The clean up of contaminated groundwater is very different from clean up of waste-water on the surface. The most obvious difference is that with groundwater clean-ups, the body of water is actually being cleaned. In waste-water(surface) cleanups, we control and treat the waste water that is entering a body of water. The water body, river or lake actually cleans itself once we stop putting pollutants into it. Groundwater is not able to clean itself hence we must clean-up the source of pollutants as well as the aquifer itself.

The mechanism of groundwater treatment starts with estimation of groundwater flow and concentration level of pollutants in the effected area.

Treatment for Organic Contaminants

Physico-Chemical methods- When a pure organic contaminant is released onto or into the ground the main force on the movement of the compound is gravity. If the ground is porous the spill will move downward. There will also be some lateral spread controlled by soil porosity. Eventually the contaminant may be absorbed on the soil particles, stopped by an impermeable layer of soil or encounters with an aquifer. On reaching the aquifer the contaminant may-

· Float on top of aquifer
· Dissolve into the aquifer
· Sink to the aquifer bottom .

For floating contaminants on top of aquifer the best treatment method recommended is Pure compound Recovery, for other possibilities the specific treatment options are discussed below:

a. Pure compound Recovery

This method is applicable for insoluble organic contaminants including most of the straight chain hydro-carbons. The treatment mechanism involves placing a well in the middle of the spill area .Water is removed and the water-level in the immediate vicinity decreases. The non-soluble compounds accumulating in the well, keep floating on top of water and are recovered through oil -water separators or through introduction of hydrophobic screens.

Other mechanism to recover pure compounds especially in areas of shallow water table is to dig a trench in the path of ground movement. When Groundwater enters the trench, the floating organic remains on top of water and is later removed by skimmers.

b. Air Stripping

It is a process employed to remove volatile compounds (usually organics) from groundwater. The basic concept of stripping is to bring the contaminated water into intimate contact with air so that the volatile compounds undergo a phase change ( from liquid phase to vapor phase). The air will then carry-off the contamination, leaving water free from these compounds . Air striping is accomplished in following ways by employing :

· Aeration tanks
· Cascade aerators
· Spray basins
· Packed towers

c. Carbon Adsorption

The method involves use of carbon for its adsorptive qualities. For removal of odors and taste , the use of activated charcoal is an established technology especially for organic compounds even though these contaminants do not exhibit traditional taste.

Carbon adsorption is particularly suited for low concentration of non-voltaic components eg. phenols, pesticides and high concentration of non-degradable compounds. For better removal efficiency of non- degradable compounds, this method should be combined with 'Bio-logical treatment' or 'Air striping'.

d. Ultra Filtration and Synthetic Resin

While ultra filtration method is used for separation of emulsion, the synthetic resin perform similar function as activated carbon, however they can be made more selective and with higher treatment capacity.

e. UV-Ozone Reactors

This technology is used in rapid oxidation of low concentration of chlorinated organics in groundwater and is rated as one of the best treatment technologies.

Biological methods

Once the contaminant has entered the aquifer , biological treatment is the most economical method of treatment. Biological method employing specific bacteria are recommended for 'in-situ' treatment as well as 'above- the- ground' treatment. In both the cases the treatment system is provided with bacteria, nutrient and other favorable condition of pH, availability of oxygen etc.. In 'in-situ' treatment applied for both organic and inorganic contaminants selected acids or alkalies are placed in the path of groundwater. In some cases an immobilized neutralizing agent is introduced underground. The contaminant plume on contact with the acid-base neutralizing agent and in presence of de-composing bio-chemical reaction under controlled environment with excess or oxygen deficient conditions based on aerobic or anaerobic bacteria employed.

'Above-ground' treatment of groundwater requires placement of well in the contaminated aquifer and withdrawal of groundwater which further undergoes biological treatment. The biological treatment above ground are analogous to conventional wastewater treatment technologies wherein, 'Suspended Growth Biological Reactors' or 'Fixed Film Reactors' are employed. The treatment configuration consists of 'Aerated lagoon', and 'Activated Sludge Process'.

Treatment for Inorganic Compounds

Unlike organic contaminants, the inorganic contaminants have, a greater chance of removal, transformation and adsorption in the unsaturated zone and hence possess lesser chance of reaching the aquifer zone. As a result of this it is observed that heavy metals moving through the soil are exchanged with cations in the soil and later removed. In certain cases of anaerobic zones in the soil, biological transformation of nitrates into nitrogen gas has been observed. The treatment methods usually recommended for inorganic constituents are discussed below. It may be noted here that the choice of method and their efficacy may involve bringing the portion of groundwater above ground or in certain cases provide ' in-situ' treatment

Chemical Addition

This is achieved by way of pH adjustment. It results in precipitation of heavy metals dissolved in groundwater. The pH adjustment (acid - to- neutral - and above ) involves passing water through limestone bed, adding caustic soda or soda ash. For adjustment of alkaline groundwater bubbling carbon di-oxide or adding strong acid is recommended. In certain , specific cases introduction of chemical reducing agents like sulphur oxides, sodium bisulphate or ferrous sulphate are used to precipitate metals, which are soluble at high pH (e.g. hexavalent chromium, arsenic etc.)

Ion Exchange

It is basically the exchange of an ion with a high ion exchange selectivity for an ion with a lower selectivity. It is the basis of water softening in which divalent ion (e.g. calcium) usually having higher ion selectivity replaces mono-valent ion (e.g. sodium) at an exchange site on an ion exchange bed., This technique though being expensive is recommended for treating wide range of dissolved inorganic e.g. chloride, nitrates, sulphates and metals.

Reverse Osmosis

Employs semi- permeable membrane and high pressure resins. The membrane rejects inorganic material and allows passage of water through it. It can remove 50-90% of in-organics.

Electro-dialisis.

The method is a combination of reverse osmosis and Ion exchange technology. It is the recommended method for treatment of groundwater with moderate to high conductivity.

Distillation

Involves evaporation of contaminated water by heating followed by re-condensation. The in-organics do not evaporate and hence are left behind. The recommendation brings out clean water free from dissolved in-organics.