Effect of Surfactant on Offsite Movement of

Preemergent Herbicides in Surface Runoff (Study 149)

June 1996

Department of Pesticide Regulation

Environmental Monitoring and Pest Management Branch

1020 N Street, Room 161

Sacramento, CA 95814 - 5624


The two California counties with the largest number of confirmed premeergent herbicide detections in well water are Fresno and Tulare. Statistical methods have been used to identify broad categories of California climate and soil conditions where residues have been found in ground water (Troiano et al., 1994), and analysis of the Fresno-Tulare County area in particular indicates that confirmed detections occur primarily in two soil types in the two counties. One soil type is a coarse soil condition where leaching may be a predominant process for pesticide movement to ground water. The second soil type is a hardpan soil condition; in these low permeability soils dry wells are sometimes used to aid in disposing of surface runoff. Concentrations of simazine, diuron, and bromacil ranging up to 1100 ppb have been detected in runoff water entering dry wells during rain events (Braun and Hawkins, 1991). This direct transport mechanism is probably an important mechanism for herbicide movement to ground water in impermeable hardpan soil areas. More than 240 wells have been confirmed positive for one or more preemergent herbicide residues in Fresno and Tulare hardpan soil areas.

Citrus accounts for approximately 75 percent of simazine, diuron, and bromacil use in the hardpan soil areas of Fresno and Tulare Counties. Citrus growers favor bare soil conditions in winter to enhance frost protection, so that more than half of the annual preemergent herbicide applications in citrus occur in late fall. Many users of residual herbicides rely on natural precipitation to incorporate these materials into the soil surface for activation. However, bare citrus orchard middles are often highly compacted with correspondingly low infiltration rates. As a result, heavy rainfall events can move preemergent residues off-site in surface runoff which can then move to dry wells.

A second potentially important situation where off-site movement of herbicides in runoff may be important to ground water quality is herbicide applications to rights-of-way (ROW). Simmons and Leyva (1994) sampled roadway storm water runoff from infiltration basin inflows, basin storage, and basin dry wells in San Joaquin County, finding five preemergent herbicides at levels ranging up to ~ 80 :g L-1. Powell et al. (1996) conducted a study of highway runoff in Glenn County where concentrations of simazine and diuron ranging up to 570 and 2800 :g L-1, respectively, were observed in simulated and actual rainfall runoff from highway shoulder plots. Because (1) ROW applications account for about 15 percent of simazine and diuron use in Fresno and Tulare Counties, and (2) maximum use rates for ROWs are generally 3-8 times greater than crop application rates, ROW applications may be an important source of herbicides reaching ground water.

Shallow mechanical incorporation has been shown to be effective in reducing the total mass of herbicide moving off-site in runoff from middles of citrus orchards (Troiano and Garretson, unpublished data). However, many growers are reluctant to disturb shallow soil in orchard middles due to concerns over root health or damaging feeder roots. In ROWs, mechanical incorporation may not be physically possible, or may be economically prohibitive.

The purpose of this study is to evaluate an alternative method of incorporation: use of nonionic surfactant applied concomitantly with the residual herbicide. Nonionic surfactants have traditionally been used to improve leaf wetting, plant uptake and rainfastness of contact herbicides (e.g., Roggenbeck et al, 1993). Experimental data for surfactant effects on efficacy and off-site movement of preemergent herbicides are relatively sparse. A study evaluating the effect of surfactants on trifluralin residual efficacy in sugarcane suggested that nonionic surfactants improved shallow incorporation of trifluralin (Loveland Industries, unpublished data). Ninety day residual weed control was effective in both shallow incorporation plots and those where nonionic surfactants had been used, whereas residual weed control in the control plots (no incorporation and no surfactant) was markedly poorer. Other studies have found that application of nonionic polymers to some soils can aid in maintaining high infiltration rates under simulated rainfall conditions (Ben-Hur et al., 1989; Letey et al., 1961), however, effects on compacted soils were not evaluated. Improved infiltration suggests superior incorporation may be observed in the case of a soil applied herbicide. A study of nonionic organosilicon surfactants in Florida found dramatically enhanced herbicidal activity of diruon and norflurazon in greenhouse pots when the surfactants are simulataneously applied with residual herbicides (Tan and Singh, 1996). The authors concluded that use of these surfactants may allow much lower rates of these herbicides when used in conjunction with surfactants.

