RUNOFF FROM CITRUS ORCHARD MIDDLES:
COMPARISON OF THREE HERBICIDES AND EFFECT
OF ORGANOSILICON SURFACTANT
F. Spurlock, C. Garretson, and J. Troiano
STATE OF CALIFORNIA
Environmental Protection Agency
Department of Pesticide Regulation
Environmental Monitoring and Pest Management Branch
Environmental Hazards Assessment Program
Sacramento, California 95814-5624
This study was partially supported by Loveland Industries who contributed
to analytical expenses and supplied the organosilicon surfactant.
The mention of commercial products, their source, or use in connection
with material reported herein is not to be construed as an actual or implied
endorsement of such product.
TABLE OF CONTENTS
Table of Contents
List of Tables
List of Figures
Materials and Methods
runoff water sampling
Evaluation of Surfactant Effects
Mathematical Description of Runoff
TABLE OF CONTENTS (CONTINUED)
Soil Texture Data
Catch Can Water Application Data
Herbicide Deposition (Application) Data
Spike Recovery Data
Repeated Measures ANOVA/Multiple Comparisons
Runoff Data: Sampling Intervals and Concentrations
Post-runoff Soil Sample Data
Model Fits to Runoff Data
LIST OF TABLES
Table 1. Selected herbicide properties
Table 2. Plot furrows and middles soil characteristics
Table 3. Summary QA/QC analytical data: control limits(%), reporting
limits, and spike recoveries (%).
Table 4. Data distribution characteristics.
Table 5. Summary of RM-ANOVAs and multiple comparisons
Table 6. Application deposition summary data
Table 7. Summary mass balance data for runoff experiments
fraction of measured application
fraction of theoretical application
LIST OF FIGURES
Figure 1. Plot layout schematic.
Figure 2. Structure of organosilicon surfactant.
Figure 3. Runoff profiles for simazine, diuron, bromacil from surfactant
Figure 4. Runoff profiles for simazine, diuron, and bromacil from nonsurfactant
Figure 5. Average runoff data and least-squares minimization fit of
Eq. 2. 28
Runoff from citrus middles can cause surface and ground water contamination
problems. A study was conducted to (1) compare the differences in runoff
of three herbicides: simazine, diuron, and bromacil; and (2) evaluate the
effect of a commercial organosilicon surfactant on reducing runoff of three
soil applied preemergent herbicides from citrus orchard middles. No effect
on herbicide runoff concentrations or herbicide remaining in the middles
was observed due to surfactant. Approximately 2.2 cm, or about 2/3, of
the 3.3 cm 1.5 h simulated rainfall water application was recovered from
the plots as runoff, emphasizing the significance of low soil infiltration
rates in driving herbicide runoff. Herbicide concentrations in the runoff
samples ranged from about 50 to 1,500 ug L-1, comparable to
field samples of actual wintertime rain runoff in and around citrus orchards
in Fresno and Tulare Counties. Although less bromacil moved off-site than
simazine or diuron, the peak bromacil runoff concentrations and the fraction
of applied bromacil recovered in runoff were still of practical environmental
significance, ranging from 400 to 1500 µg L-1 and 5% to
16 % , respectively. These data indicate that herbicide substitution based
on physical properties is not a viable approach to mitigating off-site
movement from soils where permeability is limited by low infiltration rates.
Across all herbicides, the total fraction of measured application recovered
in the surface runoff varied from 4 to 21 percent, with an overall mean
of 10 percent. An emprical mathematical relationship between herbicide
concentrations and cumulative runoff volume was derived that may have potential
as an analysis tool in future studies.
Fresno and Tulare are the two California counties with the largest number
of confirmed premeergent herbicide detections in well water. Statistical
analysis of associated soil variables indicates that herbicide detections
in these counties are typically found in either of two general types of
soil conditions. One condition is characterized by coarse soil types where
it has been hypothesized that leaching is the predominant mechanism for
pesticide movement to ground water. A second soil condition, where more
than 240 wells have been confirmed positive, is characterized by the presence
of hardpan soils. In many of the hard pan soil areas, an important pathway
for herbicide movement to ground water appears to be direct transport.
