The anthropogenic sealing of soils in urban areas

Landscape and Urban Planning 90 (2009) 1–10

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Landscape and Urban Planning journal homepage: www.elsevier.com/locate/landurbplan

Review

The anthropogenic sealing of soils in urban areas Riccardo Scalenghe a,∗ , Franco Ajmone Marsan b a b

DAAT Università degli Studi di Palermo, 13 viale delle Scienze, 90128 Palermo IT, European Union DIVAPRA Università degli Studi di Torino, 44 via Leonardo da Vinci, 10095 Grugliasco IT, European Union

a r t i c l e

i n f o

Article history: Received 11 December 2007 Received in revised form 3 September 2008 Accepted 2 October 2008 Available online 9 December 2008 Keywords: Water cycle Gas transfer Urban sprawl Urban soils Impact Europe

a b s t r a c t The sealing of soils by impervious materials is, normally, detrimental to its ecological functions. Exchanges of energy, water and gases are restricted or hampered and an increasing pressure is being exerted on adjacent, non-sealed areas. The negative effects span from loss of plant production and natural habitats to increased floods, pollution, and health risks and consequently higher social costs. Environmental Agencies produce periodical reports where the phenomenon of soil consumption by urban infrastructures is monitored with extremely sophisticated geographical tools but little specific research is available that describes the effects of soil sealing. This paper reviews some recent contributions in terms of definition, phenomenology, and conceptual and empirical modeling approaches to artificial soil sealing with a special focus to urban areas of Europe. The works about the effects of soil sealing on soil functions are then considered, in particular those that affect the energy transfer, water and gas movements and the biota. Soil sealing is also examined as a tool for protecting some environmental compartment from contamination. In general, porosity, color, geometry of the materials used in the sealing of soils, the quality of sealed soil and aspect ratio of urban infrastructures are key aspects in preserving soil functions. © 2008 Elsevier B.V. All rights reserved.

Contents 1. 2. 3.

4. 5. 6.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Geography of soil sealing in Europe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effects of sealing on soil functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Impact on energy transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Impact on water movement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Impact on gas diffusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Impacts on biota . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reducing soil sealing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Voluntary sealing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusive remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Soils perform a number of crucial functions which make them environmentally, economically and socially important. Which soil functions can be distinguished? The production, the carrier, the filter, the resource, the habitat, and the cultural function are usually recognized. Some of these functions are exclusive and in competition (EC, 2006). In earlier times, with less available technology,

∗ Corresponding author. E-mail address: [email protected] (R. Scalenghe). 0169-2046/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.landurbplan.2008.10.011

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land use was largely determined by the functions that could be performed by the natural soil. This relation between soil functions and land use has been lost to a certain extent in the course of the last century because of technological developments. Consequently, the cover of land is not simply an attribute but a concrete set of features that result largely from its use (Bouma, 2006). The use of land implies that almost always different and contrasting interests that span from agriculture to recreation, from infrastructure construction to environment preservation are in competition. A given land cover can be modified, consumed or degraded and a new type generated. As such, the consumption and formation of land cover is very similar to the transformation of capital goods. Since land can-

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Fig. 1. The sealing of soil. (I) Stop. Stamping produces compaction and erosion of the soil. Collaborate for his recuperation (Perito Moreno Glacier, Argentina, 50◦ 29 S 73◦ 03 W). (II) Anthropogenic soil sealing. (a) Temporary sealing: in agriculture as protective cover used for various purposes: to adjust soil temperature, to retain water, to control erosion to control weeds and to repel insects, (a variety of natural and synthetic materials are used); (b)–(d) permanent sealing: asphalt concrete for roads and urban surfaces (asphalt is separated from the other components in crude oil by fractional distillation). District heating pipelines: (c) the transmission of warm gases or oils through buried alters natural soil temperature regimes. System for distributing heat from a cogeneration plant in a centralized location for residential and commercial heating. The global length of pipelines is estimated to amount to almost 1.5 Mkm (Certini and Scalenghe, 2006). Amenities: (e) Olympic ski competition jump (length 140 m) in Pragelato, (45◦ 00 27 N 6◦ 56 20 E) and (f) bobsleigh facility (length 1760 m) in Cesana (44◦ 57 09 N 6◦ 48 31 E) Italy, EU. Destroying sealing: (g) the seafront skyline of Bari, had been dominated by the illegally built

