A Geographical Infomation Systems Evaluation of Thiobencarb Applications in Glenn and Colusa Counties on Rice From 1994 Through 1996, and Concentrations Detected in the Colusa Basin Drain.

CALIFORNIA ENVIRONMENTAL PROTECTION AGENCY
DEPARTMENT OF PESTICIDE REGULATION
ENVIRONMENTAL MONITORING AND PEST MANAGEMENT BRANCH
1020 N STREET, ROOM 161
SACRAMENTO, CALIFORNIA 95814-5624
August 5,1997

A GEOGRAPHICAL INFORMATION SYSTEMS EVALUATION OF THIOBENCARB APPLICATIONS IN GLENN AND COLUSA COUNTIES ON RICE FROM 1994 THROUGH 1996, AND CONCENTRATIONS DETECTED IN THE COLUSA BASIN DRAIN

I. INTRODUCTION

Thiobencarb is the active chemical ingredient of the rice herbicides Bolero® and Abolish®. Bolero® is a granular/flake herbicide which is applied to water post-flood. Abolish® is an emulsifiable concentrate herbicide, applied pre-flood directly to the soil, or post-flood, post-emergence to drained fields. Aerial application methods are primarily used to apply thiobencarb in rice growing areas of the Sacramento Valley to control annual weeds such as watergrass and sprangletop.

The Department of Pesticide Regulation (DPR) is concerned about the off-site movement of thiobencarb, during and following application, into agricultural drains and watersheds adjacent to rice-growing areas, which eventually flow into the Sacramento River. Several circumstances exist that contribute to this off-site movement. Aerial pesticide drift can occur during herbicide applications, and has been observed to coincide with some high concentrations of herbicides in agricultural drains. Unusually high rainfall and windy weather, causing levee flooding in April and May, have resulted in early emergency releases of rice field water before water-holding requirements have been met in recent years. Much of thiobencarb's dissipation depends on water being held on the rice fields following application, to allow adsorption to soil particles where it can degrade. Even when adequate holding requirements are met, residual levels of thiobencarb can exist in rice field tailwater.

DPR implemented a program beginning in 1983 to mitigate off-site movement of thiobencarb. A key provision of this program is a 30-day water holding requirement in all areas except where the grower's tailwater recovery system is part of a regional water recirculating system or where negligible amounts of field water are discharged into surface waterways. In addition, the Central Valley Regional Water Quality Control Board (CVRWQCB) established a water quality goal of 1.5 parts per billion (ppb) for all waters designated as freshwater habitat, including the Colusa Basin Drain (CBD5), an agricultural drain located near Highway 20 southwest of the town of Colusa, in Colusa County. While still below detectable levels in the Sacramento River, thiobencarb concentrations may be increasing at CBD5. Water samples are collected once or twice weekly during late April through June at CBD5 and analyzed for the presence of rice pesticides.

At CBD5 in 1994, thiobencarb was detected nine times, and the water quality goal, set by the CVRWQCB, was exceeded twice. The peak concentration was 37.4 ppb. In 1995 thiobencarb was detected 17 times and the water quality goal was exceeded four times. The peak concentration was 3.8 ppb. In 1996 thiobencarb was detected 16 times and the water quality goal was reached or exceeded 13 times. The peak detection of thiobencarb at CBD5 was 16.2 ppb.

Thiobencarb use increased approximately 47 percent in 1995 compared to 1994, and in 1996 use was up 29 percent over 1995. Total acreages treated in Colusa County increased from 23,097 acres in 1994 to 66,596 acres in 1996. In Glenn County total acres treated increased from 7,654 in 1994 to 12,211 in 1996. Abolish® use has increased because it works well with the "pinpoint flood" method of water management. Rice fields treated in this manner are flooded, then drained or allowed to dry soon after seeding to promote root growth in the rice seedling. Abolish® is then aerially applied and the field is reflooded. In addition to increased use, weed resistance problems began to occur with Londax®, another rice herbicide, used to control sedges and broadleaf weeds. Thiobencarb is now used in combination with other herbicides to control the same weeds that were at one time controlled by Londax® alone.

II. OBJECTIVE

To better quantify the relationship between thiobencarb detections at CBD5 and its application on rice fields, staff at DPR will use a geographic information systems (GIS) approach to spatially visualize the influences of different thiobencarb formulations, use patterns, application methods, and agricultural drain flows emanating from rice fields in Glenn and Colusa counties from 1994 through 1996. Staff believe this approach will enable a better understanding of the watershed upstream from CBD5 and can possibly reveal previously unidentified trends and relationships, which may lead to a better understanding of the movement of thiobencarb from its application point. This GIS project will also incorporate accurate mapping of the watershed and allow for further GIS analysis of this region.

III. PERSONNEL

This project will be conducted by the Implementation Group staff of the Environmental Hazards Assessment Program at DPR under the supervision of Roger Sava. KayLynn Newhart is the project leader. Nancy Gorder and Marshall Lee will collaborate on the project and John Steggall will act as consultant for the GIS efforts.

IV. METHODS

ArcView, a desktop GIS software program, will be used to create thematic mapping of rice acreages, adjacent agricultural drains, and waterways which receive rice tailwater runoff in Glenn and Colusa counties. Types of data that will be analyzed and presented in GIS format will include drainage water flow information available from Glenn-Colusa Irrigation District's Water Management Program. Total rice acreages and thiobencarb application by date, amount, formulation, and application method, will be obtained from DPR's Pesticide Use Reporting Database and the agricultural commissioners staff of Glenn and Colusa counties. Other data sources will include the Information Center for the Environment, Teale Data Center, United States Geological Survey, and the California Department of Water Resources.

A summary report will be written after completion of the project.

V. TIMETABLE

The completion of this project is estimated by March 1, 1998. For further information, please contact Roger Sava at (916) 324-4131.

VI. REFERENCES

California State Water Resources Control Board. 1984. Rice Herbicides: Molinate and Thiobencarb. Toxic Substances Control Program, Special Projects Report No. 84-4sp, Sacramento, CA. April 1984.

California Environmental Protection Agency, Department of Pesticide Regulation. 1986. Information on Rice Pesticides Submitted to the California Regional Water Quality Control Board Central Valley Region.

California Environmental Protection Agency, Department of Pesticide Regulation. 1995. Information on Rice Pesticides Submitted to the California Regional Water Quality Control Board Central Valley Region.

California Environmental Protection Agency, Department of Pesticide Regulation. 1994. Information on Rice Pesticides Submitted to the California Regional Water Quality Control Board Central Valley Region.