Heavy metal contamination in highway soils ... - Springer Link

Original paper

Clean Techn Environ Policy 4 (2003) 235–245 DOI 10.1007/s10098-002-0159-6

Heavy metal contamination in highway soils. Comparison of Corpus Christi, Texas and Cincinnati, Ohio shows organic matter is key to mobility Dilek G. Turer, J. Barry Maynard

235 Because of the severe adverse health effects of Pb (EPA 1999), there have been many studies of Pb contamination of soils along highways (e.g. Vandenabeele and Wood 1972; Ward et al. 1975; Wheeler and Rolfe 1979; Onyari et al. 1991; Francek 1992; Gratani et al. 1992; Sithole et al. 1993; Teichman et al. 1993; Hafen and Brinkmann 1996). These studies report very high lead levels near highways compared to background and a rapid fall-off with distance from the roadway. There is some evidence that the Pb is strongly bound to the soil. For example, Erel (1998) found that Pb was transported to soil in soluble halogenated aerosols, but was strongly bound to the soil once trapped. He calculated a residence time of Pb in the soil of >100 years. There have been a few more recent studies that included other heavy metals besides Pb. Hewitt and Candy (1990) examined levels of Pb, Cd, and Zn in soil and dust samples collected in and around the city of Cuenca, Ecuador. Pb and Zn but not Cd showed a strong relationship to vehicle traffic. Munch (1993) compared the distributions of As, Cd, Cr, Cu, Hg, Ni, Pb, V, Zn, and polynuclear aromatic hydrocarbons (PAHs) along an urban roadway in Germany. He found that Cd, Ni, Pb, V, Introduction and Zn declined exponentially with distance from the Roadside soils have long been known to contain high roadway, reaching background before 5 m. The PAHs also levels of heavy metals, especially lead. There is general decline exponentially, with values of as much as 100 times agreement that these metals decrease in concentration with depth and with distance from the roadway. What is background close to the roadway. Jaradat and Momani (1999) reported elevated Cu, Cd, Pb, and Zn for roadside less certain is the form in which the metals occur and soils in urban areas of Jordan. consequently how easy they are to remobilize. From a Contaminated roadside soils may constitute a health policy perspective, the question is, how serious is the hazard posed by these metals? A second question is, how hazard if the metals are transferred to other reservoirs. can this common, non-point source contamination of soils Studies of roadside soils in the Manoa Basin on Oahu, Hawaii (Sutherland 2000; Sutherland et al. 2000; Sutherbe distinguished from point source contamination from land and Tolosa 2001) have shown that Cu, Pb, and Zn industrial facilities or illegal dumping? Our study addresses these questions by examining the relationship have significant anthropogenic enrichments and that these between heavy metals and the other constituents in the soil contaminated soil particles can be washed into nearby water bodies where they form a potential source for bioand their distributions. accumulation of Pb. Sansalone and Buchberger (1997) sampled lateral pavement sheet flow from a study area along I-75 in Cincinnati, OH and found that Zn, Cu, Pb, Received: 23 January 2002 / Accepted: 7 June 2002 Published online: 28 August 2002 and Cd often exceeded surface quality discharge standards Springer-Verlag 2002 during rainfall events. They also found that Cd, Cu, and Zn were mostly in dissolved form, whereas Al, Fe, and Pb were D.G. Turer, J.B. Maynard (&) particulate-bound in storm water. These studies suggest University of Cincinnati, Department of Geology, that metals coming from highways can be a significant, P O Box 210013, Cincinnati, OH 45221-0013, USA unsuspected source of metal contamination in streams. E-mail: [email protected] A study of the soils at the Sansalone and Buchberger Present address: D.G. Turer (1997) site by Turer et al. (2001) showed decreases with Zonguldak Karaelmas University, depth and distance from the roadway for Cu, Pb, and Zn, Geological Engineering Department, Zonguldak, Turkey Abstract Heavy metal content of roadside soil samples from along the interstate highway systems in Corpus Christi, Texas and Cincinnati, Ohio was measured to assess the degree of contamination such soils contain and the likelihood that this contamination can be remobilized. High values of Ba, Cu, Pb, and Zn can be attributed to anthropogenic effects related to motor vehicles, whereas Cr and Ni variations are best ascribed to natural processes. The anthropogenic substances are strongly correlated to the amount of organic matter in the soil. Sequential extraction experiments, however, show that this organic matter is not extractable by agents that normally solubilize soil organic matter, so these metals are bound to an insoluble form of organic matter that is itself probably anthropogenic. The insolubility of the heavy metals and Ba indicates that these constituents are not likely to move in solution to water supplies, but they would still be subject to physical remobilization by roadway maintenance or even by grass mowing. Inhalation of small dust particles poses a potential health hazard to highway maintenance workers that needs to be assessed.

