Journal of Environmental Quality 32:78-83 (2003)
© 2003 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
TECHNICAL REPORTS
Ground Water Quality
Simazine Runoff from Citrus Orchards Affected by Shallow Mechanical Incorporation
F. Liu*,a and
N. V. O'Connellb
a Kearney Agricultural Center, Univ. of California, Parlier, CA 93648
b Cooperative Extension, Univ. of California, Tulare County, Tulare, CA 93274
* Corresponding author (fliu{at}uckac.edu)
Received for publication January 10, 2002.
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ABSTRACT
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Simazine (6-chloro-N,N'-diethyl-1,3,5-triazine-2,4-diamine) losses via runoff in California are a potential source of environmental contamination because simazine is widely used for weed control during the rainy season from November to March. This study was conducted in two citrus orchards from three rainfall events to evaluate the effects of shallow mechanical incorporation on simazine losses in runoff during the winter. Simazine losses in runoff were compared between row middles that were either undisturbed, the normal orchard practice, or subjected to shallow mechanical incorporation. Mechanical incorporation of row middles significantly reduced runoff volumes by approximately 45 and 28% for the first and second runoff events, respectively. In undisturbed plots, simazine concentrations in runoff from the first runoff event ranged from 0.62 to 0.73 mg L-1; then simazine concentrations rapidly decreased (0.03–0.35 mg L-1 ) from the second and third runoff events. In disturbed plots, simazine concentrations in runoff from the first runoff event ranged from 0.21 to 0.24 mg-1, but simazine concentrations remained relatively constant between the three runoff events. Total mass recoveries of simazine in runoff ranged from 1.93 to 2.97% and from 0.70 to 0.74% of application from the undisturbed plots and from the disturbed plots, respectively. Low water infiltration rate inhibited surface-applied herbicide incorporation into the soil matrix with natural rainfall in compacted soils. Mechanical incorporation of row middles significantly reduced runoff volumes, simazine concentrations, and mass losses in runoff after application.
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INTRODUCTION
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PREEMERGENT HERBICIDE LOSSES via runoff in California are a potential source for ground water contamination (Braden and Uchtmann, 1985; Leonard, 1990). One mechanism of preemergent herbicide movement to ground water is leaching through the soil profile to an underlying ground water aquifer (Wehtje et al., 1984; Freeze and Cherry, 1979). Recharge may result from both natural rainfall and irrigation (Bouwer, 1987). Another mechanism involved in pesticide movement to ground water is surface runoff from soils characterized by the presence of hardpan soils (Braun and Hawkins, 1991; Troiano et al., 1997). In many of the hardpan soil areas, herbicide-containing runoff enters dry wells or other drainage structures, subsequently moving directly to deeper, more permeable subsurface regions below shallow hardpan layers (Spurlock et al., 1997). Surface water resources that receive drainage from intensively farmed agricultural production areas are likely to contain higher levels of pesticides, particularly at times related to recent use of pesticide (Barer and Mickelson, 1994). Larger amounts of winter rain (November to March) occur in the southern San Joaquin Valley of California, which is believed to be associated with pesticide contamination of receiving waters (Lee, 1983; Pickett et al., 1990). Concentrations of simazine, diuron, and bromacil ranging up to 1.1 mg L-1 have been detected in rain runoff water entering dry wells in and around citrus orchards in Tulare County, California (Braun and Hawkins, 1991).
