Encyclopedia
Urban soils might be defined as soils within the administrative boundaries of municipalities or settlings, respective a territory of settlement and natural production, including rests of soils in young cities.
- urban soils
- roadside dusts
- trace metals
- metal mobilities
- platinum group metals
1. Soil Function Changes from Human Civilizations
The common approach of calculating accumulation of contaminants down the profile by comparing concentrations of a sequence of layers, cannot be used with urban soils, because they are often mixed, and contain additional extraneous materials. Estimating a city’s influence on soil contamination is made by comparing obtained data either with legislative limits, or with background values obtained from natural comparable soils [1].
Long-term changes in soil properties occur where the capacity of soil to retain contaminants is exceeded and discrete metal-mineral phases are formed [2]. The long-term input of metals to the soil can result in decreasing buffering capacity of the soil and to groundwater contamination [3].
The continuing cycle of construction, use, and renewal of urban structures leads to far higher rates of change in urban environments than it is common elsewhere [4].
23. Metal Contaminations
23.1. Metal Sources
Soil serves both as a sink and a source for trace metal contaminants in the terrestrial environment. Anthropogenically deposited metals may remain in urban soils for centuries. Main anthropogenic sources of urban dust are combustion processes (heating in winter, cooking), industrial emissions, weathering of building structures and streets, waste disposal, and traffic. Some metals can form volatile oxides and halogenides during combustion [5], leading to condensation aerosols till deposition and adsorption at dust particles. Filter systems reduced hazardous emissions from coal—and oil-fired power plants, gasworks, metallurgical, chemical and electronic plants, and motor vehicles [6].
Metal emission patterns close to roads are a function of driving activities, like deceleration and acceleration, speed, traffic density, traffic lights and roundabouts. Roadside soils are constantly loaded with both organic matter and metals [7].
In Palermo, fertilizers enhanced P-levels about four to five fold with respect to natural background soils, and additional input of Hg-Pb-Cu-Zn was indicated by significant correlations with P [8].
Urban soils have been found enriched versus mean upper crust values for Ag, Cd, Hg, Pb, Sb, Sn and Zn, particularly at historic sites, and also occasionally As, B, Co, Cu, Mo and Ni. Particularly in case of Cr, but also for V, Mn, and Ba, the data depend on the digestion method, and should not be taken to a table without this additional information. In addition, sieving to finer fractions leads to higher concentrations of pollutant metal inputs, at least for stream sediments, or as condensation aerosols, whereas coarser particles often can be traced to rock abrasions [9].
23.2. Levels Met in Roadside Dusts
Road dust is a complex mixture of particles from organics, metals, other inorganics, mold spores, pollen, animal dender etc. Its sources are traffic, industrial activities, powerplants, fossil fuel burning, waste incineration, construction and demolition activities, and resuspension [10]. Topsoils and particularly roadside dusts in urban areas are indicators of airborne particulate matter metal pollution. Most trace metals settle down as surface dust from atmospheric depositions before full incorporation into the soil matrix. Thus, the extent of atmospheric contamination may be better revealed by road dust than by soils [11][12].
Road dust is a complex mixture of particles from organics, metals, other inorganics, mold spores, pollen, animal dender etc. Its sources are traffic, industrial activities, powerplants, fossil fuel burning, waste incineration, construction and demolition activities, and resuspension [90]. Topsoils and particularly roadside dusts in urban areas are indicators of airborne particulate matter metal pollution. Most trace metals settle down as surface dust from atmospheric depositions before full incorporation into the soil matrix. Thus, the extent of atmospheric contamination may be better revealed by road dust than by soils [91,92]. Though this data compilation refers to deposited dust only in terms of mg/kg, the amount of re-suspendable particulate matter should be considered in terms of inhalation risk assessment, in particular the grain-size fractions below 2.5 µm aerodynamic diameter (PM2.5
) and below 10 µm (PM10
). In the US, the PM2.5
has been estimated at about 12% of the total dust, unless detailed investigations are available. Therefore, a detailed study of grain-size distributions was done in various roadside environments in Albuquerque NM, Atlanta GA, Birmingham AL, El Paso TX, Los Angeles CA, New York NY, and adjacent New Jersey, after sampling by a high-volume sampler and separation by a cyclone dust trap. The average composition of paved road dust from industrial sites differed from other urban regions by containing a larger fraction of elemental carbon and less-iron crustal material (Al-Ca-Si), due to coking facilities or high local emissions of trucks, trains and ships. There was slightly more diversity among urban regions than among urban site types. The portion of silicates + carbonates, as well as the PM2.5 content, was greatest in arid regions. Among 51 species determined by XRF and ion chromatography of water-solubles (detailed data not given), nine variables were identified as most important in distinguishing samples: Cr, Ni, Al, Cl, Ca, Si, Fe, Cu, and nitrate. Paved road dust had a higher content of potentially bioreactive metals and higher elemental C at urban than at rural sites [13].
