posure to high levels of this contaminant could be a significant consideration in cultural practices. Certainly this aspect of atmospheric lead contamination needs to be examined more closely. A n ancillary question is that of ground cover at that time of year. I n localities which have a snow cover on the ground during those months, lead particles that would normally reach the soil and remain there for a certain period of time would be carried away with melting snow t o streams, rivers, and quite possibly watersheds. The point t o be emphasized here is that it should be determined whether lead contamination is accentuated in different parts of the country because of climatic conditions. The second point I wish t o speak on in this discussion is the relationship between distance from the highway and lead contamination of the atmosphere and of vegetation. The relationship, of course, has been known for some time from the standpoint of accumulation by vegetation, and there is both circumstantial and real evidence for the phenomenon. One of the first papers to attract attention was published by Primault in 1958 in Switzerland. H e reported injury in the form of marginal leaf scorch and premature browning of the foliage on horse-chestnut, plane, and linden trees in Zurich, Switzerland. The injury was attributed t o lead bromide emanating from auto exhaust fumes. The greatest amounts of injury occurred along heavily traveled roads. Cannon and Bowles in this country, and Warren in Canada, have reported o n the incidence of lead in vegetation. I n Canada, trees growing within 100 yards of the main highways were found to contain upwards of 1000 p.p.m. of lead instead of the normal 10 p.p.m. In this country, as much as 3000 p.p.m. of lead were found in grass near highway intersections. These investigators also concluded that distribution of lead was due to traffic volume and prevailing winds. Most of the sampling that has been done in this and other studies, however, has been done at relatively large distance intervals, usually several yards
apart. It would seem that extensive sampling needs t o be done within 50 feet of highways. With another pollutant of a different nature but coming essentially from the same source, namely the highway and the salt that is used for snow and ice control, the main effects have been shown to be limited to a distance of 30 feet from the highway. Although most of the salt remains dissolved in the slush which is plowed back periodically during snow storms, much of this salt is whipped up in a mist by fast moving traffic and becomes an aerosol much like lead particles. Thus one may expect a similar behavior in the effects of these two pollutants. The third point I wish to speak on is the methodology and interpretation of vegetation analysis. The authors are careful in pointing to the fact that lead does not accumulate in the edible portion of the test plants with the notable exception of lettuce. Their findings may indeed be reassuring to the housewife. However, in commercial operations such as in the canning industry the leafy portions of certain crops are often sold to farmers for cattle feed. Such plant material, as well as forage and silage, is not washed before consumption. Therefore, although the authors have demonstrated that some lead is removed from the surface of plant material by washing, the lead content of unwashed samples may be a more realistic appraisal of lead contamination in certain cases.
References
(1) Cannon, H. L., Bowles, J. M., “Contamination of vegetation by tetraethyl lead,” Science 137 (3532): 765-66 (1962). (2) Lacasse, N. L., “Maple decline in New Hampshire,” M.S. Thesis, University of New Hampshire, Durham, 1963. (3) Primault, B., “Enmarge des futures auto-routes,” Schweiz. Zeitschr. For~tw.109(1): 37-43 (1958). (4) Warren, H. V., “Geology and multiple sclerosis,” Narure 184: 561 (1959). (5) “Some aspects of the relationship between health and geology,” Can. J. Public Health 52: 157-64 (1961).
D iscussion Lead in Soils and Plants: Its Relationship to Traffic Volume and Proximity to Highways E. A. Schuck Research Chemist, Department of Engineering, University of California, Los Angeles, Calif. 92502 ithout question the results presented in this paper indicate a thorough study of lead associated with various parts of consumer crops. There can be no doubt that crops grown downwind and within a few hundred feet of heavily traveled highways do exhibit contamination by the lead particulates present in automobile exhaust. Neither can there be any doubt that lead’s particulate contaminationof the portion of these crops which is harvested for human consumption is relatively small even quite close to these highways. Quite similar findings were exhibited by the Schuck-Locke findings previously discussed. However, what has not been demonstrated by these results is whether or not this particulate lead contamination leads primarily to simple surface contamination or actual absorption by the plant. This of course is a question which has little practical significance in terms of how much of this lead the consumer ingests. Yet in a strict sense it is important to differentiate between the two processes. Absorption could lead to a concentrating mechanism which would pose more of a potential danger to the consumer. The effect of simple surface contamination would be limited in that 238 Environmental Science & Technology
the amount of lead which can cling to a surface is limited. A surface contamination process would exhibit the highest contamination levels o n those plant parts which have the highest surface to volume ratio. It is of interest to note that the data of Motto, et al., do exhibit such a relationship. One miJst freely admit however that such evidence does not constitute proof of a surface contamination process. Neither, on the other hand, does it prove the existence of an absorption process. What is required of course are refinements in experimental techniques. Two examples presented at this symposium stress this need. In a study of five crops, Schuck and Locke concluded that the evidence suggested simple surface contamination. I n the Motto, et a/., paper there appears to be a slight preference for the concept of absorption. This is apparent in the data tables which report lead found in plants. In reality the results from these two studies are quite similar and leave no doubt that an interaction does occur between airborne lead particulates and certain consumer crops. From neither study can we gather other than indications as to the overall relative importance of surface contamination or absorption.
