Protocol for Sampling Selected Citrus Practices (Phase Two):

Rain runoff sampling from post emergent, mechanical incorporation, and grower standard treatments.

STUDY #158

November 1996

Department of Pesticide Regulation Environmental Monitoring and Pest Management Branch

1020 N Street, Room 161

Sacramento, CA 95814-5624

I. INTRODUCTION

Preemergent herbicide residues associated with citrus and grape production have been detected in several hundred Fresno and Tulare County domestic wells. The Environmental Hazards Assessment Program (EHAP) is conducting an educational and outreach program to mitigate the movement of preemergent herbicides to ground water in these counties. As part of this project, EHAP is conducting cooperative field studies with the University of California Cooperative Extension (UCCE) to test management practices for their ability to reduce offsite movement of preemergent herbicides.

Previous EHAP studies indicate that an important mechanism for movement of simazine, diuron and bromacil to ground water in hard-pan citrus growing areas is via surface water runoff to dry wells. In Fresno and Tulare County areas, such runoff events occur principally during the winter months, either a result of seasonal rainfall or frost protection irrigations. Therefore, late fall herbicide applications pose the greatest risk for movement of herbicides to ground water in hard pan areas. Two ways to reduce movement of soil-applied preemergent herbicides in rain runoff are to (a) reduce or eliminate fall preemergent herbicide applications, or, if preemergent herbicides continue to be used, (b) ensure that they are effectively incorporated into the soil after application.

In the former case, use of contact herbicides such as glyphosate during the fall season may be a practical solution that allows continued weed control during winter months. Glyphosate is an effective contact herbicide that is already used in citrus production. While glyphosate is foliar applied and sorbs strongly to soil, runoff samples will be taken from the glyphosate treatment in this field study to experimentally determine if late fall applications of glyphosate may pose a risk for movement in surface runoff.

Most citrus growers rely on rainfall to incorporate preemergent herbicides after fall application. However, low infiltration rates of barren compacted citrus middles prevent effective incorporation and lead to off-site movement in surface runoff. Previous EHAP studies of herbicide runoff under simulated rainfall conditions indicate that shallow mechanical incorporation can reduce herbicide runoff by 1-2 orders of magnitude in compacted hard pan soils as compared to rainfall incorporation. A comparison of runoff concentrations between rainfall incorporation and shallow mechanical incorporation will be made in this study under actual wintertime field conditions.

II. OBJECTIVES

A. Postemergent glyphosate treatment

The objective in postemergent glyphosate runoff sampling is to provide observational data of glyphosate edge-of-field runoff concentrations from noncultivated citrus orchard plots under wintertime runoff conditions.

B. Shallow mechanical incorporation/ grower standard treatments

The objective in this portion of the study is to determine the effect of shallow mechanical herbicide incorporation on simazine and diuron edge-of-field runoff concentrations relative to rainfall incorporation. The two treatments will be : (a) a control treatment consisting of a "grower standard" application of 2.5 lbs simazine plus 2.5 lbs diuron per acre followed by no incorporation other than seasonal rainfall, and (b) the identical herbicide application followed by shallow (approximately 2 inches) mechanical incorporation. Following the runoff sampling, soil cores will be collected from the upper 6 inch of soil to evaluate any treatment differences in residual soil concentrations of simazine and diuron.

III. PERSONNEL

This study will be conducted by the Environmental Hazards Assessment Program (EHAP) under the general direction of Don Weaver, Senior Environmental Research Scientist. Key personnel are listed below:

Project Leader: Frank Spurlock

Field Coordinator: Cindy Garretson

Senior Staff Scientist: John Troiano

Authorship of the final report should include, but not be limited to, Frank Spurlock, Cindy Garretson, and John Troiano. Questions concerning this monitoring program should be directed to Peter Stoddard at (916) 324-4078, facsimile (916) 324-4088.

IV. STUDY DESIGN

For these preliminary studies, three treatments will be sampled: glyphosate, grower standard application with no mechanical incoporation, grower standard treatment with shallow mechanical incoporation. Each of the three treatments will be applied at four sites, and replicated three times at each site. The sites will be selected by UCCE.

A. Glyphosate treatments.

No deposition samples will be taken for the glyphosate treatments. One grab sample will be taken from each replicate at each site from the first rain runoff event after application. Sampes will be taken from the furrow as close as possible to the furrow downstream end. The total number of glyphosate water samples will be 4 sites x 3 reps x 1 sampling event = 12 water samples.

B. Shallow mechanical incorporation and no incorporation grower standard.

Two 1 ft2 Kimbies will be randomly located in each replicate at each site immediately prior to application of simazine and diuron. The Kimbies will be used to measure simazine and diuron appplication in the treatments. After application, the two Kimbies from each replicate will be extracted in a single extraction at the laboratory to provide a composited measure of deposition in each replicate. Total deposition samples (after compositing) = 2 treatments x 4 sites x 3 replicates x 2 Kimbies/replicate = 24 deposition samples.

One grab sample will be taken from each replicate at each site from the first rain runoff event after application. Sampes will be taken from the furrow as close as possible to the furrow downstream end. Following the rain event, three 6-inch soil cores will be composited from each replicate at each site. Total water samples = 2 treatments x 4 sites x 3 replicates = 24 water samples. Total soil samples = 2 treatments x 4 sites x 3 replicates = 24 soil samples. All of the soil and water samples will be analyzed for both simazine and diuron.

V. CHEMICAL ANALYSIS / QUALITY CONTROL

All samples will be analyzed by the California Food and Agriculture Analytical Chemistry Laboratory in Sacramento.

All sampling and analytical procedures will be conducted in accordance with the current EHAP QAPP, with the following exceptions:

1. System audits. Sampling for the surface runoff is likely to occur at odd hours, therefore it is not possible that personnel be available for auditing purposes.

2. Protocol approval by U.S. EPA. Time constraints do not permit transmittal and review of this protocol by U.S. EPA.

VI. DATA ANALYSIS

Glyphosate data will be evaluated for the presence of glyphosate in runoff water. Nonzero analytical results will be compared to existing water quality standards.

A comparison will be made of simazine and diuron concentrations in runoff between mechanical incoporation plots and nonincorporated plots using normal-based statistical methods. Residual simazine and diuron concentrations in the upper 6 inches of soil after rainfall will be similarly contrasted between the two treatments.