Preemergent herbicides are usually surface-applied to commercial citrus in California. Citrus orchards account for approximately 75% of the use of preemergent herbicides in the hardpan soil areas of Fresno and Tulare Counties (Spurlock et al., 1997). Citrus growers favor bare soil conditions in winter to enhance frost protection. Bare citrus orchard middles are often highly compacted with low infiltration rates. As a result, heavy rainfall events following herbicide applications can produce preemergent residue offsite movement via surface runoff. One method of reducing herbicide runoff losses at the soil surface is to incorporate herbicides into the soil matrix by tillage, irrigation, or rainfall. Surface-applied preemergent herbicides should be incorporated into the soil matrix within days after application to be effective (Ashton et al., 1989). The maximum incorporation time is determined by several factors such as temperature, weather condition, and persistence of the herbicides. A survey showed that 91% of citrus growers usually apply rainfall to incorporate preemergent herbicides into soil in the San Joaquin Valley of California (unpublished data, 1999). However, a previous study indicated that rainfall was a poor incorporation method for preemergence herbicides in pan or compacted soils with low infiltration, regardless of the choice of herbicides (Spurlock et al., 1997). In experimental plots, mechanical incorporation, down to 76 mm via a rototiller, was effective in mitigating herbicide (simazine) runoff losses from middles of citrus orchards. Total simazine mass removed in runoff from two simulated runoff events was estimated at 6.0% of the amount applied to row middles in undistributed plots compared with only 0.8% in mechanically distributed middles (Troiano and Garretson, 1998). However, this study was conducted under two simulated rainfall events in one field with a plot size being 5.5 x 6.1 m.
Implementation and effectiveness of shallow mechanical incorporation on simazine transport have not been studied under commercial citrus growing conditions. Simazine behavior and transport under simulated runoff could greatly differ from that under a natural rainfall-runoff event. The objective of this study was to evaluate the effects of shallow mechanical incorporation of simazine applied to orchard-row middles on simazine losses in runoff that resulted from winter rainfall in commercial citrus-growing conditions. Simazine was the representative preemergence herbicide studied because it is one of the most widely detected herbicides in ground water of citrus-producing areas in Tulare and Fresno counties.
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MATERIALS AND METHODS
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Site Description
This study was conducted in two citrus groves, on a 5.5- x 6.1-m tree spacing, located in runoff-prone soils in Tulare County, California according to the statistical clustering–profiling method of Troiano et al. (1994)(1997). Soils at both sites were mapped as a San Joaquin loam (fine, mixed, active, thermic Abruptic Durixeralf) (USDA Soil Conservation Service, 1971). Slope at both sites was about 1 to 2%. The infiltration rate of the row middles was measured on-site with a cylinder infiltrometer as described by Bouwer (1986) (Fig. 1)
. Soil infiltration rates were similar at both sites. At Site 1, average bulk density of the surface 5 cm of soil from the row middles (n = 10) was 1.76 Mg m-1 and from the adjacent furrows was 1.34 Mg m-1 (Table 1) (Blake and Hartge, 1986). Average bulk density of the surface 5 cm of soil from the row middles (n = 10) was 1.74 Mg m-1 and from the adjacent furrows was 1.32 Mg m-1 at Site 2 (Table 1). High bulk density and low infiltration rate of the row middles supported field observations of compaction, a common condition in orchards where soil is kept barren due to herbicide use and is subjected to vehicular traffic (Meek et al., 1992).
Plot Preparation and Treatment Applications
Plot size was 1 middle wide and 10 trees long with 5.5- x 6.1-m tree spacing. Soil berms were built across the up-slope end and sides of each plot to contain runoff. Runoff was collected at the down-slope of each plot through a PVC pipe placed into containers situated in pits excavated into the furrow. The collection container was 25 L equipped with a battery-powered submersible pump (3875 L per hour). When runoff water in the collection container filled to the level of the float switch, the pump was activated and water flowed out through the flow meter. Flow was then divided into discharge (94% of total volume) and composite sample (6% of total volume) collected in a secondary container. The flow was divided by using a T connector with two different diameter sizes of hose (Fig. 2)
. Wire mesh was placed over the collection end of the pipe to screen out large objects such as leaves. Measured runoff volumes are listed in Table 2. The amount of rainfall related to each runoff event was obtained from a rain gauge installed at each study site (Table 2).
Simazine was applied at a grower standard rate of 2.2 kg activate ingredient (a.i.) ha-1. Selected simazine characteristics are summarized in Table 3. Off-site movement in runoff was compared between plots with and without shallow mechanical incorporation of the row middles. Experimental design was a randomized complete block with six replications at each site. Simazine was surface-applied over the entire plot area. Application was made through a CO2 gas-pressurized liquid sprayer at a nominal rate of 97 L ha-1. The application rate was monitored by placing two absorbent pads (930-cm2 surface area each) in each plot. Row middles were mechanically disturbed immediately after simazine application to a depth of approximately 5 cm with a discing and ring roller pulled down the row middles with a tractor.