not, in general terms, be created or destroyed (with the exceptions of coasts and polders), land cover change can generally be characterized in terms of different types of flows between land cover types. A key focus of land cover accounts is the understanding of the way in which the stocks of different land covers and uses are transformed over time in the connection between human societies and ecological systems (e.g. Kareiva et al., 2007; van Kamp et al., 2003; Various Authors, 2008). One area of needed knowledge is the sealing of soils, where human infrastructures and activities interrupt the connection of the soil with other ecosystem compartments. Soil seal (Duley, 1939) refers to a thin layer which limits infiltration through the (wet) soil. Various causes have been identified that can lead to the impermeabilisation of the soil surface and they include the loss of structure due to the impact of rain or soil laboring, the dispersion of colloids, the compaction. All causes impact the porosity of the soil by either reducing its amount or by modifying its pattern. The modification of macropore patterns negatively influences water infiltration, as they are fundamental in determining the rate of water intake in the soil (Bouma, 1992; Rousseva et al., 2002). The natural sealing is commonly followed by transport and deposition of detached particles (Panini et al., 1997; Singer and Shainberg, 2004; Singer, 2006). A plethora of studies on soil compaction have demonstrated that the main human activities that are responsible for soil compaction are agro-forestry, because of the large areas they affect (Van den Akker and Canarache, 2001). Contrary to natural sealing, artificial sealing is generally extensive and permanent, and entails a modification of the neighboring ecosystems (e.g., Burghardt, 2006). The significance of sealing must be extended to describe the covering of its surface by impervious materials such as, for example, concrete, metal, glass, tarmac and plastic. Soil sealing is then a common consequence of urbanization and infrastructure construction (Fig. 1). In Europe, the sprawling nature of many cities is critically important because of the consequent increased energy and soil consumption (Fig. 1, III). A report of the European Environment Agency describes the detrimental effects of expanding urban areas on the environment and an impact analysis carried out for the European Commission suggests that soil degradation may cost up to US$ 56 billion per year (EEA, 2006). The degree of sealing is related to the type of land use and to the population density. Commonly, populations exceed the carrying capacity of their ecosystems, and experience a rapid decline until conditions for growth are restored. Estimates for the next 50 years indicate that Humankind is tending to a global density of 1 person for each 0.01 km2 of reasonably biologically productive land (Certini and Scalenghe, 2006). Changes in the size of human population as well as changes in the activity of sectors such as transport and tourism may lead to urban expansion and infrastructure construction. This may not be solely the result of an increase in population but it can be the result of a change in behavior (as with urban sprawl, more extensive urban patterns are preferred). As a consequence, a certain amount of land is consumed and built-up areas increase (due to the lack of more

complex known as Punta Perotti (41◦ 06 57 N 16◦ 54 14 E). After years of legal struggling, the court ruled in favour of its demolition on 2006, April 2nd. (III) European urban areas. Urban Morphological Zones (UMZ) changes 1990–2000 as defined by Corine land cover classes considered to contribute to the urban tissue and function. UMZ expansion or shrinking areas (white). Black areas indicate no data or areas outside coverage. The land cover classes are: (i) continuous urban fabric, (ii) discontinuous urban fabric, (iii) industrial or commercial units, (iv) green urban areas, (v) port areas, (vi) airports and (vii) sport and leisure facilities. Enlarged core classes are (a) road and rail networks and (b) water courses, when neighbours to the enlarged core classes, cut by 300 m buffer. Scale of the data set: 1:100,000 [Courtesy of European Environmental Agency, web: www.eea.europa.eu].

R. Scalenghe, F. Ajmone Marsan / Landscape and Urban Planning 90 (2009) 1–10 Table 1 Component affected, effects, timing and consequences of the sealing of soils. Where symbols indicate: short-term (); medium-term ( ); and long-term (  ) effects.

Heat

Water

Effect

Time

Consequence

Decreased radiation absorption



More reflective surfaces



Heat island (HUI)



Reduced chemical reactivity Less filtering action Cracking Loss of biomass Diminishes the natural recharge of aquifers

Less infiltration

    More runoff

    

Barrier for perched water table

Gas

Biota

Landscape



Increased water through adjacent areas Increased ponding time Probability of anaerobiosis Transfer of contaminants Increased risk of flash-floods



Increased risk of anaerobiosis Release of contaminants



Risk of anaerobiosis



Partial trapping



Reduced biodiversity



Reduced carbon sink

HUI



Thermal specialization

Increased wind erosion



Increased air-borne particulate

Increased water erosion



Increased erosion of adjacent areas

Uniformity

  