In this mechanism herbicide-containing surface runoff enters drainage structures,
subsequently moving directly to deeper, more permeable subsurface regions
below shallow hard pan layers. This direct transport process effectively
bypasses the root zone where organic carbon, microbial activity, and herbicide
breakdown rates are highest. Dry wells are commonly used to dispose of
excess surface water in hard pan areas in certain areas of Fresno and Tulare
Counties; concentrations of simazine, diuron, and bromacil ranging up to
1100 ppb have been detected in runoff water entering dry wells during winter
rain events (Braun and Hawkins, 1991).
The most common preemergent herbicide residues found in ground water of
Fresno and Tulare Counties are simazine, diuron, and bromacil. Citrus accounts
for approximately 75 percent of use of these herbicides 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. Furthermore, many users of residual herbicides rely on natural precipitation
to incorporate these materials into the soil surface for activation. Bare
citrus orchard middles are often highly compacted, with correspondingly
low infiltration rates. These soil characteristics inhibit downward water
movement, limiting incorporation at the point of application. As a result,
heavy rainfall events following herbicide applications can move unincorporated
preemergent residues off-site in surface runoff where they may utimately
enter the subsurface via dry wells.
A second potentially important situation where off-site movement of herbicides
in surface runoff may be important to ground water quality is preemergent
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, and they reported residues
of five preemergent herbicides in runoff at levels ranging up to ~ 80 µg
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 µg L-1, respectively, were observed in
simulated and actual rainfall runoff from highway shoulder plots. ROW applications
are a potentially important source of preemergent herbicides in ground
water because ROW applications account for about 15 percent of simazine
and diuron use in Fresno and Tulare Counties, maximum use rates for ROWs
are generally 3-8 times greater than crop application rates, and ROW applications
are often made in fall months prior to winter rains.
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, 1997). However, many growers are reluctant
to disturb orchard middles due to concerns over crop health and/or damaging
feeder roots. In the case of ROW applications, mechanical incorporation
may be physically impossible or economically prohibitive. Therefore, alternate
strategies to reduce runoff of soil applied preemergent herbicides are
Chemical Incorporation. One goal of this study was to evaluate an
alternative method of herbicide incorporation: use of nonionic surfactant
applied concomitantly with residual herbicides. While 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 sparse. A study evaluating the effect of organosilicon surfactants
on trifluralin residual efficacy in sugarcane suggested that nonionic surfactants
improved shallow incorporation of trifluralin (Loveland Industries, unpublished
data). They found that residual weed control after 90 days 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.
For a soil applied herbicide, improved infiltration suggests that increased
incorporation may be observed, and therefore reduced herbicide movement
off-site in surface runoff. A study of nonionic organosilicon surfactants
in Florida found greatly enhanced herbicidal activity of diuron and norflurazon
in greenhouse pots when the surfactants are simulataneously applied with
residual herbicides (Tan and Singh, 1996). The authors concluded that much
lower use rates of these herbicides may be possible when used in conjunction
In California, anecdotal information indicates that organosilicon surfactants
improve residual efficacy of soil-applied preemergent herbicides in ROW
applications (Paul Washburn, Washburn Agricultural Applicators, personal
communication). Improved residual control suggests that the surfactant
may help maintain herbicides on-site - potentially due to improved herbicide
incorporation. A possible reason for this apparent phenomena is the exceptionally
low surface tensions associated with organosilicone surfactant solutions
relative to other common surfactants. Low surface tensions improve spreading
upon application, and entry and movement of low surface tension liquids
in capillary pores occurs at substantially reduced pressures relative to
solutions of higher surface tension (Hiemenz, 1986). This effect is also
observed for aqueous solutions infiltrating into capillary soil pores (Taylor
and Ashcroft, 1972). Thus, any significant increase in water infiltration
rates will improve rainfall incorporation of a herbicide into the soil.
However, the question of whether these effects are significant at economically
realistic surfactant application rates has not been investigated.
Herbicide Substitution. An alternative proposal for reducing herbicide
movement off-site in surface runoff is herbicide substitution: the use
of substitute herbicides which, by virtue of their physicochemical properties,
may have a lesser potential for off-site movement. There are little data
to support or refute this idea - especially under the conditions of interest
here - compacted, low permeability soils subjected to wintertime rainfall.
An additional objective of this study is to compare the runoff of simazine,
diuron, and bromacil under simulated rainfall conditions. Table 1 lists
the most important characteristics of simazine, diuron, and bromacil that
determine herbicide runoff potential: aqueous solubility and soil sorption.
