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
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
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
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
IV. STUDY DESIGN
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.
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 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 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
V. CHEMICAL ANALYSIS / QUALITY CONTROL
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
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
VI. DATA ANALYSIS
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
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1989. Polymers as soil conditioners under consecutive irrigations
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