WHY URBAN GEOCHEMISTRY?
PHOTO
W. Berry Lyons1 and Russell S. Harmon2
1811-5209/12/0008-0417$2.50
DOI: 10.2113/gselements.8.6.417
I
n a very short period of time, the majority of the human population has
become urban, and by 2050 two out of every three people in the world
will live in cities. Urban areas are extremely important socially, economically, and culturally, but they also have a profound impact on the environment. In that context, this issue of Elements considers the geochemical
significance of 21st-century cities and some of the unprecedented challenges
they face.
growth in this century will take
place in developing countries and,
as a consequence, many of the new
urbanites are, and will be, poor
(FIG. 1).
An annual urban growth rate of
1.7% is projected, so that by 2050
roughly 67% of the world’s human
population will reside in urban
KEYWORDS : megacities, urban geochemistry, hydrogeochemistry, urbanization,
settings (UN 2012). It is also
sustainable cities, urban areas, population growth, geochemical change
expected that, by 2025, over a
billion people will be living in
INTRODUCTION
cities with populations of 5 million or more. Megacities—
Large towns and cities disturb the natural landscape upon
urban centers with populations greater than 10 million—
which they develop, modifying watershed hydrologic will have increased from only two in 1970 to 37 by 2025,
response, altering soil character, and consequently affecting
with Africa and Asia urbanizing at the fastest rates (UN
the fate and transport of elements in urban areas. Large 2012).
cities are also “heat islands” that, among other impacts,
increase energy demands, affect water quality, cause air
pollution, are prime generators of ozone, produce greenhouse gas emissions, and disturb adjacent atmospheric
A
behavior in a way that alters downwind precipitation
patterns (Landsberg 1981). Urban pollution has multiple
origins, for example, the burning of fossil fuels, industrial
and manufacturing activity, human and industrial waste
disposal, and vehicular traffic. Urban pollutants may be
transported beyond the urban setting and can readily
impact geochemical cycles on the local, regional, and even
global scale. In addition, urban environments have
different attributes than other land-use types, and these
differences affect the fate and transport of elements/
compounds in urban areas.
The development of cities extends back more than 4000
years, with younger cities often built upon older precursors,
but high-density urbanization is a relatively new global
phenomenon (Bettencourt et al. 2007). Over the course of
the 20 th century, the world’s urban population increased
more than tenfold, from 220 million to 2.8 billion, and
today more than half the world’s population lives in urban
areas. The population of urban regions is anticipated to
reach 6.3 billion by 2050 (UN 2012). The focus of this
urbanization has largely shifted from the developed countries, in which the Industrial Revolution of the mid-18th
to mid-19 th century occurred, to Asia, the Middle East,
Africa, and Latin America. The vast majority of urban
1 School of Earth Sciences and Byrd Polar Research Center
The Ohio State University
Columbus, OH 43210, USA
E-mail: [email protected]
2 International Research Office
USACE Engineer Research and Development Center
Ruislip HA4 7HB, UK
E-mail: [email protected]
E LEMENTS , V OL . 8,
PP.
417–422
B
Today, over a billion people live in urban slums that
are typically overcrowded and polluted, and lack basic
services such as clean water and sanitation. (A) Urban slum in
Nairobi, Kenya. PHOTO © MARTIEN VAN A SSELDONK | D REAMSTIME.COM.
(B) Beleghata,Kolkata,India. PHOTO © SAMRAT35 | D REAMSTIME.COM
FIGURE 1
417
D ECEMBER 2012
Manila slum.
BBC
COURTESY OF
With the high concentration of people in urban areas comes
a concomitant high consumption rate of mineral resources,
food, water, and energy. Today, urban centers comprise only
2% of the Earth’s land area but generate some 80% of the
world’s gross domestic product. They are responsible for 70%
of global energy consumption and 80% of CO2 emissions.
