Groundwater pollution by hydrocarbons and its characteristics.

1.      What are hydrocarbons?

In organic chemistry, a hydrocarbon is an organic compound. It contains hydrogen is an organic compound. It contains hydrogen and carbon. Hydrocarbons are often referred as non-aqueous phase liquids (NAPLs). They are subdivided into two types.

(i)                 Dense NAPLs or DNAPLs – These are denser than water.

(ii)               Light NAPLs or LNAPLs – These are less denser than water

High NAPLs consist mainly of petroleum products, such as petrol, diesel, lubricants and chemical feed stocks. Denser NAPLs is consist chlorinated hydrocarbons such as carbon perchlorethene, trichloroethene and etc., pesticide manufacture, metal working industries, transformer oils. Hydrocarbons are economically important. Because major fossil fuels and things obtain from a source are hydrocarbons. 

The hydrocarbons can be divided into two classes related their chemical structure: the alkanes (n-normal, branched, and cyclo) and aromatic compounds (napthaeno; mono-, di-, and poly, i.e., PAH).

Both crude and refined petroleum vary widely in physical properties and chemical composition, depending on their origin and, in the case of refined products, the nature of the refining process. These factors also produce differences in important environmental properties such as solubilisation, volatilization, photochemical and microbial oxidation and biological toxicities (Connell and Miller, 1981a, b). 

2.      Groundwater Contamination

Many of the pollutants that affect surface water and soils also cause groundwater contamination. Groundwater contamination can be difficult to detect, control, and clean up because of its hidden nature.

Groundwater polluted by in different way of form in the subsurface. Contaminants can also occur as suspensions of colloids (sub microscopic particles) in groundwater. Petroleum is the most common LNAPL groundwater contaminant. Trichloroethylene (TCE) is an example of a DNAPL contaminant. The wide distribution of petrol stations and extensive networks of pipelines carrying petroleum. It illustrates the potential risk to groundwater quality. Physical and chemical interactions between the contaminants, the water and surrounding rock or sediment combine to determine the final of the contaminating subsurface. 

When development, effluents, or waste discharge occurs in a recharge zone, there is potential for groundwater contamination. When the contamination water reaches a discharge zone, then contamination enter the adjacent surface water body. In contaminated groundwater pollutants travel from their source in the form of a spreading mass, called a plume. 

Contaminants behaviour in the subsurface depends on whether it is lighter or denser than the groundwater. LNAPL (petroleum) will infiltrate the soil layers; upon reaching the statured zone the petroleum will float on top of the ground water. DNAPL plume will sink through the zone of aeration, and then continue to sink through the water in the saturated zone to the bedrock below. DNAPL plumes can be difficulty to monitor and control. Because DNAPL not affected by lateral flow in the groundwater. They pass easily through fractures and pollute deep-seated aquifers or seep between layers of bedrock. 

3.      Physical characteristics of hydrocarbons in groundwater

Petroleum products mainly contain light NAPLs and dense variety include chlorinated hydrocarbons widely used as industrial solvents. Chlorinated hydrocarbons can enter the subsurface as a separate non-aqueous phase liquid (NAPL). It is important to determine that how the chlorinated hydrocarbon entered the subsurface. 

Chlorinated solvents have a density greater than water. As a result NAPL forms as a dense non-aqueous liquid (DNAPL). If DNAPL released [perchloroethene (PCE)] it migrates through soil and is held in soil and aquifer pores. This DNAPL referred to as residual NAPL. It is held in soil or aquifer material under capillary forces. It will not flow under the force of gravity and will not drain into a well. DNAPL reaches the groundwater then it continues to move below the water table. If enough DNAPL is released it move depths below the surface forming ‘pools’ of free phase liquid on low permeability surfaces. It referred to as free or mobile NAPL.

The residual and mobile DNAPL serve as long-term sources of groundwater contamination. Small releases of DNAPL never reach groundwater. It remains in the unsaturated soils. Chlorinated solvents in soils are a source of vapour contamination. It can migrate to basements and start to contaminate the shallow groundwater. Mendoza and McAlary (1990) show that in sandy material a TCE vapour plume will spread rapidly by ‘molecular diffusion and possibly density driven advection. Phase transfer reactions will…………….significantly increases the size of the potential source of ground water contamination’. The relationship between contaminant concentrations in the soil-gas and concentrations in the groundwater is not straight forward and is strongly dependent on soil moisture content (McCarthy and Johnson, 1993). 