Reduced aesthetic appeal Reduced visual appearance Reduced attractiveness

Reduced/interrupted exchanges

Loss of plant cover/biomass

precise information, the built-up areas increase is used as a proxy to quantify the land taken by urban expansion although, by definition, built-up areas also include land which is not actually sealed) at the expense of other types of land use (EEA, 2001). The soil being covered will no longer be able to perform the range of environmental functions associated with it as it will be separated from the other environmental compartments (EEA, 2001) (Table 1). In addition, the proximity of unsealed areas to pollution sources such as vehicular traffic expose them to pollution (e.g., Biasioli et al., 2006; Scalenghe and Fasciani, 2008; Wolf et al., 2007). Positive effects of sealing are less obvious. Sealing landfills, waterproofing adobe, mitigating radionuclides contamination or the effects of magnetic fields are examples. The preservation of cultural heritage is another example. Ancient Troy, previously known only through the text of Homer (?)’s Iliad, or Pompeii were kept safe by the sealing. In Europe, a decade after the Swiss legislation on soil protection (Swiss Confederation, 1998), an effort has been made by the European Commission (2006) that released the Thematic Strategy for Soil Protection, in which soil sealing is identified as one of the threats. To have an idea of the amount of existing information on this subject one can put the words ‘soil’ and ‘sealing’ into

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GoogleTM . But as over two millions of pages are received back, to tackle the problem of overload in searches, Web 2.0 use the so-called ‘clustering’. A new search with one of these search engines (e.g., Clusty) over 400 clustered results is retrieved. The first seven categories are ranked in the following order (largest first): Soil Surface > European > (Soil) Protection > Method/Patent > Workshops > Stabilization > Land. This confirms that, in Europe, the threat of soil loss by impermeabilisation attracts nowadays more attention than the engineering aspects. This study presents an overview of the current issues of artificial soil sealing in Europe and reviews the recent literature on the threat that sealing poses to the conservation and functioning of the soil resource in an attempt to identify the current approaches to the problem and to highlight the needs of research. 2. Geography of soil sealing in Europe Europe is a continent where three quarter of its population live in cities of small and medium-sized. The European territory is characterized by 1595 functional urban areas (FUAs) and 76 Metropolitan European Growth Areas (MEGAs) have been identified in 29 countries (ESPON, 2006). There is a dense urban structure in the central part of Europe, stretching from the UK via the Netherlands, Belgium, western Germany and northern France, and continuing into Italy, Czech Republic, South Poland, Slovakia and Hungary (customizable interactive cartography at www.espon.eu). Scenario analysis reveals that by 2030 the configuration of the EU in terms of urbanization will not be significantly altered even though new Member will join the Union (ESPON, 2007). On the average the sealed area, the area of the soil surface covered with an impermeable material, is around 9% of the European area. The increase in artificial areas in the last decades was not due to increase in population in most of the countries, but rather to changes in population behavior (shift from an intensive to an extensive urban pattern: suburbanization). Kasanko et al. (2006) analyzed the relationship between urban land use development and population density in 15 European urban areas (Fig. 2). They

Fig. 2. Population growing and build-up areas expansion. A comparative perspective into the population and built-up areas growth in 15 European urban areas from the mid-1950s to the late 1990s (modified from Kasanko et al., 2006). Filled symbols are Porto, PO, (grey) and Palermo, IT (black) where built-up areas have grown much faster than the average, the growth has not been accompanied by equally rapid population growth. This is dramatically evident in Palermo, where during post-war the Mafia gradually infiltrated the building trades and bought their way into most government-run agencies. During the 1970s, the Mafia, albeit often indirectly, built nearly half of the new (sic!) city of Palermo by obtaining hectares of building permits (Seindal, 1998).

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Fig. 3. Humankind seals soils for shelter, transportation and other connected activities. The global area sealed by impervious surfaces probably exceeds 500,000 km2 (Elvidge et al., 2007), an area larger than the whole France. The explosion of constructed impervious surfaces depends largely on the sprawl of urban areas. The literature on sprawl is ample (e.g., Burchfield et al., 2006; Hasse and Lathrop, 2003; Peiser, 1989; Torrens, 2008; UNEP, 2008). Here we show an example of urban sprawl in the EU: the expansion of Torino (45◦ 04 N 7◦ 41 E) from XVII century, through the epoch of Camillo Benso Conte di Cavour (after the cartographies of Blaeu, Borgonio Chiapasio, and Seutter), to the present day. The city of Torino (top right) is surrounded by the Alps to the N and W and by hills to the SE. Four major rivers pass through the city: the Po and its tributaries, the Dora Riparia, the Stura di Lanzo, and the Sangone. Contemporary toponyms: (a) Corso Francia (towards Bordeaux, France), (b) Via Paolo Sacchi, (c) Via Nizza (towards Nic¸a, France). The area settled in pre-Roman times, became a Roman military camp in the first century bc. At that time the city Augusta Taurinorum reached some 5000 inhabitants. It upraised to 100,000 during the XVIII century when became the principal theatre for the legitimization of the Savoy dynasty (Pollak, 1991). In our time, the metropolitan area of Torino, including Rivoli, Grugliasco and Collegno, hosts more than 2.2 millions of residents (the whole region approximates 4.3 millions) [Courtesy of Archivio di Stato, web: www.archiviodistatotorino.it].