MATERIALS AND METHODS
Site Description. The study was conducted in a mature citrus grove
located on the California State University campus in Fresno, California.
The soil is a Hanford sandy loam, a coarse-loamy, mixed, nonacid,thermic,
Typic Xerorthorent (USDA-SCS, 1971). Summary surface soil characteristics
of the twelve experimental plots are reported in Table 2; raw data for
the individual sites are provided in appendix A. National Resource Conservation
Service tabulated properties for a Hanford sandy loam are bulk density
ranging from 1.5 to 1.6 g cm3, and a moderately rapid permeability
class of 5.1 to 15.3 cm h-1 (USDA-SCS, 1971). The measured data
demonstrate a highly compacted low infiltration surface condition, with
an average bulk density of 1.7 g cm-3 and infiltration rate
of 0.9 cm h-1 (Troiano and Garretson, 1997). Soil analysis of
cores taken in previous studies in this citrus grove show a low permeability
layer at approximately the 100 cm depth where silt and clay content increase
sharply (Troiano and Garretson, 1997).
Plot Preparation. The study was conducted on twelve 5.5 m by 3 m
plots (e.g., fig. 1). About two weeks prior to the study, glyphosate was
applied to each plot to control weeds. Row middles were leveled by hand
using a shovel in cases where extreme uneveness was evident. Trees bordering
the plots were pruned of any foliage extending over furrows or middles
to reduce interference with the simulated rainfall event. Soil berms were
built across the ends of each plot to contain the simulated rainfall water.
Runoff was collected by inserting short lengths of PVC pipe into the berms
at the downstream end of each furrow; the pipes were placed through the
furrow berms such that runoff water could be directed into 5 gallon sample
collection buckets placed in open holes immediately outside the berms.
The rainfall simulations occurred approximately 24 hours after herbicide
application to each plot. Simulated rainfall was delivered using two Rainbird
impact sprinklers (Model 2045PJ Maxi-Bird) placed at a 1.8 m height on
metal stakes at diagonally opposite corners of the plots. Sprinkler heads
were adjusted to turn in a 90 arc, allowing full coverage of the entire
plot with the simulated rainfall. Water pressure was adjusted to 25 psi
which allowed for a maximum radius of throw of several inches beyond the
sprinkler head in the opposing corner. The total water delivery of 1.0
m3 (1000 L) to the sprinklers was measured using an in-line
flow meter. Catch cans were used to measure actual water depths applied
to the plots (Appendix B). Approximately 3.3 cm of water (~ 550 Liters)
was intercepted by each plot during the 90 minute simulation, corresponding
to a water application rate of about 2.2 cm hr-1
Treatment Applications. Six plots were randomly assigned to each
of two treatment application groups: (1) simultaneous application of simazine,
diuron, and bromacil plus organosilicon surfactant, and (2) simultaneous
application of simazine, diuron, and bromacil without surfactant. All twelve
plots received a 20 to 30 minute pre-irrigation one day prior to herbicide
application to minimize plot variabilities in antecedent soil water content
for the runoff experiments. All simulated rainfall events were performed
approximately 24 hours after application. Nominal herbicide target rates
were 2 kg formulated product ha-1 for each herbicide. The surfactant
treatment group also included application of a polyalkane ether heptamethyltrisiloxane
surfactant (fig. 2) at approximately 330 mL ha-1, corresponding
to about 5 oz acre-1. Current weed control costs in citrus are
about $25 - $40 acre-1; the application would therefore represent
about an additional $7-$8 acre-1 at current surfactant costs
of roughly $1.60 oz-1. The formulations of simazine, diuron
and bromacil used in the application consisted of 90%, 90%, and 80% active
ingredient, respectively. The total application solution volume for each
plot was approximately 0.05 Liters, and the surfactant concentration in
the surfactant treatment spray solution of 0.13% (w/w) exceeded the critical
micelle concentration (CMC) of the surfactant by a factor of ~15. At concentrations
equal to or exceeding the CMC, surface tension remains at a minimum of
approximately 22 dynes cm-1 (Knoche et al., 1991), as compared
to 73 dynes cm-1 for pure water.
