 2mm, 15% clay. Sandy soil are loosed, absorb water rapidly and drain it quickly, its well aerated and can be worked on easily both moist and dry conditions.
Loamy Soils
Loam is soil composed mostly of sand (particle size > 63 µm), silt (particle size > 2 µm), and a smaller amount of clay(particle size < 2 µm). A loam soil in which the sand is dominant is referred to as sandy loam.
Silty Soils
Silty soils are usually well-aggregated, but the aggregates break down rapidly when wetted, allowing nonaggregated soil particles to be easily transported. Soils containing large stable aggregates are difficult to detach and transport and usually have greater infiltration rates. It is made up of intermediate sized particles and are considered to be fertile soils.
Properties of Soil Affecting Transport of Contaminats
Most soil properties provide important sets of reactions and interactions between soils and contaminants and it changes the transport of the contaminants within the soil (Yong, 2001). The soil properties include specific surface area, particle size distribution, bulk density, porosity, water content, Particle density and organic matter as discussed below
Particle size distribution
This can referred to as the percentage of the various grain sizes present in a soil gotten by sieving and sedimentation (BS1377-1, 1990). Particle size distribution of a material can be important in understanding its physical and chemical properties. It affects the strength and load-bearing properties of rocks and soils. It affects the reactivity of solids participating in chemical reactions, and needs to be tightly controlled in many industrial products such as the manufacture of printer toner, cosmetics, and pharmaceutical products.
Particle density (ps)
Particle density can be said to be the average mass per unit volume of the solid particles in a sample of soil (BS1377-1, 1990). It is also expressed as the ratio of the total mass of the solid particle to their total volume excluding void and water (Burke et al, 1986).
Specific Surface Area
The specific surface area of a soil reflects the surface area available for adsorption (Site, 2001). It can also be seen as a property of solids defined as the total surface area of a material per unit of mass, (with units of m²/kg or m²/g) or solid or bulk volume (units of m²/m³ or m?1). It is a derived scientific value that can be used to determine the type and properties of a material (e.g. soil or snow).
Soil Water Content
The soil water content can be seen as the mass of water which can be removed from a soil, by heating it at 1500C, expressed as a percentage of the dry mass (BS1377-1, 1990). Soil water content, we usually refer to either moisture retention between wilting point and field capacity or between any other two soil moisture constants. But water holding capacity is worked beyond field capacity. According to Ong and Lion (1991), the moisture of the unsaturated zone may range from fairly dry at surface to saturation at the capillary fringe of the water table. And for most of the unsaturated zone, it can be assumed that soils are generally at a moisture content corresponding to their ability to retain water ,called field capacity, Therefore, it is more appropriate to perform unsaturation zone study at soil water content equivalent to the field capacity . The soil water content is determined mostly by gravimetric method because it is a direct and inexpensive method.
Soil porosity refers to the amount of pore, or open space between soil particles. Pore spaces may be formed due to the movement of roots, worms, and insects; expanding gases trapped within these spaces by groundwater; and/or the dissolution of the soil parent material. Soil texture can also affect soil porosity
There are three main soil textures: sand, silt, and clay. Sand particles have diameters between .05 and 2.0 mm (visible to the naked eye) and are gritty to the touch. Silt is smooth and slippery to the touch when wet, and individual particles are between .002 and .05 mm in size (much smaller than those of sand). Clay is less than .002 mm in size and is sticky when wet. The differences in the size and shape of sand, silt, and clay influence the way the soil particles fit together, and thus their porosity.
Bulk Density
Bulk density is the mass of solid particles of soil per total volume including void (BS1377-1, 1990). It is also the weight of soil in a given volume. Soils with a bulk density higher than 1.6 g/cm3 tend to restrict root growth. Bulk density increases with compaction and tends to increase with depth. Sandy soils are more prone to high bulk density. It is expressed by Equation below
Bulk density = mass of soil
Volume of soil and void Equ. 2.1
Soil Organic Matter
Soil organic matter is the sum of all natural and thermally altered biologically derived organic material found in the soil or on the soil surface irrespective of its source, whether it is living dead or in a stage of decomposition, but excluding the above- ground portion of living plants (Summer, 2000), SOM consists of three broad classes of organic material, namely
i. Living plant, animals and microorganisms
ii. Fragments of dead plants, animals and microorganism and
iii. Highly decomposed and chemically variable organic compound, also known as humus that poor installation and movement of tanks due to land subsidence are among major factors contributing to the failure of underground storage tanks and their piping systems.
It has been estimated that about 50% of oil storage sites (Zektser et al., 2000) and about 35% of all underground storage tanks (USEPA, 1994) in the United States leaks. Unfortunately, these leaks are difficult to detect early (USEPA, 1994, Zekter et al., 2000). They are usually detected when havoc has been done to the soil and groundwater, and to the environment at large.
Soil and groundwater contamination
This is a special issue dedicated to soil and groundwater contamination and remediation. Groundwater is also one of our most important sources of water for irrigation. Unfortunately, groundwater is susceptible to pollutants. Groundwater contamination occurs when man-made products such as gasoline, oil, road salts and chemicals get into the groundwater and cause it to become unsafe and unfit for human use.
