By
Richard W. Hurst
Dept. of Geological Sciences, CSU
Los Angeles, CA 90032
And
Hurst & Associates, Inc.
9 Faculty Ct.
Thousand Oaks, CA 91360
INTRODUCTION
The paper by Lima et al. (2005) is noteworthy in that the authors utilize the Anthropogenic Lead ArchaeoStratigraphy (ALAS) Model (Hurst, 2000, 2002, and citations therein), as a reference to identify potential contributions from historic leaded gasoline combustion to the anthropogenic lead budget of Pettaquamscutt River Basin sediment. Although the ALAS Model was initially developed for the purpose of estimating the year a gasoline release occurred, the model, as realized by Lima et al., also has potential applications as an anthropogenic and/or archaeological stratigraphic tool (Hurst, 2002).
However, I take issue with the conclusion by Lima et al. (2005) that [the ALAS Model] “does not seem to function as a reliable end-member to apportion the contribution of leaded gasoline to environmental samples”, particularly for the time period after 1960 when lead concentrations in gasoline increased dramatically. In this commentary, I will clarify some issues regarding the ALAS Model used by Lima et al. and address, more thoroughly, the question regarding the reliability of the ALAS Model as an end-member for apportioning the contribution by leaded gasoline to environmental samples.
- THE ALAS MODELS
The ALAS Model employed by Lima et al.(2005) is the calculated version of the model (Hurst, 2002), rather than the model developed through the use of a suite of calibration samples comprised of archived leaded gasoline and leaded gasoline impacted sediment whose year of production or year of the gasoline release, respectively, were known (Hurst, 2000). Observed variations in comparative 206Pb/207Pb ratios between the two ALAS Models are minimal, however, averaging < 0.3% for any given year.
The calculated ALAS Model (Hurst, 2002) was an exercise to evaluate if 206Pb/207Pb ratios of the calibrated ALAS curve (Hurst, 2000) could be replicated by integrating published lead production data (U.S. Bureau of Mines Minerals Yearbooks) with published ore lead isotope ratios (e.g., Russell and Farquhar, 1960; Doe, 1970). The concordance between 206Pb/207Pb ratios of the calculated versus calibrated ALAS Models that resulted from the modeling corroborated conclusions by Hurst (2000) that lead used to produce tetraethyllead (e.g., TEL) was acquired from a well-mixed lead ore pool, derived proportionately (relative to tons produced) from domestic, foreign, and recycled lead. This conclusion has been substantiated via discussions with Ethyl Corporation, a major manufacturer of alkylleads throughout the 20th Century, and the significant correlation between 206Pb/207Pb ratios of [predominantly gasoline-derived] lead in aerosols and those of the contemporaneous ALAS Model (Hurst, 2003).
The restricted range in annual leaded gasoline 206Pb/207Pb ratios, as observed in the ALAS Model, is the reason why the model has been an effective, accurate means of age dating gasoline releases at more than 150 sites throughout the United States since 1993 (Hurst, 2003). Thus, the model should provide a foundation to apportion contributions from leaded gasoline to the anthropogenic lead budget of Pettaquamscutt River Basin sediment.
- PETTAQUAMSCUTT RIVER BASIN SEDIMENT LEAD GEOCHEMISTRY
It is important to note that the downcore profile of total leachable lead in the Pettaquamscutt River Basin sediments reflects the significant contribution expected from leaded gasoline consumption that peaked between ~ 1960 and ~ 1980; leachable lead concentrations in the sediment maximize during this time period. Lima et al. make this observation, stating that total leachable lead in the sediments post-1920 generally follows that of lead in gasoline. They further conclude that the anthropogenic lead flux into Pettaquamscutt River Basin sediments increases 4-fold increase from 1922 to 1975, then decreases 3.5-fold after 1975. These changes are attributed, respectively and correctly, to the increase and subsequent decrease in TEL usage during these time periods.