Clean Techn Environ Policy 4 (2003)

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but that the strongest predictor of metal concentration was the amount of organic matter in the soil, suggesting that individual soil components need to be studied to predict metal behavior. Partitioning of metals among soil components is normally investigated by sequential extractions (e.g. Tessier et al. 1979; Sposito et al. 1982), but few studies of roadside soils have included sequential extraction determinations. Two such studies, which both used the Tessier et al. (1979) extraction scheme, are by Lee and Touray (1998) and Norrstrom and Jacks (1998). In both cases, the highest percentage of Pb and Zn were found associated with Fe oxides, the next with carbonates, and then with organic matter, with only a minor amount in the exchangeable fraction. Organic-bound Pb ranged from 11 to 18%, while organic-bound Zn ranged from 6 to 9%. Cu by contrast was most abundant in the organic fraction, at about 30%. An important role for organic matter in controlling heavy metal distribution is suggested by the work of AlChalabi and Hawker (2000) on roadside soils of Brisbane, Australia. Their data set shows much stronger correlations between Pb and organic carbon than for traffic volume, carbonate carbon, depth, or clay content. Unpolluted soils apparently do not show this pattern. For example, Tack et al. (1997) reported low correlation coefficients for organic C versus Cu, Pb, and Zn, in the range 0.2–0.4, for unpolluted soils in Flanders. Also, McGrath (1996) reported that extractable Zn but not total Zn was significantly correlated with organic C in Irish agricultural soils. We have investigated the detailed distribution, vertically and horizontally, of heavy metals, Ba, and organic C at two sites in Corpus Christi, Texas to compare to previous work in Cincinnati. Corpus Christi provides a contrasting climate, one warm enough to preclude road salting in the winter, and has a heavy concentration of refineries north of the city that provides an opportunity to compare roadside soils from densely populated with those from highly industrialized urban areas. We also report here comparative sequential extraction results for the two cities that show the partitioning of the metals in the soils.

Study area Corpus Christi, which lies in the coastal bend area of south Texas, is a major petrochemical center with much heavy industry concentrated in an east–west band along highway I-37. An earlier study by Harrison (1987) of Pb showed high levels near highways. He analyzed the Pb content of 485 soil samples taken from parks, school playgrounds, and the edges of roadways and highways. The mean Pb values were 55 ppm for parks, 57 ppm for school playgrounds, and 250 ppm for the edges of roadways and highways. The Pb concentrations in the samples from highways were more than 1000 ppm for 6% of the samples. He estimated the baseline for this community as 13 ppm from samples obtained from an untrafficked area. For comparison, Sharma et al. (2000) reported an average value from uncontaminated soils of South Texas of 9 ppm Pb. We chose two sample sites along I-37 to test the range of soil contamination along Corpus Christi freeways to compare with our detailed study of the I-75 site in

Cincinnati (Turer et al. 2001). The first location (CC#1) at the Port Avenue interchange had a daily traffic volume of 48,000 in 1998, as reported by the Texas Department of Transportation. The second location (CC#2) is at the intersection with Up River Road and had a similar traffic volume of 45,000. Corpus Christi has an average temperature of 71.7 F (22 C) and rainfall of 30.1 inches (76.5 cm) compared to 54.1 F (12.3 C) and 39.7 inches (100.8 cm) for Cincinnati (Climate Diagnostic Center 1999). Annual snowfall is only about 0.1 inches (0.25 cm) in Corpus Christi, so road salting is not practiced, in contrast to Cincinnati where salt is used extensively in the winter for deicing. Corpus does, however, receive salt from marine aerosols. The precipitation-weighted mean concentration of chloride is >0.9 mg/l for the South Texas coast compared to only about 0.1 mg/l for Ohio (Li 2000, his figure VIII-22).