This consumption of resources not only produces manufactured products and services but also generates waste material
and pollution, thereby creating contaminated environments
that adversely affect human health while at the same time
negatively impacting ecological integrity and diversity. The
ecological footprint of a city, or the total burden that the
city places on the Earth in terms of agriculture, mining,
water usage, and pollution, can be substantial. For example,
ambient air quality has degraded to the point of causing
serious health effects in 20 of 24 megacities (Mage et al.
1996) and, in 2007, 9 of the 10 most polluted sites in the
world were industrial cities, mostly located in the developing
world (Blacksmith Institute 2007). Urban activities impact
not just the local environment but also far afield of the city
itself. In this introductory article, we draw attention to the
unique environmental character of urban environments,
introduce the concept of urban geochemistry, briefly discuss
its spatial and temporal dynamics, and identify a few
emerging issues in urban geochemistry.
mental settings. Urban geochemistry has recently been
portrayed as a “frontier” area in science (Jartun and
Ottesen 2011).
HYDROGEOCHEMICAL IMPACTS
Urbanization has profound physical, hydrological, chemical, and ecological effects on watersheds (TABLE 1, FIGS. 2, 3),
both outside and within the urban space (Paul and Meyer
2001). These effects include a decline in the natural
removal capacity of aquatic systems and altered sediment
and solute export as a consequence of channelization,
modification of natural flow dynamics, and the loss of
floodplains and wetlands. The hydrologic degradation that
typically accompanies increased watershed imperviousness
has been referred to as the urban stream syndrome (Walsh
et al. 2005), which is primarily a consequence of the expansion of impervious areas as the natural landscape is replaced
by roads, parking lots, buildings, and housing developments. As a consequence, the total area where precipitation
can infi ltrate into the ground is diminished. This, in turn,
reduces infi ltration and surface storage of precipitation
while accelerating the timing and increasing the volume
of surface water runoff, resulting in an increased likelihood
A
Typically, urbanization is accompanied by an increase in
the per capita ecological footprint. For example, a resident
of Beijing has an ecological footprint that is three times that
of the average Chinese person (Hubacek et al. 2009) and,
globally, urban residents are already responsible for more
than 70% of the fossil fuel–related CO2 emissions. But this
need not be the case, because cities with well-planned transportation infrastructure can reduce overall per capita carbon
emissions through collective transport, as is the case in New
York City, where per capita emissions are 30% less than in
the United States as a whole (Dodman 2009).
WHY URBAN GEOCHEMISTRY?
One might ask: Why is the term urban geochemistry necessary
or desirable given that environmental geochemistry is a
well-established field of the Earth sciences? What makes
urban geochemistry unique and different from the environmental geochemistry of other landscape/land-use types?
The simple answer is focus and magnitude. Because of the
very high concentrations of people and the intensity of
economic activity in urban areas, the consequent environmental impact is huge compared to that of most other
human endeavors. The enormity and scale of human impact
and the flux of chemicals into the environment in urban
regions are very large.
The term urban geochemistry was used by Iain Thornton in
the early 1990s. He viewed the subdiscipline as the “interface
of environmental geochemistry and urban pollution” and
as a field that addresses the impact of chemical dispersion
arising from urbanization on both ecosystem integrity and
human health. Geochemical phenomena within the urban
environment profoundly affect the character, distribution,
and dispersion of harmful trace metals, toxic organic
compounds, and human waste. Kaye et al. (2006) may have
been the first to use urban biogeochemistry in a paper, noting
that the then current ecosystem models did not include
human-induced routing of water and other engineered
structures, the replacement of native vegetation through
landscaping, the change from rural to urban habitation, and
the creation of urban impervious surfaces. So the enormity
of the geochemical fluxes and the human manipulation of
the landscape change the nature of transport and retention
processes and make urban areas unlike most other environ-
E LEMENTS
B
The movement of water into a soil depends on landsurface slope, vegetation, degree of surface loading,
and soil texture, structure, density and compaction. More water
moves into the soil zone on natural landscapes compared to urban
landscapes with disturbed soils. These schematic drawings compare
the generalized disposition and movement of incoming water on a
natural plant-covered landscape (A) with that in a disturbed urban
landscape (B) with limited vegetation and abundant impervious
surfaces. MODIFIED FROM SCHEYER AND H IPPLE (2005)
418
FIGURE 2
D ECEMBER 2012
A
B
C
Urban stream hydrologic impacts: (A) Lower Indian
Bend Wash, Tempe, Arizona, USA. (B) Impaired urban
stream adjacent to an industrial site in Erie, Pennsylvania.
