Seasonal changes in the lead content of pasture grass growing near a motorway

Agriculture and Environment, 5 (1980) 213--225

213

© Elsevier Scientific Publishing Company, Amsterdam .- Printed in The Netherlands

S E A S O N A L C H A N G E S IN T H E L E A D C O N T E N T O F P A S T U R E G R A S S GROlbING NEAR A MOTORWAY

D.R. CRUMP, P.J. BARLOW and D.J. VAN REST* Department of Construction and Environmental Health and *Interdisciplinary Higher Degrees Scheme, The University of Aston in Birmingham, Gosta Green, Birmingham B4 7ET (Great Britain) (Accepted 23 July 1979)

ABSTRACT Crump, D.R., Barlow, P.J. and Van Rest, D.J., 1980. Seasonal changes in the lead content of pasture grass growing near a motorway. Agric. Environm., 5: 213--225. The above-ground portion of perennial ryegrass (Lolium perenne), in fields both bordering and distant from the M6 motorway in Cheshire (Great Britain), was sampled monthly during the period October 1977--October 1978. A 10-fold seasonal variation in lead level was found at all sites. The maximum level near the motorway (285 ug/g) was found during the 'winter' months, whilst the lowest level (15 ug/g) was found from May to August. It was concluded that the seasonal variation was a result of changes in the growth form of the plant. Other factors such as seasonal changes in atmospheric lead concentration and lead uptake from the soil may be of minor significance. It is believed that present levels of lead in pasture grass, both close to and distant from the motorway, present no toxic hazard to grazing animals, but it may be advisable to restrict the use of pasture bordering major roads during the period December--April. INTRODUCTION

T h e o c c u r r e n c e o f elevated levels o f lead in the air, soil and v e g e t a t i o n adjacent t o m a j o r r o a d s is r e p o r t e d b y a n u m b e r o f w o r k e r s , including the wellk n o w n papers b y C h o w {1970), Daines et al. {1970) and C a n n o n and Bowles (1962), R a d i o i s o t o p e studies, such as t h a t o f R a b i n o w i t z and Wetherill ( 1 9 7 2 ) , have identified this lead as resulting f r o m t h e a n t i - k n o c k agents used in p e t r o l which are e m i t t e d in the a u t o m o b i l e e x h a u s t . The decreasing lead level with distance f r o m the road is well d o c u m e n t e d , and in general, p o l l u t i o n o f soil and v e g e t a t i o n , is restricted t o s o m e 50 m either side o f t h e h i g h w a y . T h e degree o f c o n t a m i n a t i o n has b e e n s h o w n to be d e t e r m i n e d b y a n u m b e r o f factors such as traffic flow, prevailing w i n d and t o p o g r a p h y ( H u n t e r , 1 9 7 6 ) . Levels o f lead in r o o t c r o p s and t h e grain kernels o f cereals are low, even w h e n g r o w n a d j a c e n t t o b u s y roads (Leh, 1 9 6 6 ; Ter Haar, 1 9 7 0 ) . In c o n t r a s t , fruit and leafy vegetables m a y have elevated lead levels as a result o f c o n t a m i n a tion b y a u t o m o b i l e emissions. T h e n o r m a l p r a c t i c e o f washing f o o d p r i o r to

214

human consumption results in a significant reduction in lead levels. However, pasture grass and fodder crops are not subjected to a washing procedure before ingestion by farm livestock and thus significant amounts of lead may be ingested by the grazing animal. There are no reports of clinical lead poisoning of livestock as a result of grazing roadside pastures, but Ward et al. (1978a) have demonstrated an increase in blood lead levels of sheep as a result of grazing near a busy highway in New Zealand. It is also reported that a considerable a m o u n t of lead may accumulate in sheep b o d y tissues as a result of ingesting fodder contaminated by automobile exhausts, and possibly reach man through the food chain (Ward et al., 1978b). Few studies have considered the possible seasonal variation in the degree of roadside lead pollution. However, there are a number of reports describing large seasonal changes in the lead concentration of grass at uncontaminated sites (Mitchell and Reith, 1966), on mine spoil wastes (Johnson et al., 1978), and grassland in the vicinity of lead smelters (Rains, 1971, 1975). McLean and Shields (1977) have also reported a change in the lead concentration of barley (Hordeum vulgare), oat (Arena sativa) and p o t a t o (Solanum tuberosum ) during the growing season at a site adjacent to a m o t o r w a y in Scotland. They suggest this variation is a result of changes in plant morphology during the season, which affects the a m o u n t of lead deposited on the plant surface. We believe that it is important to identify fully any seasonal change in lead levels of pasture before the significance of roadside lead pollution to livestock can be properly assessed. The variation of lead levels with season may be particularly important for cattle which may spend only part of the year grazing at pasture. The results reported in this paper are the first of a study aimed at such an investigation. EXPERIMENTAL WORK

