Herdsman Lake Pesticide Study – Interim Report (Davis & Garland (1986): plus subsequent studies.
(a) This study was carried out to investigate the environmental effects of spraying to control the Argentine Ant at Herdsman Lake, in March/April 1986. The Department of Agriculture’s programme to control Argentine Ants in Herdsman Lake commenced in the mid-1950’s with the use of dieldrin. Hepatchlor replaces dieldrin in 1970. The perimeter of the Lake has been treated every year except from 1983 to 1985, when public concern at the possible threat to wildlife resulted in a cancellation of the spraying programme for those seasons.
A sampling programme was carried out when the spraying programme recommenced in the summer of March/April 1986 to establish the level of organochlorines and organophosphates in the water, sediments, and two species of aquatic fauna, the mosquito fish (Gambusia affinis) and the corixicd (Micronecta robusta). Samples were taken prior to spraying, one week after spraying, and after the first heavy rains following spraying, from six sites in the Lake itself, and two from control sites in an adjoining and separate waterbody. Quantitative sampling of aquatic invertebrate was also carried out at each sampling.
The results showed that almost all components of the ecosystem of the Lake were contaminated, to varying degrees, with the organochlorine insecticides. Heptachlor and chlordane were not detected before spraying, suggesting that post-spraying levels found were due to that season’s spraying activity.
Dieldrin, and to a much lesser extent, DDT, were found in all samples, including those taken from drainage water entering the lake. This indicated that dieldrin has been present since its last use in 1970, in addition to any additional chemical entering via drains from surrounding urban areas.
Fish and insect samples contained all organochlorines when taken post-spraying, but only dieldrin and DDT pre-spraying.
Chlorpyrifos was found in post-spraying water and sediment samples only. Temephos was not detected in any samples.
Insect (corixids) numbers decreased significantly after spraying. A dead pelican was found to contain elevated levels of all organochlorines, but far below those necessary to be the cause of death.
(b) Further sampling of pesticide levels in the water, sediments, fish and invertebrate fauna of Herdsman Lake took place in October 1986, from the same sites. Additional sites, near the Herdsman Industrial Park, and three drains, were also sampled.
This sampling showed that water levels of pesticides generally fell and became more uniform between sites, since the post-rain sampling in May. The levels of heptachlor and chlordane in sediments increased dramatically, and may partially account for the decline in water levels. Levels of pesticide found in one drain were higher than the Lake itself, and this must be considered a source of chemical.
Levels of pesticides in fish and invertebrates were much lower than the post-spraying high levels of May 1986, but were still higher than the pre-spraying levels. It was not possible to draw conclusions from the apparent “mortality” amongst fish and corixids immediately post spraying as animals from other parts of the lake probably recolonised areas between May and October.
Pesticide levels in water birds were well below LD50 values for acute toxicity, but were high enough to be of concern.
P53/4 Monitoring Pesticides – A Review – A Report to the Environmental Protection Authority by Peter A Rutherford. Environment Protection Authority. Perth Western Australia. Bulletin 407. December 1989.