Soil and Runoff Water Sampling
Two background soil samples before simazine application were collected from each plot: one from row middles and one from plot furrows. Soil samples from the row middle were a composite of three subsamples. Soil samples from plot furrows were a composite of four subsamples, two taken from each furrow within the plot. Each subsample was taken to a 5-cm depth. One post-rainfall soil sample was collected from the row middle in each plot after the first runoff event. Each soil sample was a composite of six subsamples taken along the runoff path. Each subsample was taken to a 1-cm depth. Soil samples were stored frozen for a period of no longer than 16 weeks based on a simazine storage stability study (F. Spurlock, California Department of Pesticide Regulation, personal communication, 2001). Simazine content was under the detection limit of 15 µg kg-1 from the background soil samples. One runoff sample was collected in a 1-L amber glass bottle per plot per runoff for simazine determination from the secondary container during agitation (Fig. 2). Water samples were immediately refrigerated at 4°C and analyzed within 7 d to prevent herbicide degradation.
Chemical Analysis
Concentrations of simazine in water and soil samples were analyzed through enzyme-linked immunosorbent assay (ELISA). Results obtained from ELISA have shown to be equivalent to results obtained with gas chromatography (Goh et al., 1991, 1993). Water samples were analyzed directly. For soil samples, 25-g samples were extracted in a mixture of 10 mL methanol and 15 mL deionized water, then shaken on an orbital shaker for 10 min at 200 rpm. The extract was decanted and saved, and the extraction repeated. The combined extracts were filtered through a 0.2-µm nylon acrodisc. Before ELISA the filtrates were diluted 10-fold to reduce the methanol content to <4%. The double-antibody, haptenated enzyme, competitive inhibition ELISA assay was run according to Format II of Schneider and Hammock (1992). The detailed procedures were described by Troiano and Garretson (1998). For each analysis, method validations conducted on water and soil matrices showed that simazine was nondetectable. Validation consisted of spiking each medium with four replicates at three fortification levels. Overall mean recoveries ranged from 102 ± 11.8 to 103 ± 12.4% for water and ranged from 103 ± 12.3 to 104 ± 13.7% for soil samples. These data were used to calculate upper control limits and lower control limits, which was the mean ± two standard deviations. All quality control analyses yielded nondetectable concentrations of simazine. Percent recovery for fortified samples was within control limits. Minimum detection limits were 0.5 µg L-1 for water and 15 µg kg-1 for soil.
Data Analysis
This study was a randomized complete block design with six replications at each site. Statistical analyses were performed with the Statistical Analysis System (SAS Institute, 1985). Analyses of variance were conducted on simazine concentration and mass losses in runoff and collected amount of runoff. Means were separated with the Fisher Protected Least Significant Difference (LSD). All statistical tests were performed at the 
= 0.05 level of significance.
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RESULTS AND DISCUSSION
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Mechanical Incorporation on Runoff Volumes
For the first and second runoff events, mechanical incorporation of row middles significantly reduced runoff volume due to an increase in the amount of water infiltrated into the soil. Average runoff volumes measured from plots with undistributed row middles ranged from 57 to 61% of the total amount of rainfall compared with 30 to 45% of the total amount of rainfall measured from plots with disturbed row middles (Table 2). Mechanical incorporation of row middles significantly reduced runoff volumes by approximately 45 and 28% for the first and second runoff events, respectively. Low infiltration rates in undisturbed row middles resulted in significant runoff. However, incorporation of row middles has no effect on runoff volumes for the third runoff event. The measured runoff volumes ranged from 59 to 62% of the total rainfall for both treatments (Table 2). These data indicated that reduction in field runoff volume through shallow mechanical incorporation (5-cm depth) in row middles was limited to the first and second runoff events.
