1989 – 1991: Guildford (Tas) approx. Pesticide Cypermethrin, Alpha-Cypermethrin

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Buffer strips and streamwater contamination by atrazine and pyrethroids aerially applied to Eucalyptus nitens plantations. Jan L Barton and Peter E Davies. Inland Fisheries Commission, Hobart, Tasmania.

Summary

Concentrations of pesticides in streams draining 20 plantations of Eucalyptus nitens in Tasmania were examined in relation to buffer strip width. Atrazine concentrations on the day of spray in streams draining 15 plantations were significantly negatively correlated with riparian buffer strip width but not buffer quality. Concentrations following the first rain event in one month after spraying were highly positively correlated with day of spray concentrations and were only weakly correlated with other site characteristics. Streams with 30 m buffer strips had median atrazine concentrations less than 20 ug/L at all times and these buffer widths are recommended for minimising short term ecological impact on streams.

In streams draining five plantations that were aerially sprayed with the pyrethroids alpha – or cypermethrin, pyrethroid concentration and short term changes in drift (downstream movement) of stream invertebrates were highly negatively correlated with buffer strip width but with no other variable. Drift of stream invertebrates is recommended as a biomonitor for the contamination of streams with pyrethroids on the day of spray, sensitive down to 0.1ug/L. Buffer strips of at least 50 m are recommended to minimise mortality of stream invertebrates from pyrethroid spraying.

Methods

Atrazine

Stream water at 29 sites from 18 streams draining 15 Eucalyptus nitens plantations (owned by either the Forestry Commission or Australian Pulp and Paper Mills) was sampled during 1989 -1991 for the determination of atrazine concentrations…

Pyrethroids

Stream water from 7 sites in 7 streams draining 4 E.nitens plantations was sampled following operations spraying of alphamethrin in early (November – December) and 2 sites draining 1 plantation sprayed with cypermethrin were also sampled…

Results

Atrazine … Atrazine concentrations on the day of spray for all sites were significantly negatively correlated with buffer strip width, plantation catchment area, stream catchment area and length of stream within the catchment area ratio. They were not correlated with application rate and had only a marginal correlation with buffer strip quality… Median atrazine concentrations for streams <=10m, 20m and 30m buffer strip widths were 700, 58.1 and 5.4ug/L respectively…

Discussion

Only one of the plantations studied was sprayed from a light plane and this, combined with a complete lack of buffer strips and a high application rate (10kg/ha), resulted in the highest day of spray concentration found in the study (58 mg/L)…Contamination of streams draining plantations cannot be completely avoided if triazine herbicides are used. From the present study, it would appear that contamination can be minimised by the use of appropriate buffer strips. The maximum buffer strip examined in this study was 30 m. Median concentrations for all 30 m buffered streams were below 20 ug/L on all occasions following spraying, a concentration below which short term ecological effects are unilkely…

Pyrethroids

Peak (day of spray) pyrethroid concentrations were obtained on six occasions, ranging from <0.01 to 0.50 ug/L. These concentrations were significantly negatively correlated with buffer strip width, but not with any other site variable.

Discussion

Davies and Cook (1993) described the impact of a single spraying of cypermethrin on Sales Rivulet. They suggested that mayflies and stoneflies (Ephemeroptera, Plecoptera) were the most sensitive taxa, showing the greatest response in both the drift and benthos. The present study further supports their observations with drift values being the highest for these groups at pyrethroid concentrations >0.1 ug/L and having the highest correlation with concentration. It appears that contamination of streams with low concentrations of pyrethroids from spray drift results in significant responses in invertebrate drift which are generally related to the mortality of mayflies and stoneflies (Davies and Cook 1993)