Methods Twenty-two soil samples were taken along a transect perpendicular to the highway. At each station along the transect, a surface sample was taken with a steel trowel, then 10 cm core sections taken with a hand auger. Three instrumental methods of analysis were applied for different aspects of the research: wavelength dispersive X-ray fluorescence (XRF, Rigaku 3070 spectrometer) to determine bulk metal content of the samples; C–S analyzer (LECO) to measure organic C, total C, and total S; and powder X-ray diffraction (XRD, Siemens D-500) to determine clay mineralogy. For XRF analysis, samples were first dried at 100 C and then ground using a steel ball mill. Pressed pellets for XRF were prepared with 5–6 g of samples, pressed at 18 tonne of pressure for 4–5 min. Samples were run against a set of U.S. Geological Survey rock standards combined with a set of road-side soil samples previously analyzed by XRAL, Inc. of Toronto, Canada by neutron activation. To determine the amount of heavy metals that can easily be mobilized, the following procedure, which combines the first three steps of the sequential soil extraction process of Sposito et al. (1982), was applied. To 5 g of dry soil sample: 1. 100 ml of 0.5 M KNO3 was added and left on a stirrer for 16 h (exchangeable fraction) 2. 100 ml of distilled H2O was added and left on a stirrer for 2 h (absorbed fraction) 3. 100 ml of 0.5 M NaOH was added and left on a stirrer for 16–21 h (organically bound fraction). After each step, the liquid was poured into a tube and centrifuged at 15,000 rpm for 10 min. The soil that settled at the bottom of the tube was washed back into the glassware using the next extractant. When all of the three steps were completed the residue was left to air-dry and pellets were made for XRF analysis as described above. For XRD analysis, the samples were treated with hydrogen peroxide (H2O2) to break down organic coatings and with sodium dithionate (Na2S2O4) to remove iron on clays. Then they were dispersed with sodium pyrophosphate (Na4P2O7), centrifuged at 15,000 rpm for 10 min, and prepared as smears on glass slides (Moore and

D.G. Turer, J.B. Maynard: Heavy metal contamination in highway soils

Reynolds 1997). This procedure was effective for the samples from CC#2, but it was not possible to get clay dispersion from soil samples taken from CC#1, reflecting low clay proportions compared to quartz and calcite. The slides were air-dried, glycolated at 80 C for 24 h, and heated to 550 C in order to differentiate the clay minerals.

Results Two sites were chosen within Corpus Christi to reflect differences in land use within an urban setting (Fig. 1). The first was designated CC#1, which is close to the city center in an area with mostly automobile traffic and is similar in location to the Cincinnati site studied by Sansalone and Buchberger (1997) and Turer et al (2001). The second site (CC#2) has a similar traffic volume to CC#1, but is in an area with many oil refineries and heavy truck traffic going to and from the refineries. Corpus Christi site #1 (CC#1) This site is at the entrance ramp to I-37 from Martin Luther King Drive at North Port Avenue, close to the center of downtown. Daily traffic volume was 48,000 in 1998. Heavy metal and Ba concentrations in the top 15 cm of the soil are very high (except for Ni which does not show much variation with depth) compared to background values (Table 1). Our lowest values came from the sample farthest from the road at 3 m distance and a depth of 32.5 cm. Here Pb was 20 ppm, which is somewhat lower than the background measured by Harrison (1987) of about 50 ppm for soils remote from highways. Taking this sample as representative of the local background yields values of 35 ppm for Zn, 10 ppm for Cu, 50 ppm for Ni, 55 ppm for Cr, and 260 ppm for Ba. The maximum Pb value, which was in a sample from the 0–10 cm depth interval 3 m away from the road, was 820 ppm. The highest concentrations for Zn, Cu, Cr, and