One hallmark of the ALAS Model is the systematic increase in gasoline 206Pb/207Pb ratios between ~ 1968 and ~ 1980 that resulted from the increased use of radiogenic Mississippi Valley Type (MVT) ore lead (Hurst, 2000, 2002). This increase, as stated in Lima et al., is exceptionally well recorded in Pettaquamscutt River Basin sediment 206Pb/207Pb ratios, however, the authors then conclude the ALAS Model is not a viable means of estimating the proportion of gasoline derived lead in environmental samples. This contradiction can be resolved by evaluating (1) the interpretation of observed temporal variations in anthropogenic 206Pb/207Pb ratios of Pettaquamscutt River Basin sediment relative to the ALAS Model; and (2) the influence of lead concentration on anthropogenic lead isotope ratios in Pettaquamscutt River Basin sediment, particularly in sediment deposited after ~ 1960.
- Interpretation of Anthropogenic 206Pb/207Pb Ratios
The Pettaquamscutt River Basin site is not unique, in that the rapid temporal increase in anthropogenic 206Pb/207Pb ratios that is attributable to aerosol deposition of gasoline derived lead has also been recorded in: (1) sedimentary records in California (Shirahata et al., 1980; Ng and Patterson, 1982), Chesapeake Bay (Marcantonio et al., 2002), Delaware (Kim et al., 2004), Lake Erie (Graney et al., 1995), and Georgia (Okefenokee Swamp; Jackson et al., 2004); (2) Bermuda corals and surface waters (Shen and Boyle, 1987; Veron et al., 1994; Hamelin et al., 1997; Reuer et al., 2003); and (3) aerosols in California and the eastern/midwestern United States (Chow et al. 1975; Shirahata et al., 1980; Rosman et al., 1994, Sturges and Barrie, 1987; Boutron et al., 1991, 1994; Erel and Patterson, 1994). The rapid temporal increase in anthropogenic 206Pb/207Pb ratios, as defined by the ALAS Model from ~ 1968 to 1980, is not only recorded in geographically diverse environmental samples, but also serves as a stratigraphic marker for sediment deposited during this time period , when the flux of gasoline-derived lead to sediment reached a maximum. Hence, the authors statement that “high resolution sampling of the Pettaquamscutt River Basin core is exceptional at recording the rapid temporal increase in 206Pb/207Pb ratios [as observed in the ALAS Model] after 1970” runs counter to their statement that the ALAS Model is not a reliable end member for identifying gasoline lead in the environment, invoking conclusions by Kaplan (2003) that variations in annual lead isotopic ratios of TEL were the norm throughout the country. Following this line of reasoning to its logical conclusion, one would expect that anthropogenic 206Pb/207Pb ratios of environmental samples throughout the United States, including the Pettaquamscutt River Basin, should, in no way, record the systematic increase observed in the ALAS Model after ~ 1968. The fact that environmental samples from such diverse geographic locations do, in fact, record the ALAS Model’s post-1968 systematic increase in 206Pb/207Pb ratios contradicts the authors’ statement that the model does provide reasonable isotopic constraints for identifying gasoline derived lead in such samples.
3.2 Apportioning Sources of Anthropogenic Lead in Pettaquamscutt River Basin Sediment
3.2.1 The Pre-1960s Era
I fully agree with the authors’ conclusion that discrepancies between ALAS Model and Pettaquamscutt River Basin sediment 206Pb/207Pb ratios are attributable to the presence of coal derived lead, however I disagree that gasoline lead isotope ratios were highly variable. The impact of coal-derived lead on anthropogenic lead isotope ratios has been raised to explain differences between ALAS Model, i.e., gasoline, and aerosol 206Pb/207Pb ratios (Hurst, 2003). In fact, the observed convergence of, and rapid increase in anthropogenic 206Pb/207Pb ratios of Pettaquamscutt River Basin and Chesapeake Bay sediment, as well as Bermuda coral/seawater that is evident between ~ 1965 and ~ 1980 (Figure 9; Lima et al. 2005) when gasoline lead concentrations were high, would not be expected if gasoline lead isotope ratios varied dramatically, as suggested by Lima et al. It is not until the mid-1980s that the ALAS Model curve and temporal variations in anthropogenic 206Pb/207Pb ratios of environmental samples diverge, as would be expected as gasoline lead concentrations decreased dramatically.
