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which are usually experiencing fast land development, than in highly-urbanized
central cities, where the land use changes and population growth are relatively
stable.
9.6 Discussion
Urbanization is usually related to water degradation caused by various human activ-
ities, including discharges of residential, municipal, and industrial sewage, fertilizer
and pesticide use in lawns, and applications of road deicers, so higher percentage of
developed land is always associated with higher concentrations of water pollutants
in different watersheds around the world (Bowen and Valiela 2001 ; Finkenbine et al.
2001 ; Tong and Chen 2002 ; Interlandi and Crockett 2003 ;Hattetal. 2004 ; Aichele
2005 ; Brett et al. 2005 ; Deacon et al. 2005 ; Schoonover et al. 2005 ;Tuetal. 2007 ).
Likewise, in the present study, higher concentrations of water pollutants are signifi-
cantly related to higher percentage of developed land and higher population density.
Nevertheless, the application of GWR finds that the correlations vary over space and
differ among different urbanization and water quality indicators and the correlations
are not significant at all the sampling sites. Considerable spatial differences in the
correlations are observed between less- and highly-urbanized areas.
The correlations of PDLU with dissolved solid and ions become stronger as
the urbanization level of watersheds decreases, while the correlations with nutri-
ents are stronger in highly-urbanized areas. The different correlations with PDLU
between dissolved nutrients and other parameters might be explained by the dif-
ferent sources of pollutants. Compared to dissolved ions and solid, which are
contributed mainly by human activities associated with urbanization, such as urban
runoff, road deicers use, and discharges of residential, municipal, and industrial
sewage, dissolved nutrients are also largely contributed by agricultural activi-
ties, such as fertilizer application and animal waste, besides urban sources. Thus,
the sources of dissolved nutrients might differ largely between highly- and less
urbanized areas. In highly-urbanized areas without agricultural sources, dissolved
nutrients are only contributed by urban sources, and thus significant positive rela-
tionship exists between dissolved nutrients and PDLU. On the contrary, dissolved
nutrients in less-urbanized areas are contributed by both agricultural and urban
sources, and thus relatively weaker relationships were found between dissolved
nutrients and PDLU. Unlike dissolved nutrients, dissolved ions and solid are mainly
contributed anthropogenically by urban sources in both highly- and less- urbanized
areas. Their correlations with PDLU are not affected by agricultural sources and
even get stronger in less-urbanized areas.
The correlations of PD with most water quality indicators get stronger as the
urbanization level of watersheds decreases. In other words, same amount of increase
in developed land or population density causes more increase in concentrations
of water pollutants in less-urbanized areas than highly-urbanized areas, especially
for dissolved solid and ions. This result might be attributed to the low-density
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