Chapter IV, Section C, Item 1:  Contaminants: biological, radionuclide, and chemical

When most think of an environmental issue, they conjure pollution, most commonly chemical contamination. But what does it mean for something to be contaminated? Anything in excess, including water itself, can be unhealthy or even toxic at some concentration or amount. Whether the contaminating agent is chemical, biological, or radionuclide, health and environmental standards are set as concentration limits in air, soil, and water. For air pollution, the EPA has established National Ambient Air Quality Standards (NAAQS) for 6 widespread, principal pollutants, as well as National Emission Standards for Hazardous Air Pollutants (NESHAPS) to regulate hazardous chemical and radionuclide emissions from industry, while OSHA sets exposure limits in the workplace. For water pollution, the EPA has set Maximum Contaminant Levels (MCL’s) for a host of microbial, radionuclide, and chemical contaminants, expressed as maximum allowable concentrations in drinking water. For soil pollution, the EPA has guidelines called Preliminary Remedial Goals (PRG’s) that, though not legally enforceable limits, become part of legally enforceable abatement orders. These orders are largely to prevent eventual contamination of both water and air, which do have legally enforceable limits, or human exposure, which is regulated by health laws. Soil contamination is a principal media of exposure to lead and other heavy metals, making it a primary concern in environmental clean-ups for property transfers. Other than setting MCL limits on microbes in water, and providing sanitary guidelines and indoor air quality recommendations, the EPA generally leaves disease and other biological contamination issues to health departments. The Food and Drug Administration (FDA) and Center for Disease Control (CDC) set legal guidelines to limit exposure to chemicals and microbes in food and the environment.

Biological contamination is actually the most water and food resource limiting. Though toxic chemical problems are more common in industrial societies, contributing to more heightened awareness with the public, the media-sensational stories of death within days of exposure to contamination in food or water are usually examples of biological contamination. Bacterial outbreaks of e coli, typhoid, or cholera can even occur in ground water wells, as was the case recently in Canada. Bacteria and viruses are not the only form of biological contamination. Ask any hiker. In many natural, outdoor settings, the surface water is often of exceptional water quality, and could be used untreated, if not for water-borne parasites such as giardia or crypto sporidium. Unlike bacteria, they, like many other protozoan parasites, are resistant to even doubling boiling. The hiker carries a manual reverse osmosis unit, a water filtration pump with filters, a miniature version of a reverse osmosis treatment plants that utilities have to use if they draw from surface water sources. Disease is more commonly known to spread from airborne or solid surface contact, underscoring that biological contamination can involve all media of exposure. However, water is actually our most common disease vector. Eighty percent (80%) of the world’s diseases are caused by poor water supply.

Radionuclide contamination is another form not typically conjured in the context of pollution. Environmentally the nuclear issue is more commonly perceived as a waste disposal issue due to the longevity of hazardous radioactive wastes, the radiation itself sourced from the elements, not the compounds. The source of the contamination or waste is a radioactive isotope of an element. Radioactive releases from power plants typically involve radioactive isotopes of iodine, strontium, and cesium, which can be borne in air, water, or soil, and are readily absorbed into human tissues. The actual hazard from either nuclear waste or fallout is the radiation emitted from the isotopic source: alpha, beta, or gamma radiation. With radioactive decay, the source isotope transmutes into other radioactive and non-radioactive isotopes. Eventually over time, usually a considerable amount of time, the contamination may decay into a non-hazardous stable isotope. But unlike most toxic chemical compounds that degrade into more neutral forms, radioactive isotopes are environmentally persistent, a legacy shared with several non-radioactive elements as well, such as heavy metals, hazardous from chemical toxicity.

Chemical contamination is the most prevalent environmentally, due to the commercial handling of a very long laundry list of chemicals. The relative environmental liability of each on that list is more readily realized if categorized by source and type. A short list of categories incorporates all but the most unique contaminants, and includes: 1) solvents, usually chlorinated solvents, from industry, transportation, and dry cleaning; 2) petroleum hydrocarbons from industry and transportation; 3) polynuclear aromatic hydrocarbons (PAHs) and poly-chlorinated biphenols (PCBs) from industry and power transmission; 4) heavy metals from mining, smelting, and power generation; 5) agricultural and industrial salts; 6) agricultural herbicides and pesticides; and 7) potentially all of the above from waste landfills. These categories do not include nutrients, acids, caustics, and oxidizers, which usually affect water quality more generally, and are herein discussed in separate environmental classifications other than contaminants.

The energy debits to the complete energy economy from all contaminant problems are related to the energies associated with treatment. Whether the contaminated media is air, water, or soil, these treatments are generally of four types: 1) removal, usually by filtration, distillation, or ion exchange; 2) dilution; 3) destruction by dissolution in a solvent, including acids, caustics, and water; 4) destruction by oxidation, either by combustion or respiration (or hydrolysis / acid); and 5) destruction by reduction caused by an electron donor. The first two options do not destroy the contaminant. Instead it changes its media, or gets dispersed to an “acceptable” level in more of the existing media. New environmental problems are thus exchanged for the old, usually a waste disposal problem dealing with the contaminant’s new media, or a degradation of a larger volume of the existing media to attain an “acceptable” level of contamination. Though destruction of toxic compounds by the last three options is thus preferred, and is often even energetically favorable, the only remedial alternatives for elemental contaminants are removal or dilution.