The importance of coal as a source of anthropogenic lead in sediment deposited prior to the 1960s is well known (Ng and Patterson, 1982; Graney et al., 1995; Hurst, 2000), yet, as addressed in Lima et al., it is generally assumed [incorrectly] that a significant proportion of lead pollution throughout the 20th Century is attributable to gasoline combustion. The authors estimate the contribution from coal lead to Pettaquamscutt River Basin sediment to average ~ 62 + 9% from 1927 to 1969. They then conclude that ALAS Model 206Pb/207Pb ratios must be higher to reconcile apparent discrepancies. Modification of the ALAS Model, however, is not necessary.
We note that pre-1960s 206Pb/207Pb ratios of the ALAS Model and anthropogenic lead in Pettaquamscutt River Basin sediment, Chesapeake Bay sediment, and Bermuda coral/seawater exhibit restricted ranges. ALAS Model 206Pb/207Pb ratios average 1.165 + 0.009, while those of the anthropogenic component in sediment, coral, and seawater range from ~ 1.180 to 1.195 (Lima et al., 2005). Using the average 206Pb/207Pb ratios of 10 eastern coals, 1.203 + 0.012 (Chow and Earl, 1972) and the pre-1965 ALAS Model average, 1.165, as end members, then assuming that lead concentrations of coal and gasoline were approximately equal (per Lima et al., 2005), the calculated contribution to pre-1960 anthropogenic lead budget from coal to the environmental samples, when the uncertainty in coal average 206Pb/207Pb ratios is considered, ranges from ~ 40 to 80%. This result concurs with the conclusion by Lima et al. (2005), i.e., that coal-derived lead contributed significantly to anthropogenic lead prior to the 1960s, the concurrence of the results demonstrates that there is then no need to increase ALAS Model 206Pb/207Pb ratios during this time interval because pre-1960s anthropogenic 206Pb/207Pb ratios in Pettaquamscutt River Basin sediment can be modeled by increasing the contribution by coal to anthropogenic lead budgets with a concomitant reduction in the contribution by leaded gasoline. The issues of Pettaquamscutt River Basin sediment anthropogenic lead geochemical evolution and variable lead concentrations are discussed next.
- Anthropogenic Lead Geochemical Evolution
Lima et al. (2005) do not address the issue of the relative contributions from coal and leaded gasoline combustion to the anthropogenic lead budget of Pettaquamscutt River Basin sediments in the 1970s, a decade when TEL concentrations in gasoline reached their pinnacle, with some gasoline containing 3-4 grams of lead per gallon (> 1,000 ppm; Gibbs, 1990). The high concentrations of lead in gasoline, coupled with the fact that ~ 50 to 75% of the lead present in gasoline escapes through automotive exhaust emissions (Kaplan, 2003), explains: why gasoline lead became a dominant source of aerosol lead by the late 1960s; and the significant correlation between aerosol and gasoline 206Pb/207Pb ratios during this period (Hurst, 2003).
The authors have assumed that lead concentrations of leaded gasoline and coal end members that contribute to Pettaquamscutt River Basin sediment anthropogenic lead are equal, which results in a linear relationship between end member isotopic ratios but omits the concentration weighting factor. Furthermore, although contributions by post-1970s sources of anthropogenic lead in Pettaquamscutt River Basin sediments are not modeled [quantitatively] by the authors, they are mentioned with regard to their conclusion that ALAS Model 206Pb/207Pb ratios must be higher to reconcile differences in the relative contributions from gasoline and coal combustion to anthropogenic lead budgets.
Assessing relative contributions from leaded gasoline and coal to anthropogenic lead budgets is not trivial. Modeling is complicated by factors such as: (1) temporal variations in gasoline and coal lead concentrations; (2) the degradation of gasoline-derived aerosol lead, an organic phase, in the environment; and (3) potential long term release of trace elements, including lead, from the silicate-glass matrix of coal fly ash (Straughan et al., 1979). I will approach the issue of apportioning the contribution from leaded gasoline to these sediments by assessing consistencies between temporal variations in the anthropogenic lead geochemistry of Pettaquamscutt River Basin sediment and those of the ALAS Model.
In Figure 1, 206Pb/207Pb ratios of (1) the anthropogenic component of Pettaquamscutt River Basin sediments (Lima et al., 2005) and (2) the ALAS Model, i.e., leaded gasoline, have been plotted against reciprocal lead concentration. For reference, I have assumed, per Kaplan (2003), that ~ 50% of gasoline lead is released through automotive exhaust emissions. The average 206Pb/207Pb ratio of 10 eastern coals, 1.203 + 0.0012, and average lead concentration of coal fly ash, ~ 70 ppm (Chow and Earl, 1972; Hurst, unpublished) is shown as the coal end member.