used five indicator sets are to characterize urban sprawl (Fig. 3): (i) built-up areas, (ii) residential land use, (iii) land uptake, (iv) population density and (v) urban density. They found that built-up areas have grown considerably in all studied cities with the most rapid growth between 1950s and 1960s. The annual growth pace has slowed down in the 1990s to 0.75%. In Germany it is estimated that 52% of the soil in built-up areas is sealed (EEA, 2006). In Vienna (48◦ 12 N 16◦ 34 W), EU, it is estimated that, compared to 1 m2 of sealed surface for a pedestrian, a biker needs 7.7 m2 person−1 , public transport between 12 and 17.6 m2 person−1 and a car driver 60 m2 person−1 (EEA, 2002). Regions such as Mediterranean coastal areas have experienced 10% increase in soil sealing during the 1990s. Along the Mediterranean coast 3% of farmland was urbanized in the 1990s (EEA, 2006). Sprawling cities tend to consume the best agricultural lands, forcing agriculture to move to less productive areas or to upland locations (Nizeyimana et al., 2001) (Fig. 4). The expansion of the city of Guadalajara (40◦ 30 N 3◦ 10 W), EU, has led to the consumption of the richest soils along the Henares river (García Rodríguez and Pérez González, 2007). Outside Europe, the city of Nanjing (32◦ 03 N 118◦ 06 E), China, has expanded at an annual rate of seven percent between 1984 and 2003. Over the total occupied area the soils of the first and sec-

ond quality class exceeded 60% (Zhang et al., 2007). Amundson et al. (2003) conducted a quantitative analysis of disturbed and undisturbed soil distribution in the USA using a GIS-based approach. They find that a sizable fraction (4.5%) of the USA soils are in danger of substantial loss. Unsealed spaces will play a crucial role in supporting urban ecosystems, a fact recognized in public policy commitments. A study in Amsterdam (50◦ 22 N 4◦ 53 E), EU, has confirmed the positive effects of city parks on the sustainability of the urban ecosystem and on the general well-being of the population (Chiesura, 2004). A Europe-wide assessment of access to green space reported that all citizens in Brussels, Copenhagen, Glasgow, Gothenburg, Madrid, Milan, and Paris live within 15 min walk of urban green space (Stanners and Bourdeau, 1995). On the other hand, 64% of Sheffield (53◦ 23 N 1◦ 28 W), EU, households fail to meet the recommendation of the regulatory agency English Nature, that people should live no further than 300 m from their nearest green space (Barbosa et al., 2007). 3. Effects of sealing on soil functions The effects of soil-sealing on the major environmental component concern the kinetic of chemical reactions and the exchange of

R. Scalenghe, F. Ajmone Marsan / Landscape and Urban Planning 90 (2009) 1–10 Fig. 4. Human Footprint in Europe. Sanderson et al. (2002) defined human influence, Human Footprint (HF), through geographic proxies. They utilised four types of data: population density, land transformation, accessibility, and electrical power infrastructure. HF average scores (±sd) sorted by European countries: Range is 0–100, world global mean is 28 [Modified from Sanderson et al. (2002). Courtesy of Wildlife Conservation Society, web: www.wcs.org]

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water, gas, particles, and energy between the soil and other environmental compartments thereby affecting the proper functioning of the soil (Effland and Pouyat, 1997; Flores et al., 1998; Kaye et al., 2006; Pickett and Cadenasso, 2008).

In cold regions, soil sealing by roadbeds has the effect of insulation (Zhang and Wang, 2007). Perennially frozen layers (permafrost) can thaw with marked negative ecological and technical consequences.

