The Superfund program of the US Environmental Protection Agency (EPA) was created in the 1980s to address human-health and environmental risks posed by abandoned or uncontrolled hazardous-waste sites. Identification of Superfund sites and their remediation can be an expensive multistep process. As part of this process, EPA attempts to identify parties that are responsible for the contamination and thus financially responsible for remediation. Identification of potentially responsible parties is complicated because Superfund sites can have a long history of use and involve contaminants that can have many sources. Such is often the case for mining sites that involve metal contamination; metals occur naturally in the environment, they can be contaminants in the wastes generated at or released from the sites, and they can be used in consumer products, which can degrade and release the metals back to the environment. Lead as an environmental contaminant associated with mining sites provides a prime example of the complexities that are involved in identifying potential sources in that it meets all the criteria noted. Given the complexities surrounding lead contamination, Congress asked EPA to commission a study by the National Academies of Science, Engineering, and Medicine to determine whether there are techniques or methods that can help to distinguish sources of lead at Superfund sites associated with mining activities. Accordingly, the National Academies convened the Committee on Sources of Lead Contamination at or near Superfund Sites, which prepared this report.
THE COMMITTEE’S TASK AND THE SOUTHEAST MISSOURI LEAD MINING DISTRICT
In brief, the committee was asked to examine the extent to which various sources contribute to environmental lead contamination at Superfund sites that are near lead-mining areas and to focus on sources that contribute to lead contamination at sites near the Southeast Missouri Lead Mining District.1 The committee interpreted its task as a request for investigative strategies that could provide the evidence needed to identify lead sources in various environmental media. It did not consider its task to be determining lead sources; such an activity would require extensive sampling, analysis, and interpretation that the committee was not equipped to perform. As directed, the committee focused its efforts on whether sources of lead in the Southeast Missouri Lead Mining District could be distinguished by using various investigative strategies. The committee emphasizes that it did not consider exposure pathways, which would need to be considered for identifying which lead sources are contributing to human exposure.
The Southeast Missouri Lead Mining District provides a prime example of the complexities that a site might pose. It is composed primarily of three large subdistricts—the Old Lead Belt, Mine La Motte-Fredericktown, and the Viburnum Trend—and has a long history of lead mining: it began in the early 1700s and continues to this day. Some areas in Southeast Missouri are contaminated with lead, and several have been designated as Superfund sites. Determining the sources of lead at the sites, however, is complicated. First, the area is rich in naturally occurring lead-ore deposits that are at or near the surface. Second, as noted, the area has a long history of mining activities. Third, an enormous amount of wastes known as chat or tailings has been produced as a result of the mining activities; some of the waste materials have been dispersed from their original disposal sites by natural forces, such as wind and rain, and some have been intentionally transported off site. Fourth, lead has been used extensively over centuries in various products. Thus, lead at a site could result from naturally occurring deposits, from historical mining activities, from current mining activities, from consumer products, or from some combination. The following sections describe potential sources and environmental dispersal mechanisms, highlight the commit-
tee’s investigative strategies and the implications for the Southeast Missouri Lead Mining District, and provide the committee’s research recommendations.
SOURCES OF LEAD AND ENVIRONMENTAL DISPERSAL
Lead is a widespread, naturally occurring element that is found throughout the Earth’s crust and occurs in virtually all rocks, soils, sediments, and waters at low concentrations. A variety of geologic processes can cause natural lead enrichment, and, in some cases, ore deposits are formed. Whether an ore deposit is mined depends on time, economics, technology, and natural processes. If various conditions are favorable, it might be decided to mine an ore deposit. Mining begins by physically extracting the ore; the ore is then processed to obtain a concentrate of lead minerals and smelted to obtain metallic lead, which is shipped to market for use in various consumer products (see Figure S-1). At each stage, materials are discarded, and this creates an opportunity for lead to be dispersed further in the environment. Historically, waste disposal was driven by what was easiest; thus, waste was generally discarded in the most efficient manner relative to the time and place. However, by the latter part of the 20th century, waste disposal was subject to regulatory controls and handled systematically in a mine’s operating plan.
Sources of lead other than natural ones or those associated with mining can also cause environmental contamination. In the 20th century in the United States, lead was added to gasoline and paint; although those uses have been virtually eliminated, the legacy lead from them remains in soils and sediment and could be an important source of environmental contamination. Other uses of lead include plumbing materials, batteries, pesticides, agricultural amendments, ammunition, and other consumer products, such as jewelry, pottery, and glass.