The treatment solutions were applied only to the plot middles using a CO2
pressurized backpack sprayer with a handheld spray boom consisting
of 4 nozzles spaced 48.3 cm apart (1.5 m swath width). Furrows were covered
with a plastic tarp during application. The boom nozzles were equipped
with Teejet 8002 VS Driftguard Flat spray tips with 50 mesh screens. Pressure
at the boom was 40 psi and an application speed of approximately 3.2 km
h-1 was used to deliver the nominal solution application rate
of 30 L ha-1.
Deposition Samples. Three open 7.6 cm diameter jars containing exactly
50g of air-dried seived herbicide free soil were randomly placed in each
plot during herbicide application to determine deposition rate. They were
immediately removed from the plot, capped, the exterior wiped clean and
stored at -4 C. The raw deposition data are reported in appendix C.
Runoff water samples. Runoff water samples were collected in 8 gallon
increments. As previously described, runoff water from the plot middles
flowed to the furrows and was collected in calibrated 5 gallon buckets
placed at the end of each furrow. Each bucket was removed when 4 gallons
were collected and replaced with an empty bucket. Each set of two full
buckets was combined in a large container and vigorously stirred. One L
subsamples were then taken by immersing pre-labeled sample bottles into
the mixed solution. Water samples were immediately refrigerated at 4C until
Background soil samples. A furrow background and a middle background
soil sample from each plot was submitted for analysis of simazine, diuron
and bromacil. The plot middle sample was a composite of 3 individual 10
cm soil cores, while the furrow sample was a composite of 4 soil cores
- 2 taken from each furrow. All furrow background samples showed nondetectable
levels of the three herbicides (reporting limit of 12 ug kg-1
for simazine, diuron and bromacil); however, four plots were rejected due
to relatively high levels of bromacil in the plot middles (240 - 800 ug
kg-1). Substitute plots were selected as replacements after
further background sampling.
Post rainfall soil samples. Immediately after all water had drained
from the plot, soil samples were taken from the plot middles and plot furrows.
A 10 cm long by 7.5 cm diameter bucket auger was used in 2 steps to take
15 cm deep cores from both middles and furrows. The middles soil sample
for each plot was a composite of 3 individual 15 cm cores, while the furrow
sample was a composite of 4 cores, 2 taken from each furrow.
Chemical Analysis. Herbicide analyses were performed by APPL laboratories
using an HPLC/UV-VIS method previously developed for simazine, diuron and
bromacil in water (APPL SOP# ORG038) and soil (APPL SOP# ORG039). Each
extraction set included a QC blank and two matrix spike samples for each
analyte. Reporting limits, control limits, spike levels and mean spike
recoveries for the soil background and post rainfall simulation plot soils,
soil deposition, and aqueous runoff samples are summarized in Table 3.
All spike recovery data are reported in appendix D. All QC blank analyses
yielded nondetectable concentrations of herbicide. Analytical recoveries
for the matrix spike samples were generally low and somewhat variable,
especially for the deposition and water samples (Table 3).
Mass Balance Calculations. Total herbicide mass recovered from each
plot was calculated as the sum of herbicide recovered in runoff water,
furrows, and plot middles. Herbicide recovered in runoff water from each
plot was calculated by summing herbicide mass recovered from each sampling
interval (= concentration x interval volume) and summing over all sampling
intervals in a plot. Equation 1 was used to calculate post runoff herbicide
mass in the furrows and middles under the assumption that the herbicides
had moved no deeper than the soil coring sampling depth, z.
 Mass herbicide = [C ( 1+ )] x b x A x z
where C = laboratory reported analytical herbicide concentration
in furrow or middles soil samples (mass/mass, wet wt. basis), = water
content of soil samples (mass water/mass dry soil), b = soil
bulk density (mass soil/volume soil, Table 2), A = area of furrow
or plot middles, and z = sampling depth of soil cores. Values for
furrow and plot middle areas were 5.06 m2 and 16.7 m2,
respectively, and post runoff soil cores were 0.152 m in length (= z
in eq. ).
Data Analysis. The application day deposition, herbicide in runoff,
total herbicide off-site (runoff plus furrow), and post runoff middles
herbicide data were analyzed using two-way analysis of variance with repeated
measures (RM-ANOVA) on one factor (Zar, 1996; Glantz and Slinker, 1990).
The individual plots were the subjects, surfactant was a nonrepeated factor
at two levels, and herbicide was a repeated, or within subject, factor
at three levels (simazine, diuron, and bromacil). Multiple comparisons
were conducted using the Tukey multiple comparison test.
