Introduction

Organochlorine pesticides are toxic, chlorinated, hydrophobic compounds whose residues tend to bioconcentrate and bioaccumulate in wildlife. In recent years there has been a marked increase in concern over the presence of organochlorine pesticides in the environment and in our food supply. Most of these pesticides, which were used extensively in the past, have now been withdrawn from the market place and their legal uses, if any, are minor.

It was not until 1985 that most organochlorine pesticides were banned from use in Australian agriculture. However, their persistence remains as an epitaph to their former use. Residues have been found within pristine environments (Luke, et al., 1989; Bidleman, et al., 1990; Gregor, 1990) and are undoubtedly present to some extent in most, if not all, animal life forms. In a recent study, researchers at Sydney University found that 99% of the 292 human fat samples tested had residues of both DDT and dieldrin. The levels were so high that in most cases "the fat would not have been fit for human consumption if we were cannibals!" (Austin, 1989).

Because human exposure to organochlorine pesticides should be minimal, it is extremely important to be aware of the degree of contamination within food producing areas. The MIA is an important agricultural area in which organochlorines were extensively used. Most farm drainage from the MIA eventually makes its way to Barren Box Swamp (BBS) which has a target volume of 40,000 ML but can contain up to 85,000 ML. Inflows into BBS average 146,000 ML whilst outflows average 125,000 ML per annum. The difference, along with rainfall, can be regarded as having evaporated.

Assuming the yearly average BBS inflows and outflows are equal, and that a constant volume of 40,000 ML is achieved, then the water flowing through BBS has a mean residence time of approximately 3.3 months1. Combined with the swamp's size (approximately 3440 ha), this may promote high rates of sedimentation. The possible effect is that sediment particles may carry a significant burden of organochlorine pesticide contaminants.

1 Although inflows and outflows vary consierably between summer and winter, only yearly averages will be used for the purpose of this paper.

If these pesticides have a high bioavailability, biota from the area may also carry high burdens, and for edible species maximum residue limits (MRLs) could well be exceeded.

Samples of several species of fish and duck have been prepared for analysis by GC-MS. Preliminary results indicate the presence of a number of organochlorine residues, with most being below the MRLs set by the NSW Department of Health's Pure Food Act.

Ducks

Table 12 shows the results found for ducks to date. The results show that almost all residues found were below MRLs. Only two notable exceptions were apparent and both were in the same bird. Aldrin, with an MRL of 200 ug/kg, was detected at 4400 ug/kg in the fat tissue of a Grey Duck. This is a value 22 times the MRL! However, this is not really a problem since the sample was pure fat, and as fat is usually removed before eating it should not represent a serious health hazard to the consumer.

Table 1. Organochlorine residues in ducks

2 All values are in ug/kg, bracketed values correspond to the MRLs, Trace is designated as being below 10 ug/kg and ND refers to not detectable (a concentration of less than 3 ug/kg).

The same duck had an endosulfan concentration of 400 ug/kg (double the MRL) in its breast tissue. Because endosulfan is far less persistent than the other organochlorines, this result suggests that the bird has had significant exposure to the chemical fairly recently. Considering that no other bird examined had any detectable endosulfan present, it is probable that the contamination did not come from the BBS area and that the bird may only recently have flown in. This highlights the problem of using some birds as indicator species to assess the degree of contamination within an area. The Grey Teal is a nomadic species of duck (Frith, 1982). Its pesticide burden depends on the time spent in a particular region and the extent to which the area was contaminated. Conversely, both the Black Duck and Wood Duck tend to be sedentary, often remaining at the same watering hole for several months (Frith, 1982). For this reason, they would better reflect the state of BBS than the Grey Teal.

McDougall, et al. (1989) carried out similar studies using bird and fish as indicators of the level of organochlorine pollution in the Clarence and Tweed Rivers on the North Coast region of NSW for the period 1983-85. They found no BHCs, heptachlor, chlordane or aldrin residues in any samples analysed. Agriculture adjacent to the Clarence has been essentially devoted to animal production and consequently has had minimal organochlorine pesticide use. Therefore residues detected represent background levels. Results for 9 ducks for EDDT gave an average of 0.5 ug/kg and ranged from not detectable to 2.3 ug/kg. The results from the BBS study suggest that the organochlorine pesticide concentrations are well above background levels.

In comparison, the Tweed River had results similar to those found in the BBS study. This is not surprising since the area surrounding the river had substantial hectarage devoted to crops that made use of organochlorine pesticides in the past. Results for 10 ducks for EDDT ranged from 0.7 - 540 ug/kg with an average concentration of 100 ug/kg. These results are comparable with the values found in the BBS samples.

