The Persistence of Pesticides in Floodwaters and How This is Influenced by Water Management and Layout

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 ...