content, was greatest in arid regions. Among 51 species determined by XRF and ion chromatography of water-solubles (detailed data not given), nine variables were identified as most important in distinguishing samples: Cr, Ni, Al, Cl, Ca, Si, Fe, Cu, and nitrate. Paved road dust had a higher content of potentially bioreactive metals and higher elemental C at urban than at rural sites [93].Contrary to direct air dust sampling, deposited dust yields integrated contamination values over a long period of time, usually back to the last rainfall or street cleaning action. In addition to chemical toxicity, however, some particles promote catalytic reactions at their surface, which is not traceable by chemical analysis only. Direct health implications are due to the inhalable fraction, which is rather floating in the air than depositing at surfaces. Deposited dust directly imposes health hazards via consumption of unwashed food, particularly for babies who like to lick around, for dogs which like to lick and sniff at dusty surfaces, and finally the dust-washout might harm green plants in parks, as well as river systems [14].
Contrary to direct air dust sampling, deposited dust yields integrated contamination values over a long period of time, usually back to the last rainfall or street cleaning action. In addition to chemical toxicity, however, some particles promote catalytic reactions at their surface, which is not traceable by chemical analysis only. Direct health implications are due to the inhalable fraction, which is rather floating in the air than depositing at surfaces. Deposited dust directly imposes health hazards via consumption of unwashed food, particularly for babies who like to lick around, for dogs which like to lick and sniff at dusty surfaces, and finally the dust-washout might harm green plants in parks, as well as river systems [94].Permanent changes in the technologies for house construction, traffic and heating during cold seasons, as well as changes and improvements in the purification of industrial emissions, change the composition of urban dust on a long term. Within a first step, representative sampling grids have to be established to find the hotspots of contaminations, prior to rather laborious and expensive size-fractionated sampling and microlocal analysis. Contrary to direct air dust sampling, deposited dust yields integrated contamination values over a long period of time, usually back to the last rainfall or street cleaning action [15].
Permanent changes in the technologies for house construction, traffic and heating during cold seasons, as well as changes and improvements in the purification of industrial emissions, change the composition of urban dust on a long term. Within a first step, representative sampling grids have to be established to find the hotspots of contaminations, prior to rather laborious and expensive size-fractionated sampling and microlocal analysis. Contrary to direct air dust sampling, deposited dust yields integrated contamination values over a long period of time, usually back to the last rainfall or street cleaning action [3].The extent of contaminant input from traffic activities is generally determined by comparing concentration data of road deposited sediments with those of nearby uncontaminated soils [16]. Comparisons between road dust samples from these largely different urban areas might reveal common global urbanization effects, and specific rather local influences. This can be ascertained from additional data per gram dust, available from other densely populated areas. The effect of urbanization might be traceable from differences between road dust samples from Budapest and from more rural Hungary [17].
The extent of contaminant input from traffic activities is generally determined by comparing concentration data of road deposited sediments with those of nearby uncontaminated soils [36]. Comparisons between road dust samples from these largely different urban areas might reveal common global urbanization effects, and specific rather local influences. This can be ascertained from additional data per gram dust, available from other densely populated areas. The effect of urbanization might be traceable from differences between road dust samples from Budapest and from more rural Hungary [59].Vehicular traffic contributes to dust emission by release of tire and brake pads wear, releasing fibers (like copper, steel, potassium titanate, glass, organics), fillers (barium and antimony sulfate, Mg and Cr oxides, ground slag), lubricants, and abrasives. However, street dust can also contain up to 60% of particles originating from soil, like quartz, feldspars, clay minerals, chlorite and muscovite. Whereas main sources of Zn and Fe are tire wearing, brake wearing release in non-asbestos brake pads reaches up to 15%, containing Ba, Cu, Fe, Pb, Zr and Sn. Ni and Cr can originate from corrosion of cars. Technogenic magnetic particles from high temperature combustion processes have characteristic spherical shape, while those from traffic emissions and iron smelting form irregular non-spherical aggregates [18].
Vehicular traffic contributes to dust emission by release of tire and brake pads wear, releasing fibers (like copper, steel, potassium titanate, glass, organics), fillers (barium and antimony sulfate, Mg and Cr oxides, ground slag), lubricants, and abrasives. However, street dust can also contain up to 60% of particles originating from soil, like quartz, feldspars, clay minerals, chlorite and muscovite. Whereas main sources of Zn and Fe are tire wearing, brake wearing release in non-asbestos brake pads reaches up to 15%, containing Ba, Cu, Fe, Pb, Zr and Sn. Ni and Cr can originate from corrosion of cars. Technogenic magnetic particles from high temperature combustion processes have characteristic spherical shape, while those from traffic emissions and iron smelting form irregular non-spherical aggregates [25]. A compilation of road dust data easily shows that some elements are always enriched versus upper crust mean values, like As, Cd, Hg, Mo, Sb, and Sn. Though levels in road dust may be higher than background soils, Ba, Mn, P, Tl and V were found within the fluctuation of geochemical composition. Largely, but not always higher than upper crust values are Cr, Cu, Pb and Zn, and occasionally higher are Co and Ni.