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Results
of the four sampling surveys conducted during
both the wet (June 1999 and August 1999) and dry
(December 1998 and January 1999) seasons showed
that total DDT mean values for all ten stations
were above the detection limit of 0.15 mg kg-1
(dry weight). All values at or below the analytical
detection limits were halved before the statistical
tests were run, following an approach applied
in the various monitoring programmes at East Sha
Chau. Examination of the Total DDT levels of sediments
collected during the December 1998 (dry season),
January 1999 (dry season), June 1999 (wet season)
and August 1999 (wet season) surveys revealed
that mean values were below the ISQV-Low of 1.58
mg kg-1 dry weight at all sampling stations except
ES2, SS3, VS3, DS3 and WS1 (Figure
2.3a). ERL exceedances at the specific stations
and sampling times are discussed in detail below:
Sediment
from Station ES2, which is located in the outer
part of Tathong Channel, in December 1998 contained
higher Total DDT concentrations than in January
1999. ERL exceedance was observed in December
1998 but not in January, June or August 1999.
The
mean value of Total DDT was considerably higher
at Station SS3, which is located to the west of
Lamma Island, in January 1999 than in December
1998. ERL exceedance was observed in January and
August 1999 but not in December 1998. Analyses
for August 1999 sampling revealed that high concentration
of Total DDT was present in the sediments. Total
DDT levels of 1.96 mg kg-1 (dry weight) were recorded
in August and these values exceeded ERL values.
This value exceeded Total DDT recorded previously
at Station SS3 in both June 1999 (wet season)
and December 1998 (dry season) when mean concentrations
were 0.4 mg kg-1 and 0.3 mg kg-1 (dry weight),
respectively. No trend is obvious at this Station,
however, as mean Total DDT in sediments during
January 1999 (dry season) was 2.62 mg kg-1 (dry
weight).
High
mean values for Total DDT were detected in sediment
collected from Station VS3, which is located near
North Point in Victoria Harbour, during December
1998 and January 1999 monitoring. ERL exceedance
was observed in both survey months. High mean
Total DDT (5.3 mg kg-1 dry weight) concentrations
were again detected in sediment samples from Station
VS3 during the June 1999 survey. The ERL value
was exceeded for this month.
Station
WS1 which is located to the south of Tsing Yi
also had high Total DDT (1.32 mg kg-1) levels
though these did not exceed the ERL. Total DDT
in sediments sampled during August 1999 revealed
that Station WS1 exceeded the ERL and contained
DDT concentrations of 1.82 mg kg-1 (dry weight).
Total
DDT in sediments sampled during August 1999 revealed
that Station DS3 (located in Deep Bay) exceeded
the ERL and contained DDT concentrations of 1.6
ug kg-1 (dry weight).
The
amount of available information on DDT contamination
in sediments in Hong Kong is limited. Although
sampling was conducted throughout Hong Kong as
part of the SSDS Stage 1 Baseline Monitoring(8))
the detection limit used in that study was 50
mg kg-1. This value is higher than most previously
recorded DDT values from known contaminated sites
in Hong Kong (SSSP Study)(9)) and consequently
all values in the SSDS study were reported as
being below the analytical detection limit. The
surveys conducted as part of the SSSP study were
focused on areas suspected of high contamination
and reported values from marine sediments ranging
from 5 mg kg-1 in the Tuen Mun Nullah to 46 mg
kg-1 in the Rambler Channel. As evidenced by the
mean values presented in Figure
2.3a the DDT levels recorded during these
SUSDEV 21 Environmental Baseline Surveys are much
lower ranging from 5.3 mg kg-1 at VS3 (Victoria
Harbour) to below the analytical detection limit
at MS5 (Mirs Bay) during January.
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