The amount of waste produced by human society is increasing. Commercial and domestic solid waste is a great practical problem for many local governments. Industrial wastes are usually much smaller in volume but are more likely to contain hazardous materials, such as toxic chemicals, flammable liquids and asbestos. Although the total amount is less, the disposal of hazardous industrial waste has been a greater concern than of domestic waste because of the perceived hazard to health and the risk of environmental contamination.
The generation of hazardous waste has become a major problem worldwide. The root cause of the problem is industrial production and distribution. Land pollution occurs when hazardous wastes contaminate soil and groundwater due to inadequate or irresponsible disposal measures. Abandoned or neglected waste disposal sites are a particularly difficult and expensive problem for society. Sometimes, hazardous waste is disposed of illegally and in an even more dangerous manner because the owner cannot find a cheap way to get rid of it. One of the major unresolved issues in managing hazardous waste is to find methods of disposal that are both safe and inexpensive. Public concern over hazardous waste focuses on the potential health effects of exposure to toxic chemicals, and particularly the risk of cancer.
The Basel Convention passed in 1989 is an international agreement to control the transboundary movement of hazardous waste and to prevent dangerous wastes from being shipped for disposal to countries that do not have the facilities to process them safely. The Basel Convention requires that the generation of hazardous wastes and transboundary movement of the wastes be kept to a minimum. Traffic in hazardous wastes is subject to the informed permission and laws of the receiving country. Transboundary movement of hazardous wastes is subject to good environmental practices and assurance that the receiving country is able to handle them safely. All other traffic in hazardous wastes is considered illegal and therefore criminal in intent, subject to national laws and penalties. This international convention provides an essential framework for controlling the problem at an international level.
Hazardous Properties of Chemicals
Hazardous substances are compounds and mixtures that pose a threat to health and property because of their toxicity, flammability, explosive potential, radiation or other dangerous properties. Public attention tends to focus on carcinogens, industrial wastes, pesticides and radiation hazards. However, innumerable compounds that do not fall into these categories can pose a threat to the public’s safety and health.
Hazardous chemicals may present physical hazards, although this is more common in transportation and industrial incidents. Hydrocarbons may catch fire and even explode. Fires and explosions may generate their own toxic hazards depending on the chemicals that were initially present. Fires involving pesticide storage areas are a particularly dangerous situation, as the pesticides may be converted into even more highly toxic combustion products (such as paraoxons in the case of organophosphates) and substantial amounts of environmentally damaging dioxins and furans may be generated from combustion in the presence of chlorine compounds.
Toxicity, however, is the principal concern of most people with respect to hazardous waste. Chemicals may be toxic to human beings and they may also be damaging to the environment through toxicity to animal and plant species. Those that do not readily degrade in the environment (a characteristic called biopersistence) or that accumulate in the environment (a characteristic called bioaccumulation) are of particular concern.
The number and hazardous nature of toxic substances in common use has changed dramatically. In the last generation, research and development in organic chemistry and chemical engineering have introduced thousands of new compounds into widespread commercial use, including persistent compounds such as the polychlorinated biphenyls (PCBs), more potent pesticides, accelerators and plasticizers with unusual and poorly understood effects. The production of chemicals has risen dramatically. In 1941 production of all synthetic organic compounds in the United States alone, for example, was less than one billion kilograms. Today it is much greater than 80 billion kilograms. Many compounds in common use today underwent little testing and are not well understood.
Toxic chemicals are also much more intrusive in daily life than in the past. Many chemical plants or disposal sites which were once isolated or on the edge of town have become incorporated into urban areas by suburban growth. Communities now lie in closer proximity to the problem than they have in the past. Some communities are built directly over old disposal sites. Although incidents involving hazardous substances take many forms and may be highly individual, the great majority seem to involve a relatively narrow range of hazardous substances, which include: solvents, paints and coatings, metal solutions, polychlorinated biphenyls (PCBs), pesticides, and acids and alkalis. In studies conducted in the United States, the ten most common hazardous substances found in disposal sites requiring government intervention were lead, arsenic, mercury, vinyl chloride, benzene, cadmium, PCBs, chloroform, benzo(a)pyrene and trichloroethylene. However, chromium, tetrachloroethylene, toluene and di-2-ethylhexylphthalate were also prominent among those substances that could be shown to migrate or for which there was an opportunity for human exposure. The origin of these chemical wastes varies greatly and depends on the local situation, but typically elecroplating solutions, discarded chemicals, manufacturing by-products and waste solvents contribute to the waste stream.
Groundwater Contamination
Figure 1 presents a cross-section of a hypothetical hazardous waste site to illustrate problems that may be encountered. (In practice, such a site should never be placed near a body of water or over a gravel bed.) In well-designed hazardous waste disposal (containment) facilities, there is an effectively impermeable seal to prevent hazardous chemicals from migrating out of the site and into the underlying soil. Such a site also has facilities to treat those chemicals that can be neutralized or transformed and to reduce the volume of waste that goes into the site; those chemicals that cannot be so treated are contained in impermeable containers. (Permeability, however, is relative, as described below.)