(C) Flooding in a parking lot at the University of Nevada Las Vegas
following a thunderstorm on 11 September 2012. M ODIFIED FROM
IMAGES FOUND AT HTTP ://ACTIVETECTONICS .LA . ASU.EDU / IBW / IBWTOUR _
FLOODING _ 02 _ 14 _ 03/ IBW2/ PIC 00001.JPG,
HTTP ://SEAGRANT.PSU.EDU / NEMO / PHOTOS /I MPAIRED %20STREAMS /
U RBANSTREAM A LONGSIDEANINDUSTRIALFACILITY.JPG, AND AP PHOTO /L AS VEGAS
R EVIEW -JOURNAL, JOHN LOCHER
FIGURE 3
of more frequent and more severe flooding. Water quality
in urban environments is impacted through changes in
flow, water temperature, sedimentation, and biological
habitat. Additionally, the rapid transport of chemical
compounds into the aquatic system affects their behavior,
transport, and ultimate fate, thus changing the partitioning of the chemicals from the soil to a stream or retention pond.
Soils can also be strongly impacted in the urban environment. Urban soils usually do not have the classic horizon
development observed in more “natural” settings (Wong
et al. 2006), with rainwater infi ltration reduced as soildensity distributions are modified (Scheyer and
Hipple 2005).
The timing of urbanization, the age of the infrastructure,
population density and its variation through time, as well
as the amount and location of green space can all impact
urban geochemical processes and their ecological consequences. In addition to these attributes, urbanization leads
to landscape patchiness and changes in connectivity
(Grimm et al. 2008). Given the potential impact of urbanization on ecosystems, our knowledge of ecosystem
response to the increase in the number and size of urban
TABLE 1
HYDROLOGIC IMPACTS RESULTING FROM THE INCREASE IN
IMPERVIOUS SURFACES IN URBANIZED AREAS (after Horner et al. 1994).
Resulting Impacts
Habitat loss
Increased
imperviousness
leads to:
Flooding
Increased volume
•
Increased
peak flow
Increased peakflow duration
(e.g. inadequate
substrate,
loss of
riparian
areas, etc.)
Erosion
Channel
widening
Streambed
alteration
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Increased stream
temperature
•
Decreased
base flow
•
Changes in sediment loadings
•
•
E LEMENTS
areas is based on very few studies. Some have argued that
urban areas can be viewed as large point sources of emissions and that continued high fluxes of potentially toxic
elements, such as metals, impact the long-term sustainability of many urban regions (Wong et al. 2006). Others
have assigned urban areas a “metabolism,” which is in part
a measure of a city’s environmental impact (e.g. Kennedy
et al. 2007). Scaling laws have also been developed that
indicate increases in urban populations actually decrease
per capita material usage and, by analogy, decrease waste
production (Bettencourt et al. 2007). Modification of the
environment on a massive scale, large population densities,
and subsequent waste production make urban geochemistry
a meaningful term.
TEMPORAL AND SPATIAL DYNAMICS
OF URBAN GEOCHEMISTRY
The advance of human culture during the Holocene has
had a profound effect on the natural environment. In fact,
some historians have argued that the invention of the
industrial city occurred ~250 years ago and that urbanization is a key theme in world history. Others have defi ned
this time as the beginning of the Anthropocene, or a period
when humans have had a significant global impact on the
Earth’s ecosystem (e.g. Crutzen 2002). The flow of energy
and materials into and out of a fixed unit area has increased,
as the organization of human society has evolved from
nomadic hunting and gathering groups to agrarian communities, then to small manufacturing towns, to more industrialized and populated urban centers, and fi nally to
modern cities and even megacities—megapolitan regions
with urban, suburban, and exurban components. In turn,
waste generated by human activities has become progressively concentrated in and adjacent to cities.