In California, anecdotal information indicates that organosilicon surfactants may significantly improve residual efficacy of preemergent herbicides in ROW applications (Paul Washburn, Washburn Agricultural Applicators, personal communication). The improved residual control, if true, suggests superior rainfastness under right-of-way conditions, hence improved herbicide incorporation at the time of application. A possible reason for this apparent phenomena is the exceptionally low surface tensions associated with organosilicone surfactant solutions. Low surface tensions will improve spreading upon application, and should also improve water infiltration rates into capillary soil pores (Taylor and Ashcroft, 1972; Hiemenz, 1986) leading to improved soil incorporation. However, the question of whether these effects will be significant at economically realistic surfactant application rates remains unknown.


The objective of this study is to compare concentrations and total mass of simazine, diuron and bromacil in runoff from citrus orchard middles to the furrow under simulated rainfall conditions for herbicide applications with and without organosilicon surfactant.


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

Laboratory Liaison/Quality Assurance: Cindy Garretson

Experimental Design/Data Analysis: Terri Barry

Authorship of final report should include, but not be limited to Frank Spurlock, Cindy Garretson, Terri Barry, and John Troiano.

Questions concerning this monitoring program should be directed to Peter Stoddard at (916) 324-4078 and FAX (916) 324-4088.


This study will be conducted in a mature citrus grove on the Fresno California State University farm. Study design and methods will be similar to previous EHAP studies that have examined herbicide movement in surface runoff from citrus middles (e.g., Sano, 1996). Both treatments will be replicated six times in 18' x 10' experimental plots for a total of 2x6 = 12 plots.

(1) application of simazine, diuron, and bromacil followed within 24-48 hours by a simulated rainfall event, and

(2) application of simazine, diuron, bromacil, and Silwet-77 organosilicone surfactant followed within 24-48 hours by a simulated rainfall event.

Background soil samples

Two background soil samples will be collected from each plot: one taken from the row middle and one from the plot furrows. The row middle soil sample will be a composite of three individual 10 cm cores. The plot furrow soil sample will be a composite of four soil cores, two taken from each furrow within the plot. The total number of background soil samples for chemical analysis will be 2 samples per plot x 12 plots = 24 samples; these composite samples will be obtained from (12 x 3, middles) + (12 x 4, furrows) = 84 individual soil cores.

Herbicide/surfactant application

The spray solution rate will be 46 L hectare-1 (30 gallons acre-1 , nominal). Each treatment will include application of simazine, diuron, and bromacil at 1.8 Kg hectare-1 (2 lbs a.i. acre-1) each. Treatment 2 will also include simultaneous application of 40 mL Silwet-77 hectare-1 (3.3 oz. Silwet-77 surfactant acre-1 , approximately 10 oz. surfactant per 100 gallons spray solution). Applications will be made with a calibrated pressurized backpack sprayer and a hand boom similar to the method used by Sano (1996). Plot furrows will be covered by a plastic tarp during all applications to avoid spray drift deposition to the furrow area. Herbicide deposition rates will be measured in each plot using three deposition rate jars. The half-pint mason deposition rate jars, each filled with 50.0 grams of clean (simazine,diuron and bromacil-free) sand, will be placed on the soil surface within the plot row middle.

Simulated rainfall

Simulated rainfall events at each plot will be conducted using 2 impact sprinklers located in opposing corners of each plot. The nominal water application rate will be 2.5 cm hr-1; the total water application to each plot will be approximately 650 L (~ 2 cm). Prior to the simulated rainfall water application, distribution uniformity measurements will be conducted in plots of similar dimension at the approximate pressure and flow rate anticipated for applications to the experimental plots. During water application, actual water applied in each plot will be measured using flow meters.

Runoff collection

Runoff water from the simulated rainfall events will be collected immediately past the downstream end of the plot furrows using removable sample collection buckets placed in holes excavated for this purpose. The furrows will be diked at each end of the plot, and runoff directed through drain spigots inserted in the dike. Water flow will be directed through the spigot and into the sample collection buckets similar to the method used by Sano (1996). Total runoff volume collected in each bucket will be recorded, and 1L water subsamples from each filled bucket will then be collected (unfiltered) and stored refrigerated in 1L amber bottles until analysis. Based on results of Sano (1996), it is expected that, on the average, each plot will yield approximately 7 water samples over the duration of a runoff event. The projected total number of water samples for this study will be 7 x 12 = 84 water samples.

