and bioaccumulation have been used, in combination with toxicity, for the purpose of hazard categorization. However, for insoluble or sparingly soluble chemicals such as metals and metal compounds, persistence and bioaccumulation are neither appropriate nor useful. Unfortunately, this is not always recognized by regulators or even by scientists.
The use of persistent, bioaccumulative and toxic (PBT) criteria for chemicals was developed to address the hazards posed by synthetic organic chemicals. In fact, the criteria and test methods to evaluate persistence (i.e., the lack of degradability of a chemical) and bioaccumulation (the dispersion of a chemical through knowledge of the water-octanol partition coefficient) were developed to be used in combination with toxicity in order to reduce the importance given to the use of toxicity data alone. These test methods were based on an understanding of the chemistry of chemicals of concern at the time and of the biological interactions that the chemicals would have with the surrounding biota. Specifically, it was realized that if some chemicals exerted high intrinsic toxicity under standardized laboratory test conditions but did not persist or bioaccumulate, the environmental hazard of such chemicals would be lower.
As mentioned above, persistence is measured by determining the lack of degradability of a substance from a form that is biologically available and active to a form that is less available. This applies to many substances – metals and metal compounds tend to be in forms that are not bioavailable. Only under specific conditions would metals or metal compounds transform into a bioavailable form. Thus, rather than persistence, the key criterion for classifying metals and metal compounds should be their capacity to transform into bioavailable form(s). Furthermore, although bioavailability is a necessary precursor to toxicity, it does not inevitably lead to toxicity. Although metals and metal compounds stay in the environment for long periods of time, the risk they may pose generally decreases over time. For example, metals introduced into the aquatic environment are subject to removal/immobilization processes (e.g., precipitation, complexation and absorption).
Similarly, the use of bioaccumulation has significant limitations for predicting hazard for metals and metal compounds. Generally, either bioconcentration factors (BCFs) or bioaccumulation factors (BAFs) are used for this purpose. A bioconcentration factor is the ratio of the concentration of a substance in an organism, following direct uptake from the surrounding environment (water), to the concentration of the same substance in the surrounding environment. A bioaccumulation factor considers uptake from food as well. In contrast to organic compounds, uptake of metals is not based on lipid partitioning. Further, organisms have internal mechanisms (homeostasis) that allow them to regulate (bioregulate) the uptake of essential metals and to control the presence of other metals. Thus, if the concentration of an essential metal in the surrounding environment is low and the organism requires more, it will actively accumulate that metal. This will result in an elevated bioconcentration factors (or bioaccumulation factor) value which, while of concern in the case of organic substances, is not an appropriate measure in the case of metals.
The primary determining factor of hazard for metals and metal compounds is therefore toxicity, which requires consideration of dose (indeed, the fundamental tenet of toxicology is the dose makes the poison). Historically, it has been the practice to measure the toxicity of soluble metal salts, or indeed the toxicity of the free metal ion. However, in different media, metal ions compete with different types or forms of organic matter (e.g., fish gills, suspended solids, soil particulate material) to reduce the total amount of metals present in bioavailable form. Toxicity of the bioavailable fraction (i.e., as determined through transformation processes) is the most appropriate and technically defensible method for categorizing and ranking the hazard of metals and metal compounds.
The relative proportion of hazardous constituents present in any collection of chemicals (crude oil-derived products included) is variable and rarely consistent because of site differences. Therefore, the extent of the contamination will vary from one site to another and, in addition, the farther a contaminant progresses from low molecular weight to high molecular weight the greater the occurrence of polynuclear aromatic hydrocarbons, complex ring systems (not necessity aromatic ring systems) as well as an increase in the composition of the semi-volatile chemicals or the non-volatile chemicals. These latter chemical constituents (many of which are not so immediately toxic as the volatiles) can result in long-term/chronic impacts to the flora and fauna of the environment. Thus, any complex mixture of chemicals should be analyzed for the semi-volatile compounds which may pose the greatest long-term risk to the environment.
Heavy metals are common chemical pollutants. The most common heavy metals found at contaminated sites, in order of abundance are Pb, Cr, As, Zn, Cd, Cu, and Hg. Those metals are important since they are capable of decreasing crop production due to the risk of bioaccumulation and biomagnification in the food chain. There is also the risk of superficial and groundwater contamination. Knowledge of the basic chemistry, environmental, and associated health effects of these heavy metals is necessary in understanding their speciation, bioavailability, and remedial options. The fate and transport of a heavy metal in soil depends significantly on the chemical form and speciation of the metal. Once in the soil, heavy metals are adsorbed by initial fast reactions (minutes, hours), followed by slow adsorption reactions (days, years) and are, therefore, redistributed into different chemical forms with varying bioavailability, mobility, and toxicity (Shiowatana et al., 2001). This distribution is believed to be controlled by reactions of heavy metals in soils such as (i) mineral precipitation and dissolution, (ii) ion exchange, adsorption, and desorption, (iii) aqueous complexation, (iv) biological immobilization and mobilization, and (v) plant uptake (Levy et al., 1992). The toxicity of metals varies greatly with pH, water hardness, dissolved oxygen levels, salinity, temperature and other parameters.
The specific type of metal contamination found in a contaminated soil is related to the operation that occurred at the site. The range of contaminant concentrations and the physical and chemical forms of contaminants will also depend on activities and disposal patterns for contaminated wastes on the site. Other factors that may influence the form, concentration, and distribution of metal contaminants include soil and groundwater chemistry and local transport mechanisms.
Finally, in order to evaluate the impact of a chemical that has been released to the environment, the chemical must be characterized in terms of the transport and transformation in that system (atmosphere, water, or land) and the potential for the transport of the chemical from one system to another or from one system to the other two. The assessment should focus on areas with which a released chemical is most likely to have contact. For a meaningful characterization, the environment must be viewed as a series of interacting compartments and it must be determined whether a chemical will remain and accumulate in the local area of the origin of the chemical. The potential for the chemical to be physically, chemically, or biologically transformed in the system of its origin (such as by hydrolysis, oxidation, or other transformation; Chapter 8) or be transported to another system such as by volatilization or by precipitation. The chemical could also be transferred by deposition and runoff to surface water that provides drinking water.
Each of these scenarios defines a pathway from the air emission to contact with a person, and each pathway has an associated route of contact. The true potential for exposure cannot be quantified until the pathways and routes that account for a substantial fraction of the intake and uptake for the receptor population have been identified. The likelihood of any pathway depends on the chemical properties of the substance released, where and how it is released, and environmental conditions. Sometimes the exposure increases along a pathway (such as bioaccumulation), but more often the exposure may decrease.
Thus, characterizing transportation pathways begins at the source of the agent release. In some situations, the source may be obvious and can be defined and characterized from air or soil concentrations. In many cases, such as contamination of water supplies, sources and emissions may be multiple and poorly characterized. However, classification of a potential transportation route should, as much as possible, be based on the released volume, duration of the release, and the rate of emission.
In order to fully understand the impact of
