FEATURE
The Lead Fingerprints of Gasoline Contamination Isotopic analysis of the lead additives in gasoline can improve estimates of the ages of leaks and spills. RICHARD W. HURST,
TERRY E. DAVIS, BARBARA D.
A
ccidental gasoline releases are a major concern for federal, state, and local regulatory agencies. Important sources of such leaks are the more than 2.5 million underground storage tanks. Attributing legal and financial liability for the remediation of these sites is often difficult. In many cases, the long-term use of a particular site, for example, a service station in operation since 1950 under many owners has made accurate allocation of remediation expenses all but impossible. Although methods such as gas chromatographic fingerprinting have been developed to identify the type of hydrocarbons present in contaminated soils and groundwater, they have been only partially successful in determining the time of a release (i). In the best situations, the use of gas chromatographic fingerprinting can determine the time of a release to within 5 to 10 years. But a new method, which uses stable lead isotope ratios, can constrain the time of a release to within 1 to 5 years (2-6). The method is based on the observation that the lead compounds added to gasoline reflect the characteristic and distinct stable lead isotope ratios of the ores from which the lead was obtained. For gasoline additives in the United States, the use of one particular source with distinctive, high isotopic ratios increased progressively from the late 1960s to the late 1980s. The increased use of this ore is manifested as systematically higher stable isotope ratios for gasolines during this time. The Anthropogenic Lead Archaeostratigraphy (ALAS) method calibrates these changes to date and fingerprint gasoline releases and provides an independent age estimate that can be used in conjunction with historical and environmental data to resolve liability issues A changing mix of additives Methods used by environmental consultants and site remediators throughout the 1980s to estimate the timing of the most common hydrocarbon release, 3 0 4 A • VOL. 30, NO. 7, 1996 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS
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gasoline, have relied on either the history of use of specific gasoline additives or degradation rates of petroleum hydrocarbons in the environment. The latter approach is more qualitative because degradation rates are influenced by soil conditions, which may change markedly over time (7). Chronologies based on gasoline additives and characteristics of gasoline hydrocarbon compounds are more quantitative, providing age resolutions of 5 to 10 years in the best cases (Figure 1). These methods have been successful in cases in which free gasoline was available and resolution of the time of a release to closer than five years was not necessary In some cases, it is sufficient to know that a release either pre- or postdated a specific year. Gasoline additive chronologies and fingerprinting characteristics can sometimes resolve such questions. For example, before 1985 the proportion of the major C10+ hydrocarbons, which contain 10 or more carbon atoms, was greater than the amount of normal propyl benzene. The chronology of gasoline additives has been the most common source of age diagnostic information. Oxygenates such as methyl-terf-butyl ether (MTBE) were added after 1980 and 1990 to eastern and western U.S. gasolines, respectively. Antiknock additives, such as the alkylleads (e.g., tetraethyl lead) and manganese compounds (2-methyl cyclopentadienyl manganese tricarbonyl, MMT), also have age significance. The speciation of lead additives has changed, with tetraethyl lead being the only alkyllead additive in leaded fuels after 1980. Hence, analyses of alkyllead species present in free products can help date the time of a release. Although MMT can be age diagnostic its absence in gasoline does not necessarily indicate a post-1978 gasoline because it was not routinely added by all manufacturers Lead scavengers ethylene dibromide and ethylene dichloride were added to leaded fuels to prevent accumulations of metallic lead in the engine and are time correlative with leaded gasolines 0013-936X/96/0930-304AS12.00/0 © 1996 American Chemical Society
Lead levels in gasolines have dropped significantly since die early 1980s; current part-per-billion levels are attributable to natural crude oil sources plus any minute amount of lead added during the refining process. Although one is tempted to date gasolines on the basis of their lead content, selective adsorption of lead by soils or other gasoline-soil matter exchange can alter the lead content, compromising the results and producing a possibly erroneous age. Gasoline additive chronologies can be important, cost-effective remedies in some site remediation and insurance cost recovery operations. However, the relative rates of degradation and selective solubility in water or hydrocarbons of each additive must be taken into account to estimate ages. Lead's persistent signature The lead additives that were used in gasoline and that persist in the anthropogenic lead fallout from automobile emissions provide the basis for using stable lead isotopes to date gasoline releases. Different lead ores have characteristic and different stable lead isotope ratios, which provide a signature of the source when the lead is incorporated into gasoline additives. For gasoline additives in the United States, the