The initial development of urban areas was made possible
by the domestication of grains and stock animals, thereby
leading to land conversion and changes in biogeochemical
fluxes. Increased technological development has led to
larger agricultural yields, which caused both a division of
labor and the separation of manufacturing from agriculture. These changes, in turn, have led to the concentration
of people in cities, progressively higher and higher population densities, and consequent urban growth. Long before
the creation of our modern megacities, cities needed to
maintain an environmental balance in terms of water, air,
and food (Mumford 1956). The largest cities in the 19th
century were less than 1 km 2 in size, but today, cities can
cover thousands of square kilometers or more. As urban
populations grow, they need resources, and these must
come from ever-greater distances. Think of the food supply
of a large city today. Most, if not all, food is grown or
produced at some substantial distance from the city, but
the elements incorporated into the food products and their
packaging are later discharged as waste within the urban
area. This greatly changes biogeochemical cycles. For
example, agricultural produce is taken from a farmer’s field
419
D ECEMBER 2012
and packaged in paper, plastic, or metal produced from
raw material sources far from the city. Subsequently,
residual waste components are redistributed into urban
streams, treatment plants, or landfi lls. Recent research
inventories suggest that humans mobilize ~50% of the total
mass of metals involved in global surficial cycles today
(Rauch and Pacyna 2009). However, ice core and sediment
records indicate that human activities, such as mining,
smelting, and the general utilization of metals, caused
significant mobilization of metals from the Bronze Age
through Roman into medieval times (Hong et al. 1996).
Vehicular traffic can be a major contributor of toxic metal
accumulations in soils and sediments within the urban
environment (Li et al. 2001).
Urban areas are dynamic in both time and space. Population
growth, economic development, and technological change
drive this dynamic. Urban geochemical problems,
processes, and issues have not been static through time
and space, and the scale of geochemical impact has varied.
For example, as urban growth has accompanied industrialization, environmental degradation in terms of air and
water quality has tracked this change (FIG. 4), from the
Industrial Revolution in Britain between the mid-18th and
A
B
(A) Political drawing of the Thames River in 1848 when
London suffered repeated epidemics of cholera and
typhoid caused by dumping increasing amounts of raw sewage and
waste into the Thames. BY PUNCH MAGAZINE (PUBLIC DOMAIN), VIA WIKIMEDIA
COMMONS (B) Firefighters battling a blaze on the Cuyahoga River in
Cleveland, Ohio, in 1952. The river first burned in 1936 when a spark
from a blowtorch ignited debris and oil floating on the river surface.
The river was set ablaze several more times during the following
three decades, including a major fire in 1969 that helped create the
U.S. federal Water Pollution Control Act (1972), commonly known as
the ”Clean Water Act.” IMAGE FROM NOAA (HTTP://OCEANSERVICE.NOAA.GOV/
EDUCATION /KITS /POLLUTION /MEDIA /SUPP _ POL02D.HTML)
FIGURE 4
E LEMENTS
the mid-19th century, to North America from the late 19th
to the mid-20 th century, and most recently to Asia during
the late 20 th and early 21st centuries (FIG. 5).
Activities to remediate urban environmental problems
began in earnest with the undertaking of large urban civil
engineering efforts by cities like London to provide potable
water and wastewater removal during the latter part of the
19th century. With the onset of the fi rst environmental
regulations in the 1880s and the advent of the environmental movement from the mid-1950s to the 1970s came
policies, laws, and changes in behavior that led to a
decrease in industrial emissions and the disposal or introduction of many potentially hazardous elements and
compounds within the urban environment. Progressively
better and more focused waste treatment, such as provided
by incineration and sewage-treatment facilities, also aided
in the removal of many point sources of toxic elements
and nutrients.