Lead can be dispersed in the environment by natural processes and through intentional transport. A primary natural transport mechanism is via water. Although lead concentrations in water are generally lower than those of other dissolved species, lead can be transported in natural waters attached to colloidal or suspended particles. Depending on environmental conditions, sediment-associated lead in the river bank can be reintroduced into the river over both short and long periods. Depending on the prevailing weather, airborne transport has the greatest potential to disperse pollutants on regional and global scales because air is not confined by geographic barriers. Furthermore, transport rates of contaminants are often higher by air than by groundwater and surface waters. Intentional transport of mining materials and wastes has been an important mechanism for dispersal of lead in the environment. In the Southeast Missouri Lead Mining District, mine wastes (waste rock, tailings, and slags) were
transported for use as construction fill, roadbed materials, agricultural amendments, and building materials.
INVESTIGATIVE STRATEGIES FOR LEAD-SOURCE ATTRIBUTION
Over the years, a wide array of analytic techniques have been used to determine the relative contribution of lead from natural and anthropogenic sources to soil, sediment, water, and air with varied success. Most techniques rely on a fingerprinting approach in which unique physical or chemical properties of the lead or lead-associated materials are used to distinguish sources from one another. The committee reviewed the techniques and provides investigative strategies for determining lead sources in soil, sediment, water, and air in Chapter 4 of this report. The approaches rely on systematic collection, analysis, and interpretation of multiple lines of evidence; an investigation continues until there is sufficient confidence that an accurate lead-source attribution has been reached. The committee emphasizes that its strategies are illustrative and that the sequence of steps will depend on site-specific conditions, availability of analytic techniques, and cost and time considerations.
The investigative strategies for soil, sediment, and water begin with a site characterization.2 The goal here is to develop a site conceptual model that can guide investigations toward likely lead sources. Ideally, site characterization should document the source locations, the form of lead and other materials associated with sources, pathways and processes by which lead could be transported from a source, and, for anthropogenic sources, the history of lead use or processing. Once basic questions have been answered, a detailed sampling protocol can be developed for the specific site, and decisions can be made as to the best analytic approaches to pursue.
For the strategies noted, the committee concluded that lead isotopic analysis should be considered first for laboratory analysis of samples. Lead-isotope ratios in geologic materials vary owing to the production of lead isotopes from the radioactive decay of uranium (U) and thorium (Th). On geologic timescales, 235U decays to 207Pb, 238U decays to 206Pb, and 232Th decays to 208Pb; 204Pb is primordial and has no radioactive parent. Various factors, such as geologic conditions and ages, result in a wide array of lead-isotope ratios among lead-bearing ore deposits and other geologic materials. Consequently, ore deposits can have unique lead-isotope ratios (signatures) that distinguish them from each other. When lead is purified from the ores and used in industry, the lead-containing commercial products retain the lead-isotope ratios of the ore deposits used to make the products. Thus, the technique works well if there is an isotopic contrast between the sources that one wishes to distinguish, but it can be equivocal if there are overlapping isotopic ratios in the various potential sources. In most cases, some potential sources can be eliminated and thus focus the investigation on fewer possible sources.
If the isotopic analysis does not yield unique signatures or if additional confidence is needed to reach conclusions, the committee suggests other analytic techniques and methods, such as ones that rely on analysis of mineralogy, particle size, chemical speciation, or other source-associated elemental ratios or isotopes. Each method has the potential to provide additional evidence to identify lead sources. However, the committee emphasizes that even after extensive sampling and analysis it still might not be possible to attribute lead to particular sources with confidence.
IMPLICATIONS FOR THE SOUTHEAST MISSOURI LEAD MINING DISTRICT
Isotopic analytic techniques might be especially promising for distinguishing non-native ores (or products made from non-native ores) from the natural or locally mined materials in Southeast Missouri. The committee tested the possibility by compiling and comparing 206Pb/207Pb ratios of North American lead ores, gasoline, and paints from around the world. As noted, various ore deposits can have different lead-isotope ratios, and gasoline producers and paint manufacturers used lead mined from different ore bodies. Therefore, it is expected that in some specific cases the lead isotopic composition of gasoline and paint might differ from that of local ore bodies; in other cases, they might be the same and therefore not useful for source attribution. The lead ores from Southeast Missouri have an extremely unusual lead isotopic composition, and the isotopic composition of lead mined from Southeast Missouri (and adjacent areas in the tristate region) appears to be distinct from that of nearly all other major lead mines (and products made with lead from them) in North America.
Figure S-2 clearly shows that Southeast Missouri ores, especially those from the Old Lead Belt, have higher 206Pb/207Pb ratios than other ore deposits, gasoline, and paints used in the United States. The only exceptions are the lead ores from the Upper Mississippi Valley District, which were probably not produced in substantial quantities in the 20th century compared with those from Southeast Missouri, and a single sample from a house within 0.5 miles of the Herculaneum lead smelter that could be the result of lead contamination from the smelter. Thus, the committee concludes that in the case of the Southeast Missouri Lead Mining District, it is likely that lead-isotope ratios can be used to attribute the sources of lead in
2 The investigative approach for air is different from that for the other environmental media in that it uses a tiered approach that focuses primarily on modeling for source attribution, although similar techniques to analyze airborne lead particles are noted.
environmental media between mining materials and gasoline and paint in most cases. If there are non-native ores or other lead-containing consumer products at the site—such as plumbing materials, batteries, or pesticides—lead-isotope studies of those materials might establish their potential role as a source of lead contamination at a site.
