to the one reported by other authors (Lagerwerff and Specht, 1970; Chow, 1970; Page et al., 1971; Milberg et al., 1980). Figures 3 and 4 show the disitribution of ...
LEAD AND ZINC
LEVELS
ROAD-SIDE
AND CHEMICAL
SOILS
OF CARACAS,
FRACTIONATION
IN
VENEZUELA
J. GARCIA-MIRAGAYA, S. CASTRO, and J. PAOLINI Centro de Ecologia, I.V.I.C., Apdo. 1827, Caracas 1010-A, Venezuela
(Received 16 July, 1980; Revised 25 September, 1980) Abstract. Twenty five soil samples located nearby highways and streets of Caracas, Venezuela were collected and chemical, physical and mineralogicalanalysis were done to characterize them. The soils have light textures, neutral or slightlyalkaline pH, medium to high organic matter content, and all of them have carbonate. Kaolinite and mica were the dominant clay minerals in all soil samples. Lead and Zn were extracted with 1N-HNO3 to investigate the levels of these polluting heavy metals in these roadside soils. A very high level of Pb was found in the soils (average enrichment factor of 151.4), while Zn levels were much lower (average enrichment factor of 5.25), but still higher than normal soils' levels. These results are indication of strong metal pollution (especially by Pb) of Caracas' roadside soils, due to heavy transit of motor vehicles as well as to the exclusive use of highly leaded gasoline in Venezuela. Nevertheless both metals accumulate only on the surface layers of those soils located within 5 m from the roadside. These facts were taken as evidenceof the low vertical and horizontal mobilityof the metals, which was mainly attributed to the high pH of the studied soils (between 7.5 and 7.8). Three surface soil samples having high Pb and Zn levels were selected for chemical fractionation by McLaren and Crawford's (1973) methodology. It was found that less than 1% Pb and below 5.5~ Zn were in exchangeable form in these soils. Therefore, Pb and Zn are predominantly present in non-exchangeable forms in the studied soils. These non-exchangeable metals tend to become associated with different soils materials. Lead is mainly associated with the organic fraction, as well as to the inorganic and residual fractions, and Zn is mainly associated to the inorganic and residual fractions, and also the iron oxides, being practically absent in the organic fraction.
Introduction The growing pollution of the e n v i r o n m e n t by heavy metals due to u r b a n and industrial growth has increased the interest arid need to study their levels a n d chemical behavior in soil-water systems. Lead a n d Z n c o n t a m i n a t e soils, plants, water, a n d the atmosphere especially in areas of heavy automobile traffic (Lagerwerff a n d Specht, 1970; John, 1971; Page et al., 1971; Laxen a n d Harrison, 1977). I n developing countries, like Venezuela, the growing n u m b e r of cars in big u r b a n agglomerations like Caracas (300 000 m o t o r vehicles, Ministerio de M i n a s e H i d r o c a r b u r o s , 1975), together with the high Pb c o n t e n t of gasoline ( m a x i m u m permited limit of 1.3 l g 1-2, c o m p a r e d with 0.15 and 0.55 for G e r m a n y and E n g l a n d respectively, Laxen a n d H a r r i s o n , 1977), are very likely to c o n t a m i n a t e the city e n v i r o n m e n t a n d especially roadside soils. Since there are no studies of Pb a n d Z n levels a n d chemical forms in Caracas roadside soils, it was considered of interest to u n d e r t a k e an investigation on the levels of Pb a n d Z n , as well as their chemical forms in roadside soils from several sites of heavily traveled highways in Caracas, Venezuela. Water, Air, and Soil Pollution 15 (1981) 285-297. 0049-6979/81/0153-0285501.95. Copyright © 1981 by D. Reidel Publishing Co., Dordrecht, Holland, and Boston, U.S.A.
