Once an aquifer is contaminated, the process of restoring the quality of water is generally time-consuming and expensive, and it is often more cost effective to locate a new source of water. For these reasons, the restoration of an aquifer is usually evaluated on the basis of these criteria: 1) the potential for additional contamination; 2) the time period over which the contamination has occurred; 3) the type of contaminant; and 4) the hydrogeology of the site. Restoration techniques fall into two major categories, in-situ methods and conventional methods of withdrawal, treatment, and disposal.
Remedies undertaken within the aquifer involve the use of chemical or biological agents which either reduce the toxicity of the contaminants or prevent them from moving any further into the aquifer, or both. One such method requires the introduction of biological cultures or chemical reactants and sealants through a series of injection wells . This action will reduce the toxicity, form an impervious layer to prevent the spread of the contaminant, and clean the aquifer by rinsing. However, a major drawback to this approach is the difficulty and expense of installing enough injection wells to assure a uniform distribution throughout the aquifer.
In-situ degradation, another restoration method, can theoretically be accomplished through either biological or chemical methods. Biological methods involve placing microorganisms in the aquifer that are capable of utilizing and degrading the hazardous contaminant. A great deal of progress has been made in the development of microorganisms which will degrade both simple and complex organic compounds. It may be necessary to supplement the organisms introduced with additional nutrients and substances to help them degrade certain insoluble organic compounds. Before introduction of these organisms, it is also important to evaluate the intermediate products of the degradation to carbon dioxide and water for toxicity. Chemical methods of in-situ degradation fall into three general categories: 1) injection of neutralizing agents for acid or caustic compounds; 2) addition of oxidizing agents such as chlorine or ozone to destroy organic compounds; and 3) introduction of amino acids to reduce PCBs (polychlorinated biphenyls ).
There are also methods for stabilizing an aquifer and preventing a contaminant plume from extending. One stabilizing alternative is the conversion of a contaminant to an insoluble form. This method is limited to use on inorganic salts, and even those compounds can dissolve if the physical or chemical properties of the aquifer change. A change in pH , for instance, might allow the contaminant to return to solution. Like other methods of in-situ restoration, conversion requires the contaminant to be contained within a workable area.
The other important stabilizing alternatives are containment methods, which enclose the contaminant in an insoluble material and prevent it from spreading through the rest of the aquifer. There are partial and total containment methods. For partial containment, a clay cap can be applied to keep rainfall from moving additional contaminants into the aquifer. This method has been used quite often where there are sanitary landfills or other types of hazardous waste sites. Total containment methods are designed to isolate the area of contamination through the construction of some kind of physical barrier, but these have limited usefulness. These barriers include slurry walls, grout curtains, sheet steel and floor seals . Slurry walls are usually made of concrete, and are put in place by digging a trench, which is then filled with the slurry. The depth at which these walls can be use is limited to 80–90 ft (24–27 m). Grout curtains are formed by injecting a cement grout under pressure into the aquifer through a series of injection wells, but it is difficult to know how effective a barrier this is and whether it has uniformly penetrated the aquifer. Depth is again a limiting factor, and this method has a range of 50–60 ft (15–18 m). Steel sheets can be driven to a depth of 100 ft (30 m) but no satisfactory technique for forming impermeable joints between the individual sheets has been developed. Floor seals are grouts installed horizontally, and they are used where the contaminant plume has not penetrated the entire depth of the aquifer. None of these methods offers a permanent solution to the potential problems of contamination, and some type of continued monitoring and maintenance is necessary.
Conventional methods of restoration involve removal of the contaminant followed by withdrawal of the water, and final treatment and disposal. Site characteristics that are important include the topography of the land surface, characteristics of the soil , depth to the water table and how this depth varies across the site, and depth to impermeable layers. The options for collection and withdrawal can be divided into five groups: 1) collection wells; 2) subsurface gravity collection drains; 3) impervious grout curtains (as described above); 4) cut-off trenches; or 5) a combination of these options. Collection wells are usually installed in a line, and are designed to withdraw the contaminant plume and to keep movement of other clean water into the wells at a minimum. Various sorts of drains can be effective in intercepting the plume, but they do not work in deep aquifers or hard rock. Cut-off trenches can be dug if the contamination is not too deep, and water can then be drained to a place where it can be treated before final discharge into a lake or stream.
Water taken from a contaminated aquifer requires treatment before final discharge and disposal. To what degree it can be treated depends on the type of contaminant and the effectiveness of the available options. Reverse osmosis uses pressure at a high temperature to force water through a membrane that allows water molecules, but not contaminants, to pass. This process removes most soluble organic chemicals , heavy metals , and inorganic salts. Ultrafiltration also uses a pressure-driven method with a membrane. It operates at lower temperatures and is not as effective for contaminants with smaller molecules. An ion exchange uses a bed or series of tubes filled with a resin to remove selected compounds from the water and replace them with harmless ions. The system has the advantage of being transportable, depending on the type of resin, it can be used more than once by flushing the resin with either an acid or salt solution. Both organic and inorganic substances can be removed using this method.
Wet-air oxidation is another important treatment method. It introduces oxygen into the liquid at a high temperature, which effectively treats inorganic and organic contaminants. Combined ozonation/ultraviolet radiation is a chemical process in which the water containing toxic chemicals is brought into contact with ozone and ultraviolet radiation to break down the organic contaminants into harmless parts. Chemical treatment is a general name for a variety of processes that can be used to treat water. They often result in the precipitation of the contaminant, and will not remove soluble organic and inorganic substances. Aerobic biological treatments are processes that employ microorganisms in the presence of dissolved oxygen to convert organic matter to harmless products. Another approach uses activated carbon in columns—the water is run over the carbon and the contaminants become attached to it. Contaminants begin to cover the surface area of the carbon over time, and these filters must be periodically replaced.
These treatment processes are often used in combination. The methods used depend on the ultimate disposal plan for the end products. The three primary disposal options are: 1) discharge to a sewage treatment plant; 2) discharge to a surface water body; and 3) land application. Each option has positive and negative aspects. Any discharge to a municipal treatment plant requires pre-treatment to standards that will allow the plant to accept the waste. Land application requires an evaluation of plant nutrient supplying capability and any potentially harmful side-effects on the crops grown. Discharge to surface water requires that the waste be treated to the standard allowable for that water. For many organics the final disposal method is burning.
[James L. Anderson ]
Freeze, R. A., and J. A. Cherry. Ground Water. Englewood Cliffs, NJ: Prentice-Hall, 1979.