3.1. Impact on energy transfer

3.2. Impact on water movement

The sealing of the soil with exogenous materials can have a great influence on the heat exchange with the atmosphere (Table 1). The thermal properties of a soil vary with soil type and its moisture content (Sandholt et al., 2002). Because heat conducts very slowly in soils, soil temperature anomalies on shorttime scales in the topsoils are released to the atmosphere via surface exchange and cannot be distributed to deeper layers. Only persistent anomalies in the surface heat budget can propagate to deep soil layers and affect temperature variations in those layers (Hu and Feng, 2004). Temperature affects the chemical processes of organic molecules adsorption and desorption onto mineral surfaces as well as aggregate formation, which in turn physically protects organic compounds (Sollins et al., 1996). Among others, the turnover of organic matter can be modified as a significant fraction of relatively labile organic matter is subject to temperature-sensitive decomposition (Davidson and Janssens, 2006). The temperature in a sealed soil would depend on the albedo, emissivity, and thermal properties of the sealing material and is generally higher than in a non-sealed soil. Also, the thermal conductivity decreases as temperature increases (Sakaguchi et al., 2007). The sealing of soils under cities would in turn modify the local climate, leading to even higher temperatures. The differentiation of the city climate is most evidently manifested in the increase of temperature of the air close to the sealed soil relative to the air temperature outside the city. This phenomenon is called the urban heat island, UHI (Howard, 1833) and has additional causes in the thermal properties of surface materials and the waste heat from air conditioning, industry, and other sources also contributes to the UHI. Many urban surfaces such as roadways and roofs have a relatively low albedo (reflectivity to solar radiation). While some of the radiation reflected from these surfaces leaves the urban environment, much of it is intercepted and partially absorbed by other urban surfaces. As a result cities tend to have lower effective albedos than their surroundings. Takebayashi and Moriyama (2007) found that the daytime temperature of a cement surface, a surface with gray paint, a bare soil surface, a green surface and a surface with white paint was in descending order. Measured solar reflectance were 0.17, 0.15, 0.37 and 0.36, respectively, for soil, roofs (Munsell: hue green), concrete (Munsell: 2 < value <6, 2 < chroma < 1) and highly reflective surfaces (Munsell: value > 6, chroma < 0). In München (48◦ 6 N 11◦ 42 E), EU, Bründl and Hoppe (1984) reported that annual heating degree days were 14% lower in a central urban area compared to a suburban one. In Łódz´ (51◦ 71 N 19◦ 40 E), EU, the intensity of the UHI (T in ◦ C) is as much as 12 ◦ C and depends on the building development compactness (Kłysik and Fortuniak, 1999). Giridharan et al. (2004) and Ali-Toudert and Mayer (2007) revealed that non-sealed soil is among the most critical variables to mitigate UHI with respect to urban design. While Hardin and Jensen (2007) demonstrated that, in an urban area, for every unit increase in leaf area surface temperature decreased by 1.2 ◦ C. Weng et al. (2007) have detected a higher surface temperature for impervious surfaces in the city of Indianapolis (39◦ 47 N 86◦ 8 W), USA, indicating that unsealed soils are essential for temperature regulation. Xiao and Weng (2007) in China (26◦ 08 –41 N 105◦ 51 –106◦ 52 E) revealed that a change in land use towards urban impervious surfaces brought about an increase of air temperature in the Guizhou province.

Water flow in a porous system such as the soil is generally described by Darcy’s law, which consists of the gradient driving the water flow and the resistance or conductivity respectively, which controls the permeability. Several factors affect the hydraulic conductivity and can influence both the hydrological and the mechanical behavior of the soil (Baumhardt et al., 1990; Bonsu, 1992; Giakoumakis and Tsakiris, 1991; Assouline, 2004). The most obvious observation is that artificial sealing of the soil surface makes it impermeable to water flow (Table 1). This, together with the thermal impacts described above, implies that the water regime of the underlying soil is severely altered. In addition to the general decrease in soil moisture content, there is a lowering of water tables in urban areas and this would, in turn, decrease the rate of chemical reactions. In the case of a perched water table, however, the ponding time would be longer and the risk of anaerobiosis higher. The sealing of a surface has evident consequences also on the neighboring areas. It increases the amount and the speed of runoff that arrives on the unsealed surface thus increasing the risk of ponding and erosion in unsealed areas. Despite high rainfall in many temperate cities, the water budget is so altered that much less water is readily available to plants and for the base flow of streams. Bhaduri et al. (2001) found a linear relationship between the size of impervious area and average annual runoff later confirmed by Assouline and Mualem (2002). Haase and Nuissl (2007) have conducted a study on the city of Liepzig (51◦ 20 N 12◦ 23 E), EU, and found that surface runoff had more than doubled in the city area between 1940 and 2003 due to the increase of impervious surfaces. These authors found a corresponding decrease in the overall evapotranspiration from the soils of the urbanized area. Simulation studies in USA by Choi and Deal (2007) confirm the increase in surface flow and sediment runoff under a population growth scenario and an increase of impervious surface area. In Leeds (53◦ 47 N, 1◦ 32 W), EU, Perry and Nawaz (2008) calculated a 12% increase in surface runoff as a result of a 12.6% increase in soil paving. Intensive impermeabilisation of urban surfaces, increasing the amount and speed of runoff and exerting a greater pressure on the sewerage system, can also seriously increase the risk of flooding (Natale and Savi, 2007). The increase of impervious surfaces can have consequences also on the quality of water, due to the reduced filtering capacity of the soil (Bhaduri et al., 2001; Gaffield et al., 2003). Although Brun and Band (2000) had reported a threshold of 20% impervious cover for a dramatic change in runoff, Conway (2007) reported that water characteristics such as pH and salinity are affected when as little as 2% of the surface is sealed. In addition, the pollutants load on the impervious surface is discharged onto the contiguous, unsealed soil. Pollutants tend in fact to accumulate on impervious surfaces and are then washed off with rain events (Hope et al., 2004). An accumulation of metals and other contaminants in highway runoff was observed by Kayhanian et al. (2007) in various locations in the USA. A survey in Korea (36◦ 21 N 127◦ 23 E) revealed that the first flush after a rain event affected contaminant constituents in the stormwater runoff in the order: suspended solids > organics > nutrients (Kim et al., 2007). A recent investigation on urban land use change and its impact on the water quality in the Tampa Bay urban area (27◦ 45 N 82◦ 31 W), USA, showed a strong association between most pollutant loadings and

