Bioremediation: Applied Microbial Solutions for Real-World Environmental Cleanup Edited by Ronald M. Atlas and Jim C Philp © 2005 ASM Press, Washington, D.C.

water, which recharges an aquifer, is polluted or when hazardous substances soak through the soil into the groundwater. Groundwater begins to accumulate within the unsaturated zone of soil, a layer that contains air and water filling the pores. Below this layer lies the saturated zone, in which all the pores and rock fractures are filled with water. The top of the saturated zone is referred to as the water table. Soil overlying the water table provides the primary protection against groundwater pollution. Some potential pollutants nevertheless reach the groundwater.

The potential vulnerability of an aquifer to groundwater contamination is in large part a function of the susceptibility ofits recharge area to infiltration. Areas that are replenished at a high rate are generally more vulnerable to pollution than those replenished at a lower rate. Unconfined aquifers that do not have a cover of dense material are susceptible to contamination. Bedrock areas with large fractures are also susceptible by providing pathways for the contaminants. Confined, deep aquifers tend to be better protected with a dense layer of clay material.

There are three general approaches to cleaning up contaminated soil: (i) soil can be excavated from the ground and be either treated or disposed of (ex situ treatment), (ii) soil can be left in the ground and treated in place (in situ treatment), or (iii) soil can be left in the ground and contained to prevent the contamination from becoming more widespread and reaching plants, animals, or humans (containment and intrinsic remediation). Containment of soil in place is often done by placing a large plastic cover or concrete barrier over the contaminated soil to prevent direct contact and keep rainwater from seeping into the soil and spreading the contamination. In situ and ex situ treatment approaches can include flushing contaminants out of the soil by using water, chemical solvents, or air; destroying the contaminants by incineration; encouraging natural organisms in the soil to break them down; or adding material to the soil to encapsulate the contaminants and to prevent them from spreading.

Bioremediation is one of the technologies that can be applied by each of these general approaches. In some cases, one relies upon the intrinsic biodegradative capabilities of the indigenous microbial communities and monitors the movement and progressive slow decline in contaminant concentrations. This monitored natural attenuation (MNA) approach is valuable when there are no acute threats to human health and where the impact is not spreading rapidly. In other cases, active remediation is needed to curtail the impact. Depending upon the nature of the problem, it may be necessary to excavate the contaminated soil and move it to a site for its safe disposal or treatment. This ex situ approach to bioremediation is analogous to what is done for traditional sewage and solid waste treatment. Bioremediation may also be conducted in situ, for example, by bioventing, in which air is used to move the contaminants from the groundwater into a phase where evaporation and biodégradation can occur simultaneously. Depending upon the nature of the pollutant, there may already be sufficient populations of microorganisms to degrade the contaminant—in which case stimulation of those microbial populations by environmental medications (e.g., addition of fertilizer or oxygen) may be all that is required. In other situations, it may be beneficial to consider augmenting the indigenous microbial populations by seeding with specialized microorganisms, including possibly with genetically modified microorganisms. In this chapter, we review the various bioremediation technologies and the situations to which they are applicable.

Because groundwater is a hidden resource, it is too easy to forget that its misuse is a hidden problem. For the purpose of context, the global importance of groundwater needs to be emphasized. As many as 2 billion people rely directly on aquifers for drinking water, and 40% of the world's food is produced by irrigated agriculture that relies largely on groundwater (220). At least 12 megacities (populations of over 10 million) could not function without groundwater, and typically at least 25% of the water for these cities comes from aquifers.

China alone has over 500 cities, and two-thirds of the water for them comes from aquifers.

Despite this importance, the number of instances of groundwater contamination due to accidental spills or unsatisfactory disposal is beyond counting. Up to a certain point, natural processes, especially biodégradation, can attenuate contamination. In this regard, the biological active zone is the vadose (unsaturated) zone, where attenuation rates are highest. Contaminant removal continues in the saturated zone but usually at much lower rates, and migration of contaminants to the saturated zone can have the effect of dispersion of the contaminants. While bringing about dilution, the latter process often cannot be relied upon for complete decontamination.

The easy availability of groundwater and its vast supply (95% of the freshwater on the planet, apart from the locked water of the polar ice caps, is groundwater) have been its undoing. A great deal of it lurks fairly close to the surface, but intrusive disturbance of the subsurface has very high potential for destroying its flow and distribution. Therefore, techniques for remediating contaminated groundwater should operate by minimal disturbance. The sheer scale of the problem dictates that the remediation technologies should be as inexpensive as possible, and it is for that reason that bioremediation is often considered.

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