Theoretical and measured herbicide application rates showed substantial
differences, recoveries from the QC spike samples were low and variable,
and herbicide mass balances were generally low and variable between plots.
Therefore statistical analyses were conducted on the post-runoff analytical
herbicide data expressed in three forms: using concentrations expressed
on a (1) mass basis, (2) fraction of measured application basis,
and (3) fraction of theoretical application basis. The three sets
of statistical analyses yielded generally consistent conclusions.
Formal assumptions of between-cell homogeniety of variances and within-cell
normality were evaluated using the Levene test for homogeniety of variances
and the Anderson-Darling normality test, respectively (Minitab statistical
software, v. 10.2). There were indications that selected group data may
be non-normally distributed (e.g., fraction of measured bromacil application
recovered in runoff, Table 4). The null hypothesis - that there was no
difference in off-site movement of the 3 herbicides - was evaluated using
a "weight-of evidence approach" based on 9 RM-ANOVAs (II, III, IV in Table
5) because nonparametric analogues for two way RM-ANOVA are not available,
and ANOVA is generally considered robust to nonnormality (Zar, 1996).
In RM-ANOVA an additional condition - sufficient but not necessary - for
the F test statistic confidence levels to coincide with those of the theoretical
F distribution is compound symmetry (Glantz and Slinker, 1990). Compound
symmetry is met when group data are equally correlated across the subjects
and display equal equal variances (Zar, 1996; Glantz and Slinker, 1990).
Pair-wise correlation coefficients among the three repeated measures groups
(e.g., simazine, diuron, and bromacil) are reported along with results
from the Levene and Anderson-Darling tests in Table 4. RM-ANOVA results
are summarized in Table 5; full ANOVA tables are given in appendix E.
Applications. Deposition results are summarized in Table 6. While
all individual herbicide mean depositions for the surfactant treatments
were lower than the nonsurfactant treatments, differences in mean total
herbicide deposition to the two surfactant treatment groups was not highly
significant (p = 0.09, Table 5). Across all 12 plots, mean simazine deposition
was greater than either diuron or bromacil (p < 0.01), the latter two
herbicides not being significantly different at the p = 0.05 level (Table
5). The percent active ingredient in the formulations were 90, 90, and
80 percent for simazine, diuron, and bromacil, respectively. The theoretical
application rates of 3370 mg/plot (simazine, diuron) and 3000 mg/plot (bromacil)
corresponded to application of 2 lb formulated herbicide per acre. The
12 plot mean measured applications were 71%, 64%, and 73% of theoretical
application for simazine, diuron, and bromacil, respectively.
Evaluation of Surfactant Effects. The runoff profiles of the herbicides
from the plots are shown in figure 3 (surfactant treatment) and figure
4 (nonsurfactant treatment). Table 7 summarizes the runoff and soil data.
All raw runoff water and post-runoff soil concentrations are reported in
appendices F and G. Surfactant did not significantly effect volume of runoff
water collected from the plots, herbicide mass recovered in runoff, total
herbicide mass recovered off-site (= runoff + furrows), or herbicide mass
remaining in the middles after runoff (Table 7). It might be argued that
the water application was large enough to mask any surfactant effect over
the time course of the experiment due to dilution, however, there was no
significant difference in herbicide recovered in the first sampling interval
between the surfactant and nonsurfactant treatments (p = 0.69, 0.23, 0.68
for herbicide mass, fraction of measured application, and fraction of theoretical
application basis, respectively; Table 5). Practically speaking, this suggests
that any surfactant effect would probably also be minimal for shorter rainfall
events of comparable intensity as the rainfall simulation.
Differences between Herbicides. There was a statistically significant
difference between the mean mass and mean fractional amounts of bromacil
moving off-site relative to simazine and diuron (Table 5). This result
is not entirely unexpected; the solubility of bromacil is much greater
than either diuron or simazine (Table 1), suggesting a greater bromacil
mass incorporated into the soil per depth of water infiltrated. However,
from a practical standpoint the measured concentrations and fraction of
bromacil application moving off-site are still important. Typical detection
limits for bromacil in ground water samples are 0.05 - 0.1 ppb, approximately
10,000 times less than the mean initial bromacil runoff concentration (~
1,000 ppb); even a relatively small runoff volume entering the subsurface
may have a potentially large impact on ground water. The mean fraction
of the bromacil applications recovered off-site was about six to eight
percent, depending on assumptions of application rate. These data indicate
that herbicide substitution for the purpose of mitigating off-site herbicide
movement in surface runoff is probably an ineffective strategy, particulary
over the range of KOC and solubility represented by simazine,
diuron, and bromacil.
