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Review the indicators
at a glance
System Dimensions (continued)
- How large are patches of “natural” lands (forests,
grasslands and shrublands, and wetlands), which provide green
space and wildlife habitat? Natural lands are important for
urban and suburban recreation and quality of life, and they are
also important as wildlife habitat. The value of these patches
for both people and wildlife can be affected by their size. About
half of all natural lands in urban and suburban areas are in patches
smaller than 10 acres. A progressively smaller percentage are
found in larger patches—nationally, less than 5% of urban/suburban
natural lands are found in patches of 1,000 acres or more.
- How much of urban and suburban areas is covered with buildings,
concrete, asphalt, and other “hard,” or impervious,
surfaces? Places that have a higher percentage of impervious
surfaces often have more, and dirtier, runoff, than places with
less “hard” surface. Data are not adequate for national
reporting on this indicator.
- What fraction of urban and suburban stream banks are vegetated?
Vegetation along streams can reduce the effects of runoff and
serve as wildlife habitat. This indicator requires further development.
Chemical and Physical Conditions
As is the case for several of the other systems, many of the indicators
of chemical and physical condition of urban and suburban areas focus
on streams. Because streams receive runoff from the land surface,
they are powerful indicators of conditions on that land surface.
Two indicators track concentrations of nitrate and phosphorus, nutrients
that can, in excess, cause problems. A third indicator tracks contaminants
such as pesticides, PCBs, and heavy metals in stream water and soils,
while a fourth tracks urban air quality, particularly concentrations
of ozone, a key component of smog. Finally, one indicator, requiring
further development, tracks the differences in temperature between
cities and their surrounding regions.
- How much nitrate is there in urban and suburban streams?
Nitrate is an important plant nutrient, but it also contributes
to water quality problems. Nitrate in drinking water is a health
threat for young children, and it must be removed at significant
cost by municipalities that rely on river water. Sources of nitrate
include sewage treatment plants, animal wastes, and fertilizers.
About 60% of urban and suburban stream sites tested have concentrations
of nitrate below 1 part per million (ppm); all samples were below
the federal drinking water standard of 10 ppm. No trend data are
available, but nitrate levels are lower in urban/suburban streams
than in streams in agricultural areas, but higher than in forest
streams.
- How much phosphorus is there in urban and suburban streams?
About two-thirds of urban and suburban stream sites had phosphorus
levels of at least 0.1 part per million, the upper limit recommended
by the Environmental Protection Agency to prevent nuisance algae
growth in fresh waters. Phosphorus concentrations in urban and
suburban streams are similar to those in farmland streams, and
much higher than those in forest streams. Sources of phosphorus
in urban streams include sewage treatment plants, animal wastes,
some detergents, and fertilizers.
- How common are air pollution (ozone) levels that exceed
federal guidelines in urban and suburban areas? In 2004, 11% of monitoring stations in urban and suburban areas had
at least 4 days of high ozone levels. Federal air quality standards
are violated when an area experiences high ozone levels, on
average, for 4 or more days per year over three consecutive
years. From 1990 through 2004, the number of stations reporting
high levels has fluctuated around an average of about 45%, but dropped to just 11% in 2004.
During the same period, the number of monitors recording high
levels on 25 or more days per year declined, to about 1% in 2004 (all of these monitors were in Southern California).
- What levels of contaminants (primarily artificial compounds
and heavy metals) are found in stream water and soil? All
urban and suburban stream sites had at least one chemical that
exceeded guidelines for protection of aquatic life, and 5% of
sites had contaminants that exceeded human health standards or
guidelines. About 85% of stream sites in urban and suburban areas
had an average of at least five detectable contaminants throughout
the year. Data are not adequate to report on contamination in
soils in urban and suburban areas.
- How much hotter are urban and suburban areas than less developed
areas nearby? Asphalt, concrete, and other constructed materials
in developed areas absorb solar energy, often leading to higher
temperatures than in undeveloped areas. This can raise summertime
cooling costs and cause human health problems where air conditioning
is not available. This indicator requires further development.
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