The study area The present investigation involves a detailed study of lead levels in grass and soil at one site adjacent to the M6 m o t o r w a y , near Sandbach in Cheshire (O.S. map sheet No. 118 ref. 762636). This section of m o t o r w a y is level, runs in a NNW--SSE direction at field height and has an undulating surrounding topography. Average weekday traffic flow is some 40 000 vehicles per 24 h of which approximately 66% are petrol driven (Department of Transport, 1977). The area is described as being relatively free of industrial pollution (Cheshire Planning Department, 1977). Adjacent to each of the three lane carriageways of the m o t o r w a y is a 10 m wide grass verge, separated from the permanent pasture of perennial ryegrass by a standard 4-bar w o o d e n m o t o r w a y fence. This section of m o t o r w a y has been open to traffic since 1962, and forms the border between two farms at the study site. Farm A, which lies to the west of the m o t o r w a y is grazed by

215 cattle and used for silage and hay production, typically between the months of March and November. The land eastward of the m o t o r w a y (Farm B) is additionally grazed by horses and sheep. Three other areas of pasture land, which are some distance from the motorway, but owned by the same farmers, were studied as control sites. These are situated 1500 m west of the m o t o r w a y and 450 and 2000 m east of the motorway. There are no main roads within 2000 m of these background sites and all three were at least 100 m from farm access roads. All study sites were grazed by farm livestock at some time during the sampling period. There is no known mineral source of lead in the area, the soils having formed upon Keuper Marl (Evans et al., 1968). Investigations by the authors have shown the clay c o n t e n t (< 180 p) of the top 5 cm to be 70--85% of the total inorganic weight of the soil.

Sample collection The above-ground portion of perennial ryegrass (Lolium perenne) was sampled m o n t h l y during the period October 1977--October 1978 at each study site. In fields bordering the motorway, grass within I m square quadrats was collected at intervals along a transect perpendicular to and extending 100 m either side of the M6. At the three background sites grass was taken from within 1 m square quadrats placed at random within a 10 × 10 m area. Samples of the top 5 cm of soil were taken from the centre of each quadrat. All grass and soil samples were placed in polyethylene bags and stored at --18°C until time of analysis.

The analysis of lead in grass and soil samples Grass samples were dried in an oven at 80°C for 24 h and cut into small pieces using stainless steel scissors. Approximately 1 g of the dried grass was accurately weighed into a 100 ml conical flask. The grass was then digested with 10 ml of concentrated nitric acid (Analar) by simmering for 1 h on a hot-plate. The sample was then filtered using Whatman No. 1 filter paper and made up to 25 ml with deionised water. Soil samples for the m o n t h of October 1977 were prepared by air drying for 14 days at 30°C and lightly ground using a pestle and mortar. Approximately 1 g of the fraction passing a 2 mm sieve was accurately weighed into a kjeldahl flask and boiled with 10 ml of concentrated nitric acid for 1 h. The sample was then filtered and the resulting filtrate made up to 25 ml with deionised water. The lead c o n t e n t of the samples was determined by atomic absorption spect r o p h o t o m e t r y using the 283.3 nm spectral line. A Perkin-Elmer (model 360) instrument was used either in the flame or flameless mode. The flameless mode consisted of a Perkin-Elmer HGA-74 Graphite furnace. All results were recorded on a Perkin-Elmer 056 chart recorder.

216 Recovery tests were carried o u t using Standard Reference orchard leaves (U.S. National Bureau of Standards, Material 1571), the m e t h o d of analysis and determination being the same as for the grass samples. A recovery of 92% was obtained. Various characteristics of the soil samples taken during the study period (October 1977--October 1978) were determined such as percentage water, organic carbon c o n t e n t and soil pH.