3.2.1 Temporal Variations 1930s – 1960s
Pettaquamscutt Basin River sediment deposited from the 1930s through 1950s exhibit similar lead geochemical characteristics: 206Pb/207Pb ~ 1.190; Pb ~ 65 – 125 ppm (PRBS fields in Figure 1). The data points plot in close proximity to the coal datum point rather than the field for contemporaneous gasoline indicating a higher contribution from coal derived lead during this period as proposed by Lima et al. (2005) who calculate the contribution from coal to the anthropogenic lead component in these sediments to average 62 + 9%. This result corroborates conclusions based upon lead isotope analyses of anthropogenic lead in contemporaneous sediments/aerosols throughout the United States (Ng and Patterson, 1982; Graney et al., 1995; Hurst, 2000, 2003).
During the 1960s, the anthropogenic 206Pb/207Pb ratios of Pettaquamscutt River Basin sediments remain similar to these of the 1930s to 1950s, ~ 1.190, but leachable, anthropogenic lead concentrations continue to increase steadily from 111 ppm in 1962 to 150 ppm in 1969. Assuming again, per Lima et al., that gasoline and coal lead concentrations are equal, the calculated gasoline derived lead component in Pettaquamscutt River Basin sediment increases from 24% in 1962 to 41% by 1969.
In summary, the anthropogenic lead geochemistry of Pettaquamscutt River Basin sediment deposited from the 1930s through 1960s: (1) reflects the contemporaneous increase in gasoline lead concentrations during this period (Gibbs, 1990); (2) exhibits restricted 206Pb/207Pb ratios that reflect the restricted ranges observed in both coal and leaded gasoline sources; and (3) can be used to model contributions from leaded gasoline to the anthropogenic lead budget of the sediment without modifying 206Pb/207Pb ratios of the ALAS Model as proposed by the authors. Again, contrary to the conclusion by Lima et al., unmodified ALAS Model 206Pb/207Pb ratios serve as viable end members for estimating contributions by leaded gasoline combustion to the anthropogenic lead budget of Pettaquamscutt River Basin sediments prior to the 1960s.
3.2.2 General Temporal Variations: 1968 – Present
From the early 1970s to 1980, ALAS Model 206Pb/207Pb ratios increase dramatically from
~ 1.175 to ~ 1.211; gasoline lead concentrations remain high through the mid 1970s but begin to decrease by the late 1970s (Figure 1). Pettaquamscutt Basin River sediment anthropogenic lead record this temporal increase, exhibiting a dramatic shift in 206Pb/207Pb from ~ 1.190 in the 1960s, to ~ 1.200 in the 1970s; anthropogenic lead concentrations also increase in these sediments during this time period. Again, these changes recorded in the anthropogenic lead component of Pettaquamscutt River Basin sediment are totally consistent with those observed in the lead geochemistry of leaded gasoline as represented by the ALAS Model and lead concentrations in gasoline during this time period.
By the late 1970s and into the 1980s, lead concentrations in gasoline decline, but gasoline 206Pb/207Pb ratios increase (> 1.210). As observed in Figure 1, the “1980s Gasoline Trend” now indicates that gasoline-derived lead does not contribute significantly to anthropogenic lead in Pettaquamscutt River Basin sediments. Although 1980s leaded gasoline has lead concentrations similar to those observed in Pettaquamscutt River Basin sediments, the 206Pb/207Pb ratios of the contemporaneous ALAS Model are too radiogenic. The 1980s Pettaquamscutt River Basin [PRBS] Trend can be extrapolated back to that of the coal end member as proportions of coal, + industrial lead sources, increase relative to that of gasoline with much lower lead concentrations. The ALAS Model, therefore, remains consistent, indicating a negligible contribution from leaded gasoline to Pettaquamscutt River Basin sediment during the 1980s when lead concentrations in gasoline were drastically reduced (Gibbs, 1990).