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the extent of impervious surface (Xian et al., 2007). Contaminants tend to accumulate in finer particles (Ajmone-Marsan et al., 2007; Herngren et al., 2005; Vaze and Chiew, 2002). Nehls et al. (2006) investigated samples taken from pavements adjacent to roads in Berlin (52◦ 31N 13◦ 25 E) and Warsaw (52◦ 15 N 21◦ 00 E), EU. They found that the volume of the nanometer-size pores depends almost solely on the amount of deposited carbon, while the millimetersize pores are mainly influenced by the grain sizes of the sand fraction. The volumes of the micrometer-size pores seem to be related to both above-mentioned factors. Therefore, the seam materials have different physical properties and ecological functions, e.g., an improved water-storage capacity compared to the original sandy seam filling. The available water capacity increases by 0.05–0.11 m3 m−3 , as compared to the original sandy seam filling. 3.3. Impact on gas diffusion The state of gaseous components in the soil is driven mainly by the biological activity and by dissolution and sorption mechanisms (Smagin, 2006). Temperature plays always a crucial role. Land-use change is also an important driver of soil–atmosphere gas exchange (Table 1). Kluitenberg et al. (1991) conducted an experiment in which the air permeability of two soil materials was measured at a series of soil matric potentials after being sealed with rubberized asphalt or paraffin. The results indicated that the asphalt was able to maintain a better seal than the rigid paraffin as the soil materials dried and shrank. Seal integrity of the asphalt was maintained as matric potentials were reduced from −50 kPa to −750 kPa for soils of moderate and high shrinkage capacity. In contrast, the air-permeability data indicate that the paraffin failed to provide an adequate seal at −50 kPa, even before the soil was desorbed. Wiegand and Schott (1999) studied the influence of a sealed soil on soil–gas migration by using 222 Rn as a natural tracer, focusing on anthropogenic sealing by asphalt or concrete. On the basis of their findings from measuring at locations with various degree of imperviousness they proposed a model for the influence of soil sealing on soil–gas migration (Fig. 5). Accumulation of tracer gas is maximum at low depth when the degree of sealing of the surface is 99% but at a low degree of sealing the meteorological

Fig. 5. Gases and soil sealing. The model for the intensity and range of the influence of soil sealing on soil–gas migration in the dependence on the degree of sealing. The intensity of influence of sealing is high in the black areas and decrease proportionally. The dotted line sorts the region of influence of sealing from the region of dominating meteorological conditions [Modified from Wiegand and Schott, 1999. Courtesy of Società Italiana di Fisica, web: www.sif.it].