Environmental data It has been shown that a number of environmental factors, such as wind characteristics and rainfall, can influence the lead concentration of pasture adjacent to major roads (Page et al., 1971; Davies and Holmes, 1972). To investigate these factors, data for the percentage frequency of wind direction and speed and the a m o u n t of rainfall was obtained for the study region. Unfortunately the nearest wind measurements are for the Keele Weather Station (O.S.maosheet 1 18 ref. 819446) which is some 20 km SSE of the study site. The rainfall data obtained was for three sites approximately 9 km from the study area (O.S. map sheet 118 refs. 853635, 843577, 776542). In addition, some measurements of rainfall pH in the study region were obtained. RESULTS

Grass lead levels The lead concentration of pasture grass bordering either side of the motorway was elevated in comparison to the control sites in all m o n t h s of the year as shown in Table I. For example, to the east of the road mean lead levels within 10 m of the m o t o r w a y fence are 197 and 25.3 pg/g in March and July, respectively. At a distance o f 2000 m the corresponding values are 26.7 and 5.0 ~g/g. Fig. 1 compares the pattern of lead contamination in July 1977 and March 1978, these m o n t h s showing the typical pattern of lead distribution. Both m o n t h s show a decrease in lead levels with distance from the motorway fence, but levels in March are notably greater than during July. The pas;ure in March contains approximately 10 times the lead concentration of July grass at all distances within the 100 m transect each side of the motorway. Highest levels of c o n t a m i n a t i o n are within 20 m of the road. The eastern side of the m o t o r w a y shows particularly high levels and is probably a reflection of the prevailing westerly winds. At sites distant from the m o t o r w a y a similar difference in lead levels between the 2 m o n t h s is apparent. Table II shows the calculated mean lead concentration and standard deviation for grass at five random points at the background sites for July and March 1978. The March values are some five to eight times greater than those for July at each site, even though the sites are a considerable distance from the motorway.

217 TABLE I M e a n lead c o n c e n t r a t i o n a n d s t a n d a r d d e v i a t i o n of p a s t u r e grass w i t h i n 10 m of t h e m o t o r w a y f e n c e a n d at c o n t r o l sites (ug/g d r y wt.) East

West

Date of sampling .

.

.

.

.

2000m . . . . Mean S.D. .

.

24 Oct. 1977 21 Nov. 1977 20 Dec. ] 9 7 7 25 Jan. 1 9 7 8 3 March 1978 4 April 1 9 7 8 10 May 1978 31 M a y 1978 13 J u l y 1978 10 Aug. 1 9 7 8 6 Sept. 1978

.

.

.

.

.

.

26.7 15.3 2.3 2.3 5.0 5.5 9.7

.

.

450 m . . Mean

. .

.

.

.

.

.

.

.

7.8 15.4 25.8 27.7 18.5 2.4 2.6 6.3 8.8 11.4

5.3 1.8 0.9 0.1 1.0 2.0 **

. S.D. .

0--10 m 0--10 m 1500 m . . . . . . . Mean* Mean* Mean S.D.

.

.

20 63.5 100 216 197 148 16.8 26 25.3 38.7 55

** 3.2 6.3 4.4 ** 0.6 0.4 0.7 3.2 1.5

9 12.9 46.9 71 138.5 41.7 9.2 24.8 12.4 21.6 13.5

1.98 5.7 10.7 26.9 26 15.2 2.9 2.1 3.3 3.8 10.4

0.35 0.8 1.9 3.6 3.1 3.3 1.6 0.3 0.8 0.9 1.1

* M e a n o f s a m p l e s t a k e n at 0 m (2}, 5 m (2) a n d 10 m (2). * * O n l y t h r e e samples analysed. S.D. s t a n d a r d d e v i a t i o n o f five samples. 3C'0

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Fig.1. V a r i a t i o n in grass lead c o n c e n t r a t i o n w i t h d i s t a n c e f r o m t h e M6 m o t o r w a y in M a r c h a n d J u l y 1976.

