Rainfall runoff from urban roadways often contains elevated amounts of heavy metals in both particulate and dissolved forms (Sansalone and Buchberger, 1997). Because metals do not degrade naturally, high concentrations of them in runoff can result in accumulation in the roadside soil at levels that are toxic to organisms in surrounding environments. This study investigated the accumulation of metals in roadside soils at a site for which extensive runoff data were also available. For this study, 58 soil samples, collected from I-75 near Cincinnati, Ohio, were examined using X-ray fluorescence, C-S analyzer, inductively coupled plasma spectroscopy, atomic absorption spectrometry and X-ray diffraction. The results demonstrated that heavy metal contamination in the top 15 cm of the soil samples is very high compared to local background levels. The maximum measured amount for Pb is 1980 ppm (at 10-15 cm depth) and for Zn is 1430 ppm (at 0-1 em depth). Metal content in the soil falls off rapidly with depth, and metal content decreases as organic C decreases. The correlation to organic C is stronger than the correlation to depth. The results of sequential soil extraction, however, showed lower amounts of Pb and Zn associated with organic matter than was expected based on the correlation of metals to % organic C in the whole soil. Measurement of organic C in the residues of the sequential extraction steps revealed that much of the carbon was not removed and hence is of a more refractory nature than is usual in uncontaminated soils. Cluster analysis of the heavy metal data showed that Pb, Zn and Cu are closely associated to one another, but that Ni and Cr do not chow an association with each other or with either organic C or depth. ICP spectroscopy of exchanged canons showed that only 4.5% of Pb, 8.3% of Zn, 6.9% of Cu and 3.7% of Cr in the soil is exchangeable. Combined with the small amounts of metals bound to soluble organic matter, this result shows that it is unlikely that those contaminants can be remobilized into water. At this site, clays are not an important agent in holding the metals in place because of low amounts of swelling clays. Instead, insoluble organic matter is more important. Mass balance calculations for Pb in soil showed that most of the Pb came from exhausts of vehicles when leaded gasoline was in use, and that about 40% of this Pb is retained in the soil. This study shows that, highway environments being a relatively constant source of anthropogenic organic matter as well as heavy metals, heavy metals will continue to remain bound to organic matter in-situ unless they are re-mobilized mechanically. Removal of these heavy metals as wind-blown dust is the most likely mechanism. Another possibility is surface run-off carrying the metals into surface drainages, bypassing the soil. This study also shows that for those countries still using leaded gasoline, important reductions in Pb contamination of soils can be achieved by restricting the use of Pb additives.
Mvua ya mvua kutoka barabara za mijini mara nyingi ina kiasi kikubwa cha metali nzito katika aina zote mbili za chembechembe na zilizomwagika (Sansalone na Buchberger, 1997). Kwa sababu metali haziharibiki kwa asili, viwango vya juu vya metali katika maji ya maji yanaweza kusababisha mkusanyiko katika udongo wa kando ya barabara kwa viwango ambavyo ni sumu kwa viumbe katika mazingira ya karibu. Utafiti huu ulichunguza mkusanyiko wa metali katika udongo wa kando ya barabara katika tovuti ambayo data kubwa ya mtiririko pia ilikuwa inapatikana. Kwa ajili ya utafiti huu, sampuli 58 za udongo zilizokusanywa kutoka I-75 karibu na Cincinnati, Ohio, zilichunguzwa kwa kutumia X-ray fluorescence, C-S analyzer, inductively coupled plasma spectroscopy, atomic absorption spectrometry na X-ray diffraction. Matokeo ya utafiti huo yanaonyesha kwamba uchafuzi wa metali nzito katika sehemu ya juu ya sampuli za udongo ni wa juu sana ikilinganishwa na viwango vya asili vya eneo hilo. Kiasi cha juu cha Pb ni ppm 10 na ppm 10 katika kina cha cm 10 na kwa Zn ni ppm 14 katika kina cha cm 1. Kiasi cha chuma katika udongo hupungua haraka kadiri kina kinavyozidi, na kiasi cha chuma hupungua kadiri C ya kikaboni inavyozidi kupungua. Uhusiano na C ya kikaboni ni nguvu kuliko uhusiano na kina. Hata hivyo, matokeo ya uchimbaji wa udongo wa mfululizo yalionyesha kiasi cha chini cha Pb na Zn inayohusishwa na vitu vya kikaboni kuliko ilivyotarajiwa kulingana na uhusiano wa metali na asilimia ya C ya kikaboni katika udongo wote. Kupima C ya kikaboni katika mabaki ya hatua za uchimbaji wa mfululizo ilifunua kwamba kaboni nyingi hazikuondolewa na kwa hivyo ni ya asili ya moto kuliko kawaida katika udongo usio na uchafuzi. Uchambuzi wa data ya chuma nzito ulionyesha kwamba Pb, Zn na Cu zina uhusiano wa karibu, lakini Ni na Cr hazina uhusiano na kila mmoja au na C ya kikaboni au kina. ICP spectroscopy ya canons kubadilishana ilionyesha kwamba tu 4.5% ya Pb, 8.3% ya Zn, 6.9% ya Cu na 3.7% ya Cr katika udongo ni kubadilishana. Pamoja na kiasi kidogo cha metali iliyounganishwa na vitu vya kikaboni vinavyoweza kuyeyuka, matokeo haya yanaonyesha kuwa haiwezekani kwamba uchafuzi huo unaweza kuhamishwa tena ndani ya maji. Katika eneo hili, udongo si wakala muhimu katika kushikilia metali mahali kwa sababu ya kiasi kidogo cha udongo kupanuka. Badala yake, vitu visivyoweza kuyeyuka vya kikaboni ni muhimu zaidi. Utafiti wa Pb katika udongo ulionyesha kwamba Pb nyingi zilitoka kwa kutolea nje ya magari wakati mafuta ya mafuta ya mafuta yalipokuwa yakitumiwa, na kwamba karibu 40% ya Pb hii inahifadhiwa katika udongo. Utafiti huu unaonyesha kwamba mazingira ya barabara kuu ni chanzo cha kudumu cha vitu vya kikaboni vya binadamu na metali nzito, na metali nzito zitaendelea kushikamana na vitu vya kikaboni in situ isipokuwa zinahamishwa kwa njia ya mitambo. Uondoaji wa metali hizi nzito kama vumbi linalopeperushwa na upepo ndio utaratibu unaoonekana zaidi. Uwezekano mwingine ni maji ya juu yanayopeleka metali hizo kwenye mifereji ya juu, bila kuingilia udongo. Utafiti huu pia unaonyesha kwamba kwa nchi ambazo bado zinatumia petroli yenye risasi, kupunguzwa kwa kiasi kikubwa kwa uchafuzi wa Pb wa udongo kunaweza kupatikana kwa kuzuia matumizi ya nyongeza za Pb.