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conditions predominate and accumulation of Rn is observed below 0.5 m. Methane is more effective than CO2 in trapping the thermal radiation reflected by the Earth. Its emissions from soils range from 0.02 to 200 mg m−2 d−1 (Smagin, 2000). Among the gaseous products of the nitrogen cycle, nitrogen monoxide (N2 O) constitutes the highest environmental hazard (Smagin, 2000). Unsealed patches within a urbanized region of the USA, Great Plains, occupied 6.4% of a 1578-km2 area, but contributed up to 5% and 30% of the soil CH4 consumption and N2 O emission (Kaye et al., 2004). These small portions of the landscape sandwiched between sealed surfaces may contribute significantly to regional soil–atmosphere gas exchange. Urban soils appear to play a role, although to a lower degree with respect to their rural or natural counterparts, as a sink for carbon. Urban soils have the potential to store a considerable amount of carbon, particularly in arid climates, although this may not be significant in terms of atmospheric CO2 enrichment (Kaye et al., 2006). Urbanization, however, may induce large decreases in carbon pools, as the soils beneath impervious surfaces contain less carbon and have a reduced C sequestration capacity (Pouyat et al., 2006). 3.4. Impacts on biota The condition of urban soils underpins the functioning of urban ecosystems. The impact of soil sealing on a landscape is a function of the original composition of the natural soil but it always results in biodiversity loss. Urbanization is in fact considered a key factor of biological homogenization. Native ecosystems, when sealed under cities, are replaced by pavement and buildings and what is left of the natural soil is covered with green areas, which are often dominated by non-native ornamental species (e.g., Pauchard et al., 2006). Urbanization is primarily fragmenting large areas, extending its influence over the entire landscape. The effect of this fragmentation occurs at all spatial scales and affects all organisms. Fragments of natural vegetation may be too small or even too isolated to support some species. Savard et al. (2000) divided the concerns about biodiversity into three groups: (i) those related to the impact on adjacent ecosystems; (ii) those dealing with how to maximize biodiversity within the partially sealed ecosystem and (iii) those related to the management of undesirable species within the ecosystem. Temperature increases related to UHI causes changes in plant phenology (Neil and Wu, 2006) shifting towards earlier springtime flowering in urbanized areas compared to surrounding rural areas (Roetzer et al., 2000; Wilby, 2006). UHI and elevated soil and air temperatures have been exerting an evolutionary pressure on organisms. Fungi that inhabit the soil are useful subjects for assessing the evolutionary consequences of anthropogenic sealing for the thermal physiology of ectotherms. Fungi cannot thermoregulate, and are thus subject to the full range of temperatures observed in soil. McLean et al. (2005) measured thermal reaction norms of chitinolytic fungi and found differences between urban and rural isolates suggesting that thermal specialization and countergradient variation in the fungal community. If genotypes that are specialized for growth at high temperatures are absent within a population, selection for faster growth at high temperatures might produce counter-gradient variation instead. In Adelaide (34◦ 55 S 138◦ 35 E), an isolated city in South Australia with a Mediterranean climate, the metropolitan area has a significant diversity of both native and introduced flora and fauna (Tait et al., 2005). There has been a dramatic change in species composition, with an increase in total species numbers of 30% (plants by 46%). More than a 100 native species of plants and animals have become locally extinct, and a minimum of 600 introduced species has arrived. Fifty percent of the native mammal species were lost, birds where

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totally replaced by new species, while amphibians and reptiles showed no net change. A less obvious impact of the sealing could be on the propagation of seismic waves. The amplification of these waves in some specific sites can be very important. Scattering of waves near the surface, at layers interfaces, often strengthen the consequences. The thickness of the surface layer, its mechanical properties, its general shape as well as the seismic wave type involved have a great influence on the maximum amplification and the frequency for which it occurs. The liquid present in the pores plays an important role (Crampin, 1987). Soil sealing, by trapping water, may lead to a local amplification of seismic motion. Numerical analysis of seismic wave amplification in Nice (43◦ 42 N 7◦ 16 E), EU, confirms a significant amplification, between 1 and 2 Hz, in the highly sealed areas (Semblat et al., 2000). Similarities in the distribution of earthquakes damage in Kobe (34◦ 41 N 135◦ 12 E), Japan, Loma Prieta (37◦ 6 N 121◦ 50 W), USA, and Mexico city (19◦ 24 N 99◦ 7 W), Mexico, were inferred by Lomnitz (1997) to be caused by similarities in emplacing artificial fill on soft mud. 4. Reducing soil sealing In paved areas, semipervious pavement systems could be adopted to limit the consequences of sealing. They are constructed to be highly conductive by using a material which is weak in retention, but its upper layer, the dark seam material, which is rich in organic carbon, may act as a filter. Weaknesses are that with increasing age, the original seam filling becomes less conductive due to accumulations of different materials (foliage, dust, oil, etc.). So, compared to impervious soil sealing, (e.g., concrete or tar) seams allow at least little exchange between the sealed soils and their environment including gas exchange, water infiltration, and solute fluxes (Nehls et al., 2006). In unpaved areas, the application at the soil surface of rock fragment cover could protect soil against sealing by raindrop impact (Meyer et al., 1972; Kochenderfer and Helvey, 1987), especially if the size is fine-medium (Valentin, 1994). The consent on these is not diffused, some others reported a negative consequence (Blackburn, 1975; Casenave and Valentin, 1992). Other systems, adopted from agricultural techniques are amendments (e.g., gypsum; Norton et al., 1991; Singer and Shainberg, 2004), shallow tillage (e.g., disrupting seals and returns infiltration rates to pre-crusted levels; Singer and Warrington, 1992). 5. Voluntary sealing There are different types of voluntary sealing of soil: (i) physical, (ii) chemical and (iii) biological. Physical soil sealing is the clogging and plugging of soil pores to reduce the hydraulic conductivity. Chemical sealing is the changing in the soil structure caused by chemical reactions. Biological sealing is the clogging of soil pores with microbial products or by-products. Sealing landfills. Soil chemical treatments based on the dispersion and flocculation of clay minerals, as well as the mobilization and precipitation of humic compounds (e.g., Smith and Fey, 1996). Temporary storage of manure. Manure cannot be applied on frozen land it must be stored for several months in many countries as thousands million ton are produced yearly. Without a proper storage, ground water quality becomes a key concern. As a low cost technique, a bacterial culture (Bacillus licheniformis) can produce a non-viscose water insoluble polymer to be used as a plugging