218

TABLE II M e a n l e a d c o n c e n t r a t i o n a n d s t a n d a r d d e v i a t i o n at b a c k ~ o u n d s i t e s i n M a r c h a n d J u l y 1978 Month {1978)

1500 m

450 m

2000 m

March Mean S.D.

25.96 3.11

27.7 4.43

26.7 5.37

July Mean S.D.

3.3 0.78

6.3 0.75

5.0 1.04

Seasonal change in grass lead levels To emphasise this observed seasonal change in grass lead levels the mean value was calculated for the background site, situated 1500 m west, and for the area within 10 m of the m o t o r w a y fence. The resulting values are shown in Fig. 2. The graph shows the existence of a winter maximum and summer minimum in grass lead levels, both near and distant from the motorway. In autumn, lead levels increase almost linearly and,, after the winter peak, decrease rapidly during spring. The seasonal pattern has also been found at a second study site, adjacent to the M5 m o t o r w a y in Gloucestershire, and at three farms bordering the M6 m o t o r w a y in the English Midlands (Vick, 1976). Pasture lead levels east of the road are greater than those to the west.

Surface soil lead concentration Data for the surface soil lead content in October 1977 shows elevated levels on farmland bordering the m o t o r w a y . These values, however, are not outside the normal rahge (3--106 ~g/g) for British softs given by Alloway and Davies (1971). Maximum levels (50 pg/g dry weight) occurred eastward of the road, which is proably a consequence of the prevailing westerly winds influencing the dispersion of lead emitted by automobiles on the m o t o r w a y . The area of contamination was restricted to 20 m from the m o t o r w a y fence. Samples taken distant from the m o t o r w a y have shown the normal levels of total soil lead in this area to be 20 + 5 ~ g/g dry weight.

Surface soil properties The pH of the softs in the study area was found to be between 6.5 and 7.5. Soft near the m o t o r w a y was noticeably more alkaline, possibly due to road building materials. There was little change with season at all sites. Soil organic carbon c o n t e n t is between 8 and 14% of inorganic weight, and

219 220

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Fig.2. Seasonal change in the lead concentration of grass both near to and distant from the M 6 m o t o r ~ a y during ] 9 7 7 / 1 9 7 8 .

no seasonal trend was found. The water c o n t e n t of the soil was minimum in June {approximately 20% of mineral weight) and generally highest in early spring and autumn (60--70%). DISCUSSION

The investigation has confirmed the occurrence of lead contamination of roadside soil and pasture. A rapid decrease in lead levels with distance from the road and the influence o f prevailing winds is evident. This is in agreement with the findings of a number of other workers w h o have identified this pattern in different regions of the world (Sommer et al., 1971; Hopkinson et al., 1972; Ward et al., 1975; Belal and Saleh, 1 9 7 8 ) . This study, however, has also s h o w n the importance of season on the degree of pollution, a factor that has to date been poorly investigated.

220 A number of reports have suggested that fodder crops grown adjacent to major roads should not be fed to livestock because of possible adverse health effects to the animals, and the possibility that lead may reach man though the food chain in meat and milk products (Wiklander, 1971; Fowles, 1976). The levels of lead in pasture bordering the M6 motorway do not appear to be high enough to be toxic to livestock when compared with the lethal concentrations of lead in forage reported by Arenson {1975). Horses appear to be most suscept. ible to lead poisoning, showing clinical poisoning as a result of ingesting 80 t~g/g of forage in the total diet. Cattle and sheep do not show symptoms of clinical lead poisoning until approximately 200 and 350 pg lead/g of total diet, respectively, are ingested. Fortunately, highest lead levels in pasture grass bordering the motorway occur during the winter months when horses and dairy cattle are normally stall-fed. This study shows that during summer the lead concentration is less than that found at the background sites during winter months. Further research is in progress to assess the significance to the grazing animal of the lead levels reported. Also of interest are the factors which may cause the observed seasonal change in grass lead concentration. Several workers have reported that the availability of lead to plants is affected by soil pH and organic carbon content {Cox and Rains, 1972; Reddy and Patrick, 1977). This study does not show any significant seasonal variation in these factors. Some seasonal variation in soil moisture is evident, but no correlation to grass lead levels exists. Different levels of lead contamination between sites have been related to traffic flow. Generally, the greater the number of vehicles, the higher the level of pollution of soil and vegetation adjacent to the highway (Ruhling and Tyler, 1968; Goldsmith et al., 1976). Traffic flow on British motorways peaks in August and is minimum in winter months (Department of the Environment, 1976); thus the seasonal variation in the lead concentration of the pasture grass bordering the M6 motorway is not a result of changes in traffic flow. Rainfall data shows that the greatest rainfall occurs during winter months when lead levels are highest. The effectiveness of water in washing lead off plant surfaces has been reported as varying from 30% (Kloke and Riebartsch, 1964) to 80% {Schuck and Locke, 1970) of the total lead. Davies and Holmes (1972) report higher levels of lead in grass adjacent to a busy road during dry periods, suggesting rainfall results in significant losses of lead from the plant surface. Studies at this laboratory have shown that running water does not remove a significant amount of lead from the grass. It would appear that the effectiveness of washing is dependent upon the surface characteristics of each plant species (Fidora, 1972). It is possible that rainfall pH might be an important factor affecting loss of lead from the plant surface. Measurements taken at this laboratory and data from other sources (Martin, 1978) show the rainfall to be acidic (typically pH 3.5--6.0) in the study region. Little (1973) reports that an acid solution is more effective in removing lead from leaf surfaces. At the M6 study site a

