Heavy-Metal Pollution from Automotive Emissions and Its Effect on Roadside Soils and Pasture Species in New Zealand Neil 1. Ward, Robert R. Brooks*, and Edward Roberts' Department of Chemistry, Biochemistry and Biophysics, Massey University, Palmerston North, New Zealand
Colin R. Boswell Computing Service Centre, Victoria University, Wellington, New Zealand
rn Concentrations of cadmium, chromium, copper, lead, nickel, and zinc were determined by atomic absorption spectrophotometry in soils and pasture species along a major motorway in New Zealand. Enhanced levels of all elements were found and correlated well with traffic densities. Concentrations in soil profiles decreased with depth and showed that the source of pollution was aerial deposition from motor vehicles. About a third of the aerial burden of each element was removable by washing of vegetation species. Highest accumulations of heavy metals were usually found on white clover, and the lowest on Paspalum grass. Elemental concentrations in pasture species were usually well correlated with traffic densities, particularly in the case of the grass, Yorkshire fog. Plant/soil correlations for individual heavy metals were in general poorer than for relationships involving traffic densities and concentrations in plants and soils. Lead particulate pollution from automotive emissions and other industrial sources has been extensively investigated in recent years (1-7). Apart from lead, very little attention has been paid to the possibility of pollution from other heavy metals derived from automobiles. However, Lagerwerff and Specht (8)reported the presence of cadmium, nickel, and zinc (as well as lead) in soils and grasses at roadsides. These elements were presumably derived from motor vehicle exhausts and, for a site with 48 000 vehicles per day, had the following range of concentrations (pg/g dry weight) for surface soils in the interval 8-32 m from the road: cadmium, 0.94-0.24; lead, 540-140; nickel, 7.40-2.40; and zinc, 162-114. Corresponding values for grasses were: cadmium, 0.75-0.48; lead, 51.3-18.5; nickel, 3.8-1.3; and zinc, 40-30.3. The toxicity of all the above elements toward humans has been well documented (9),and because of the paucity of data on pollution by these elements (except in the case of lead) from motor vehicle exhausts, there is clearly a need for further studies on these and other heavy metals. This need is further reinforced by the fact that neither of the previous studies had included chromium or copper (both of which are components of motor vehicles), and neither had attempted to correlate elemental concentrations with traffic densities. In the work presented in this paper, we have studied concentrations of cadmium, chromium, copper, lead, nickel, and zinc in soils and pasture species along a busy motorway in Auckland City, New Zealand, and have correlated concentrations with traffic densities at each collection locality. Experimental The Auckland Motorway is situated to the south of New Zealand's largest city, Auckland (population approximately 700 000). The sampling sites were selected a t 17 interchanges on a 1-m-wide, grassed median strip located in the center of the motorway. Surface soils (0-1 cm) were collected a t all 17 sites, and soil profiles were sampled at nine sites by means of a 0.5-cm-diam Agronomy Department, Massey University
core extractor. The cores were subsequently divided into twenty 0.5-cm sections. Background soil samples were taken from the same soil type at distances well away from the effect of motor vehicle emissions. Soils were then dried for three days at 60 "C and sieved to -60 mesh. Digestion of 0.1-g samples of soils was carried out with 10 cm3 of a 1:l mixture of concentrated nitric and hydrofluoric acids contained in 50 cm3 polypropylene squat beakers. The solutions were taken to dryness over a water bath, and the residues were redissolved in 2 M hydrochloric acid. The pasture species sampled from the median strip of the Auckland Motorway were: Lolium perenne L. (perennial ryegrass), Trifolium repens L. (white clover), Poa annua L., Dactylis glomerata L. (cocksfoot), Holcus lanatus L. (Yorkshire fog), Bellis perennis L. (daisy), Paspalum dilatatum L., and "flatweeds" of the genera: Plantago, Bellis, and Crepis. Pasture samples were divided into unwashed and washed portions for analysis, the washing being carried out by agitation in about 20 L of slowly running water. Samples weighing about 5-10 g were oven-dried at 150 "C, ashed at 450 "C, and dissolved in 2 M hydrochloric acid in the ratio of 5 cm3 of acid to 0.050 g of ash. Further dilutions were carried out when required. All solutions were analyzed by flame atomic absorption by use of a Varian Techtron AA-5 instrument fitted with a BC6 background corrector to correct for nonatomic absorption. Wavelengths (nm) of the absorption lines used were: Cd 228.8, Cr 357.9, Cu 324.8, Ni 232.2, P b 217.0, and Zn 213.9. Concentrations were in most cases sufficiently high to obviate the need for separation or preconcentration procedures.
Results and Discussion The mean concentrations of cadmium, chromium, copper, lead, nickel, and zinc in surface soils sampled from the various interchanges (grouped according to traffic density) are presented in Table I. All six elements show significantly higher levels along the motorway compared with background areas. Although elemental levels are considerably higher than background for all elements, there was a possibility that in some cases, the soil had been transported from elsewhere in the city during the construction of the motorway and that the high levels were due to naturally high concentrations of some elements in the basaltic soils of the Auckland area. This could apply to chromium and nickel, though not to the other elements which are not usually enriched in basaltic substrates. To confirm the true origin of heavy metals in the soils, a number of soil profiles were analyzed. Decrease of elemental concentrations with depth would indicate surface contamination from motor vehicles, whereas unchanging concentrations would show that anomalous heavy-metal levels were a function of the substrate itself. Figures 1 and 2 show concentrations of six elements in soil profiles from nine interchanges divided into three groups according to traffic densities. It is clear from these figures that elemental concentrations invariably decrease with depth at Volume 11, Number 9, September 1977
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Table 1. Mean Concentrations (pg/g Dry Weight) of Heavy Metals in Surface Soils and Vegetation (Unwashed) from 17 Interchanges (Grouped According to Traffic Density) of Auckland Motorway, New realand Materlal
Mean traffic
No. of
density,
Interchanges
vehlclesl24 h
Soilsa
>50 000 40000-50000 20000-39000 10000-19000
5 5 4 3
...
Background
i
Vegetation
Concentratlons of pollutant
>50 000 40000-50000 20000-39000 10000-19000
Cd
Cr
cu
NI
Pb
Zn
2.2 0.7 0.4 0.3