Post-simulated rainfall soil samples.

Two post-rainfall simulation soil samples will be collected from each plot: one taken from the row middle and one from the plot furrows. The row middle soil sample will be a composite of three individual 10 cm cores. The plot furrow soil sample will be a composite of four soil cores, two taken from each furrow within the plot. The total number of post simulation soil samples for chemical analysis will be 2 samples per plot x 12 plots = 24 samples; these composite samples will be obtained from (12 x 3, middles) + (12 x 4, furrows) = 84 individual soil cores.


Total number of field samples for chemical analysis will be:

SOIL: background (24) + deposition (36) + final (24) = 84 soil samples

WATER: runoff = 84 water samples

APPL Laboratories in Fresno will develop and validate a method for analyzing soil and water samples for simazine, diuron and bromacil using an HPLC/UV method that has been developed and validated.

The analytical quality control program will include the following: A solvent blank and two matrix spikes will be analyzed with each extraction set. Results of matrix spikes must fall within established warning and control limits currently established. This study will be done in accordance with EHAP SOP QAQC001.001.


Data collected will include (1) total runoff volume from each plot, and (2) simazine, diuron and bromacil concentrations in the runoff water. Together these data also provide a measurement of total simazine, diuron and bromacil that move off the plots in runoff. Normal-based statistical methods, including ANOVA, will be used to compare the effect of surfactant on treatment means of total simazine mass, total diuron mass, and total bromacil mass transported off-site in runoff. Soil furrow and middle samples will be used to estimate mass balance and determine surfactant effects, if any, on herbicide redistribution after simulated rainfall.


Sample Collection July-August 1996

Herbicide Analysis August-September 1996

Data Analysis September-November 1996

Final Report February 1997


Ben-Hur, M., J. Faris,. M. Malik, and J. Letey. 1989. Polymers as soil conditioners under consecutive irrigations and rainfall. Soil Sci. Soc. Am. J. 53:1173.

Braun, A.L. and L.S. Hawkins. 1991. Presence of Bromacil, Diuron, and Simazine in Surface Water Runoff From Agricultural Fields and Non-Crop Sites in Tulare County, California. report PM91-1. Environmental Monitoring and Pest Management Branch, California Department of Pesticide Regulation.

Hiemenz, P.C. 1986. Principles of Colloid and Surface Chemistry, 2nd Ed. p. 338. Marcel Dekker, New York.

Letey, J., R.E. Pelshik and J. Osborne. 1961. Wetting Agents. Calif. Agric. October 1961, p. 8.

Powell, S, R. Neal, and J. Leyva. 1996. Runoff and Leaching of Simazine and Diuron used on Highway Rights-Of-Way. report EH96-03.Environmental Monitoring and Pest Management Branch, California Department of Pesticide Regulation.

Roggenbuck, F.C., D. Penner, R.F. Burrow, and Bryan Thomas. 1993. Study of the enhancement of herbicide activity and rainfastness by an organosilicon adjuvant utilizing radiolabelled herbicide and adjuvant. Pest. Sci. 37:121.

Sano, B. 1996. The Redistribution of Simazine Within a Citrus Orchard Floor. M.S. Thesis, California State University, Fresno.

Simmons, S.E. and J.J. Leyva. 1994. Presence of Soil-Applied Herbicides in Three Rights-Of-Way Basins in San Joaquin County, California. report EH 94-01. Environmental Monitoring and Pest Management Branch, California Department of Pesticide Regulation.

Tan, S., and M. Singh. 1996. Weed control efficacy of diuron and norflurazon as affected by adjuvants. submitted to Florida Agricultural Experimental Station Journal.

Taylor, S., and G. Ashcroft. 1972. Physical Edaphology: The physics of irrigated and nonirrigated soils. p. 342. W.H. Freeman and Co., San Francisco.

Troiano, J., B. Johnson, S. Powell, and S. Schoenig. 1994. Use of cluster and principal component analyses to profile areas in California where ground water has been contaminated by pesticides in California. Environmental Monitoring and Assessment, 32:269-288.