use of Mississippi Valley ores with distinctive, high isotopic ratios increased progressively from the late 1960s to the late 1980s. The ALAS method translates the changes in the values of stable lead isotopes into a calibrated history that can be used to date the timing of gasoline releases. Each lead ore contributes to the distinctive lead isotopic signature of the additive and thus the gasoline. Using high-precision lead isotope ratios, the mixing of leads from multiple sources, even at concentrations below one part per million (ppm) (8) can be quantified (6). A variety of samples are amenable to stable isotope analysis, including free products, soils, and dissolved lead in groundwater (to partper-billion levels). Because lead is retained in soils over time and environmental processes do not fractionate the ratios, the signature provides a lasting and reliable means of dating gasoline spills and leaks. This persistent signature also means that samples can be taken prior to remediation and analyzed later. Isot o n p ratios are also used to distinguish anthropogenic lead from natural background lead The use of stable lead isotopes to date gasoline releases is based on Claire C. Patterson's research, which focused on global lead pollution (9-11). In his quest to quantify the global effects of lead, Patterson observed that systematic, statistically significant increases occurred in 206Pb/207Pb ratios of the anthropogenic fraction of well-dated marine sediments collected immediately offshore of Los Angeles near oil refineries in the San Pedro area (9). The increases exceeded analytical errors by a factor of 30 and were attributed to increased use of lead from the Mississippi Valley region, which is characterized by high lead isotope ratios (Table 1) Subsequent research showed that these increases continued through the late 1980s (12-14) paralleling increased use of Mississippi Valley lead ores whose contributions exceeded 80% of the domestic supply. Results of these studies also demonstrated that average stable isotope ratios of leaded gasolines were
iFIGURE 1
Chronology of gasoline additives and fingerprinting characteristics The history of gasoline additives provides general indications of their age with resolutions of 5 to 10 years. Time ranges of known use of the various additives, characteristics, and gasoline lead concentrations are shown.
Source: National Institute of Hydrocarbon Fingerprinting, Albuquerque, N.M.. ,1995
relatively uniform over intervals of one year because the major lead additive producers, Ethyl and DuPont, used similar starting mixtures of ores for alkyllead products (9-12). This is evident in the ALAS model curve (Figure 2), time-dependent lead isotopic variations from gasoline combustion published in the literature (9-14), and agreement of the ALAS model curve with aerosol lead isotope trends controlled by leaded gasoline combustion (3-5). Such systematic variations would not result if the additive companies had randomly selected different mixes of lead ores over time In 1991 we began developing the ALAS model with the acquisition of gasoline-contaminated sediment with well-documented ages and archived, refined gasolines. Sample ages were documented (< 1 year uncertainty) by using one or more of the following methods: sediment chronology, radioactive isotope dating, and documentation by reliable institutions and agency response records of spills at sites. The development of new extraction techniques and modified EPA protocols were required to evaluate the existence of multiple anthropogenic lead sources in soils, differentiate each anthropogenic source from the natural background lead in the soil, and provide data for lead-mixing models. For example, clay-rich soil extractions might begin with an organic solvent to remove organolead, if present, folVOL. 30, NO. 7, 1996/ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS " 3 0 5 A
FIGURE 2
The ALAS model calibration curve One Anthropogenic Lead Archeostratigraphy (ALAS) Model calibration curve that uses a delta notation is shown. In this example the measured '"Pb/^'Pb ratio of a sample is compared to that of an internal laboratory standard whose 206 Pb/207Pb ratj 0 is known precisely. The difference between the measured anthropogenic (i.e., gasoline-derived) lead isotope ratio of the sample relative to that of the standard is then plotted against the known or documented age of the gasoline release in a soil sample or the gasoline so that:
The lack of scatter in the lead isotope results for any given year is attributable to alkyllead producers acquiring their lead from the same sources, in approximately the same proportions over the course of any given year.
a single source if data points form a cluster, from multiple sources if several clusters result, or from mixtures of sources if data form linear arrays. It is then possible to identify the lead isotopic and concentration characteristics of each source and quantify the relative contribution of each source to a mixture. ALAS calibration curves plot changes in gasoline lead isotope ratios relative to a lead isotopic standard, for example, A 206 Pb/ 207 Pb over time (Figure 2). The changes generally conform to the observed lead isotopic variations originally described by Patterson. Lead isotopic ratios of gasolines were low in the 1960s, giving way to rapid increases through the 1970s, followed by moderate increases through the late 1980s at the end of the leaded gasoline era. Highprecision lead isotope ratio analyses (