Mechanical Incorporation on Simazine Runoff Losses
There were no significant differences in simazine amounts applied between the two treatments at both sites, indicating consistent application rates (Table 4). The first runoff occurred 5 d after simazine application at Site 1 with 22.0 mm of rainfall and 10 d at Site 2 with 16.0 mm of rainfall (Table 2). In undisturbed plots, the first runoff event following simazine application produced high concentrations (0.62 to 0.73 mg L-1), then concentration decreased rapidly with subsequent rainfall events (0.03–0.35 mg L-1) (Table 4). The ranges of concentrations for the first runoff from the undisturbed plots were similar to concentrations measured in the Braun and Hawkins (1991) study. In disturbed plots, simazine concentrations in runoff ranged from 0.21 to 0.24 mg L-1 from the first runoff event; simazine concentrations from the second and the third runoff events ranged from 0.08 to 0.17 mg L-1 (Table 4). Mechanical incorporation significantly reduced simazine concentration in runoff for the first runoff event by a factor of three. Incorporation caused soil mixing and subsequent redistribution of the herbicides. Simazine content in surface soil (0–1 cm) was significantly higher in the undisturbed row middle than that in the disturbed row middle after the first runoff event (Table 4). It is apparent that low water infiltration rates inhibit simazine incorporation in the undisturbed row middle. Pesticide concentration in runoff has been shown to be correlated with pesticide content in surface soil (0–1 cm) samples (Leonard, 1990). For the three runoff events, simazine concentrations remained relatively constant in disturbed plots compared with undisturbed plots.
At both sites, average simazine recoveries in runoff from undisturbed plots were 1.8, 0.7, and 0.2% of the application, respectively, for the three runoff events; simazine mass recoveries from distributed plots were estimated at 0.4, 0.2, and 0.2%, respectively, for the three runoff events (Table 4). For the first and second runoff events, simazine mass losses in runoff from undisturbed plots were significantly higher than those from disturbed plots. However, simazine mass losses were unaffected by incorporation at the third runoff event (Table 4). Data showed that simazine available for runoff losses is greatest at the start of the runoff event, especially in undisturbed plots. Previous studies showed that a layer of surface soil called the "mixing zone," often assumed to be about 10 mm thick, interacts with runoff partitioning and enrichment (Leonard et al., 1979). The decrease in herbicide losses in successive rainfall events is probably due to a reduction in the amount of herbicide in the shallow mixing zone over time. Processes potentially responsible for decreases in residues are degradation, volatilization, and/or downward movement with infiltrated water.
Total mass recoveries in runoff from the three runoff events ranged from 1.93 to 2.97% of the application from undisturbed plots and ranged from 0.70 to 0.74% of the application from disturbed plots (Table 4). This indicated that for both treatments, mechanical incorporation in row middles and nonincorporation, simazine loads in runoff were not a large fraction of application. However, simazine residues ranged between 0.1 and 1.0 mg L-1 in runoff, which are about three to four orders of magnitude greater than those commonly detected in ground water (Maes et al., 1992). Use of drainage wells for disposal of runoff, containing residues at these levels, subsequently moving directly to deeper, more permeable subsurface regions below shallow hardpan layers, represents a significant source of ground water contamination.
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CONCLUSIONS
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In the southern San Joaquin Valley of California, many commercial citrus groves are located on soils that have a low water infiltration rate. A low infiltration rate inhibits surface-applied herbicide incorporation into the soil matrix with natural rainfall, so that even moderate rains could result in significant off-site movement of herbicides. Any approach to mitigating herbicide movement from soils with low infiltration rates should seek to mix the herbicide into the soil matrix or/and reduce runoff volumes. Mechanical incorporation of row middles significantly reduced runoff volumes, herbicide concentrations, and consequently, mass losses in runoff. Even though mechanical incorporation is a common agricultural practice in other crops, there is resistance to use of mechanical incorporation in citrus because of perceived deleterious effects on root health. This study indicated that shallow mechanical incorporation (up to 5 cm) could be an alternative method to mitigate herbicide off-site movement in commercial citrus-growing conditions.
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ABSTRACT
INTRODUCTION
