nowitz and Wetherill as to the sources of lead poisoning are clearly undermined by the historic and biological facts that they failed to consider.
form the basis of the authors’ conclusions are too small to justify their conclusions. Other scientists using isotope ratios for identifying sources of lead have also drawn erroneous conclusions. For example, Chow (1970) concluded from his work that lead in gasoline iB of the Tertiary or older model age, and t h a t little of it is made from Missouri ores. Yet manufacturers’ records show that a substantial portion of lead ore used by a t least one U.S. antiknock manufacturer derives from Missouri, and the same is probably true of the other manufacturers! In the Benicia case, the contamination of the soils with smelter emissions for nearly a century has no doubt caused wide variations in isotopic composition of lead in those soils, depending on t h e sources of ore refined, the wind directions, and the extent to which soils in those areas have been turned over. A few measurements of the isotopic composition of soil lead or grass lead cannot accurately reflect the exposure of horses who have been grazing there for extended periods of time. In general, the assumption t h a t there are fixed, distinct isotopic ratios for gasoline-derived lead or soil lead is fallacious. Techniques using isotope ratios certainly have application, but only in limited cases. The Benicia case is not one of these situations, and the conclusions of Rabi-
Literature Cited Bureau of Mines, Rept. of Selby Smelter Commission, Bull. 98, Washington, D.C., 1915. Chow, T. J., “Isotopic Identification of Industrial Pollutant Lead,” Presented at Second Intern. Clean Air Congress of IUAPPA, December 6-11, 1970. Hammond, P. B., Aronson, A. L., Annals New York Acad. Sci. 111, pp 595-611, 1964. Lawther. P. J., Commins, B. T., McK. Ellison, J., Biles, B., “Airborne Lead and Its Uptake by Inhalation,” presented at Zoological Society of London, January 27, 1972. National Research Council, National Academy of Sciences, “Airborne Lead in Perspective,” Washington, D.C., pp 67, 180, 1972. Rabinowitz, M. B., Wetherill, G. W., Enuiron. Sci. Technol., 6, 705-9 (1972). Schmitt, Ii., Brown, G., Devlin, E. L., Larsen, A. A , , Douglas, E., Saville, J. W., Arch. Enuiron. Health, 23, 185-95 (1971). E. B. Rifkin’ Gary Ter Haar Research & Development Department
Sir: We are well aware of the historical and widespread problem of lead poisoning of livestock from smelter effluent, as discussed by Rifkin and Ter Haar. There is no doubt that lead smelters are capable of introducing to t h e surrounding environment lethal levels of lead contamination, without significant contributions from other sources. The purpose of our short discussion was to evaluate the usefulness of lead isotopes in distinguishing between different sources of lead contamination. We investigated whether in this particular case and for these particular horses, this smelter was the sole source of lead t o t h e poisoned horses or whether it was one of a number of isotopically distinguishable sources. Our d a t a demonstrate that in this locale the smelter was one of two or more contributing sources. As discussed in our paper, we recognized t h a t neither the smelter emissions, nor local T E L have fixed isotopic compositions and certainly did not make the assumption, as stated by Rifkin and Ter Haar, t h a t “there are fixed, distinct isotopic ratios” for these sources. I t was necessary to establish these isotopic compositions for the time period under consideration. As explained previously, this was done by analysis of the lead isotopic composition and concentration in samples of annual grass selected to represent essentially pure smelter lead or automotive ( T E L ) lead. Since the samples were selected in this way, their lead concentrations cannot be used to estimate the average amount of lead consumed by the horses from these sources. The three horses studied had not grazed on these pastures for “extended periods of time,” but only seven to nine months, almost entirely just prior to our sampling. Therefore the isotopic composition measured may be expected to represent t h a t contributed by the smelter during that interval, regardless of whether the smelter pro-
duced lead of different isotopic composition a t some time in the past. As found by Dedolph et al. (1970), and confirmed by Rabinowitz (1972) for this particular species, the contribution of soil lead to t h e edible portion of the grass was negligible relative to t h e airborne component for the grass and soil lead concentrations under consideration. In considering the Benicia pasture grasses in particular, Mueller and Stanley (1970) and Rains (1971) also concluded that very little of t h e lead in the tops of the grasses is from the soil. Our isotopic analyses of grass were determined on the entire plant eaten by the horses, including the stems. The concentrations of lead in the soil and the grass at the pastures are similar. Therefore, although horses forage in such a way so as to ingest some soil, they consume more grass than soil, and the possible contribution of older lead from the soil is not very important. The biological questions raised by Rifkin and Ter Haar go beyond the initial scope of our investigation, but we believe our published d a t a and subsequent analyses may help to answer these questions. The quantity of lead in these horses is not as great as Rifkin and Ter Haar’s estimates would suggest. Isotope dilution measurements of total lead content of bones, livers, and kidneys indicate 6-16 Wgjg on a fresh weight basis. Isotopic abundance ratios identify about half of this lead as coming from the smelter and eaten by the horses with their pasture grass. Measurements of a control group of horses which died of other causes in suburban Los Angeles, and which probably were exposed to similar quantities of automobile exhaust as the Benicia horses, showed concentrations of 1-5 fig/g. This is about one quarter as much as the poisoned horses and it is plausible t h a t this represents about as much lead as was in the horses’ bodies before they were subjected to the additional exposure a t these pastures. So, in effect, the biological question raised by Rifkin
556
Environmental Science 8, Technology
Ethyl Corp. Ferndale, Mich. 48220
To whom correspondence should be addressed.
