’Koch, H.-J., Bieling, H., Thormann, K., East Ger. Pat, 68896, International Patent Classification CO’lg (Sept. 20, 1969). Law, S. L., Science, 174,285 (1971). Reeves, W. A., Guthrie, J. D., Textile Res. J., 23,522 (1953). Roberts, E. J., Rowland, S. P., ibid., 41,864 (1971). Roberts, E. J., Wade, C. P., Rowland, S. P., Carbohydrate Res., 17,339 (1971).
Rowland, S. P., Roberts, E. J., Wade, C. P., Textile Res. J., 39, 530 (1969).
Scholten, H. G., Prielipp, G. E. (to Dow Chem. Co.), U. S. Pat. 3,085,859 (April 16,1963). Wade, C. P., Roberts, E. J., Rowland, S. P., Textile Res. J., 42, 158 (1972).
Received for review October 2, 1972. Accepted February 1, 1973. T h e Southern Regional Research Laboratories are one of the facilities of the Southern Region, Agricultural Research Service, U. S. Department of Agriculture.
CORRESPONDENCE
Identifying Sources of Lead Contamination by Stable Isotope Techniques Sir: In a recent paper, Rabinowitz and Wetherill (1972) claim t h a t horse fatalities near Benicia, Calif., were due in part to lead residues from smelter operations and in part to atmospheric lead contributed from automotive exhausts. This conclusion is untenable, based on historic and biological considerations. Environmental contamination in the Benicia area has been the subject of controversy since the last century. By 1913, the matter had reached such a point t h a t the U.S. Bureau of Mines was asked to investigate complaints by local residents about air quality and to find the reason for fatalities among horses grazing in the area. The Bureau of Mines (1915) attributed the problem to the smelter operations a few miles to the west a t Selby. Since 1883, this plant had emitted large concentrations of lead and other toxic substances, resulting in contamination of the air and soil and vegetation in t h a t area. Obviously leaded gasoline played no part in causing the problem. In recent years, the same problem has recurred, with additional horse fatalities reported. I t seems obvious that the soils in this area have been heavily contaminated with lead and other heavy metals from decades of continuing exposure to smelter emissions. I t is known t h a t soils very high in heavy metals can yield vegetation with relatively high metal content. An animal whose main source of food is such vegetation will obviously ingest large quantities of these metals, and fatalities may result. Similar problems have occurred elsewhere in the vicinit y of smelters. Examples are Trail, B.C. (Schmitt et al., 1971) and St. Paul, Minn. (Hammond and Aronson, 1964). Here, too, horse fatalities have been reported. Careful investigation has shown t h a t vegetation containing lead emitted by the smelters caused the problem. Claims that leaded gasoline is fully or partly responsible for these occurrences are refuted by the fact t h a t horse fatalities of this type never occur except in the vicinity of smelters or other industrial sources of high lead emissions. If automotive exhausts were contributors to the problem, one would expect to observe such problems near busy highways, far from smelters. Yet no such problems exist. Further, if automotive exhaust contributed dangerous amounts of lead to the atmosphere, the health of other animals and humans would be threatened. Yet no such observations have ever been made, despite recent massive efforts to identify hazards due to exposure of humans to automotive exhaust lead. T h a t the authors have reached an unjustified conclusion is also clear from biological considerations. Simple calculations show t h a t lead burned in gasoline cannot be a con-
tributor to the Benicia problem. The authors report that concentrations of lead in grass in the pastures near the smelter range from about 700-6000 ppm (dry basis). Assuming 3000 ppm as an average, and recognizing that a horse eats daily about 21 kg of dry forage/kg body weight (NRC, 1972), we see t h a t a horse grazing in this pasture would ingest about 60,000 pg of lead/kg of body weight. This amount is 35 times the toxic dose (NRC, 1972) and would easily explain the horse deaths. The amount that could reasonably be absorbed from the air because of lead emitted from automobiles is completely insignificant in comparison. Although the authors surprisingly omit lead-in-air data for the pasture areas, we can use a n estimate of 2 pg/m3 of lead in air due to cars, which would almost surely be on the high side. A horse’s breathing rate is about 0.3 m3/kg of body weight per day. If 3% of the lead inspired is absorbed (NRC, 1972)-a more accurate estimate of this figure we believe would be 15% (Lawther et al. 1972)-only 0.2 pg lead/kg of body weight would appear in the horse’s blood due to breathing airborne lead from cars. On the other hand, the amount of lead absorbed into the blood daily owing to eating the contaminated forage mentioned above would be 1200 pg/kg of body weight. (This assumes t h a t 2% of the 60,000 pg is absorbed through the gut-10% is estimated for humans.) Thus the relative contribution of smelter lead vs. automotive lead to the horse’s blood would be 1200/0.2, or 6000 to 1, hardly the equivalent of the 1 to 1 ratio deduced by the authors. In order to approach the 1 to 1 ratio, the lead in air value would have to reach 12,000 pg/m3. Clearly this is beyond the realm of possibility. The author’s conclusion that the lead in the horses comes half from the smelter and half from gasoline apparently derives from the following data:
Horse lead Smelter lead Automotive lead
206/204 ratio 17.7 17.3 17.9
But the authors also show that gasoline lead (measured in Missouri) has a ratio of 18.5. There is no reason to suppose that gasoline sold in Missouri has a 206/204 ratio substantially different from gasoline sold in California, since lead antiknock plants in the U.S. are located with little relation to the source of ore or the area where the product is sold. Given the variability of gasoline lead between 17.9 and 18.5, it appears t h a t the variations which Volume 7, Number 6, June 1973
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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-
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 pro556
Environmental Science 8, Technology
nowitz and Wetherill as to the sources of lead poisoning are clearly undermined by the historic and biological facts that they failed to consider. 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 Ethyl Corp. Ferndale, Mich. 48220
To whom correspondence should be addressed.
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
