Removal of Pollutants from Groundwater

The following points highlight the top two methods used for the removal of pollutants from groundwater. The methods are: 1. Active Remediation 2. Natural Attenuation and Monitoring.

Method # 1. Active Remediation:

The predominant groundwater remediation strategy is the application of the so-called ‘pump-and-treat’ technology in which mainly physico-chemical techniques are used to remove the pollutants in-the above- ground treatment units, via, for example air stripping and activated carbon. Biological reactors are used in fewer than 10% of cases (Fig. 31.3).

Probably due to limited experience and demonstration data, limited acceptance of the technology, and the failures to achieve the clean-up levels required.

To data, probably most experience with full scale ex situ and in situ applications of bioremediation has been acquired for the biodegradation of petroleum hydrocarbons, comprising straight and branched chain, saturated, unsaturated and cyclic aliphatics to mono-, di- and polyaromatic hydrocarbons.

Recently, however, new types of bioreactor designs have been developed that eliminate poly-chlorinated solvents and aromatics as well. For example, UASB reactors seeded with granular methanogenic sludge have been shown to completely (>99%) dechlorinate tetrachloroethylene present at 4 mg^-1 in polluted groundwater.

Acetate was used as carbon source and electron donor and process costs were competitive (US $1.2 [per m³ treated). The UASB reactor technology is also being upgraded with granular sludge combining both anaerobic and aerobic bacteria.

The ‘pump-and-treat’ strategy fails achieving the clean-up targets in most of the cases and moreover requires long clean-up times. Of the 77 pump-and-treat sites evaluated by a committee under the auspices of the US National Research Council (NRC) in 1992, only eight had reportedly reached the clean-up goals, which in all cases were the maximum contaminant levels for constituents regulated under the Safe Drinking Water Act. Of the eight successful sites, six were polluted with petroleum hydrocarbons which would also have been eliminated via natural attenuation.

The conclusion of the NRC Committee has been that the pump-and-treat methods were quite limited in their ability to remove contaminants from the sub-surface because of sub-surface heterogeneities, presence of fractures, low-permeability layers, strongly adsorbed compounds, and slow mass transfer in the sub-surface.

Even with the best extraction methods, usually only a small fraction of soil -bound contaminants can be mobilised, leaving a large residual fraction in the soil.

As a result of this failure, remediation policy and technical developments are shifting toward increased use of in situ containment practice, e.g., bio-fencing (Fig. 31.3), rather than full treatment scenarios. In those cases where full treatment is necessary, less stringent clean-up goals are set, based on risk assessment taking into account the type of land use.

Aside from the much-studied genetic compounds discussed above there is a host of toxic compounds normally present at trace level and whose fate remains poorly studied. Example of this type are the poly- chlorinated dioxins and furans which are formed as by-products of chemical synthesis processes.

They are also produced by combustion of garbage, waste oils, soils polluted with oils, chemical wastes containing PCBs, and by various other high temperature processes. Because of the high toxicity of some dioxins and furans, these compounds are of major eco- toxicological concern.

Research and development work has attempted to minimise their production in incinerators and emission via fly ashes. Yet, the biological breakdown of these compounds in the environment is of considerable significance.

Indeed, they are often present in wastes which are extremely difficult to treat properly by incineration (e.g. polluted soils and river sediments). They are also present in fly ashes of incinerators which are deposited in landfills and, usually contaminate landfill leachates.

Method # 2. Natural Attenuation and Monitoring:

Recently a lot of interest is generating in new monitoring techniques due to several factors. One such factor is the fact that remediation technologies are often insufficient to meet stringent cleanup targets. This limitation is making legislators reassess the target pollutant levels and making them consider the use of risk-based end-points in place of absolute end-point values.

The new concept of risk-based end-points requires the development of new analytical tools. The latter assess the bioavailable rather than the total pollutant concentration by relying on bioassays. It is because the traditional analytical methods fail to distinguish pollutants available to biological systems from those that exist in inert, or complexed, unavailable forms.

When a polluted soil is subjected to a period of intensive microbial activity, the toxicity is reduced by a factor of 5 to 10. This eco-toxicological information can be easily deduced by running a simple bioassay with soil leachates. One type of bioassay is based on the inhibition of the natural bioluminescence of the marine organism Photobacterium phosphoreum, which is used, for example, in the Microtox. Lumistox and Biotox tests.

These assays are, however, not specific since light inhibition will occur upon exposure to any toxicant. This limitation is circumvented in a new class of bacterial biosensors that are specific to certain types of toxicants.

For convenience, biosensors able to detect bioavailable metals, were constructed by placing lux genes of Vibrio fischeri as reporter genes under the control of genes involved in the regulation of heavy metal resistance in the bacterium Alcaligenes eutrophus.

The recombinant strains, upon mixing with metal-polluted soils or water, emit light in proportion to the concentration of specific bioavailable metals. Light emission is easily measured spectrophotometrically.

Another factor that generates interest in new monitoring tools in recent times is the high cost and slow pace of remediation technologies. Natural attenuation (or intrinsic bioremediation) is being advocated as more pragmatic remediation approach. Intrinsic bioremediation is based on natural processes to remove, sequester or detoxify pollutants without human interference.

Intrinsic bioremediation has been observed most frequently with groundwater contaminated with hydrocarbons. If evidences suggest that a site is improving because of intrinsic bioremediation, and that the pollution does not pose a threat to human health, the environmental regulating agency may grant that site ‘monitoring only’ status.

This strategy simply needs remote monitoring in order to follow sub-surface contaminant concentrations in situ. Remote monitoring can be carried out with a ground-penetrating radar which monitors contaminant breakdown in the sub-surface based on the increase of solution conductance which accompanies the breakdown of hydrocarbons or chlorinated solvents.

Another technique used in remote monitoring employs genetically engineered microorganisms (GEMs) which emit light in response to the presence of specific contaminants.

As these microorganisms are attached on a photocell connected to a radio chip, the light signals get converted into radio waves which are detected at a distance. These sensors can be scattered throughout polluted sites to monitor the progress of pollutant breakdown.