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Peryman Journals Ltd.

LEAD ANALYSIS IN URBAN DUST ON ST. THOMAS, U.S. VIRGIN ISLANDS

D. R. BROWN

Division of Science and Mathematics, College of the Virgin Islands, St. Thomas, U.S. Virgin Islands 00802, U.S.A.

Abatrrct-Tbe average concentration of Pb in urban dust in central Charlotte Amalie, St. Thomas, U.S. Virgin isknds, was found to be 803 ppm. On tbe hilts in the town the Ievel was higher, over 2000 ppm. The Pb content ofdust was found to depend on the distance from the street of the sampling site, but the dependence was less than expected Urban dust samples taken on the neighbouring island of St. Croix, where there is lower traffic density, showed an average Pb level of 327 ppm.

Key word index: Lead, dust, urban, automobile, U.S. Virgin IsIands.

St. Thomas is typical of many tropical and sub-tropical islands throughout the world tbat have experienced dramatic increases in tourism over reamt decades and a concomitant rapid economic growth. Narrow streets, that were adequate for the tramc of 30 y ago, are now overcrowded with cars and taxis. Tbe health risk associated with Pb and other automobile-generated pollutants has never been assessed in an environment quite like St. Thomas, where it has tradition- ally been assumed that such pollutants are rapidly dispersed by ocean breezes. A sketch map of St. Thomas is shown in Fig. 1.

We have identified many locations on St. Thomas where people live, work and go to school in extremely close proximity to passing heavy tratlic. It is in these areas, we believe. that a significant health risk from Pb may exist. In the first phase of our assessment of this risk we have analyzed roadside dust for Pb, this being a recognized indicator of the abundance of Pb in the local environment (Duggan, f980), as well as being a significant direct source of ingested Pb for young children (Schmitt er al., 1979; Stephens, 1981).

Studies in Puerto Rico on soil have shown that the Pb content ofsoil typically decreases by a factor of ten on moving from 3 m to 10 m distance away from a busy road (Rodriguez- PIores and Rodriguez-Castellon, 1982). If the Pb content of dust shows similar variation with distance it would strongly

suggest that the extreme proximity to passing traffic of o&ices, houses and schools in St. Thomas contributes to an elevated health risk from Pb to those living and working in such locations. For this reason, one of the objectives of this work has been to examine the way in which Pb in dust varies with distance from the road.

EXPERIMENTAL

Samples of dust were collected over a 3-month period during the summer of 1984. Sampfing sites were chosen to avoid the incorporation of extraneous material, especially paint chippings. In general, sites were selected in which dust had been allowed to settle and had remained undisturbed for some time. Ledges, built into the lower parts of walls, window ledges and other recesses, iron gates, door hinges and other surfaces proved suitable. The dust was swept into Iabelled envelopes with small paint brushes. Samples were dried at 100°C for 24 h. passed through a coarse sieve and ground to a fine powder. Duplicate samples of 0.5-1.0 g were used for each analysis. These samples were refluxed in 40 ml 2N HNO, plus 5 ml 30% H&r for 30 min. The solutions were fittered and made up to 100 ml with water. They were analyzed using an Instrumentation Laboratories Single Ream Atomic Absorption Spectrophotometcr with deuterium arc lamp background correction. M~urements were made at

ATLANTIC OCEA

e Hill 1275 m)

N

CARXEBEAN SEA

Fig. 1. St. Thomas, U.S. Virgin Islands.

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the 283.3 nm absorption line in the Pb spectrum, comparing with standard solutions containing 3,10,20,30 and 40 ppm Pb. Results arc quoted as ppm Pb based on the dry weight of the dust sample. When duplicate analysts did not fall within aeecptable experimental error of each other the nsults from that sample were dhrdcd.

RESULTS AND DlSCUSSION

The results are summarized in Table 1. where they arc compared with the Pb contents of dust taken by other workers elsewhere. The lcvcls of Pb found in dust arc gcncrally distributed lognormally (Stark et al., 1982, and refcrenccs the&n) so gcomctric means and gcomctric start- dard deviations are reported.

On and around Main Street, in arcas of the town idcntificd as potentially at risk, 18 samples wcrc collcctcd. These samples were taken from ledges and other surfaces close to the ground and within 1 m of the road. The avcragc Pb content of these samples was very similar to that found in urban dust in Urbana, Illinois and, at 803 ppm, was close to the IO00 ppm figure which is generally considered un- desirably high (Day et al., 1975). A further 24 samples were taken in other streets, less busy than Main Street. As cxvcd, the Pb content of these was lower than those from the busiest streets.

On Mafolie Hill, the busiest of the hills on the island, cigbt samples wcrc taken Two unoccupied houses, built dircctty adjacent to the road and with no intervening sidewalk, wcrc chosen as the sites for these samples. Thick dust was rcmovcd from the doorsteps, the window lcdgcs and the glass louvrcs in the windows. These samples all contained very high concentrations of Pb.

A further 30 mmplcs were taken in areas where traRic is light and where there is less shelter from prevailing winds. The Pb content of tbcsc samples were similar to those of samples taken in rural locations in England (Day et at., 1975).

