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Rice Pesticide Concentrations in the Colusa Basin Drain and the Sacramento River, California, 1990–1993

Water Resources Division, U.S. Geological Survey, 6000 J Street, Placer Hall, Sacramento, CA 95819.

^* Corresponding author (kcrepeau{at}usgs.gov).

ABSTRACT

The pesticides molinate, thiobencarb, and carbofuran are applied to rice (Oryza sativa L.) fields in the Sacramento Valley, California, each year during April through June. These pesticides are of concern because of their adverse effects on water quality and their potential adverse effects on aquatic life. Therefore, the California Regional Water Quality Control Board (CRWQCB) mandated the holding of irrigation-return water in rice fields to increase pesticide degradation and dissipation before the water is released to the Sacramento River. The CRWQCB also established performance goals to maintain drinking water quality for the city of Sacramento and to protect the habitat for aquatic life. The objectives of this study were to determine the effects of increased irrigation-return water holding times on rice pesticide concentrations and loads in the Colusa Basin Drain and the Sacramento River. Dissolved pesticide concentrations were measured in water samples collected in May through July during 1990–1993 at the Colusa Basin Drain at Road 99E near Knights Landing (Colosa Basin Drain) and at the Sacramento River at Sacramento. Pesticide concentrations and loads at both sites showed a decrease from 1990–1992 and an increase in 1993. This trend cannot be explained by the yearly holding time regulations but can be explained by the amount of yearly emergency releases. The yearly changes in pesticide concentration detected demonstrate that continued monitoring and management of pesticides is necessary to maintain water quality.

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