Figure 1. Cross-section of a hypothetical hazardous waste site
Chemicals may escape by leaking if the container is compromised, leaching if water gets in or spilling during handling or after the site is disturbed. Once they permeate the liner of a site, or if the liner is broken or if there is no liner, they enter the ground and migrate downward due to gravity. This migration is much more rapid through porous soil and is slow through clay and bedrock. Even underground, water flows downhill and will take the path of least resistance, and so the groundwater level will fall slightly in the direction of flow and the flow will be much faster through sand or gravel. If there is a water table under the ground, the chemicals will eventually reach it. Lighter chemicals tend to float on the groundwater and form an upper layer. Heavier chemicals and water-soluble compounds tend to dissolve or be carried along by the groundwater as it flows slowly underground through porous rock or gravel. The region of contamination, called the plume, can be mapped by drilling test wells, or bore holes. The plume slowly expands and moves in the direction of groundwater movement.
Surface water contamination may occur by runoff from the site, if the top layer of soil is contaminated, or by groundwater. When the groundwater feeds into a local body of water, such as a river or lake, the contamination is carried into this body of water. Some chemicals tend to deposit in the bottom sediment and others are carried along by the flow.
Groundwater contamination may take centuries to clear by itself. If shallow wells are used as a water source by local residents, there is a possibility of exposure by ingestion and by skin contact.
Human Health Concerns
People come into contact with toxic substances in many ways. Exposure to a toxic substance may occur at several points in the use cycle of the substance. People work in a plant where the substances arise as waste from an industrial process and do not change clothes or wash before coming home. They may reside near hazardous waste disposal sites which are illegal or poorly designed or managed, with opportunities for exposure as a result of accidents or careless handling or lack of containment of the substance, or lack of fencing to keep children off the site. Exposure may occur in the home as the result of consumer products that are mislabelled, poorly stored and not child-proof.
Three routes of exposure are by far the most important in considering the implications for toxicity of hazardous waste: inhalation, ingestion and absorption through the skin. Once absorbed, and depending on the route of exposure, there are many ways in which people can be affected by toxic chemicals. Obviously, the list of possible toxic effects associated with hazardous waste is very long. However, public concern and scientific studies have tended to concentrate on the risk of cancer and reproductive effects. In general, this has reflected the profile of chemical hazards at these sites.
There have been many studies of residents who live around or near such sites. With a few exceptions, these studies have shown remarkably little in the way of verifiable, clinically significant health problems. The exceptions have tended to be situations where the contamination is exceptionally severe and there has been a clear pathway of exposure of residents immediately adjacent to the site or who drink well water drawing on groundwater contaminated by the site. There are several likely reasons for this surprising absence of documentable health effects. One is that unlike air pollution and surface water pollution, the chemicals in land pollution are not easily available to people. People may live in areas highly contaminated by chemicals, but unless they actually come in contact with the chemicals by one of the routes of exposure mentioned above, no toxicity will result. Another reason may be that the chronic effects of exposure to these toxic chemicals take a long time to develop and are very difficult to study. Yet another reason may be that these chemicals are less potent in causing chronic health effects in humans than is usually supposed.
Notwithstanding the human health effects, the damage of land pollution to ecosystems may be very great. Plant and animal species, soil bacteria (which contribute to agricultural productivity) and other ecosystem constituents may be irreversibly damaged by degrees of pollution that are not associated with any visible human health effect.
Control of the Problem
Because of population distributions, land use restrictions, transportation costs and concern from society over environmental effects, there is intense pressure to find a solution to the problem of economical disposal of hazardous waste. This has led to increased interest in methods such as source reduction, recycling, chemical neutralization and secure hazardous waste disposal (containment) sites. The first two reduce the amount of waste that is produced. Chemical neutralization reduces the toxicity of the waste and may convert it into a more easily handled solid. Whenever possible, it is preferred that this be done at the site of production of the waste to reduce the amount of waste that must be moved. Well-designed hazardous waste disposal facilities, using the best available technologies of chemical processing and containment, are needed for the residual waste.
Secure hazardous waste containment sites are relatively expensive to build. The site needs to be selected carefully to ensure that pollution of surface water and major aquifers (groundwater) will not readily occur. The site must be designed and built with impermeable barriers to prevent contamination of soil and groundwater. These barriers are typically heavy plastic liners and layers of tamped clay fill under the holding areas. In reality, the barrier acts to delay breakthrough and to slow the permeation that eventually does occur to an acceptable rate, one that will not result in accumulation or significant pollution of groundwater. Permeability is a property of the material, described in terms of the resistance of the material to a liquid or gas penetrating it under given conditions of pressure and temperature. Even the least permeable barrier, such as plastic liners or packed clay, will eventually allow the passage of some liquid chemical through the barrier, although it may take years and even centuries, and once breakthrough occurs the flow becomes continuous, although it may occur at a very low rate. This means that groundwater immediately below a hazardous waste disposal site is always at some risk of contamination, even if it is very small. Once groundwater is contaminated, it is very difficult and often impossible to decontaminate.