Table I. Lead Concentrations Near Benicia, Calif. Miles
Description Liver Bone TEL residue
Atomic
from smelter
Lead conc., PPm
5.3 5.3 6.2
6.5 16.4 557,000
206/204 17.86 17.75 18.02
ratios 206/207 1.183 1.184 1.185
and Ter Haar is t h a t of the source of the remaining one quarter of the lead found in the bodies of these horses. As discussed in a recent report by the California Air Resources Board (1972), the most recent study of this problem in the area was initiated by a large number of horse deaths on the Wesner ranch, 8.5 km downwind from the smelter. This report states that residues from the disposal of tetraethyllead were dumped a t two sites, one 1.9 km and the other 2.8 km from the Wesner pasture. It is also stated t h a t representatives of the Ethyl Corp. measured organic lead vapor concentrations of more than 2000 wg/m3 a t the dumping sites and 248 and 428 pg/m3 a t the Wesner pasture a t the time the horse deaths occurred. One of the three horses studied in our previous report was from this pasture. We have subsequently measured (Table I) additional samples of a horse, diagnosed as having died of lead poisoning a t the Wesner ranch, and T E L residues from one of these disposal sites. The horse results are similar to those reported previously; the T E L data are similar to the automotive lead data. Therefore, a t least for the Wesner horse deaths, our previous suggestion appears to be substantiated t h a t the source of the additional component of nonsmelter lead found in the horse tissues was either finer particles of automotive exhaust or T E L vapor. We have been unable to obtain sufficient data on the previous lead exposure history of the two non-Wesner horses to identify the source of their nonsmelter component. It may be surprising t h a t these relatively low tissue concentrations are associated with such high values of exposure to smelter and tetraethyllead. This, however, gets into problems of physiology and veterinary medicine entirely beyond our competence, and probably beyond that of present knowledge. In collaboration with Joel Kopple, we are currently carrying out controlled studies of human lead metabolism. Interpretation of these horse tissue concentrations would require similar studies of horses. It is
certainly not our intention to take an alarmist position, as much harm can result from overreaction to hypothetical environmental hazards. Nevertheless, the relatively small factor (-4) found between concentrations of lead in suburban Los Angeles horses and those in the Benicia area, together with the typical high concentrations of lead in grass (200 pg/g dry wt.) found 300 meters from any roads in the Los Angeles area, suggests t h a t further consideration of possible incipient toxic effects, other than those associated with smelters, should be considered. For example, horse owners could be advised to supplement the diet of their animals with forage from nonurban areas. We find the statements by Rif’kin and Ter Haar that there is no reason why gasoline lead in Missouri should be different from t h a t in California difficult to reconcile with their (correct) statement that gasoline lead has no fixed and distinct isotopic composition. The fact is t h a t the isotopic composition of gasoline lead does vary both in time and place, certainly as a consequence of T E L manufacturers’ making use of different sources of lead. We have no access to the manufacturers’ records which would be needed to predict what should be found a t a given time and place. We can only measure what is there. It would be most interesting if relatively pure Missouri lead with its characteristically high Pb206/Pb204 ratio were to be used as the principal source of gasoline lead in some area, as this would provide a more easily identifiable source than those found heretofore. Literature Cited California Air Resources Board, “A Joint Study of Lead Contamination Relative to Horse Deaths in Southern Solano County,” September 1972. Dedoloh. R.. Ter Haar. G.. Holtzman. R.. Lucas. H.. Enuiron. Sci.’Technol. 4,217, 1970. Mueller, P., Stanlev. R.. State of California Deuartment of Public Health, Air and -Industrial Hygiene Laboratory, Rept. No. 87, 1970. Rabinowitz, M . , Chemosphere, 4, 175, 1972. Rains, D. W., Nature, 233, 210, 1971. M. 6. Rabinowitz G. W. Wetherill’ Department of Planetary and Space Science and Institute of Geophysics and Planetary Physics University of California Los Angeles, Calif. 90024 1 To
whom correspondence should be addressed.
Volume 7, Number 6, June 1973
557
