Changes in Rice Pesticide Use and Surface Water Concentrations in the Sacramento River Watershed, California

By James L. Orlando and Kathryn M. Kuivila



Scientific Investigations Report 2004-5097

Sacramento, California 2004

In cooperation with the

Central Valley Regional Water Quality Control Board

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    Pesticides applied to rice fields in California are transported into the Sacramento River watershed by the release of rice field water. Despite monitoring and mitigation programs, concentrations of two rice pesticides, molinate and thiobencarb, continue to exceed the surface-water concentration performance goals established by the Central Valley Regional Water Quality Control Board. There have been major changes in pesticide use over the past decade, and the total amount of pesticides applied remains high. Molinate use has declined by nearly half, while thiobencarb use has more than doubled; carbofuran has been eliminated and partially replaced by the pyrethroid pesticide lambda-cyhalothrin.

    A study was conducted in 2002 and 2003 by the U.S. Geological Survey to determine if the changes in pesticide use on rice resulted in corresponding changes in pesticide concentrations in surface waters. During the rice growing season (May-July), water samples, collected weekly at three sites in 2002 and two sites in 2003, were analyzed for pesticides using both solid-phase and liquid-liquid extraction in combination with gas chromatography/mass spectrometry. Analytes included lambda-cyhalothrin, molinate, thiobencarb, and two degradation products of molinate: 2-keto-molinate and 4-keto-molinate.

    Molinate, thiobencarb, and 4-keto-molinate were detected in all samples, 2-keto-molinate was detected in less than half of the samples, and lambda-cyhalothrin was not detected in any samples. At two of the sites sampled in 2002 (Colusa Basin Drain 1 and Sacramento Slough), concentrations of molinate were similar, but thiobencarb concentrations differed by a factor of five. Although concentrations cannot be estimated directly from application amounts in different watersheds, the ratio of molinate to thiobencarb concentrations can be compared with the ratio of molinate to thiobencarb use in the basins. The higher concentration ratio in the Sacramento Slough Basin, compared with the ratio in the basin area feeding the Colusa Basin Drain 1, is consistent with the higher use ratio, suggesting that differences in application amounts can explain the observed concentration differences. The samples from the downstream site (Tower) sampled in 2002 had the lowest concentrations of pesticides. Performance goals were exceeded for either molinate or thiobencarb in six samples from the upstream sites, but not in any samples from the downstream Tower site. In 2003, concentrations at upstream sites were much lower than the previous year with only one sample containing thiobencarb at a concentration above the performance goal. Lower concentrations could be partially due to delays in rice planting and pesticide application owing to spring rainstorms.

    Historical data is available on peak concentrations of molinate and thiobencarb measured at Colusa Basin Drain 5 (one of our sites in 2003) since 1981. Implementing holding times for pesticide-treated rice field water in the early 1980s succeeded in decreasing concentrations in surface waters. Detailed pesticide use data is available since 1991 and changing use patterns for molinate and thiobencarb can explain some, but not all, of the trends in peak pesticide concentrations. A stronger relationship is seen between the lengths of time that performance goals were exceeded and the amount of a pesticide applied within a basin.

    Different extraction and analytical techniques were used to improve the recovery and lower the method detection limit for lambda-cyhalothrin. Recoveries of lambda-cyhalothrin from solid-phase extraction cartridges typically vary, so subsamples were processed by liquid-liquid extraction. The advantage of using a larger sample volume (3 L instead of 1 L) to lower detection limits was offset by poor recovery during the cleanup step using an activated carbon column. Results suggest that as the concentrations of dissolved organic carbon in the sample increase, the recovery goes down. Limiting the sample size to 1 L and eliminating the activated carbon cleanup step improved the analysis for lambda-cyhalothrin.

    Suspended sediment samples were collected at two sites in July 2002 for analysis of molinate, thiobencarb, and lambda-cyhalothrin. Molinate was detected in both samples, thiobencarb was detected and quantified in one sample, but could not be quantified in the other, and lambda-cyhalothrin was not detected in either sample. Sediment concentrations of both molinate and thiobencarb were higher than predicted by calculated equilibrium partitioning using concentrations in the surrounding aqueous phase.






Pesticide Use Patterns

Project Design

Purpose and Scope


Study Design and Sampling

Study Area

Selection of Sampling Sites

Sample Collection

Surface Water 2002

Suspended Sediment 2002

Surface Water 2003

Sample Processing and Analytical Methods

Dissolved Pesticides

Quality Assurance and Quality Control

Pesticides Associated with Suspended Sediments

Analysis of Suspended Sediments and Dissolved Organic Carbon

Analytical Results

Pesticide Concentrations

Suspended Sediment and Dissolved Organic Carbon Concentrations

Sampling Method Comparison

Comparison of Pesticide Concentrations

Dissolved Pesticide Concentrations 2002

Dissolved Pesticide Concentrations 2003

Performance Goals and MCLs

Historical Peak Concentrations

Analysis of Lambda-Cyhalothrin

Matrix Recoveries in 2002

Additional Method Testing in 2003

Dissolved Organic Carbon

Pesticide Partitioning Between Water and Sediments

Sediment-Associated Pesticide Concentrations

Equilibrium Partitioning


References Cited

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