Ba came from surface sampling 2 m away from the road and were 390 ppm, 180 ppm, 140 ppm, and 720 ppm, respectively. It is also important to note the negative relationship between depth and heavy metal concentrations. As depth increases, the amount of heavy metal in the soil decreases (Fig. 2). In addition, heavy metal and Ba concentrations generally decrease as the distance from the road increases. Previous work at the Cincinnati site (Turer et al. 2001) had shown that only a small proportion of the heavy metals were easily remobilized into solution. To test for easily remobilized metals in the Corpus Christi samples, the first three steps of the sequential soil extraction process of Sposito et al. (1982) were applied to three samples taken from surface, 0–10 cm and 10–20 cm depth at the location 2 m away from the road. The results showed that the mobility of these metals is again low. As can be seen in Table 2, the extraction process, which took out the heavy metals and Ba that are in the exchangeable fraction, the absorbed fraction, and the fraction bound to soluble organic matter, produced decreases in Pb, Zn, Cu, Ni, and Ba concentrations of 11–38%. Cr is not reported because of a high Cr equipment blank. One may also notice a decreasing trend in the % mobilized with depth for Pb, Zn, and Cu, but an increase for Ni and Ba. The results of the LECO analysis (Table 1) showed that CC#1 soils contain an average of 1.45% organic carbon and 1.70% carbonate C. By comparison, the Cincinnati site averaged 3.33% organic C and 3.65% carbonate C. For CC#1, there is a positive correlation between heavy metal concentrations (except for Ni) and the amount of organic C in the soil samples. As organic C increases, the amounts of Pb, Zn, and Cu increase with r2 values of 0.79–0.85 (Fig. 3). The same kind of trend is also seen for Ba, but not for Ni or Cr (r20.6. The usual negative relationship between depth and organic C exists with r2=0.82. The sequential extraction test for element mobility gave similar results to CC#1. Application of the first three steps of the soil extraction process to the samples taken from 0–5 cm, 5–15 cm, and 15–20 cm depth intervals at a location 0.8 m away from the road showed average % decreases of 10–30%, with Cu again being the most mobile element (Table 2). Trends of extractability with depth are not well developed compared to CC#1. Cluster analysis showed that Cr, Zn, and Ba have a strong association with each other, while Cu and Pb form another group with a lower level of clustering (0.67 versus 0.87). Samples were also taken from surface soils near the truck entrances to three of the refineries in this neighborhood. The results (Table 1) show values that are somewhat lower than for the highway soils, but with metal contents still as much as 10 times background. An exception is Ba, which was considerably higher in the refinery street soils than in the highway soils. XRD patterns of samples taken from 0.8 m away from the road were studied to identify the clay mineralogy of


239

Fig. 2. The distribution of heavy metal and Ba concentrations with respect to depth and distance from the road (CC#1)

the soil. Following the methods of Biscaye (1965) and Johns et al. (1954) for quantitative analysis of clay minerals, 60% of the clay is calculated as illite (47% of which is smectite as mixed layers) and the rest is kaolinite. For comparison, the Cincinnati soils had 76% illite (30% of which is smectite as mixed layering) and the rest was chlorite and kaolinite. Thus about 28% of the clay in the Corpus Christi soil consists of expandable (smectite-type) layers compared with 23% for Cincinnati. When the amount of total clay in the soil is taken into account (Table 3), the Cincinnati soils have about 7.8% expandable clay, whereas CC#1 and CC#2 average 5.7 and 6.0%.

Discussion The heavy metal concentrations in CC#1 and CC#2 are very high compared to the average background values reported for United States soils (Shacklette and Boerngen 1984) or to values that Sharma et al. (2000) reported for south Texas (Table 1). Pb concentrations in the surface samples collected in this study are also above the average of 250 ppm reported for the edges of roadways and freeways in Corpus Christi by Harrison (1987). However, the highest Pb concentration we measured (820 ppm at CC#1) is substantially less than the highest value reported by Harrison (1987) which was 2,970 ppm, on an embankment of the Crosstown Expressway.


Table 2. Extractability of metals

Samples CC#1 CC#1 CC#1 CC#1 CC#1 CC#1

240

2 2 2 2 2 2

0 0 0-10 0-10 10-20 10-20

Average % change CC#2 0.5 0-5 CC#2 0.5 0-5 CC#2 0.5 5-15 CC#2 0.5 5-15 CC#2 0.5 15-20 CC#2 0.5 15-20

Pb % Change Zn % Change Cu % Change Ni % Change Ba % Change (ppm) (ppm) (ppm) (ppm) (ppm) extraction extraction extraction

