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 surface-water concentration performance goals established by the Central Valley Regional Water Quality Control Board. There have been major changes in rice pesticide use over the past decade, 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.

Orlando, J.L., and Kuivila, K.M., 2004, Changes in rice pesticide use and surface water concentrations in the Sacramento River watershed, California: U.S. Geological Survey Scientific Investigations Report 2004–5097, 28 p.

Kuivila, K.M. and Jennings, B.E., 2007, Input, flux, and persistence of six select pesticides in San Francisco Bay, Int. J. Environ. Anal. Chem. 87:13, 897 – 911.

Crepeau, K.L., and Kuivila, K.M., 2000, Factors controlling rice pesticide concentrations in the Colusa Basin Drain and the Sacramento River, California, 1990-1993, J. Environmental Quality, vol 29(3), pp 926-935.