Understanding Soils in Urban Environments

By Pam Hazelton and Brian Murphy

With an ever-increasing proportion of the world’s population living in cities, soil properties such as salinity, acidity, water retention, erosion and pollution are becoming more significant in urban areas. While these are known issues for agriculture and forestry, as urban development increases, it is essential to recognise the potential of soil properties to create problems for the environment as well as structural concerns for buildings and other engineering works.

Understanding Soils in Urban Environments explains how urban soils develop, change and erode. It describes their physical and chemical properties with a focus on specific soil problems that cause environmental damage, such as acid sulfate soils, and also affect the integrity of engineering structural works. This fully revised second edition addresses contemporary issues, including an increase in the use of green roofs and urban green space as well as manufactured soils in a variety of urban environments.

Understanding Soils in Urban Environments provides a concise introduction to all aspects of soils in urban environments and will be extremely useful to students in a wide range of disciplines, from soil science and urban forestry and horticulture, to planning, engineering, construction and land remediation, as well as to engineers, builders, landscape architects, ecologists, planners and developers.

• Language: English
• Publisher: CSIRO PUBLISHING
• Release date: Nov 1, 2021
• ISBN: 9781486314034

Pam Hazelton

Dr Pam Hazelton has been a practising soil scientist for more than 35 years. She graduated in Science from the University of Sydney and gained her PhD for her work on semi-arid soils from the University of NSW. She was a consultant to the Soil Conservation Service, a soil surveyor in the Department of Conservation and Land Management and has worked with environmental consultants. She has been a lecturer at a variety of universities and in the Faculty of Engineering and IT at the University of Technology Sydney specifically focused on the environmental and engineering problems of urban soils. She is a former President of Soil Science Australia and also Vice President of the International Union of Soil Scientists Commission for Education in Soil Science.

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Soils in an urban environment

Introduction

Soil assessment and management have generally focused on agriculture, horticulture and forestry, but more recently have given increased attention to urban soils, no doubt as a consequence of the increasing urbanisation of the world’s population. In 1950 globally only 30% of the world’s population lived in urban areas. By 2016 urban population had increased to 54% with a projection of 66% in urban areas in 2050, with some nations having 80 to 90% of the population living in urban areas (Burghardt et al. 2015; Lal and Stewart 2018). Soil underlies most of our cities and urban hinterlands. Hence this shift in population density and change in land use increases the necessity to understand and manage the functions of soil properties to adequately sustain day-to-day urban activities as the land surface is sealed by buildings, concrete, carparks or low-density urbanisation (Prokop et al. 2011; Gardi et al. 2015; Hendawy et al. 2019; Wadduwage et al. 2017).

In the urban environment soils can be in their natural condition or be slightly disturbed, completely disturbed or, in some cases, manufactured, as in human-made landscapes (Leake and Haege 2014; Wessolek and Toland 2017). Soils and these properties can be modified when the land use is changed, whether from a more or less natural state in a rural environment to an urban condition. Disturbed soils especially differ from soils formed in situ because their natural profile layers have been mixed, destroyed or removed. Under human influence, natural soils may be buried under fill, or chemical and waste materials may have been added. These activities can result in large changes in the physical, chemical and engineering properties of the natural soils at a site. Subsoils can be exposed or mixed with topsoils and compaction also may have occurred. Sealing the soil surface can lead to changes in the soil function (EC 2012).

The natural properties of the soils, or the changes to their properties, can determine whether the soils are able to carry out the physical, chemical and biological, hydrological, environmental and engineering functions required for the urban environment. Failure of the soil to function as expected can have significant environmental and economic consequences on the urban landscape, resulting in serious impacts, including damage to buildings and roads, land slip, poor water quality, soil contamination, dryland salinity and degraded ecosystems (Scheyer and Hipple 2005; Hicks and Hird 2007).

This book is an introduction to understanding the importance of fundamental soil properties, their functions and their behaviour in a variety of urban environments.

Nature of soils

Soil sometimes occurs as weathered in situ material derived from bedrock. In other situations, it comprises materials that have been transported to or from sites by various agents, including water, wind, gravity, ice and – as frequently occurs in urban areas – human activity.

The soil and its properties result from the interaction of chemical, physical and biological activities. Soil is a unique resource and a distinctive identifiable part of the environment. The soil type is influenced by environmental factors, including the parent material from which it is derived, vegetation, climate, topography and availability of water. Soil is, in effect, the ultimate interface between the geosphere, the atmosphere, the hydrosphere and the biosphere (Rimmer 1998; Scheyer and Hipple 2005; Lal and Stewart 2018). The effects of human impact on soils are linked to the way in which the soil is used and the land managed (Scheyer and Hipple 2005).

Soils in the urban environment

Soil lies beneath the feet of urban dwellers – it nourishes gardens and parklands, supports building foundations, underlies transport corridors and is often used as a sink for effluent and waste disposal (see Figures 1.1 and 1.2 and Table 1.1). It is the medium in which vegetation grows, often to remediate sites which have been scarred by change in the landscape (Hazelton 2006). A variety of professions require urban soils data: urban and land use planners; engineers; builders and architects; urban farmers and park, forest and habitat managers; and stormwater and water quality managers (USDA 2019).