Materials from the land's surface can move through the soil and end up in the groundwater. For example, pesticides and fertilizers can find their way into groundwater supplies over time. Road salt, toxic substances from mining sites, and used motor oil also may seep into groundwater. In addition, it is possible for untreated waste from septic tanks and toxic chemicals from underground storage tanks and leaky landfills to contaminate groundwater.
Oil as a source of contamination
Oil spills may originate in natural or anthropogenic causes.
Natural causes – such as oil that seeps from the bottom of oceans which enters the marine environment. Crude oil is formed during long periods of time through natural processes involving organic matter from dead organisms. Thus, oil exists in many environments and may be naturally spilled due to various factors (including climatic conditions, disturbance, etc.). Such natural oil spills may occur in oceans, due to eroding of sedimentary rocks from the bottom of the ocean (the effect may be similar with that of an accidental oil spill from human drilling in oceans such as the recent BP oil spill from the Gulf of Mexico).
Anthropogenic causes- including accidental oil spills (such as the recent BP oil spill in the Gulf of Mexico) as well as leaks and spills due to a large variety of human activities related to oil refining, handling and transport, storage and use of crude oil and any of its distilled products.
Thus, it is evident that a variety of sources for oil spills and a variety of ways the oil could be spilled exist. While various anthropogenic and natural sources of oil spill pollution determine the type and amount of oil spilled, as well as the location of the oil spill, the type of the oil spill pollution is important for the fate and transport of the spilled oil and its impact on humans and the environment. For example: a sudden oil spill involving large amounts spilled (thousands or even millions of gallons – such as that from an oil tanker failure or due to accidents in offshore drilling) could have disastrous effects due to the high concentrations of released contaminants and the difficulty to remediate such big spills. At the same time, an oil spill involving small but continuous releases such as those from leaking pipelines or road runoffs may have little visible effect (they are naturally attenuated usually due to microbial degradation as well as due to many chemical-physical processes).
The type, amount of oil discharged and its location will dictate the oil spill cleanup efforts, which could involve deployment of adsorbent booms, controlled burning, bioremediation, emulsification using detergents for increased degradation.
Even though numerous climate factors and natural disturbances can generate oil spills, the main causes of oil spill pollution are usually of anthropogenic origin. The most commonly encountered anthropogenic sources are the following:
Accidental Spills
Accidental spills may occur in various circumstances, most often during the following activities:
i. Storage – oil and oil products may be stored in a variety of ways including underground and aboveground storage tanks (USTs and/or ASTs, respectively); such containers (especially USTs) may develop leaks over time
ii. Handling – during transfer operations and various uses
iii. Transportation – these could be large oil spills (up to million and hundreds of million gallons) on water or land through accidental rupture of big transporting vessels (e.g., tanker ships or tanker trucks). For example, the Exxon Valdez spill was a massive oil spill off the Alaskan shoreline due to ship failure which happened in late 1980’s– oil spill pollution residuals from that spill are still affecting our environment – or smaller oil spills, through pipelines and other devices also happens and their impact is mainly due to a large number of usually minor spills
iv. Offshore drilling
v. Routine maintenance activities – such as cleaning of ships may release oil into navigable waters. This may seem insignificant; however, due to the large number of ships even a few gallons spilled per ship maintenance could build up to a substantial number when all ships are considered
vi. Road runoff – oily road runoff adds up especially on crowded roads. With many precipitation events, the original small amounts of oil from regular traffic would get moved around and may build up in our environment
Intentional Oil Discharges
Intentional oil discharges are not necessarily malevolent. Most of them occur in the following circumstances:
i. Through drains or in the sewer system. This include any regular activities such as changing car oil if the replaced oil is simply discharged into a drain or sewer system
ii. Indirectly through the burning of fuels, including vehicle emissions; they release various individual components of oils and oil products, such as a variety of hydrocarbons (out of which benzene and PAHs could pose serious health risks).
Response Surface Methodology
Response surface Methodology (RSM) is a collection of mathematical and statistical techniques useful for the modelling and analysis of problems in which a response of interest is influenced by several variables and the objective is to optimize this response (Montgomery, 2005). It explores the relationships between several explanatory variables and one or more response variables. The method was introduced by George E. P. Box and K. B. Wilson in 1951. RSM is an important branch of experimental design. RSM is a critical technology in developing new processes and optimizing their performance. The objectives of quality improvement, including reduction of variability and improved process and product performance, can often be accomplished directly using RSM.
The main idea of RSM is to use a sequence of designed experiments to obtain an optimal response. Box and Wilson suggest using a second-degreepolynomial model to do this. They acknowledge that this model is only an approximation, but they use it because such a model is easy to estimate and apply, even when little is known about the process.
Contaminant Transport Modelling
These shows how petroleum product are transported by groundwater. Different models will included in the description of different physical processes, which implies that they will need different extent of input data and thus the accuracy of their output result will be different.