221

seasonal trend in rainfall pH is n o t distinct, although in general the rain is slightly more acid during winter. Thus, as conditions for greatest removal of lead from the pasture grass occur in winter, when the grass lead concentration is at a maximum, it may be concluded that rainfall volume and pH do not account for the observed seasonal variation in grass lead content. Some workers have reported up to a two-fold seasonal variation in atmospheric lead concentrations during winter months due to meteorological conditions (Bullock and Lewis, 1968; Lawther et al., 1972; Butler et al., 1975}. However this seasonal variation appears not to be sufficient to explain the observed ten fold change in grass lead levels reported here.

Wind speed and direction Wind speed and direction are reported to be important factors determining atmospheric lead concentrations some distance from the pollutant source (Blemel, 1967; Daines et al., 1970; Butler et al., 1975}. It was decided to look at the possible influence of wind speed and direction on the lead concentration of the grass by comparing the percentage time for which a greater than 3 k n o t wind blew to one side of the m o t o r w a y in excess of that blowing over the other. This was calculated from the Keele Weather Station data, and the values for October 1977--May 1978 are shown in Table III. Changes in prevailing wind direction during different m o n t h s can be related to both the time of the change in grass lead levels and the rate of this change. The rate of change in grass lead levels is shown in Table III. The values represent lead concentration against time for each m o n t h period as shown in Fig. 2. Thus a high positive value (e.g. +60 ° ) reflects a rapid increase in grass lead levels and a high negative value (e.g. --70 ° ) a fast decrease with time. Farm B, eastward of the road, shows an increase in pasture lead level during TABLE Ill Comparison of prevailing wind direction and frequency with rate of change in grass lead concentrations between sampling periods Month

Farm towards which wind prevailed

Excess time for which wind prevailed towards farm (%)