Thirty-one samples were taken down the alleys in St.

Thomas off Main Street, at varying distances from the street. The log of the Pb content of thcsc samples is plotted against distance in Fig 2. It was hoped that the variation of Pb content with distance could bc matched to an cxponcntial cxprcssion of the type dcvclopcd for the way in which the Pb content of soil va&s with dii from the road (Ward et al., 1975; Wheeler and Rolfc, 1979). but the spr& of the data prccludcs a meaningful analysis of this type. However. a correlation eoc&icnt of - 0.66 was found, with the following expression describing the relationship:

Pb(ppm) = 812~““~~~*~‘~(~).

The most noti#rsbk feature of this plot is that the dramatic drop off of Pb content seen in soils over the first 5 m is not men in the dust. The Pb content drops off more gradually. The tentative conclusion must bc that environmental Pb kvels arc not elevated enormously, as was feared, in locations within 5 or 10m of heavy traffic when compared with locations 20 or 30 m from the same traffic.

On the nearby island of St. Croix, 27 samples of dust were taken during a single sampling visit. St. Croix is much larger than St. Thomas and is less congested with tragic. Sampling sites were chosen to be as similar to those on St. Thomas as possible. Most of the samples were taken in the capital, Christians&ad, close to heavy traffic, and the others were taken in areas of low traffic density. The overall Pb content in these samples was substantially lower than in equivalent samples taken on St. Thomas.

In conclusion, WC have established that there are loealixed arcas of St. Thomas where Pb is abundant in the environment. Based upon the Pb content of dust in busy central areas, it would seem that the distance one is from the roadside has a real but relatively small effect on the oecurrcnec of the metal. Persons working and living within very short distances of passing traffic may k at less risk from Pb than was feared. Air sampling will bc initiated in areas thought to beat risk,and on the basis of the eombincd dust and air results, it will bc dccidcd whether clinical trials involving blood Pb testing is ncecssary.

Table 1. Pb conecntrations in urban dust on St. Thomas, U.S. Virgin Islands and elsewhere

Sampling site Ocomctric mean Geometric S.D.

No. samples Pb concn (ppm) (ppmf

st. Thomas Main Street Streets other than Main h&folk Hill Site A Site B Rural locations

St. Croix Urban locations Rurai locations

Urbana, IlIinois*

London, U.K.,

Manchester. U.K.$

Urban New Jcrsey§

Christchurch, N. 21

RuraI locations,t U.K.

I8 803 179

24 597 407

3 2467 835 5 2104 199

30 40 66

19 327 142 8 46 34

- ~~(*200)

- &Ymon)

-

- 970 -

- 2alo-2500 -

- 634ofl - 85 -

l Hopke er al.. 1980. t Millar and Cooncy. 1982 *Day et al., 1975. OWilbcr and Hunter, 1979. IFergusson and Simmonds, 1983. $Samplcd at a busy intersection.

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Distance (m)

Fig. 2. Effect of distance from street on Pb concentration in urban dust.

Acknowledgements-The assistance and advice of Roy Watlington. Associate Professor of Physics at the College of the Virgin Islands. is greatly appreciated.

REFERENCES

Day J. P., Hart M. and Robinson M. S. (1975) Lead in urban street dust. Nature 253, 343-345.

Duggan M. J. (1980) Lead in urban dust: an assessment. War Air Soil Pollur. 14, 309-321.

Fergusson J. E. and Simmonds P. R. (1983) Heavy metal pollution at an intersection involving a busy urban road in Christchurch, New Zealand-l. Lcvcls of Cr, Mn. Fe, Ni. Cu. Zn. Cd and Pb in street dust. N. 2. J. Sci. 26,219-228.

Hopke P. K., Lamb R. E. and Natusch D. F. S. (1980) Multielemental characterization of urban roadway dust. Envir. Sci. Technol. 14, 166172.

Millar 1. B. and Cooncy P. A. (1982) Urban lead-a study of environmental lead and its significance to school children

in the vicinity of a major trunk road. Atmospheric Environment 16,61%20.

Rodriguez-Flares M. and Rodriguex-Castellon E. (1982) Lead and cadmium levels in soil and plants near highways and their correlation with traffic density. Enuir. Pollut. Ser. B. 4, 281-290.

Schmitt N., Philion J. J., Larsen A. A., Harnedck M. and Lynch A. J. (1979) Surface soil as a potential source of lead exposure for young children. Can. Med. Ass. J. 121, 1474-1480.

Stark A. D., Quah R. F.. Meigs J. W. and DeLouisc E. R. (1982) The relationship of environmental lead and blood- lead levels in children. Enuir. Res. 27, 372-383.

Stephens R. (1981) Human exposure to lead from motor vehicle emissions. Inc. J. Enuir. Stud. 17. 73-8.

Ward N. 1.. Reeves R. D. and Brooks R. R. (1975) Lead in soil and vegetation along a New Zealand state highway with low traffic volume. Enuir. Polk 9, 243-251.

Wilber G. W. and Hunter J. V. (1979) Distribution of metals in street sweepings, stormwater solids, and urban aquatic sediments. J. Wt. Pollut. Control Fed. 51, 2810-2822.