The net water infiltration was low in these plots; about 2/3 of the applied
water was recovered as runoff (Table 7). It is apparent that the compacted
soil in this orchard has a low infiltration rate. By difference between
applied water and runoff water recovered, the maximum average infiltration
rate observed here over the course of the simulated runoff events was about
0.7 cm hr-1. The actual average middles infiltration rate is
somewhat less due to water infiltration in the furrows during the experiment.
A low infiltration rate inhibits incorporation, so that even moderate rains
could result in significant off-site movement in runoff - regardless of
herbicide solubility. Infiltration rate is therefore a dominant factor
in this soil - relative to herbicide properties - in determining the amount
of herbicide runoff.
Mass Balance. Total herbicide recoveries were calculated by summing
herbicide mass recovered in runoff water, post-runoff herbicide mass in
the soil furrows, and post-runoff herbicide mass in the plot middles. The
calculated recoveries were somewhat low, ranging from about 60 to 75 percent
of measured application. Two factors that contribute to the low recoveries
are (1) low analytical recoveries as indicated by the ongoing quality control
soil and water matrix spikes (Table 3), and (2) inaccurate water content
data for the soil samples. The raw soil herbicide analytical data were
reported by the laboratory on a wet weight basis [ug herbicide (kg wet
soil)-1 ] along with laboratory determined soil water content
(mass water/mass dry soil), the latter used along with soil bulk density
to calculate total herbicide mass recovered in soil (eq.1). The laboratory
reported soil moisture content of middles soil samples were uniformly low,
with a mean of about 8.9%. This average water content is unrealistically
low for soil sampled immediately after a water application, and accounts
for at least a portion of the low herbicide total recoveries.
Comparison to previous studies. The runoff herbicide concentrations
illustrated in figures 3 and 4 ranged from about 50 to 1500 ppb, similar
in magnitude to simazine and diuron concentrations reported by Bruan and
Hawkins (1991) in their study of actual wintertime runoff in and around
citrus groves. Their highest concentration samples were collected about
2 1/2 months after herbicide application; the simulated rainfall events
in this study were conducted within 36 hours of herbicide application.
The Braun and Hawkins wintertime study reported much lower bromacil concentrations
than observed here due to the fact that most citrus applications of bromacil
occur in the spring, simazine and diuron being the most common fall-applied
Infiltration rates are a dominant factor influencing the runoff process.
In a previous study conducted in this orchard (Troiano and Garretson, 1997),
a similar water application resulted in a smaller fraction of applied water
running off the plots (~38% vs 68% observed here). The only known differences
in conditions or procedures between the two experiments is that a longer
pre-irrigation was conducted in the current study, 30 - 45 minutes vs.
about 15 - 20 minutes in the previous study (C. Garretson, personal communication).
Because infiltration rates are inversely related to soil moisture content,
it is possible that a higher antecedent soil water content contributed
to the greater fraction of applied water as runoff in the current study.
A consequence is that a larger portion of applied herbicide was recovered
in runoff in the present study; the average fraction of simazine recovered
in runoff here was about 10% of applied (Table 7), as compared to 4.3%
reported by Troiano and Garretson (1997).
Mathematical description of the runoff process Chemical movement
offsite during individual runoff events has been modeled as a function
of time with some limited success (Ahuja and Lehman, 1983; Ahuja et al.,
1981), however no time measurements were made during runoff in the current
study. Alternately, the relationship between concentration and runoff volume
was evaluated. An empirical first-order relationship was found between
herbicide concentrations and the square root of runoff volume.
where V = measured cumulative runoff volume (L), C = measured concentration
in runoff, Co is a parameter estimate of the initial herbicide
runoff concentration at V = 0 (at the moment runoff just begins to leave
the plot), and a concentration decline parameter k (L-1/2).