Distinguishing lead from natural (background) sources and lead from local mining materials or waste is much more challenging in contaminated soil. Natural and mined materials from the same region will typically contain lead that exhibits similar lead isotopic composition, and this would hinder the use of lead isotopic analysis to determine source. However, one might be able to use other methods alone or in combination—such as analysis of grain size, structure, and composition of soil particles, and depth distributions of metals in soils—to attribute potential sources. For example, smelter-derived particles in soils often contain spherical grains that are composed of sulfides, oxides, or glassy particles, and their spatial distribution is also likely to vary as a function of wind direction and distance from the smelter. With time, however, weathering of minerals derived from primary ore or smelter sources in soils might alter or dissolve them and potentially disperse them in the environment. The committee emphasizes that the selection of specific properties to be used for source attribution must be based on physical, compositional, mineralogic, or geochemical differences in the materials that make up the various source materials that need to be distinguished. Furthermore, it might be necessary to use multiple methods to differentiate between sources. In some cases, even with multiple analytic approaches, source identification might not be definitive in distinguishing natural materials from local mined materials or wastes, especially in soils and sediments.
Additional research will provide opportunities to enhance the analytic techniques and methods that make up the committee’s proposed investigative strategies and to reduce uncertainty in their implementation. The committee finds that characterizing potential sources has high priority for research and lists below several topics that deserve attention. The research needs are not listed in order of priority.
- Improved characterization of lead isotopic composition of historical paints. To evaluate the utility of lead isotopic analysis for distinguishing lead sources, additional study of lead isotopes will be necessary. There appear to be few data on the lead isotopic composition of house paint in Southeast Missouri, and the committee recommends systematic sampling and lead isotopic analysis of paint in houses built before 1980 throughout the Southeast Missouri Lead Mining District. Such study would give a much clearer indication of the lead isotopic composition of paint used in the area and would clarify the degree to which lead isotopes provide a method for attributing some lead sources.
- Improved characterization of lead-bearing geologic materials and mining wastes. Because lead isotopic analysis will probably not allow distinguishing natural materials from mining materials or wastes, physical and chemical properties of the latter materials need to be characterized better so that the properties can be used to help to distinguish sources. The goal is to identify fine-scale variability in the sources on the basis of isotopic composition, elemental chemistry, mineralogy, grain size, or texture.
- Capability to model metal concentrations in soils as a function of depth. The science of modeling sediment distributions in river floodplains is relatively advanced. In contrast, modeling of metal movement through soils is less developed. Studies could be completed on soils near known metal-emitting sources to map and model how the metals have moved, if at all, through the soils as a function of time.
- Characterization of size distribution of suspended particles in air samples. Tailings impoundments, chat piles, and smelter emissions are important sources of airborne lead in a mining district, and little is known about the size of suspended particles and their mobility. Size-selective sampling of air samples in the Southeast Missouri Lead Mining District should be conducted to determine the particle-size distribution. Size-selective sampling can be conducted upwind or downwind of a source, such as a smelter or tailings impoundment, to evaluate contributions from the source.
- Characterization of source strength of airborne particles from mining wastes. Little is known about rates of emission of airborne lead from stockpiles of mining wastes. Portable wind tunnels can be used to estimate the airborne particle emission rates and dust production from stockpiles of mining wastes and to measure the threshold wind velocity at a specific site that results in particle suspension and later transport downwind. That information can help to establish whether airborne lead from mining wastes is an important source of lead contamination downwind.
- Investigation of industrial archeology as a tool for source identification. Industrial archeology is the systematic study of material evidence associated with historical industrial activity. Although historical industrial processes were generally on a smaller scale than modern industry, they operated less efficiently, were more likely to use processes and materials that produced hazardous byproducts, and lacked pollution controls. An understanding of historical practices could shed light on their relative
importance as a source of legacy pollution that persists long after the industries have disappeared.
Although the committee was not asked to identify possible exposure pathways or methods for determining which lead source at a contaminated site might be the most important for human exposure, it did consider whether its investigative approaches could help to answer such questions. Blood lead is often used as a measure to determine whether (and how much) lead exposure has occurred. The committee’s recommendation for using lead isotopic analysis also applies to investigating sources of blood lead. Although additional study would be needed to determine where and under what conditions lead isotopic analysis could provide unambiguous lead-source attribution for people who have high blood lead concentrations, the approach appears promising for the Southeast Missouri Lead Mining District and possibly other mining districts, such as Coeur d’Alene, Idaho.