696 614 701 613 220 203

at CC#1 extraction extraction extraction

237 256 556 466 189 164

Average % change at CC#2

12 13 8

496 375 335 262 90 86

11 –8 16 13 7

25 22 4

58 26 35 20 16 13

17 400 295 361 273 100 89

26 24 11 20

The Pb, Zn, and Cu concentrations in soils along I-75 in Cincinnati are higher than the ones measured along I-37 in Corpus Christi (Table 3). This might be expected because of the difference in daily traffic volumes between the two cities, about 160,000 per day on I-75 near Cincinnati and about 45,000 per day for the Corpus Christi sites. By contrast, Ni and Cr concentrations were similar for both cities and show little relationship to other variables. We conclude that Ni and Cr in the soils of both cities are controlled by parent material, whereas the concentrations of Pb, Zn, Cu, and Ba are controlled largely by anthropogenic processes. A similar association of Ni+Cr with parent material and Pb+Zn+Cu with human activity was reported by Wilcke et al. (1998) for urban soils of Bangkok. For the two downtown sites, Cincinnati and CC#1, we see two strong trends: (1) as depth increases, the heavy metal concentration decreases sharply; (2) as organic C content of the soil increases, heavy metal contamination also increases. CC#2 follows these trends for Zn and Ba, but not for Pb and Cu. Comparing the mobility of the heavy metals in Corpus Christi and Cincinnati, using the results of the first three steps of the sequential soil extraction process, showed that the potential for mobility of Pb is very low for both sites. Cu is higher than Pb in mobility for both sites. For Zn, however, the mobile proportion percentage is appreciably higher in the Corpus Christi samples than in the Cincinnati samples (Table 3). Consistent trends with depth in the degree of extractability were not found for either Corpus Christi or Cincinnati. One of the key agents that would hold these heavy metals in place was thought to be swelling clays in the soils. Although the amount of smectite in Corpus Christi soils is slightly higher than in Cincinnati soils, 28 versus 23% of the clay-sized fraction, there is no change in smectite amount with depth or distance from the roadway, so the differences in the amounts of metals and their extractability that are seen with depth and distance cannot be attributed to differences in the clay component of the soil. Our results from both Cincinnati and Corpus Christi indicate instead a much stronger role for organic matter, based on generally strong correlations of metal amounts to % organic C. The sequential soil extraction procedure, however,

55 41 18

26 32 29 29 56 48

38 35 24 31 19 15 11

30 39 23

–22 –1 14

1045 945 901 782 444 345

–3 34 37 32 24 26 22

31

–10 24 17 10

10 13 22 15

648 662 639 556 335 297

–2 13 11 7

shows that very little Pb and Zn and modest amounts of Cu are mobilized by the NaOH step, which is designed to liberate normal soil organic matter. For the Cincinnati samples, we found that this step did not in fact solubilize the organic matter as expected. Only about 15% of the organic carbon in the samples was removed (Turer et al. 2001). Thus the organic matter in these roadside soils is dominated by a component that is not extracted by conventional techniques. The discovery that the majority of the organic carbon is not in fact extracted shows that much of this carbon is in a more refractory form than is normal in uncontaminated soils. The similarity in behavior of the two Corpus Christi sites to each other and to the Cincinnati sites suggests that non-point highway-related sources are responsible. The two likely sources for this insoluble organic matter are vehicle exhaust emissions (Kleeman et al. 2000) and asphalt paving materials (Faure et al. 2000). The Pb and Zn, and to some extent the Cu, in these roadside soils appear to be largely concentrated in this insoluble organic fraction. A test of the importance of this organic matter is given by the difference in extractability of metals for the two cities. Cincinnati, with soil organic C at 3.33%, shows a higher retention of Pb and a much higher retention of Zn and Cu than does CC#1 with 1.45% organic C (Table 3). CC#2, with 1.42% organic C, is essentially identical to CC#1 except for lower Cu extractability. The results of the sequential extractions suggest that metals, especially Pb, are relatively immobile in roadside soils. An estimate of how much metal has been remobilized can be obtained knowing the amount of excess Pb in the roadside soils and the amount of Pb coming from vehicle exhausts. To calculate the amount of lead coming from vehicle exhaust, 40 lg Pb per meter of roadway per day per vehicle (Ward et al. 1975) was used for the years before 1970 when leaded gasoline use was widespread. For the years after 1970 when Pb usage was gradually declining, Pb emission rates were obtained from the equation y ¼ 0:001933568x4 þ 0:6557864x3 82:71838x2 þ 4591:503x 94423:44 where x is year since 1900 expressed as 70, 71, 72, ... and y is the average Pb exhausted in tonnes (Turer et al.