The parameter k is analogous to a rate constant, describing the extent
to which herbicide concentration in runoff decreases with increasing cumulative
runoff volume. Large values of the decline constant k correspond to situations
where rapid decreases in herbicide concentration are observed as the runoff
Figure 5 illustrates the observed 12 plot average data for each herbicide
and the respective fits of the model based on nonlinear least squares minimization
of eq. 2. Because the number of sampling intervals varied from plot to
plot (i.e., total runoff volumes varied among the 12 plots), only sampling
intervals for which at least half of the plots reported data were used
to create the average runoff data set (see figure 5 for further discussion).
A total of 138 of the 148 total runoff samples taken from the 12 plots
are represented by the average data. The correlation coefficients for eq.
2 fit to the average data exceeded 0.99 for all 3 herbicides. Although
eq. 2 was somewhat less succesful in describing the individual plot data
(0.78 < r < 0.99), the majority of fits were reasonable as indicated
by a mean correlation coefficient for all 36 (3 chemicals x 12 plots) runoff
profiles of 0.93 (Table 8). In addition, no systematic deviations of the
runoff profile data from eq. 2 were evident.
An additional comparison between simazine, diuron and bromacil runoff behavior
was made using the nonlinear least-squares minimization best fit values
for the "initial" runoff herbicide concentration C0 and the
concentration decline constant k (L-1/2) from the 36 runoff
profiles (Table 8). Plots of the observed data and individual fits are
given in appendix H. The best fit C0 values for the 3 herbicides
are not statistically different. However, the mean bromacil decline constants
(k) were significantly different than those for simazine and diuron (P
< 0.0001, Table 5), while the k values of the latter 2 herbicides were
not significantly different at the P = 0.05 level. It is apparent that
the more rapid decline in bromacil concentrations as the experiments progressed
relative to simazine or diuron contributed to a smaller fraction of bromacil
moving off-site in runoff previous discussed. However, the difference between
herbicides may have only limited practical significance. Both observed
concentrations and fraction of application moving off-site were substantial
for all 3 herbicides. Under actual rainfall conditions in a given field,
a herbicide's runoff tendency will be a function of both rainfall depth
and duration. The water application rate of about 2.2 cm h-1
for 1.5 h simulated an intense rainfall event.
In spite of the relatively coarse surface soil in these plots (~ 75 percent
sand), it is apparent that the very low infiltration rate is the dominant
factor driving herbicide movement off-site in runoff. The low infiltration
rate is probably related to compaction (surface bulk density ~ 1.7 g cm3,
Table 2). Moreover, the cultural practices that lead to surface compaction
and correspondingly low infiltration rates (complete weed control, no cultivation)
are typical of citrus culture in Fresno and Tulare Counties. These observations
further indicate that practically, in low permeability soils, herbicide
substitution is not a satisfactory method for mitigating off-site movement.
1.The organosilicon surfactant applied at a nominal application rate of
5 oz acre-1 had no effect on the fraction of simazine, diuron,
or bromacil recovered in runoff during a simulated rainfall event in a
low permeability citrus orchard soil.
2. The percentage of herbicide recovered off-site was substantial under
these conditions, ranging from about 4 to 21 percent of the initial herbicide
application. The overall mean fraction of measured herbicide application
recovered in runoff across all plots and all 3 herbicides was 10 percent.
Runoff concentrations ranged from about 50 - 1500 µg L-1
in runoff water samples.
3. Bromacil had a lesser tendency to move off-site in runoff than simazine
or diuron. This difference was attributable to bromacil's much greater
solubility, providing a modicum of increased downward movement with simulated
rainfall into the profile.
4. These results indicate that natural rainfall is a poor incorporation
method in pan or compacted low permeability soils - regardless of the choice
of herbicide. Substantial quantities of all 3 herbicides were recovered
in the runoff water. Any approach to mitigating herbicide movement from
low permeability soils in runoff should seek to either (i) significantly
improve water infiltration rates into the soil, or (ii) mix the herbicide
into the soil, rendering it unavailable to move with surface runoff, or
(iii) apply sufficient water for complete incorporation at rates lower
than the infiltration rate of the soil.
6. An empirical relationship was found between runoff concentration and
cumulative runoff volumes under the conditions studied - low permeability
soil exposed to short-term constant intensity simulated rainfall. Concentrations
were found to decrease exponentially with the square root of runoff volume,
the decline characterized by a depletion constant. This relationship may
provide an additional evaluation tool in future studies.
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