The energy of treatment by removal with filtration or distillation was previously discussed in terms of the desalination of water, part of the assessment of the energy value of water. Though desalination involves distillation from, or filtration of, various salts, these processes are available for many other contaminants. The principles are the same: either heat energy is used to separate the water from the contaminant by a phase change, or work energy is used to pressurize water through a membrane. With ion exchange, an undesired contaminant is replaced with a desired one, usually substituting one for the other in a crystal or resin. A familiar example is home water softening, where calcium in the water is replaced by sodium from a salt source. Sometimes “undesirable” versus “desirable” is a matter of perspective: calcium is removed because it is hard to clean off of fixtures, but the added sodium in the water can cause hypertension, while the chloride is often released in the environment from septic systems. Along with road salt, the chloride adds to a growing problem, the increased salination of groundwater. For all three contaminant removal processes, as in the example of desalination, a geometric increase in the amount of energy is required for an increase in water purity.

The energy demands and associated costs of contaminant removal can quickly become so prohibitive that “going the other way,” diluting the contaminant rather than concentrating it into a residuum, is the only feasible option. As the old saying goes, “dilution is the solution for pollution.” Though it may reflect an old philosophy, dilution is still considered part of the solution, not only in waste disposal, but also in meeting water supply standards. A salient example became political football recently: the lowering of the MCL for arsenic in drinking water from 50 ppb to 10 ppb. The lowering of any MCL results in increased costs to water utilities to meet the stricter standard. Treatment to the new standard is nearly impossible without some measure of “blending,” a type of dilution where the contaminant concentration is reduced by blending uncontaminated water from another source into the contaminated source. Blending sacrifices a little water quality from one source for the sake of a greater water supply, and is feasible where the water supply is sufficient to provide the uncontaminated mixing component. With the arsenic example in arid regions, however, dilution is not an option, and the cost of treatment by removal methods is costly to water utilities. Dilution of contaminants into the environment generally is still a method of waste disposal, especially into the oceans. The question is how much the environment can absorb, and at what cost.

Dissolution can be a method of contaminant destruction, but at the very least, a byproduct of the contaminant remains in a solvent, and if the byproduct or the combined solution is also hazardous, another waste treatment requirement arises. The most common solvent is water, but many different solvents exist for different substances. Trichloroethylene, a very environmentally persistent chlorinated solvent, was used to dissolve oils and greases from metal parts. In this case the solvent became a major contaminator of ground water supplies. Sometimes the solvent may destroy the contaminant, but more often, the contaminant exists in solution and may precipitate from solution. In this way, the dissolution becomes merely a means of dilution and not destruction of the contaminant. Some substances that are not soluble in water may be soluble in another substance such as fat. Fat soluble contaminants that are also insoluble in water can become very troublesome, accumulating in animal tissue without a way to be rid from the body. The concentrations of these bioaccumulators actually increase up the food chain, which as why such examples as mercury and PCB may occur at high concentrations in fish, even though the concentrations may be relatively low in the water.

Oxidation is the most common way to destroy contaminants. Oxidation-reduction is a specific way to describe a class of reactions in chemistry, but in fact, all reactions can be expressed as the oxidation of one species, losing electrons, followed by the reduction of another species, gaining them. Energy is used to break existing chemical bonds between elements–ignition–then electrons are re-arranged and energy is re-released to form new bonds in new compounds. The released energy may be more or less than the energy used to get it all started, but ignition always requires some initial energy investment. In the process, the reducing agent is oxidized and the oxidizing agent is reduced, but given the confusion that can arise, it is easier conceptually to just track the electrons, referring to one reactant as an electron donor and the other an electron acceptor. In near-surface environments including shallow groundwater, the most common electron acceptor is oxygen. If the contaminant is a willing electron donor, oxidation will occur, usually in the form of either combustion or respiration. In a waste incinerator, we add the ignition energy needed to get combustion started. In the environment, most reactions get going with the help of microbes, i.e. they are catalyzed by microbes that use the exchange of electrons in respiration to draw energy. Though oxidation reactions are usually energetically favorable, the reactions often need to be stimulated by providing nutrients to the microbes and increasing dissolved oxygen levels. These result in treatment costs.

Reduction is required to get rid of some of our most stubborn contaminants, including oxidized metals, other inorganic compounds, acids, excess nutrients, and chlorinated solvents. Unlike oxidation reactions, these reduction reactions are not energetically favorable, and require reducing conditions, an environment devoid of oxygen uncommon near the Earth’s surface. The contaminants require an electron donor to breakdown, usually in the form of reduced metals, other inorganic compounds, bases, organic matter, petroleum hydrocarbons, and hydrogen gas. An electron acceptor pecking order for those electrons also exists in the environment, electrons usually going first to ubiquitous oxygen, then nitrate, iron, and sulphate before reducing chlorinated solvents and organic matter to methane.

But sometimes an electron donor can wreak havoc. A particularly notorious electron donor, reduced sulfur, is unfortunately ubiquitous in most of our fossil fuels, particularly in coal. Sulfur also occurs as metal sulfides in exposed bedrock, and as sulfide metal ore deposits. Sulfide has the unfortunate knack of creating acid when oxidized.

Table of Contents