241

Fig. 3. Organic C versus heavy metals and Ba at CC#1

2001, their Fig. 6). The 1998 vehicle counts, which are bidirectional, were divided by 2 to get 1998 traffic volumes. Previous years back to 1963 were calculated using the known average rate of traffic increase of 2.5% per year. Using this approach the amount of Pb coming from exhausts of the vehicles before 1998 was calculated as 3 kg/m at CC#1, and 2.5 kg/m at CC#2. The excess amounts of heavy metals and Ba in the soil are calculated for a 1 m·6 m·0.2 m volume (which approximates the area sampled that had values above background). First, the weighted averages of heavy metals and Ba were calculated for each sampling point down to the depth of 0.2 m and then multiplied by the area defined by the half

distance between sampling points and 1 m along the roadside. The highest excess amounts of the heavy metals and Ba come from CC#2 except for Pb, which was highest for CC#1 (Table 4). The results showed that about 40% of Pb coming from vehicle exhaust remains in the soil at CC#1 and 28.4% for CC#2. The 40% value calculated for CC#1 is very close to the one calculated for Cincinnati soils, which was 43%. The difference between the amount of Pb exhausted and the excess amount of Pb in the soil can be attributed to removal of Pb as wind-blown dust or carriage of primary exhaust more than 250 m from the road (Ward et al. 1975). Another possibility is surface run-off carrying Pb into surface


242

Fig. 4. The distribution of heavy metal and Ba concentrations with respect to depth and distance from the road (CC#2)

drainages, bypassing the soils (Sansalone and Buchberger 1997). Comparing results of all the analysis done for soils from the two different locations in Corpus Christi, it can be seen that CC#1 has a very similar heavy metal concentration profile in the soil to the one in Cincinnati soils and also a very similar retention of Pb. This could be because of both sampling sites being very close to downtown and being away from any secondary contamination source. The CC#2 site is also similar in many ways to Cincinnati, but does not show the rapid fall-off with distance from the roadway that is reported in almost all studies of roadside soil contamination. In addition, the relationship to organic

matter is weaker at CC#2 than for CC#1 or Cincinnati. Such differences could be a result of nearby industrial activity overprinting the normal pattern of vehicular contamination. A survey of residential soils in the northern section of Corpus Christi by the Texas Natural Resources Conservation Commission (unpublished data on file in the Corpus Christi office) shows some areas of anomalous metal enrichment, as high as 1000 ppm Pb. For residences in the immediate vicinity of the Upriver Road intersection, the maximum found was 200 ppm and the average about 60 ppm, similar to the numbers we found for the refinery entrances. Zn averaged 130 ppm and reached a high of 240 ppm, again similar to our numbers


243

Fig. 5. Organic C versus heavy metals and Ba at CC#2

Table 3. Organic and clay contents compared

Location

Cinti CC #1 CC #2 Cinti CC #1 CC #2

Traffic (thousands/day)

% Clay

156 48 45

33.7 21.4 20.2

% Expandable Clay

% Organic carbon

Highest metal values (ppm)

7.7 6.0 5.7

3.33 1.45 1.41

1980 1430 440 820 390 180 650 360 90 Percent easily extractable 9 5 31 11 17 38 7 20 31

Pb

Zn

Cu

Ni

Cr

Ba

90 70 50

140 140 220

720 470

24 –3 10


Table 4. Excess metals in roadside soils compared to exhaust emissions (values expressed as kg of metal per m of roadway)

Pb

244

CC#1 Excess (kg/m) 1.2 Exhaust (kg/m) 3.0 CC#2 Excess (kg/m) 0.71 Exhaust (kg/m) 2.5 Cincinnati Excess (kg/m) 4.9 Exhaust (kg/m) 11.5