Slope of lead concentration vs. time (degrees) Farm A

Oct. 1977 Nov. 1977 Dec. 1977 Jan. 1978 Feb. 1978 March 1978 April 1978 May 1978

B B A B A B A A

6.5 76.5 16.0 32.5 14 53 31 12

Farm B

+ 22

+ 78

+ 77

+ 74

+ 58

+ 86

+ 79

-- 57

--

8 5

--

56

--

7 0

--

86

+66

+ 55

222 November while at Farm A, westward of the road, little change is occurring. This is during a time when the wind is prevailing westerly and thus carries most of the pollution over Farm B. In December, when a sharp increase in pasture lead levels occurs on Farm A, westward of the road, the prevailing wind has changed such that it is towards Farm A. A similar relationship exists if the month of peak pasture lead concentration is considered. Peak grass lead concentration on Farm A occurs in late January, a month where the wind is prevailing westerly. In contrast, on Farm B, peak levels are reached in early March after a month where an easterly wind prevailed. If the rate of decline in grass lead levels in spring is considered, by examination of Table III, it can be seen that the most rapid decrease occurs at different times each side of the motorway. The period of most rapid fail in lead levels is when the prevailing wind is blowing the pollution away from that field. It would appear, therefore, that prevailing wind directions which influence dispersion of the pollution cloud over the m o t o r w a y , may determine the exact time and rate of change in pasture lead levels adjacent to the M6 m o t o r w a y in Cheshire. However, some other factor(s) must explain the general occurrence of the seasonal changes which involve nearly a 10-fold variation in lead levels both near and distant from the motorway. As no significant difference in lead levels was found 1500 m west and 2000 m east of the road, it would suggest these two sites are not influenced by pollution from the motorway. Yet these two sites also exhibit large seasonal changes in pasture lead levels. Previous workers investigating contaminated sites (Rains, 1971) and fairly remote sites (Mitchell and Reith, 1966} suggested seasonal changes in lead levels were determined by growth of the plant itself. Mitchell and Reith (1966) propose an increase in translocation of lead from roots to s h o o t in autumn. There is some evidencein the literature which suggest this may occur (Jones et al,, 1973). Rains (1971) explains the variation by seasonal changes in plant morphology. It is suggested that lead builds up on the non-growing plant during autumn and winter, and is diluted by new rapid growth in the spring. Another possible mechanism of seasonal variation is discussed in the review paper by Chamberlain (1970) which suggests that fine particulates may adhere to wax surfaces which are discarded and replaced in the growing plant. A recent review by Zimdahl (1976) shows that it is still unclear whether significant amounts of lead enter the plant either from the soil or through the foliage. It would appear from this study that as growth slows in autumn, lead deposits build up on grass surfaces, perhaps tightly bound to a wax layer. The increase is fairly linear over a period of some 4 months and greatest downwind of the motorway. A rapid decrease occurs in spring, largely during a 2 month period, probably due to dilution of the lead by new growth. In the summer, lead levels on the grass remain fairly constant during which time a balance of lead input versus losses to the grazing animal and soil exists. Translocation of lead from roots to shoot may account for only a small frac-

223

tion of the increase in plant lead during autumn in the grass adjacent to the motorway. However, at the background site where the level of airborne lead is low, translocation is likely to be a far more important contributor to the total lead level. CONCLUSION

A large seasonal variation in grass lead concentrations has been identified in permanent pastures, both near and distant from the M6 m o t o r w a y in Cheshire (Great Britain). Highest levels occur during winter months and minim u m concentrations in the summer. Grass lead levels are elevated adjacent to the m o t o r w a y in all months of the year. A number of environmental factors have been investigated to study the cause of the seasonal variation in lead levels. Rainfall volume and acidity, traffic flow, soil pH and organic matter content have been shown not to be important. It thus appears that the seasonal variation is a result of changes in plant form and leaf surface characteristics, which significantly determine the rate of retention of lead particulates by the grass. At sites adjacent to the m o t o r w a y the prevailing wind directions influences the timing and rate of change in grass lead concentration. Translocation of lead within the plant may also be an important mechanism of the seasonal change in lead levels, particularly at the background sites where the lead levels are low. Fortunately, levels of contamination in fields bordering the m o t o r w a y are least during the period of most active grazing by farm livestock. Thus it is unlikely that animals grazing near m o t o r w a y s in the U.K. are ingesting toxic levels of lead. However, it may be advisable to restrict grazing of roadside pastures to the months of May--November as during this period lead levels are at their lowest. ACKNOWLEDGEMENTS

The authors acknowledge financial support from Associated Octel Co. Ltd., the Wolfson Foundation and a research studentship from the SRC/SSI~C Joint C o m m i t t e e through the Interdisciplinary Higher Degrees Scheme (IHD). Acknowledgement is also given to Mr. A. MacLennan for his technical assistance and to Dr. C.M. Vick for advice in planning this work. We are also grateful to the farmers who allowed unlimited access to their land and gave much valuable time and assistance. REFERENCES Alloway, B.J. and Davies, B.E., 1971. Heavy metal content of plants grown on soils contaminated by lead mining. J. Agric. Sci. Camb., 76: 321--323. Aronson, A.L., 1975. Sources and pathways of lead in domestic animals. 68th Annu. Meet. Air Pollut. Control Assoc., Boston, Mass., June 15--20.

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