agent of the pores of high permeability soils (Ghaly et al., 2007). Driving tunnels. Ground freezing could be chosen as an initial support method during construction of tunnels. The main objective of ground freezing is to provide a strong support structure to resist soil and groundwater pressures. This is normally achieved by creating a watertight, frozen-ground barrier (Woodward, 2005). To freeze the soil, freezing processes involve either liquid nitrogen, carbon dioxide, chilled calcium chloride brine. These methods are employed often in combination with other soil stabilization measures. Increasing water-holding capacity. A layer of asphalt placed below the plant root zone (half a meter in depth) in coarse textured soils can increase their water holding capacity. Marked increase in yield with certain high value crops on droughty sand soils with the asphalt barrier was documented by Hansen and Eickson (1969). Waterproofing adobe. Water-repellent treatments of uncooked soil bricks with natural fibers. In these cases, key factors are: (i) the adhesive properties of the applied substance with fiber and soil, (ii) the water-repellent property of substance and (iii) the low cost of the material (e.g., Ghavami et al., 1999). Mitigate contamination. Antifiltration ‘wall-in-the-soil’ techniques aimed at the protection of groundwaters and the underground runoff of water bodies from possible metals and organics or radioactive contamination. For instance, when combating the consequences of the Chernobyl nuclear plant accident (Onishi et al., 2007). Recycling by-products. Hydrophobized clays deriving from production of mineral oils could be used as waterproofing material (Lyakhevich et al., 2006). Apparently safely. Electromagnetic fields. A hidden and little known anthropic influence on soils is that of magnetic fields (e.g., Scalenghe, 2007). Application of waterproofing plasticizers that operate under external electric fields can significantly improve their dielectric properties when additives are chlorinated paraffins (Novakov et al., 2007). Expansive soils ‘slackening’. Different remedies were used to meet the Vertisol geotechnical challenge. Moisture change causes the expansive soil to swell or shrink. Minimizing the moisture change and the volumetric change is minimized. One material being used is the geomembrane (e.g., Steinberg, 2000).

6. Conclusive remarks The future of most part of Humankind will be urban and sealing will go along with urbanization at a scale unprecedented in human history. Is the solution Eutropia or Olinda, invisible cities envisioned by Italo Calvino (1972)? Eutropia, made up of many cities, all but one of them empty, and that its inhabitants periodically travel to the next; Olinda that contains the Olinda-yet-to-be in embryo which, as a meristem, grows out. In our visible cities, it is impossible to think a development of the Humankind separate from urban growth. And it would be unrealistic to go ‘back to the future’ of unpaved roads. The sealing of soil can lead to decrease of water permeability, in the loss of biodiversity, and in the reduction of the capacity for the soil to act as a carbon sink. Although it is evident that soil sealing has a strong impact on the soil resource, little direct evidence was found in the literature that can help quantify its influence. We were able to collate a remarkable amount of research that deals, mostly indirectly, with the effects of sealing on some ecosystem parameters but a major research effort is needed to identify some key ecological points. The alteration of the carbon cycle, and the related CO2 emissions, the effects on water quality, the potential increase of desertification and salinisation are but the main effects of soil sealing that require an adequate quantitative approach.

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