Sediment

The Effects Range Low (ERL) value used in this baseline survey for Total DDT was previously determined through examination of a range of international values. The value of 1.58 mg kg-1 (dry weight) was based on a study used to categorise sediment quality criteria in the US(29) and applied to Hong Kong by EVS(30).

EPD does not gather data on Total DDT in the routine monitoring of sediments in Hong Kong waters. Data are, however, available from past studies in Hong Kong's marine sediments and have been compared to the data collected in the present baseline Survey.

Highest Total DDT in sediments during this baseline survey were found at Station VS3 where mean concentrations of 5.3 ug kg-1 were detected. This value is considerably lower than values previously reported for this station. For example, Connell et al recorded a value of 14.7 ug kg-1 (dry weight) at station VS3 (31)) . Total DDT concentrations as high as 22.0 ug kg-1 have been recorded in sediments from the contaminated mud pits at East Sha Chau (32)) and levels in reportedly clean sediments in the UK contained Total DDT in the range 5 - 13 ug kg-1 (33). Typhoon shelters in Hong Kong which owing to poor flushing rates, are known to accumulate contaminants in sediments to high concentrations, have recorded Total DDT ranging from 56 - 97 ug kg-1(34).

In the 1996 Strategic Sediment Sampling Programme (SSSP) study conducted in Hong Kong by EVS, Total DDT was measured from various contaminated sites(35) . The SSSP study recorded the highest Total DDT in sediments from the Rambler Channel and Kai Tak Nullah where concentrations were 46.0 ug kg-1 and 31.0 ug kg-1, respectively. The reference area used in the SSSP study was located in the West Lamma Channel (close to station SS3 in the present Study) and was initially thought to be representative of uncontaminated sediment. The mean concentration of Total DDT recorded at station SS3 was 38.0 ug kg-1 which is considerably higher than the levels reported in this baseline survey.

Using different analytical methodology to the procedures used in this Study, Hong et al recorded Total DDT in Hong Kong sediments ranging from 1.38 to 97.0 ug kg-1 with the highest concentrations in typhoon shelters (Stations VS14 adjacent to Kai Tak and VS16 located in southwest Kowloon)(36) . Fowler (1990) (cited in Hong et al)(37) reported that DDT concentrations in near-shore surface sediments world-wide ranged from < 0.1 to 44.0 ug kg-1 . The highest Total DDT concentrations recorded in Victoria Harbour in the present baseline Study (ie 5.3 ug kg-1) were thus in the lower range and indicate that Hong Kong marine sediments are not particularly contaminated with this organochlorine pesticide.

The DDT concentrations recorded in Hong Kong sediments during this baseline Study have been compared with sediment quality criteria values from other jurisdictions. Co-occurrence analysis (CoA) involves calculating the mean chemical concentrations with literature-derived chemistry and toxicity or benthic community alteration data to derive effects range-low values (ERL) or threshold effects levels (TEL) below which effects are unlikely, and an effects range-medium (ERM) or probable effects (PEL) above which effects are probable(38) . The TEL (low and high end) values set for sediments by Environment Canada (cited in EVS 1996)(39) are 3.89 and 51.7 ug kg-1 (dry weight), respectively. The highest Total DDT recorded in sediments during this baseline survey were found at Station VS3 where mean concentrations of 5.3 ug kg-1 were detected. This value slightly exceeds the TEL-low but is well below the TEL-high. Impacts on the biota inhabiting sediments in Station VS3 would, therefore, be expected. It is possible, however, that marine assemblages at stations exceeding the ERL, are tolerant of these Total DDT levels. However, this cannot be determined under the present Study and would require synoptic sampling for sediment chemistry, sediment toxicity and benthic assemblage analyses before any conclusions could be drawn. Mean Total DDT levels at the other stations (ie MS15, MS5, NS4, PS6, TS2) were lower than the Environment Canada TEL-low value and thus effects in biota at these stations are possible. The spatial variation of Total DDT in sediments recorded in this baseline Study and reported previously in Hong Kong are shown in Figure 4.1a.