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At
present, there are no official evaluative criteria
for concentrations of organic contaminants in
Hong Kong sediment. Draft sediment quality criteria
have, however, been specified for the classification
of sediment under the Planning, Environment and
Lands Bureau and Works Branch Joint Draft Technical
Circular. The classification system comprises
two criteria for a broad range of Contaminants
of Concern. The lower criterion is referred to
as the Lower Chemical Exceedance Limit (LCEL),
whereas the upper criterion is referred to as
the Upper Chemical Exceedance Limit (UCEL). No
LCEL and UCEL values have yet been set for Total
DDT and TBT.
While
not yet being formally applied in Hong Kong, the
Effects Range Low (ERL) which is equivalent to
the LCEL and Interim Sediment Quality Values (ISQVs)
proposed by EVS(6)) are considered to provide
the most appropriate means of grading concentration
of organics such as Total DDT in sediment. ERL
values provide a lower limit below which sediments
are considered 'uncontaminated', whereas Effects
Range Medium (ERM or ISQV-High) values provide
an upper limit above which sediments may be considered
'highly contaminated'. These values together with
the detection limits agreed for the present study
are presented in Table 2.3a for reference. The
ERL (ISQV-Low) levels are used as reference values
in this report to assess the importance of Total
DDT present in marine sediments as no ISQV-High
level is available. No ISQV levels have, however,
been set for evaluating the importance of TBT
concentrations in sediment.
It
is acknowledged that the ERL for DDT was determined
through examination of a wide range of international
values without the benefit of grading these values
against ambient values in Hong Kong. The value
of 1.58 mg kg-1 was drawn from an exhaustive study
by Long et al (7) on sediment criteria in the
US. The applicability of this value to Hong Kong
and, therefore, the implications of the values
encountered during the Environmental Baseline
Surveys, is difficult to assess. The routine monitoring
dataset gathered by EPD does not include measurements
of Total DDT levels. An appropriate LCEL value
for Hong Kong sediments can only be determined
when more local data are gathered.
| Table
2.3a ERL, ISQV levels and Detection Limits
for Total DDT and TBT in Sediments |
| Contaminant |
ERL
or ISQV-Low (LCEL) |
ERM
or ISQV-High (UCEL) |
Detection
Limit(1) |
| Total
DDT (ug kg-1 dry weight) |
1.58 |
na |
0.15 |
| TBT
(ug Sn kg-1 dry weight) |
na |
na |
20 |
Note:
(1)
The detection limits shown here are those presented
and agreed upon by the ESMG members.
na
No value has been set at present.
|
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