Many hazardous waste disposal sites are regularly monitored with collection systems and by testing nearby wells to ensure that pollution is not spreading. The more advanced are built with recycling and processing facilities on-site or nearby to further reduce the waste that goes into the disposal site.
Hazardous waste containment sites are not a perfect solution to the problem of land pollution. They require expensive expertise to design, are expensive to build, and may require monitoring, which creates an ongoing cost. They do not guarantee that groundwater contamination will not occur in the future, although they are effective in minimizing this. A major disadvantage is that someone, inevitably, must live near one. Communities where hazardous waste sites are located or proposed to be located usually oppose them strongly and make it difficult for governments to grant approval. This is called the “not in my back yard” (NIMBY) syndrome and is a common response to the siting of facilities considered undesirable. In the case of hazardous waste sites, the NIMBY syndrome tends to be especially strong.
Unfortunately, without hazardous waste containment sites, society may lose control of the situation entirely. When no hazardous waste site is available, or when it is too expensive to use one, hazardous waste is often disposed of illegally. Such practices include pouring liquid waste on the ground in remote areas, dumping the waste into drains that go into local waterways and shipping the waste to jurisdictions that have more lax laws governing the handling of hazardous waste. This may create an even more dangerous situation than a poorly managed disposal site would create.
There are several technologies that can be used to dispose of the remaining waste. High-temperature incineration is one of the cleanest and most effective means of disposing of hazardous waste, but the cost of these facilities is very high. One of the more promising approaches has been to incinerate liquid toxic waste in cement kilns, which operate at the necessary high temperatures and are found throughout the developing as well as the developed world. Injection into deep wells, below the water table, is one option for chemicals that cannot be disposed of in any other way. However, groundwater migration can be tricky and sometimes unusual pressure situations underground or leaks in the well lead to groundwater contamination anyway. Dehalogenation is a chemical technology that strips the chlorine and bromine atoms from halogenated hydrocarbons, such as PCBs, so that they can be easily disposed of by incineration.
A major unresolved issue in municipal solid waste handling is contamination by hazardous waste discarded by accident or intent. This can be minimized by diverting disposal into a separate waste stream. Most municipal solid waste systems divert chemical and other hazardous wastes so that they do not contaminate the solid waste stream. The separate waste stream should, ideally, be diverted to a secure hazardous waste disposal site.
There is a pressing need for facilities to collect and properly dispose of small quantities of hazardous waste, at minimal cost. Individuals who find themselves in possession of a bottle or can of solvents, pesticides or some unknown powder or fluid usually cannot afford the high cost of proper disposal and do not understand the risk. Some system for collecting such hazardous waste from consumers is needed before it is poured on the ground, flushed down the toilet or burned and released into the air. A number of municipalities sponsor “toxic roundup” days, when residents bring small quantities of toxic materials to a central location for safe disposal. Decentralized systems have been introduced in some urban areas, involving home or local pick-up of small quantities of toxic substances to be discarded. In the United States, experience has shown that people are willing to drive up to five miles to dispose of household toxic wastes safely. Consumer education to promote awareness of the potential toxicity of common products is urgently needed. Pesticides in aerosol cans, bleaches, household cleaners and cleaning fluids are potentially dangerous, especially to children.
Abandoned Hazardous Waste Disposal Sites
Abandoned or insecure hazardous waste sites are a common problem worldwide. Hazardous waste sites that need to be cleaned up are great liabilities to society. The ability of countries and local jurisdictions to clean up major hazardous waste sites varies greatly. Ideally, the owner of the site or the person who created the site should pay for its clean-up. In practice, such sites have often changed hands and the past owners have often gone out of business, the current owners may not have the financial resources to clean up, and the clean-up effort tends to be delayed for very long periods by expensive technical studies followed by legal battles. Smaller and less affluent countries have little leverage in negotiating clean-ups with the current site owners or the responsible parties, and no substantial resources to clean up the site.
The traditional approaches to cleaning up hazardous waste sites are very slow and expensive. It requires highly specialized expertise that is often in short supply. A hazardous waste site is first evaluated to determine how serious the land pollution is and whether the groundwater is contaminated. The likelihood of residents coming into contact with hazardous substances is determined and, in some cases, an estimate of the risk to health that this poses is calculated. Acceptable clean-up levels must be decided upon, the extent to which exposure must ultimately be reduced to protect human health and the environment. Most governments make decisions about clean-up levels by applying various applicable environmental laws, air pollution standards, drinking water standards, and based on a hazards assessment of health risks posed by the particular site. Clean-up levels are therefore set to reflect both health and environmental concerns. A decision must be made on how the site is to be remediated, or how best to achieve this reduction in exposure. Remediation is a technical problem of achieving these clean-up levels by engineering and other methods. Some of the techniques that are used include incineration, solidification, chemical treatment, evaporation, repeated flushing of soil, biodegradation, containment, removal of soil off-site and pumping out groundwater. These engineering options are too complex and specific to the circumstances to describe in detail. Solutions must fit the particular situation and the funds available to achieve control. In some cases, remediation is not feasible. A decision then has to be made on what land use will be permitted on the site.