Fish

Table 2 shows results for organochlorine pesticide residues detected in the muscle of carp (Cyprinus carpio). All residues determined are well below the MRLs. Results given by McDougall, et al. (1989) for the Tweed River are again comparable with those found for BBS. They found that during 1983-84, Mullet3 (Mugil cephalus) gave an average concentration for EDDT of 30 ug/kg with a range from not detectable to 270 ug/kg for the 17 fish examined. In comparison, results for the Clarence River for the EDDT gave an average of 2 ug/kg with a range of not detectable to 50 ug/kg for 39 samples. The results for BBS and the Tweed River for fish also appear to be above background levels.

Table 2. Organochlorine residues in carp muscle

Conclusion

The past use of organochlorine pesticides in the MIA has resulted in the accumulation of several of these pesticides in the biota within BBS. The main contaminants found were EDDT with heptachlor, aldrin, endosulfan and EBHC also being found in some samples. The results suggest that the bioavailability of organochlorines within BBS is limited and/or that only a small amount of contamination exists. Residue levels were generally well below the MRLs set by the NSW Department of Health Pure Foods Act indicating that the samples examined do not represent a human health hazard. It would appear that the past use of organochlorine pesticides in the MIA has not significantly contaminated BBS and, because most uses are now illegal, existing levels should decrease further with time.

3 The fish examined by McDougall, et al. (1989) did not include carp. Even though the physiological characteristics of mullet may well be different to those of carp it has been selected for the comparison. This is a result of the residue burden carried by the mullet being higher than the other fish examined in North Coast study (i.e. the worst case situation).

Further Research

Surface sediments tend to be loose and well mixed whilst deeper sediments are generally more closely packed together and inaccessible (Reuber, et al., 1987). Contaminants associated with deeper sediments are no longer accessible to the aquatic environment. However, if periodic flooding or bioturbation disturbs the sediment profile, the contaminants in the deeper areas may become resuspended and become available to biota. Further research should be aimed at establishing the quantity of organochlorine pesticides buried within the sediments and likelihood of them becoming mobilised within the environment. Work is currently being undertaken to determine sedimentation rates within the swamp and the effects, if any, of bioturbation.

Work requiring the use of the half-lives of organochlorine pesticides must rely upon figures derived overseas. The reliability of results and conclusions drawn from their use is questionable as they may not accurately predict the pesticides decay behaviour under Australian conditions. Studies should be carried out to determine decay constants for these pesticides in Australia so they can be safely applied to the work undertaken here.

References

1. Austin, N. (1989). The Killing Fields - How much poison did you eat today? The Bulletin, Oct. 3: 44-49, 1989.

2. Bidleman, T.F., Patton, G.W.,Hinckley, D.A., Walla, M.D., Cotham, W.E. and Hargrave, B..T. (1990). Chlorinated Pesticides and Polychlorinated Biphenyls in the Atmosphere of the Canadian Arctic. In: Long Range Transport of Pesticides. Ed. D.A. Kurtz. Lewis Pub. Inc., Michigan, USA, 347-372.

3. Department of Water Resources (1989). Murrumbidgee Irrigation Areas Integrated Drainage Scheme Investigations. Interim Report: 1-60.

4. Frith, H.J. (1982). Waterfowl in Australia. Australian Natural Science Edition, Angus and Robertson Pub., Sydney.

5. Gregor, D.J. (1990). Deposition and Accumulation of Selected Agricultural Pesticides in Canadian Arctic Snow. In: Long Range Transport of Pesticides. Ed. D.A. Kurtz. Lewis Pub. Inc., Michigan, USA, 373-386.

6. Luke, B.G., Johnstone, G.W. and Woehler, E.J. (1989). Organochlorine Pesticides, PCBs and Mercury in Antarctic and Subantarctic Seabirds. Chemosphere 19(12):2007-2021.

7. McDougall, K.W., Ahmad, N., Harris, C.R. and Higginson, F.R. (1989). Organochlorine Insecticide Residues in Fish and Birds from Three River Systems on the North Coast Region of New South Wales. Bull. Environ. Contam. Toxicol. 42:884-890.

8. Reuber, B., Mackay, D., Paterson, S. and Stokes, P. (1987). A Discussion of Chemical Equilibria and Transport at the Sediment-Water Interface. Environ. Tox. and Chem. 6:731-739.