Urban soil is an important ecological asset (Sauerwein 2011) and a key component of the urban ecosystem and the natural infrastructure of urban land (Scheyer and Hipple 2005; Hecht and Sanders 2007). Soils affect urban ecosystem functions by:

•absorption of rainfall to prevent flooding and runoff – the urbanisation of rural land can significantly increase runoff because of larger paved, sealed and roofed areas;

•absorption and filtering of effluent and pollutants such as oil-based pesticides, herbicides and heavy metals, thereby preventing them from entering water supplies – the concentration of human activity increases the local output of effluent and pollutants, which increases the potential for contamination;

•nourishment of gardens, parklands and sporting grounds – in urban areas there is an increased need for revegetation growth for recreation and environmental rehabilitation; soils store and supply both water and nutrients for use by plants;

•natural habitat protection of conservation areas, especially endangered ecological communities

•the use of soils as foundation materials for buildings and transport corridors.

The impact of urbanisation on the environment is so critical that the extent of impervious surfaces and urban/suburban developed land are key indicators of the health of water and terrestrial ecosystems (Hecht and Sanders 2007). Although knowledge of soil management has always been essential in rural environments, it is equally important, or even more, with the global growth in urban population to understand and manage soils in an urban environment for sustainable environmental management (see Figure 1.3).

Figure 1.1: The distribution of activities across an urban landscape, indicating the range of soil functions in an urban environment.

Figure 1.2: The interaction of soils with other aspects of the urban environment, including the human and biophysical environments (after Bridgeman et al. 1995).

Table 1.1. Soil interactions with an urban environment

Figure 1.3: The impacts of urbanising soils. Note the large areas of bare soil, burial of the existing soil materials, exposure of subsoils, the digging of ditches, placement of subsoils on top of surface soils, burial of pipes and service access lines in the soil, and the sealing of the surface by concrete and bitumen.

The specific pedogenic processes and characteristics of urban soils are closely related to the history of a city and its hinterland (Sauerwein 2011). Natural, artificial, cultural and social environments determine the dominant features of soils in urban areas

In many regions, urban areas have expanded into rural hinterland, including valuable horticultural land such as vegetable gardens and flower gardens. These once-productive areas have been transformed into residential, commercial and industrial sites. The waste from these sites has often been disposed of in the soil. It is important to identify these remaining areas of land suitable for and capable of agriculture and conservation before the planning of a development (Hazelton 2018).

Types of urban soil materials

Some urban soils can be highly variable, as the degree of ground disturbance varies with urban land uses. There are often human-made soils (Pouyat et al. 2007), Anthroposols (Isbell et al. 2016) or Technosols (Schad 2018; Charzynski et al. 2017) (see Table 1.2). Therefore, features from both traditional agriculture and modern urbanisation can be observed in the soil of urban areas and cities.

Soil classification systems have identified different urban soil materials. For example, the Australian Soil Classification system (Isbell et al. 2016) has identified different materials within the Anthroposol soil order on the basis of the types of materials and the processes used in its formation. Included are Hortic, Garbic, Urbic, Dredgic, Spolic and Scalpic soil materials (see Table 1.2). The Australian classification also recognises the need to consider the possibility of toxic soil materials occurring. Likewise, the internationally accepted World Reference Base (Schad 2018) has recognised potential urban soil materials that contain significant amounts of artefacts such as bricks, pottery, glass, crushed or dressed stone, wooden boards, industrial waste, garbage, processed oil products, bitumen, mine spoil and crude oil. Technic hard material is also considered diagnostic and examples of technic hard material are asphalt, concrete or a continuous layer of worked stones. As for the Australian classification, the WRB recognises a range of soil materials unique to urban soils (see Table 1.2). The need to recognise soil contamination and toxic soil materials is accepted, but the problems of setting specific critical limits for toxicity prevents the definition of specific values for toxicity limits.

Table 1.2 Some classifications or categories of disturbed soil materials in an urban environment

Soil Taxonomy (Soil Survey Staff 2014) also recognises specific soil materials for urban soils. Included are buried materials with the potential to produce methane, including garbage, wood-mill pulp and sewage plant products (Methanogenic soil materials); layers with large pieces of asphalt (Asphaltic soil materials); layers with large pieces of concrete (Concretic soil materials); layers with large pieces or amounts of synthetic gypsum products including plaster (Gypsifactic soil materials); layers with coal combustion by-products and fly ash (Combustic and Ashfactic soil materials); layers with pyrolysis by-products (Pyrocarbonic soil materials); layers dominated by non-soil artefacts (Artefactic and Pauciartefactic soil materials); layers dominated by human transported material that was water transported (Dredgic soil materials); layers dominated by human transported material (Spolic soil materials); and layers that are comprised of soils that have been mechanically mixed and reorientated (Araric soil materials).

It is clear that in describing urban soil materials it is necessary to identify the original soil material, the processes to which the original soil material has been subjected and the nature of any materials that have been added to the soil.

Some of the specific changes to soils following urbanisation are summarised by Stroganova et al. (1997) and shown in Table 1.3.

Urban land use is often intense and can have a high environmental impact. This is illustrated in Figure 1.1 and in Tables 1.1 and 1.3. Urbanisation alters soils in dramatic ways. Urban land uses alter plant nutrients; chemicals and heavy metals affect soil physical properties in unique ways. Restoring urban soils to their initial condition is either impossible or requires very different methods from those used in non-urban environments (Pavao-Zuckerman 2008).

Table 1.3. Changes in environmental functions of urban soils