Zn

Cu

Ni

Cr

Ba

0.34

0.13

0.02

0.06

0.49

0.72

0.16

0.03

0.37

0.58

2.2

1.2

0.13

0.40

heavy metals at both sites decreases with depth, but increases with increasing organic C. There is also a decrease in heavy metal contamination with distance from the roadway. For CC#2, the relationship to organic matter was weaker, and the decrease with distance from the roadway was not observed. We conclude that the similarity in behavior of CC#1 and Cincinnati is related to contamination coming dominantly from gasoline-powered vehicles, whereas the CC#2 site receives a considerable contribution from nearby refineries or from heavy diesel-powered vehicles going to and from the refineries. Sequential extraction of metals from the Cincinnati and Corpus Christi soils shows that most of the Pb and Zn and much of the Cu is tightly bound, mostly to an insoluble form of the organic matter in the soil. Consequently, it is unlikely for these heavy metals to be remobilized into solution. From a policy perspective, the risk to public health from dissolved heavy metals coming from roadside soils and entering either groundwater or surface water is very small. The health risk from particulates derived from these soils is not, however, trivial. One likely hazard that has been recognized is transport of metal-bearing particulates to nearby water bodies. We believe that a greater, unrecognized hazard is dust generated by maintenance operations. It is possible that a serious health risk from dust inhalation exists for maintenance and construction workers along urban highways. Disruption of these soils during highway maintenance, including mowing and excavation for resurfacing, will cause suspension of these heavily contaminated soils as small dust particles in the air. Studies are needed of the metal content of the dust created by reconstruction and by mowing to assess the seriousness of this risk. Other policy implications from our study are that the strong correlation between heavy metals and organic matter only applies to the pure non-point source case. Soils in which there is no or only a very weak association should be suspected of having an additional point-source contribution of metals. Furthermore, the highest Ba values in this study were found along streets in the immediate vicinity of petroleum refineries, so we suggest that Ba be routinely sought as a tracer of petrochemical contamination. Finally, Ni and Cr appear not to be anthropogenic in origin and could be de-emphasized in future work.

and considerably higher than background. Based on these relatively high values for Pb and the other heavy metals, we conclude that the nearby refinery operations have an indirect effect on the roadside soils because of much heavier than normal truck traffic on these streets. Trucks emit more metal contamination than cars (Al-Chalabi and Hawker 2000). Also Kleeman et al. (2000) found that particles emitted from gasoline-powered vehicles consist mostly of organic compounds (largely in the form of polynuclear aromatic hydrocarbons), whereas particles emitted from diesel engines contain roughly equal amounts of these organic compounds and elemental carbon, which would be chemically inert in the soil. This difference in the structure of the organic matter in the vehicle emissions could be the reason for the weaker correlations of heavy metals to organic C found at CC#2. Very few other studies of roadside soil contamination have included Ba with the heavy metals, yet its common occurrence in geologic environments with Pb and Zn (e.g. Maynard 1983) suggests that it could well have a similar behavior to these metals in soils. We found the depth and distance trends for Ba to be similar to those for Pb and Zn, as was the relationship to % organic C. In addition, the covariance with Pb reported by Monaci and Bargagli (1997) was also found for Corpus Christi. The correlation coefficients between Ba and Pb concentrations for CC#1 and CC#2 are 0.78 and 0.57. Thus Ba should be routinely sought in studies of roadside contamination in addition to the more usual heavy metal suite. By the same token, it would seem reasonable to exclude Cr and Ni from such studies, because they seem always to be dominated by non-anthropogenic processes. Ba is particularly characReferences teristic of oil field operations (e.g. Fisher 1995), and so its Al-Chalabi AS, Hawker D (2000) Distribution of vehicular lead in presence in roadside soils in high concentrations may roadside soils of major roads of Brisbane, Australia. Water Air Soil Pollut 118:299–310 prove to be a good indicator of contamination from Biscaye E (1965) Mineralogy and sedimentation of recent deep-sea petroleum refining or waste disposal. clay in Atlantic Ocean and adjacent seas and oceans. Geol Soc Am

Conclusions Heavy metal contamination in soils from two different localities with different styles of urban development along I-37 near Corpus Christi, Texas, is very high in the top 15 cm of the soil compared to local background values for Pb, Zn, Cu, and Ba. In contrast, Ni and Cr appear to be controlled by the original soil material rather than by anthropogenic sources. Soils from CC#1 are very similar to soil samples taken along I-75 near Cincinnati, Ohio. Both sites are close to the city center. The concentration of

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