The stage and location of urban development greatly affect
the quantity of resource used and the subsequent amount
of waste produced, as well as how the waste stream is
managed. Hence, today, there are developed, high-technology cities in Western Europe and the Far East; decaying,
legacy industrial cities in Eastern Europe; modern, fastgrowing industrial cities in China; and infrastructure-poor
megacities in Southeast Asia and Africa, each having its
own kinds and magnitudes of environmental impacts.
Thus, geochemical problems will be different in any particular urban area at any specific stage of its development, as
well as in different cities at the same time (e.g. in developing versus industrialized countries). The specifics will
also vary from region to region because of differences in
climate, geological setting, and growth pattern, but the
development trajectory through time should generally
follow the same pattern, although it will be influenced by
factors such as culture, confl ict, climate, and technological
innovation. The stages in this continuum of urban growth
and development do have many things in common,
however. Water pollution issues are a common problem in
both early and modern cities (Radkau 2008). The issue of
human-waste removal and processing continues to be a
major concern, particularly E. coli bacterial contamination
in storm runoff. In developing regions of the world, the
urban “sanitation crisis” is not only still with us but in
many instances is getting worse. Harmful factory emissions
and toxic dusts are characteristic of the early stages of
industrial development in urban areas. Contaminants that
have not previously been present in the natural environment, like pharmaceuticals and endocrine-disrupting
chemicals, are emerging problems in many developed
urban areas.
Urban areas are major drivers of economic growth, being
centers of industrial production, but also consumers of a
high percentage of global resources. The quantity of waste
products introduced into an urban setting is mitigated
when environmental controls exist. Kennedy et al. (2007)
compared the per capita water generation and consumption
of resources in 8 cities from 1965 into the 1990s and found
that in most of them there had been an increase in per
capita water, energ y, and material usages and
wastewater generation.
EXAMPLES OF EMERGING URBAN
GEOCHEMICAL PROBLEMS
Human activities have affected every elemental cycle on
the planet. Examples include the introduction of trace
metals through smelting and manufacturing and an
increase in nitrogen and phosphorus fluxes through fertil-
420
D ECEMBER 2012
have elevated concentrations of Li. This Li is thought to
be from pharmaceutical use, and, along with Gd, it is an
excellent tracer in urban aquatic systems (Barber et
al. 2006).
Urban Halo Effect
With 279 micrograms of polluting particles per cubic
meter recorded in the ambient atmosphere (World
Bank 2011), Ulaanbaatar typifies the air pollution of rapidly developing cities in Asia. Ulaanbaatar is not only the capital of Mongolia,
but it also serves as the country’s transport and industrial center:
nearly everything, from textiles to processed foods to cement, is
manufactured there. The coal mines that power these industries
are located nearby. SOURCE : R EUTERS /C ARLOS BARRIA
FIGURE 5
izer addition. In fact, human activities likely dominate, or
have strongly perturbed, the global cycles of most elements
other than the alkali metals, alkaline earths, and halogens
(Klee and Graedel 2004). But even these elements can have
significant anthropogenic sources in urban areas. For
example, the addition of deicing salt to urban and suburban
roadways and its subsequent entry into municipal wastewater can greatly impact the sodium and chlorine concentrations in urban streams and rivers (Kaushal and Belt
2012). Below, we present a few noteworthy problems related
to urban geochemical processes and elemental fluxes. We
do not mean that these are the most important issues facing
urban regions, but instead we use them to illustrate the
diversity of current problems in urban geochemistry.
Gadolinium and Lithium from
Pharmaceutical Sources
Gadolinium (Gd) chelates have been used in the biomedical
field as a contrasting agent for magnetic resonance imaging
(MRI) since 1988. Since the mid-1990s, Gd has been
observed in urban aquatic systems and more recently in
urban water supplies. The enrichment of Gd relative to
other normalized rare earth element concentrations in
these waters indicates that the enhanced concentrations
of Gd are from an anthropogenic source. In the western
districts of Berlin, the Gd concentration in drinking water
is ~33 times higher than the natural background concentration (Kulaksız and Bau 2011). Gadolinium is a good
tracer of urban anthropogenic influence on rivers and
streams as it is not effectively removed by natural processes
or municipal water treatment. Although these relatively
low concentrations do not pose an immediate health
threat, the presence of Gd in drinking water may serve as
a useful indicator of the presence of other emerging
contaminants, like pharmaceutical compounds (Kulaksız
and Bau 2011).
