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http://hdl.handle.net/2123/130
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| Title: | The Persistence of Pesticides in Floodwaters and How This is Influenced by Water Management and Layout |
| Authors: | Quayle, Wendy C |
| Issue Date: | 2005 |
| Series/Report no.: | Program 1 |
| Abstract: | The detection of pesticides in drainage water from rice in the MIA that frequently exceeded
NSW Environmental Protection Agency guidelines for the protection of aquatic ecosystems
led to rice growers with-holding water for up to 28 days. This project investigated the effects
of different irrigation management systems on the dissipation of these chemicals after they are
sprayed onto the free water of rice fields and accumulation of salt when water containing
pesticides was retained on farm. A model was evaluated for its ability to simulate the fate of
the pesticide molinate and hence predict the load in rice floodwaters prior to drainage. The
aim of the project was to provide improved methods to minimize off-farm pesticide
contamination.
Dissipation of molinate and thiobencarb was determined in a 27 ha commercial rice field
using a ‘static’ bankless channel irrigation system. The half-life was found to be 2.7 days for
molinate (@2 L/ha), and 3.6 days for thiobencarb, (@3.75 L/ha). The time required for the
highest average concentration to dissipate to EPA Notification Levels of 3.4 μg/L for
molinate and 2.8 μg/L for thiobencarb would be 24.4 days and 21.5 days, respectively.
Subsequently, the dissipation of molinate, thiobencarb, clomazone and chlorpyrifos was
determined in a small plot trial consisting of 12 individual plots. The half-life for molinate
was 4.7 days @2L/ha and 4.2-5.6 days @3.75 L/ha. The half-life for thiobencarb ranged
between 3.4 - 4.1 days when applied at 3.75 L/ha. The half-life of clomazone applied at
0.5L/ha was 2.9-7.2 days. The half-life for chlorpyrifos applied at 0.1L/ha was 5.4 days.
The persistence for molinate ranged from 33.6 to 42 days; for thiobencarb ranged from 22.2
to 29.6 days and for chlorpyrifos ranged from 48.6 to 59.4 days to reach EPA notification
levels. Although there are no set EPA Notification Levels for clomazone, it would take 14
days for values to dissipate to 3μg/L. These data would suggest that the current withholding
period of 28 days is adequate for complete dissipation of thiobencarb and clomazone but in
some situations is inadequate for dissipation of molinate and chlorpyrifos.
The effect of sampling position within bays and proximity of bays to water inflow points on
pesticide concentration was studied in a commercial rice field with a bankless channel and
zig-zag water flow pattern. Sampling position within a bay had a significant (P<0.05) effect
on the concentration of the insecticide chlorpyrifos and the herbicide molinate but only in the
bay which was closest to the water inflow point, not in other bays. The concentration of both
molinate and chlorpyrifos increased significantly (P<0.05) in a direction distal from the
bankless channel. There was no significant effect on the proximity of levees to sampling
position. Concentrations of molinate and chlorpyrifos were significantly lower (P<0.05) in
the top bay compared with two other bays located down the fall of the field, further away
from the supply of water. The herbicide benzofenap was not affected by the sampling position
within any of the bays, but the concentrations in the bay nearest the supply were all
significantly (P<0.05) lower than in other two bays. The highest Cl, EC, TDS and TSS
measurements and lowest pH were found in bays furthest from the water inflow ... |
| URI: | http://hdl.handle.net/2123/130 |
| ISBN: | 1 876903 36 8 |
| Appears in Collections: | Research Papers
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