3.2.3 Modeling Leaded Gasoline Contributions: 1969 to 1975
Lima et al. do not quantify contributions from leaded gasoline and coal to anthropogenic lead in Pettaquamscutt River Basin sediment after 1969, but have implied the ALAS Model can not be used reliably to estimate the relative contribution from leaded gasoline to environmental samples. Using the mixing equations of Lima et al (2005)., and by assuming, as the authors have, that leaded gasoline/coal lead concentrations are equivalent, ~ 100 ppm, calculated contributions from leaded gasoline to anthropogenic lead in Pettaquamscutt River Basin sediment for 1969, 1971, and 1975 are, respectively, 41%, 24%, and 33%. As Lima et al. indicate, this result runs counter to our thinking, that leaded gasoline combustion was a major source of anthropogenic lead during this time period.
If, however, the gasoline lead concentration is reduced to 175 ppm (~ 0.5 gm/gal), and a 100 ppm average for coal lead is used, the 1969, 1971, and 1975 gasoline contributions in the sediment increase, respectively, to 70%, 55%, and 84%. Gasoline contributions increase another 5 – 10% for each of these years if the average coal lead concentration drops to 70 ppm as assumed in Figure 1. These results are consistent with modeling based solely upon measured lead concentrations in downcore, sedimentary profiles and lead concentrations concur with observed soil lead concentrations in suburban to urban environments (Filippelli et al., 2005; Hurst, unpublished). This exercise demonstrates the importance of the concentration weighting factor on modeling, and also indicates, in light of the earlier discussions, that the ALAS Model can be used as a reliable tool to constrain the proportion of lead derived from gasoline combustion in environmental samples.
- CONCLUSIONS
One important conclusion advanced by Lima et al. as a result of their investigation into the causes of temporal variations observed in the lead geochemistry of Pettaquamscutt River Basin sediments is that, prior to the 1960s, a significant fraction of anthropogenic lead in some areas was related to coal, rather than gasoline, combustion. This observation corroborates work by many other research groups and needs to be re-emphasized given the ad hoc assumption that is often made about the dominance of lead pollution created by gasoline combustion throughout a major portion of the 20th Century.
My disagreements with the authors’ conclusions are two-fold. First, the need to increase 206Pb/207Pb ratios of the ALAS Model prior to 1970 in order to reconcile differences in lead input is not necessary when contributions from coal combustion to the anthropogenic lead budget of Pettaquamscutt River Basin sediments is considered. Second, the authors failed to evaluate the use of the ALAS Model after ~ 1970, when concentrations of lead in gasoline increase, then decrease dramatically. Rather, they presented a broad statement about the inability of the ALAS Model to be used effectively as a means of identifying contributions from leaded gasoline combustion to anthropogenic lead budgets of environmental samples.
A re-evaluation of the authors’ lead geochemical data from Pettaquamscutt River Basin sediment corroborates the authors’ conclusion that prior to the 1960s, coal derived lead is a significant source of anthropogenic lead in these sediments, averaging 62 + 9% (Lima et al., 2005) and ranging from 40 to 80% (this work). However, as shown herein, this can be accomplished without modifying 206Pb/207Pb ratios of the ALAS Model as suggested by the authors. Furthermore modeling contributions from coal versus gasoline derived lead in Pettaquamscutt River Basin sediment deposited between 1969 and 1975, the time period when gasoline lead concentrations maximized, again requires no modification of ALAS Model 206Pb/207Pb ratios, but by using reasonable estimates of gasoline lead concentrations it is possible to show that contributions from gasoline derived lead during this time period exceed those of coal lead, ranging from a minimum of ~ 55 to 65% to a maximum of 85 to 95%.
Finally, by the late 1970s through the 1980s, coal and/or other industrially derived lead again become the major sources of anthropogenic lead in Pettaquamscutt River Basin sediments due to the dramatic reduction in gasoline lead concentrations. In all cases, ALAS Model data can be employed to identify, constrain, and model contributions from leaded gasoline combustion in these sediments, and therefore, contrary to the conclusion of Lima et al. (2005), the model can be used to constrain, within reasonable limits, contributions from leaded gasoline to anthropogenic lead in environmental samples. Portions of the ALAS Model calibration curve, such as the rapid temporal increase in [gasoline] 206Pb/207Pb ratios between ~ 1970 and 1980, can be used as a chronostratigraphic marker of this time period when observed in the anthropogenic lead isotope geochemistry of sedimentary sequences in the United States.
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