Lithium (Li) carbonate is prescribed as a medication for
individuals with bipolar disorder. A 2011 study indicated
that 2.4% of the global population will be diagnosed with
bipolar disorder sometime in their lifetime. In the United
States the percentage is higher, at 4.4%. Because of its solubility, Li is difficult to remove from solution, and urban
wastewater-treatment effluent has been demonstrated to
E LEMENTS
Waste and emissions from cities are not confi ned to the
urban areas themselves. A certain percentage is transported
beyond the urban limits. The dispersion depends in part
on the physicochemical form of the compound, its volatility and solubility, and the way it is introduced into the
environment. This export of chemicals to other areas has
been termed the urban halo effect (Diamond and Hodge
2007). Material can be transported from urban regions in
both the atmosphere and flowing water. Transport distances
can be regional and even hemispheric. A case in point is
that, after the banning of tetraethyl lead as an antiknock
additive in gasoline in the United States, a rapid and significant decrease in lead concentrations was observed in
Greenland snow and Sargasso Sea surface seawater (Boyle
et al. 1994). The impact of metal pollutants on urban waterways, both past and present, has also been well documented. Although numerous studies have demonstrated a
decrease in downstream concentration of pollutants introduced from large urban areas in response to environmental
legislation (e.g. Sañudo-Wilhelmy and Gill 1999), urbangenerated pollutants are still an important downstream
issue in many rivers and downwind of urban
airscapes globally.
SUSTAINABLE CITIES AND
GEOCHEMICAL PROCESSES
Urban design plays a significant role in urban geochemistry. For example, in the 1990s it was demonstrated that,
for 120 cities with populations over 100,000, urban areas
grew faster than urban populations (Angel et al. 2005).
This ratio of area to population growth in cities in developing countries was increased by a factor of about two,
compared to a factor of about five in cities in industrialized
countries, but the total urban area expanded more rapidly
in the cities in developing countries (Angel et al. 2005).
In addition, the population density in urban regions has
an impact on urban geochemical cycling, especially as it
relates to transportation (Kennedy et al. 2007). Therefore,
the planning of urban expansion has great consequences
in terms of future urban geochemistry. The development
of infrastructure, the protection of sensitive and perhaps
environmentally mitigating green space, and the plans for
transportation have important consequences on pollutant
introduction and control. Clearly, urban areas are complex
systems with many codependent components (Bettencourt
and West 2010). Geochemical attributes need to be introduced into discussions on future urban development. In
order to deal with present and future urban challenges,
including biogeochemical ones, a diverse, multidisciplinary, integrated group of social and biophysical scientists, engineers, urban planners, and policy makers is
needed to develop truly sustainable urban growth and
promote ecosystem and human well-being.
IN THIS ISSUE
Cities of the 21st century face unprecedented challenges,
which include both emerging and legacy problems caused
or exacerbated by the unprecedented spatial scale of
humanity’s current global footprint. Cities must now
adapt, not only to complex urban dynamics, like excessive
population density and the spontaneous growth of
421
D ECEMBER 2012
informal settlements, but also to natural disturbances and
disasters and to global climate change. This issue of
Elements considers this challenge.
Because of the broad and interdisciplinary nature of urban
geochemistry, it would be unrealistic to try to cover all
aspects of this topic in a few articles. Albanese and Cicchella
(2012) discuss the legacy of past activities that cause environmental concern in urban areas today. Wong et al. (2012)
review the impact of urbanization on both physical and
chemical hydrology. Bain et al. (2012) characterize biogeochemical changes in urban systems using recent data from
the urban Long-Term Ecological Research program in the
United States. Filippelli et al. (2012) review the geochem-
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