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SECTION
5 ASSIMILATIVE CAPACITY CAPITAL STOCK
| 5.2 |
Assimilative
Capacity of Marine Waters |
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| Marine
Water Quality in Hong Kong
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| 5.2.1.7 |
Since
1986 EPD has conducted routine monitoring
to evaluate marine water quality and to
assess the success of a series of pollution
abatement measures (EPD 1998a). The programme
currently involves sampling at 93 marine
water stations located within ten gazetted
Water Control Zones (WCZs) in open water
and typhoon shelters. Maps showing the boundaries
of the WCZs and monitoring locations are
included in Annex
E. Statutory water quality objectives
(WQOs) have been determined for a number
of pollutants in each of these areas.
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| 5.2.1.8 |
A
comparison between the values of pollutants
measured in each WCZ and the applicable
WQO indicates the degree of compliance of
the waterbody, as well as the level of concern
associated with each value. While EPD measure
a large number of parameters, several parameters
have been selected to represent the range
of water pollutant concerns as follows:
- Dissolved
Oxygen (DO) - represents the general state
of the water body and reflects the extent
of organic pollution;
- Total
Inorganic Nitrogen (TIN) - represents
the nutrient enrichment of marine waters
and the potential for eutrophication (TIN
consists of the sum of nitrate nitrogen,
nitrite nitrogen and ammoniacal nitrogen);
- E.
coli - is a representative pathogen which
indicates the presence of sewage in marine
waters.
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| 5.2.1.9 |
Data
on compliance with WQOs (as used for reporting
in EPD's annual marine water quality reports)
are discussed below to indicate trends and
the extent to which each waterbody is polluted.
Compliance for each of the ten WCZs for
these three parameters between 1993 and
1997 is summarised in Annex
F1 to provide an indication of general
water quality in each area. The data show
that the levels of compliance with the WQOs
in Tolo Harbour, Southern, Port Shelter
and Mirs Bay WCZs generally remain steady
in the past five years. On the other hand,
the WQO compliance rates of Junk Bay, Deep
Bay, Northwestern, Western Buffer, Eastern
Buffer and Victoria Harbour WCZs have shown
some decline during the same period.
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| 5.2.1.10 |
Compliance
with WQOs for individual pollutants are charted
in EPD's annual marine water quality reports.
For DO, the trend in compliance between 1994
and 1997 was one of improvement (except in
Inner Deep Bay and Tolo Harbour) and overall
compliance for DO across all WCZs was close
to 90% in 1997. However, data for 1998 show
that overall compliance for DO declined to
52% principally due to steep declines in dissolved
oxygen in Port Shelter, Junk Bay, North Western,
Western Buffer, Eastern Buffer and Victoria
Harbour. Compliance with the WQO for TIN has
varied over recent years with consistently
low compliance in Southern, Deep Bay and Victoria
Harbour WCZs such that overall compliance
across the WCZs was only 50% in 1998. This
drastic reduction of compliance with the DO
and TIN objectives in 1998 may be relate to
unusual weather or hydrological conditions
in an El Nino year. Compliance with WQOs for
unionized ammonia (NH3-N) and E. coli have
generally been at, or close to 100% for all
WCZs between 1993 and 1998. |
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| 5.2.1.11 |
Another
marine water quality concern is the level
of suspended solids. Measures of suspended
solids indicate the degree of light transmittance
and thus the potential for inhibition of photosynthesis.
An analysis of long-term water quality trends
for the years 1982-1995 (EPD 1996) examined
a related parameter, ie turbidity, at 82 stations.
This analysis found that there was a statistically
significant trend of increasing turbidity
at 10 of the stations which were located in
the Tuen Mun, Ma Wan, Rambler Channel, South
Tsing Yi, Stonecutter's Island, Green Island,
Tseung Kwan O and Chai Wan areas. Suspended
solids and turbidity levels are often determined
by localised elevations due to dredging, dredged
material disposal or ship wakes, and it is
interesting to note that the areas highlighted
in the trend analysis have been sites of major
development projects during the late 1980s-early
1990s. Assessment of suspended solids and
turbidity trends can be complicated by naturally
occurring fluxes in particulates in marine
waters (eg sediment transport during heavy
outflows from the Pearl River). Therefore,
high levels of suspended solids or turbidity
are not necessarily an indication of pollution
although they may point to significant erosional
activity which may be a separate concern.
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