The saturation of a DNAPL in soil depends on various characters of water and the DNAPL. The wettability of the media (subsurface), capillary pressure, relative permeability of the formation, and the solubility, volatility, density, and viscosity of the NAPL (Mercer & Cohen, 1990). The interaction of DNAPL and soils can lead to DNAPL in the water table. Therefore a significant contaminant mass occur between the soil and water table interface. This happens because interfacial tension and capillary pressures cause a DNAPL pressure in vertical movement until the DNAPL is able to penetrate the water table. 

A release of LNAPL (Solvent mixtures) to the soil will tend to move through soil and significant amount of LNAPL held in the soil. If enough LNAPL is released, it will float on the water table and will tend to create a shallow groundwater plume near the source.

 Aqueous mixtures contain chlorinated hydrocarbons. As contaminated water, such as land fill leachate, enters the subsurface and it moves through soil or unsaturated zone. If there is an enough volume it enters the ground water. The hydraulics of the flow system and the retardation characteristics of the contaminants will determine movement of dissolved phase contaminants in the groundwater. 

Chlorinated compounds which dissolved in shallow groundwater can volatilize and present a threat of vapour intrusion to indoor air. Light NAPLs are more easily controlled if they float on the water table and occur at relatively shallow depths. But dense NAPLs move under the influence of their density and penetrate to considerable depths. 

4.      The remediation of contamination groundwater 

If groundwater contaminated there are two major steps can be taken in remediation. First concern to determine the contaminant distribution in the aquifer, weather it is in the unsaturated zone or the saturated zone or both. Next are reduction in the inflow pollutants in to the water body and a reduction of their concentration in the water. Remedial procedures often require scientific input, long periods of time and enormous financial resources. 

There are three basic ways of dealing with the pollutant; (i) containment of the pollution, (ii) contaminant withdrawal and (iii) in-situ treatment of contaminants. Contaminant involves prevent the contamination by using physical barriers. Such as stopping the use of low permeability material to constructing slurry walls and promote the uses of sheet piles, cement grouts and impermeable membranes. Interlock steel sheet piles into the ground and use geo membranes to prevent the groundwater contamination.

For example, modern landfills are carefully constructed. They lined with clay and flexible synthetic membranes intended to prevent groundwater contamination. 

Contaminant withdrawal involves pumping process. When a contaminated groundwater pumped, it flow more permeable horizons and consequently these are cleaned relatively easily and quickly. However, contaminants penetrated into less permeable horizons and the smaller pore spaces that which are unaffected. When pumping stops, contaminants from this zone scatter into permeable zones that have been cleared. Pump – and – treat system has been uses over the past twenty years. This type of system is expensive to operate. This system has under gone criticism because of its inefficiency and its cost to maintain. 

In situ treatment of contamination includes biological or chemical degradation techniques. It contains bioremediation, bioventing and phytoremediation. The bioremediation is a multidisciplinary field. Hydrocarbons naturally degrade in the subsurface due to microbial – mediated reactions. But here reaction rates are slow because electron acceptors are quickly depleted in contaminated groundwater and are slowly recharged. In treatment, encourage the bacteria growing in contaminated zone by adding the nutrients. Organic matter degraded into simple compounds such as water and carbon dioxide. Specific bacteria can metabolise a particular contaminant.

                        Source: http://whatliesbeneath.wetpaint.com/page/Groundwater+Remediation

Bioventing is a process of remove the volatile fraction of a contaminant. This method can use to remove the contaminants from an aquifer which are present at less than the residual saturation level. Phytoremediation includes uses the trees and plants to absorbing the contaminants. The plants are positioned that their roots are grow directly to the water table. Once they absorbed the contaminants after they removed and disposed. This method is only use to clean the shallow groundwater. 

                 Source: http://whatliesbeneath.wetpaint.com/page/Groundwater+Remediation

We can’t restore the groundwater with in a time and at a reasonable cost. We need to minimise all form of pollution. This is our valuable gift for our future generation.

References:

(1)   Barbara W. Murck (2005); Environmental Science; Wiley publication; USA.

(2)   Des W. Connell, Gregory j. Miller (1984); Chemistry and Ecotoxicology of pollution; A Wiley - Inter science publication; USA.

(3)   Kevin T. Pickering & Lewis A. Owen (1997); An Introduction to global environmental issues; Routledge; London.

(4)   http://i2massociates.com/content/ground-water-remediation-design-operation-and-maintenance

(5)   http://whatliesbeneath.wetpaint.com/page/Groundwater+Remediation