286
J. GARCIA-MIRAGAYA E T A L .
2. Materials and Methods Twenty five soil samples from soils near the highways and streets of Caracas, Venezuela were collected in November 1977, Most of the samples were superficial but several were taken at lower depths in the soil profile. Most of them were located at less than 1 m from the roadside, but a few were at higher distances. More details are given in Table I. All the soil samples were air dried and passed through a 2 mm aperture mesh. The following chemical, physical, and mineralogical analyses were performed on all the collected samples: Particle size analyses by the hydrometer method (Day, 1965); mineralogy of the clay fraction by X-ray diffraction, following the procedures of Whittig (1965); pH in distilled water (1 : 1), and 1N-KC1 with a Radiometer pH-meter; organic matter content (O.M.) by the Walkley and Black method (Jackson, 1970); cation exchange capacity (C.E.C.) by saturation with 1N-MgCt2, washing the excess of salt with 1 x t0 -3 N-MgC12 and displacement of Mg with 1N-NaC1, Mg was determined by atomic absorption spectrophotometry (AAS) with an air-acetylene TABLE I
Location of the Soils Sample Number 1 2 3 4 5 6 7
8 9
Location
El Valle Freeway
Roche Avenue St, Juan Bosco Av. Colon Blvd, O. Leafy Square,
dust sample O. Leary Square Botanical Garden Botanical Garden East Side Park
l0
Boyac~ Av,
11
Caracas-Valencia Freeway E1 Valle Freeway, nearby Coche bridge
12 13 14 15 16 17 18 19 20 21
" " "
< < <
5 cm) Pb (gg g-l)
Zn (lag g-l)
140 30 121 38 18 74 18 10 1602 81 29
43 4 31 10 5 16 13 4 178 34 23
Y _+ cr = 218 + 520 (C.V. = 238.5~o) for Pb, _+ cy= 34.9 ___55.4 (C.V. = 158.7~o) for Zn. s a m p l e s is 1514 ~tg g - I a n d t h e c o e f f i c i e n t o f v a r i a t i o n (C.V.) is q u i t e high ( 1 2 3 . 2 3 ) , r e f l e c t i n g m a i n l y t h e d i f f e r e n t l o c a t i o n o f t h e s t u d i e d soils. T h e s a m e c o m m e n t is v a l i d for t h e a v e r a g e Z n c o n t e n t o f t h e s u r f a c e soils s a m p l e s . T h e s a m p l e w i t h t h e h i g h e s t P b c o n t e n t ( N o . 11) has a v e r y h i g h c o n t e n t (6292 g g g - 1 ) , w h i c h is h i g h e r t h a n any v a l u e o f P b c o n t e n t in soils c o n t a m i n a t e d by m o t o r v e h i c l e s r e p o r t e d in t h e l i t e r a t u r e ( Z i m d a h l a n d A r v i c k , 1973). T a k i n g 10 lag g-1 as t h e a v e r a g e P b c o n t e n t in n o n - c o n t a m i n a t e d soils ( A l l a w a y , 1968), w e c a n see t h a t t h e r e is an a v e r a g e e n r i c h m e n t f a c t o r
290
J. GARCIA-MIRAGAYAE T AL.
(E.F.) of 151.4 for the surface soil samples, with a maximum of 629.2. For the subsurface soil layers the average value of the enrichment factor was 21.8, with a maximum of 160.2. For Zn we find an average E.F. of 5.25 (taking 50 pg g-1 as the average Zn content of non-contaminated soils, according to Allaway, 1968), with a maximum of 18.94. The average E.F. of the subsurface soil layers was 0.698 indicating no contamination of the subsurface soil layers with Zn. However, it should be noted that sample 22 has a value of 178 ~tg g-I of Zn which is more than three times higher than the average. These results indicate that these roadside soils are highly contaminated with Pb, and less so with Zn. Also the results suggest that these metals tend to accumulate in soil's surface layers. The high degree of Pb contamination of these soils was expected, considering the high concentration and heavy traffic of motor vehicles and the exclusive use of highly leaded gasoline in Caracas. 3.2. V A R I A T I O N OF LEAD AND ZINC CONTENT WITH D I S T A N C E FROM R O A D S I D E A N D SOIL DEPTH
Figure 1 shows the 1 N - H N O 3 extractable Pb content at different distances from the roadside of one selected site. As can be seen, the Pb content decreases drastically with distance from the roadside. This is especially noticeable in the 0 to 5 cm layer, as well as.in the 5 to 15 one, being practically unnoticeable in the 15 to 25 cm layer due to the relatively low Pb content of this soil layer even at a distance of 1 m from the roadside. Figure 2 shows the same kind of plots for Zn and we can see that it follows a similar sort of pattern, although showing lower metal contents. One would expect this type of pattern if the motor vehicles were the polluting source, and it is similar
DEPTH
400 A
,,
300
.e. ~'.2OO
I00
~ 1
I
, ~ I
I
3
I
I
5
m. I
I
7
I
I
9
I
I
I
II
I
15
~
Ei]
15
Distonce from roodside (meters) Fig. 1.
Variation of Pb content of soils (extracted by 1N-HNO3) with distance from roadside. (Soil samples 12 to 20).
291
HEAVY METALS IN VENEZUELAN ROADSIDE SOILS
DEPTH I00
:
~
,0-
5era.
.......a 5-t5 ul5-25 ,'
75 z~
