Natural Attenuation of Hazardous Contaminants

by Jim V. Rouse

A casual reading of the news consistently brings awareness of the increased presence of hazardous contaminants in the natural environment, and the danger this presents to human health and well-being. Much of this contamination resulted from the past release of materials from past industrial activity, in times when the nature and behavior of such materials was not well understood. Increasingly, evidence has been developed that the extent of such contamination would be far greater were it not for the presence of natural measures, which prevent or slow the spread of such material--an evidence for the design of the earth as a habitation for mankind. Further, experience has shown that remedial measures that mimic the natural attenuation reactions are much more effective than conventional engineered techniques that ignore or conflict with the natural approaches.

The various contaminants can be divided into three general classes:

1. naturally-occurring inorganic contaminants such as heavy metals and radionuclides, used in industrial manufacturing operations,
2. hydrocarbons such as gasoline, needed for our way of life, and
3. complex volatile and semi-volatile organic molecules, commonly chlorinated, intentionally developed for their longevity in the natural environment, and used as solvents and pesticides.

Examples of the first class include arsenic, uranium and radium, hexavalent chromium, and cyanide. While each of these is a known toxicant, capable of causing harm to the human body in excess concentrations, and commonly invoked in murder mysteries, they are naturally formed, and have been in the human environment throughout man's history. Arsenic is the twentieth most common element, and is found in virtually all soils. Higher-than-normal concentrations are present in phosphate rocks used to manufacture phosphate fertilizers, and in gold ores. Uranium and radium, one of the products of uranium radioactive decay, is present in most marine shales, and in coal deposits, as well as in the phosphate ores. The naturally-occurring trivalent form of chromium is needed to support life as a micronutrient, but the hexavalent form, used for plating, timber preservation, and other industrial applications, is a known toxic. Cyanide is a high-energy combination of nitrogen and carbon, both of which are needed to support life. It is formed under natural conditions during combustion, which explains why it is a common constituent in cigarette smoke.

Hydrocarbons used in our present life style are refined from crude oil, and separated on the basis of the number of carbon atoms in the molecule, for the intended use. Gasoline includes a number of ring-structure molecules, such as benzene, known to be carcinogens, while oils such as lubricating and fuel oils are primarily straight-chain in nature.

Volatile and semi-volatile organics are primarily produced to have long life in the environment, and commonly are not naturally occurring. Many of these molecules were intentionally developed with the thought that they would not degrade in the environment.

Just as there are various classes of contaminants, there are also a number of classes of reactions, which operate to prevent or slow the migration of contaminants in the environment, or to degrade them. These involve both inorganic geochemical reactions between contaminants and the soil, and microbiological reactions, in which specific groups of bacteria and fungi consume organic contaminants or react with other ions such as sulfate or nitrate, to produce geochemical conditions which "fix" contaminants onto soil, or change their geochemical mobility.

In evaluating the inorganic geochemical reactions, it is helpful to consider a conceptual geochemical model of the behavior of contaminants in the subsurface, as shown in figure 1. When contamination is released into the environment, it commonly moves vertically through the unsaturated soil to the water table, that surface below which all pore space is filled with ground water. It then begins to move laterally as dissolved contamination in the ground water, and forms three zones. The first of these, the core zone, is an area in which the finite attenuation capacity of the subsurface has been depleted, and the water is similar to the water in the source. The second, active zone, is the area in which the various geochemical mechanisms such as sorption and neutralization are active. The third, neutralized zone is an area where the geochemical attenuation capacity of the subsurface is intact, but where more conservative contaminants, and the major ions of the source can be detected.

Many contaminants tend to be sorbed onto clay minerals or iron hydroxide coatings of soil particles, thereby retarding the movement of the contaminants, in comparison to the velocity of the ground water carrying the contamination. This is true for both heavy metals and radionuclides, and for organic contaminants. This sorptive ability can be quantified by laboratory tests of soil material, or by comparing the length of plumes of contaminants to the distance traveled by the ground water carrying the contaminants, for example the ratio of the active and neutralized zones shown in the drawing.

Many sources of inorganic contamination include either acidic or alkaline aqueous solutions. These are neutralized by the buffering ability of soil or aquifer materials. As the water becomes more neutral (as measured by its pH, a measure of acidity or alkalinity), many metals tend to form precipitates. The order of metal removal is known and can be used in evaluating the attenuation of contaminant plumes using common geochemical "fingerprinting" techniques. In this way, the heavy metals in industrial wastes are prevented from migration.

Microbiological reactions involve either the direct consumption of the contaminants, or other geochemical changes, which immobilize them. Bacteria can use hydrocarbons and many volatile molecules as sources of carbon for cell growth and energy. In such reactions, the organisms consume the contaminants, under either aerobic or anaerobic (septic) reactions. For example, spills of gasoline have been shown to commonly reach stable conditions, as the microorganisms adapt to the presence of the potential food supply, with the limit to their growth formed by the oxygen supply. Studies have shown that many molecules, previously thought to be immune to bacterial degradation, actually are consumed by bacteria that adapt to the conditions in relatively short time periods. Cyanide is easily consumed by bacteria, which value it for its supply of both carbon and nitrogen. Rarely is it the case that some form of microorganism cannot adapt to a specific contaminant.

Prior efforts at environmental cleanup commonly involved "brute force" measures, such as excavation of soil or "pump and treat" ground-water removal, at great cost, with little beneficial results. Increasingly, environmental remediation efforts are aimed at understanding and mimicking the natural remedial reactions. This approach commonly has been much more effective. For example, hexavalent chromium plumes, which resisted years of "pump and treat" efforts, have been eliminated in weeks to months, by the introduction of chemicals to reduce the hexavalent form to the naturally-occurring trivalent form (EPA/625/R-00/005, October, 2000). Natural populations of bacteria can be aided in their consumption of hydrocarbons or cyanide, by the simple expedient of adding oxygen to the ground water, by blowing compressed air into the ground water. Complex hydrocarbon molecules are oxidized to the constituent elements by the subsurface introduction of common oxidants such as hydrogen peroxide or potassium permanganate. Man has once again found that understanding and using the design of the earth is much better than trying to forge ahead without such understanding.

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