U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting Charleston South Carolina March 8-12, 1999--Volume 2 of 3--Contamination of Hydrologic Systems and Related Ecosystems, Water-Resources Investigations Report 99-4018B

Herbicide Concentrations in the Sacramento-San Joaquin Delta, California

By Kathryn M. Kuivila, Holly D. Barnett, and Jody L. Edmunds

This paper is available in pdf format: CA-0218.pdf

ABSTRACT

The Sacramento-San Joaquin Delta watershed in California encompasses agricultural areas that receive intensive applications of various herbicides, including some designed to inhibit photosynthesis. This study is to determine whether herbicides impair phytoplankton primary productivity in the Sacramento-San Joaquin Delta. The sampling strategy contrasted conditions in May-June, a time of expected high herbicide concentrations, with conditions in October-November, a time of expected low herbicide concentrations. Water samples from May through November 1997 were analyzed for herbicide concentrations and phytoplankton primary production rates. Thirteen herbicides were detected in one or more water samples. Herbicide concentrations varied considerably spatially and temporally. Diuron, metalachlor, and diethatyl-ethyl had the highest concentrations in the study. Two sites, Paradise Cut at Paradise Road and French Camp Slough at McKinley Road, had the most frequent detections and highest concentrations of herbicides.

The highest concentrations of molinate and thiobencarb were detected at the site receiving input from the Sacramento River watershed, following application of these herbicides on rice in May. The highest use of EPTC is in the San Joaquin River watershed and the highest concentrations were detected at the site representing this watershed. In contrast, the source of the other herbicides could not be attributed to a single watershed. Diuron and metolachlor had widespread detections that can be explained by their relatively high use in all the watersheds, whereas diethatyl-ethyl primarily was detected at the one site near the highest application in the Delta. The distributions of 2,4-D and hexazinone were more complex, and the amounts and timing of application do not readily explain the pattern of occurrence.

The results of this part of the study illustrate the complexity of herbicide concentrations in the Sacramento-San Joaquin Delta. In particular, the occurrence of diuron and hexazinone needs to be studied in more detail to determine their influence on primary production and phytoplankton species composition.