Urban land use has both direct and indirect impacts on water resources. Some impacts result from the direct modification or destruction of streams, lakes and wetlands. Other impacts occur primarily offsite due to changes in the quality and quantity of runoff from urban development and construction activities (Dreher and Price 1992).
Urban areas generate both nonpoint and point sources of contaminants. Point sources that have an impact on surface water include industrial and municipal waste discharges; those that affect groundwater quality include leaky underground storage facilities, as well as miscellaneous accidental spills of organic or inorganic contaminants. Groundwater contamination by volatile organic compounds (VOCs) is more common in urban settings due to the heavy use of solvents and fuels. Nonpoint sources include runoff and/or infiltration of water from roads, industrial areas, and golf courses. Contaminants include metals, industrial organic chemicals, nutrients, and pesticides.
Areas of intensive use with much of the land covered by structures are
considered urban or built-up lands according to Anderson and others (1976).
Subcategories include residential, commercial, industrial, and transportation
uses. Population data is often used to refine the residential subcategory
of urban land use. This component of urban land use increased the most
(50 percent) during the 1970's, according to Vesterby and others (1994).
(source:
http://wwwrvares.er.usgs.gov)
Urban land uses have varying degrees of intensity and are related to the potential pollution. While the relationship between urban land use and water quality is complex, a fairly reliable correlation has been defined between the annual loading rates for various pollutants and land use based mainly on density. The most widely used measure of density is imprevious cover. Studies have shown that pollutant loading from stormwater increases with the percentage of impervious surfaces (Marsh). Through proper site design and land use management, these impacts can be reduced.
Urban land uses can have an impact on both water quality and quantity. These impacts often occur concurrently with an increased need for infrastructure. For example, with a greater demand for water, the need for reservoirs increases. With greater settlement, more flood protection is needed, resulting in dams and embankment construction. With greater congestion, the supply of municipal and industrial wastewater is increased, requiring additional wastewater treatment facilities.
Secondary effects of urbanization include the changes in weather due to contaminants in the air, solar radiation, temperature, visibility, humidity, wind speed and direction, cloudiness, precipitation, atmospheric electicity, severe storms, and fronts. Urban areas are wetter and warmer than rural areas. Rainfall is often higher in urban areas, by a margin of five to ten percent, because of greater turbulence above urban areas and a high concentration of dust particles which can serve as nuclei for raindrops (Mather 1986 p. 176). Urban areas differ from similar land uses that are more dispersed because the potential for infiltration and interception by trees is much lower.
Removal of trees and
vegetation may lead to decreased evapotranspiration and interception, and
increased stream sedimentation.
Initial
construction of urban structures and infrastructure such as roads and drainage
ways can lead to decreased infiltration and lowered groundwater table,
increased storm flows and decreased base flows during dry periods.
Generally,
built-up urban areas have higher sediment yields than areas with undisturbed
natural vegetation. With construction, the sediment yield can increase
several times.
Complete
development of residential, commercial or industrial areas may lead to
increased imperviousness. Such developments can lead to increases in runoff
volume, flood damage potential, and increased peak discharges.
Construction
of storm drains and channel improvement may lead to local relief from flooding.
However, concentration of floodwaters may aggravate flood problems downstream
(Kibler 1982).
Urban
land uses are marked with impermeable land cover elements such as roofs,
roads, sidewalks, and parking lots, resulting in more runoff and negligible
infiltration. The conversion of pervious land cover to impervious surface
results in increases in the rate of volume of storm runoff and reductions
in groundwater recharge.
Discharge
of runoff waters directly to watercourses may adversely affect the quality
of the receiving water body. For example, a range of organic and inorganic
pollutants as well as heavy metals may be carried from road surfaces.
In
urban areas, collection in combined sewerage systems may impose increased
hydraulic and pollutant loads on conventional wastewater treatment facilities
(Perry and McIntyre 1986).
Areas
where urban infiltration can occur include lawns, parks, and golf courses.
These land uses are often associated with practices that utilize fertilizers
and chemicals, thus increasing the potential for groundwater contamination.
Storm water quality is variable in urban areas due to differences in volume and intensity of storm events. At low intensities, soluble and light particles are discharged. A higher intensity storm event may carry solids and other heavier materials with it.
Human
waste and some industrial waste are normally carried by sewers and removed
in sewage systems. Early sewer systems were designed to carry both storm
water and sewage, eventually discharging into rivers and streams. As urbanization
and population increased, the need for wastewater treatment grew, and the
shortcomings of the combined sewer system was quickly apparent. During
storms, stormwater mixes with sewage and overloads wastewater treatment
plants leading to combined sewer overflows that are usually discharged
with no treatment into receiving water bodies. Many older cities are currently
faced with the challenge of separating sanitary and storm water sewer systems,
which is a costly endeavor.
Usually, the greatest pollutant load to the system occurs during the beginning of a storm, due to the 'first flush' effect. Increased levels of suspended solids (SS) and organic materials are transmitted downstream, posing a threat not only to the receiving water body but to its inhabitants and to human health.
Uncontrolled erosion from construction activities can generate enormous quantities of sediment, from 20 to 200 tons per acre per year. In comparison, typical erosion rates for cropland range from 1 to 20 tons per acre per year (Dreher and Price 1992). Erosion of construction sites and the conveyance of sediment offsite may cause:
water
quality impairment - sediments can increase turbidity, degrade habitats,
and limit photosynthesis.
sediments
transfer nutrients and other pollutants downstream
loss
of flood conveyance and storage - exacerbating draining and flood problems
The primary purpose for controlling erosion and offsite sediment washoff from construction sites is to prevent the above mentioned associated problems (Dreher and Price 1992). Soil erosion and sediment control measures include:
minimizing
the area disturbed and the time of disturbance by following natural contours
and protecting sensitive features.
stabilizing
disturbed soils as soon as possible, including methods such as seeding,
mulching, and/or non-vegetative measures such as erosion blankets.
controlling
sediment through the use of such methods as a basin, filter and/or trap
to prevent eroded sediments from leaving the site:
controlling
runoff onto and through construction sites, for example, through construction
of a stabilized channel and a stabilized outlet for diverting and/or conveying
offsite runoff.
routinely
inspecting construction sites and maintaining control measures
Residential land uses range from high-density multiple-unit structures,
such as apartment buildings, to low-density single-family houses on large
lots typical of suburban areas. In residential areas, septic tanks, sewage
disposal systems, runoff from driveways and parking lots, and fertilizers
and pesticides applied for lawn care can affect water quality. Although
housing unit density (dwelling units per acre) often is used for land-use
classification, population (people per square mile) is considered to be
a more direct measure of the potential impact on water quality."
(source:
http://wwwrvares.er.usgs.gov)
Appropriate site planning and design is critical for minimizing the impacts of residential land use on water resources. The overall objectives of residential land use development include minimizing construction and development in sensitive areas and preserving the natural hydrologic conditions and pollutant filtering mechanisms. Steep slopes, stream corridors, shorelines, wetlands and woodlands should be avoided in the site plan so the effect of residental land use on sensitive areas is minimized. In order to help achieve natural hydrology preservation, natural drainage features should be preserved and natural depressional storage areas should be protected.
Minimizing imprevious surface area also plays an important role and can be achieved by:
reducing
building setbacks, thus reducing driveway, entryway, and walkway length
reducing
street widths
reducing
sidewalks to one side in low traffic areas
utilizing
cluster development
using
permeable paving materials
