| Regional
Influences on Natural Assimilative Capacity |
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| 5.2.1.1 |
The
marine waters of Hong Kong can be separated
into three areas based on hydrographic conditions.
Western waters around Lantau Island are
under the estuarine influence of the Pearl
River, whereas those in the east are under
the oceanic influence of the South China
Sea and those in the central area are transitional.
The assimilative capacity of Hong Kong's
marine waters is determined primarily by
two factors: the local hydrography and the
ambient pollutant load. Both of these factors
are discussed below.
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| 5.2.1.2 |
The
hydrographic mixing and flushing characteristics
of the offshore waterbodies surrounding
Hong Kong are governed by:
- residual
offshore currents which generally flow
toward the northeast in the wet season
and toward the southwest in the dry season;
- tidal
currents which produce small-scale variations
on the predominant seasonal flows;
- discharges
of freshwater from the Pearl River which
vary substantially by season and year;
and
- high
energy events such as typhoons which may
override seasonal and tidal current patterns
and increase turbulent mixing.
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| 5.2.1.3 |
These
factors result in temporal variability in
water quality parameters, in the physical
structure of the water column, and in the
nature of the biological community.
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| 5.2.1.4 |
Nearshore
hydrographic conditions are strongly influenced
by the configuration of the landform which
can affect retention time (the time a pollutant
is held within an enclosed bay or other water
mass) or flushing capacity (the rate of exchange
between inshore and offshore waters). One
of the most obvious examples of a constrained
waterbody is Tolo Harbour. Although the previous
and present dilution capacity of the waterbody
have not been quantitatively compared, its
naturally constrained capacity is likely to
have diminished with time due to progressive
reclamation. |
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| 5.2.1.5 |
As
oceanic waters are generally less polluted
than nearshore waters, oceanic exchange acts
to transfer wastes away from Hong Kong waters,
whereas flows from the Pearl River generally
intensify pollutant loadings in Hong Kong
waters. A recent report on marine water quality
in the Pearl River Estuary cites oil and organochlorine
pesticides/herbicides as the principal pollutants
and documents bioaccumulation of these contaminants,
as well as heavy metals such as lead, in marine
organisms (Anon 1997). A recent dry season
survey of marine water quality throughout
the Pearl River Estuary and surrounding oceanic
waters found that the Pearl River is the main
source of nutrients and biological oxygen
demand in the area. However, the survey also
concluded that due to oceanic-estuarine circulation
rates in the region, the area has a strong
self-cleaning capacity (EPD, 1997a). |
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5.2.1.6
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The
estimation of assimilative capacity of Hong
Kong's marine waters can therefore be approached
by addressing the specific conditions and
pollutant loadings for the ten different Water
Control Zones which have been designated by
EPD (see Paragraph 5.2.1.7). Each WCZ has
a series of water quality objectives (WQOs)
for different marine water pollutants, and
by comparing existing levels of pollutants
in these water bodies to the quality objectives,
a proxy discussion of assimilative capacity
is possible. For example, such an approach
was taken in the Deep Bay Water Quality Regional
Control Strategy Study (Hyder 1998) undertaken
for EPD in which WQO compliance for Deep Bay
was taken as being representative of the assimilative
capacity and through water quality modelling,
a series measures (including pollution control,
enhanced wastewater treatment and export of
wastes) were proposed to reduce pollutant
loadings such that compliance could be reached.
The following section therefore discusses
marine pollutant levels with reference to
WQOs in order to assess assimilative capacity,
although quantitative evaluation of the assimilative
capacity of each WCZ is beyond the scope of
this study. |
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