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a LFR Levine·Fricke, 1413 Woodlawn Avenue, Wilmington, DE 19806
b Monsanto Company, 800 North Lindbergh Boulevard, St. Louis, MO 63167
c Dow AgroSciences LLC, 9330 Zionsville Road, Indianapolis, IN 46268
d Klein & Associates, 12917 Topping Estates Drive, Town & Country, MO 63131
e Syngenta Ltd., Jealott's Hill Research Station, Bracknell, Berkshire, England RG42 6ET
* Corresponding author (andy.newcombe{at}lfr.com)
Received for publication November 11, 2004.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: ARP, Acetochlor Registration Partnership • DT50 and DT90, times for 50 and 90% of the initial residues to dissipate, respectively • ESA, ethanesulfonic acid • LOD, limit of detection • LOQ, limit of quantitation • OXA, oxanilic acid • PGW, prospective ground water
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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The USEPA granted the conditional registration of acetochlor to the Acetochlor Registration Partnership (ARP) on 11 Mar. 1994 (USEPA, 1994). The ARP consisted originally of Monsanto and Zeneca, Inc., and as of fall 2000, Monsanto and Dow AgroSciences. The USEPA imposed several restrictions and conditions on the use of acetochlor to limit potential risks to human health and the environment, including ground water and surface water monitoring programs, which were designed to ensure the continued protection of ground water and surface water resources.
One of the USEPA required monitoring programs was the acetochlor PGW program. The regulatory objective of the acetochlor PGW program was to evaluate the potential for acetochlor and its two principal soil degradation products [acetochlor ethanesulfonic acid (ESA) and acetochlor oxanilic and (OXA)] to move through the vadose zone to shallow ground water under a full range of vulnerable and typical soil textures. The ARP agreed to restrict or cancel acetochlor if the USEPA determined that the results established a "pattern of movement" of acetochlor to ground water. A "pattern of movement" was defined as detections of acetochlor greater than 0.10 µg L–1 in ground water or greater than 1.0 µg L–1 in soil-pore water collected from suction lysimeters at a depth of 2.7 m below ground surface, but only when such soil-pore water detections were consistent with the downward movement of a conservative tracer and acetochlor detections in suction lysimeters nearer the soil surface.
The USEPA required that the ARP conduct eight PGW studies across a range of soil textures and geographical locations. The distribution of soil textures was weighted toward coarser soil types (more vulnerable to pesticide leaching), while also including the finer-textured soils that dominate most corn-growing areas. The eight states chosen (Wisconsin, Ohio, Minnesota, Nebraska, Iowa, Indiana, Pennsylvania, and Delaware) were representative of areas of significant acetochlor use.
Large-scale surveys have detailed the presence of agricultural chemicals, particularly corn and soybean [Glycine max (L.) Merr.] herbicides, in ground water and drinking water derived from ground water (Holden et al., 1992; Barbash and Resek, 1996). Surveys of ground water were expanded to include acetochlor after its introduction (Barbash et al., 1999, and citations contained therein). The Barbash et al. (1999) review also included results for major degradates of corn herbicides. The results of these numerous, multisite and multiyear studies show that soil-applied corn herbicides and their soil degradates are found in shallow ground water generally at low levels (<1.0 µg L–1) in areas where the products are used. Generally, detections in drinking water wells are less frequent and occur at lower concentrations than in shallow monitoring wells. For the chloroacetanilide class of herbicides, the principal soil degradates generally occur more frequently and at higher concentrations than the parent herbicide.
This publication describes the methods, results, and interpretations of the acetochlor PGW monitoring program. The acetochlor ground and surface water monitoring programs are described in two companion publications (De Guzman et al., 2005; Hackett et al., 2005).
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Site Characterization
Site characterization activities included surface soil and subsoil characterization, aquifer characterization, and the conduct of a site survey (Tables 1 and 2). Surface soil (0–15 cm) was collected from each site to assess variability of surface-soil texture, pH, organic matter, cation exchange capacity, and disturbed bulk density. Soil samples were collected using a stainless steel trowel or hand auger and shipped to a contract laboratory for characterization.
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Shelby tube sampling was conducted to obtain relatively undisturbed soil samples for the measurement of vertical saturated hydraulic conductivity (Black et al., 1965) and undisturbed bulk density. Soil samples were collected using 8-cm-o.d., 76-cm-long steel Shelby tubes. A hollow-stem auger-drilling rig was used to advance the Shelby tube into the soil profile. Samples were scheduled to be collected in 61-cm increments from land surface to ground water; however, the presence of coarse materials (cobbles and stones) in the vadose zone prevented the collection of continuous cores at two sites.
Aquifer properties were assessed by observations made during piezometer and monitoring-well drilling activities, and by measurements recorded after instrumentation. Aquifer characterization included the types of materials encountered below the water table, depths to ground water, ground water flow direction, hydraulic gradient, hydraulic conductivity, porosity, and pore-water velocity.
Depths to ground water were recorded to assess ground water flow direction and hydraulic gradients at each study location. Monitoring wells were instrumented with dedicated submersible pumps; consequently depths to ground water were only measured in the piezometers located at the corners of the test plot and on the periphery of the study location. Depths to ground water were measured manually from a fixed surveyed point on the top of the casing of each piezometer.
Hydraulic conductivity of the aquifer was estimated by conducting rising or falling-head slug tests in randomly selected monitoring wells located in the test plot. The slug test data were used to calculate the hydraulic conductivity of the aquifer in the vicinity of the well, using standard formulae for monitoring wells screened in unconfined aquifers (Bouwer and Rice, 1976; Bouwer, 1989).
Porosity of the aquifer material was not measured directly, but was estimated empirically based on the types of sediments (Driscoll, 1986) encountered below the water table during monitoring well installation. Pore-water velocity values were calculated using hydraulic conductivity, hydraulic gradient, and porosity data.
Study Design and Instrumentation
Each site was instrumented in accordance with the USEPA draft guidance document on the conduct of PGW studies (USEPA, 1995, 1998) with the exception of the Wisconsin and Ohio studies, which were initiated before the issue of USEPA's 1995 guidance document. The USEPA agreed to the instrumentation configuration for these studies before instrumentation.
Each site consisted of an approximately 1.2-ha test plot adjacent to a 0.2-ha control plot (Fig. 1) . The control plot was located hydrogeologically upgradient from the test plot. The test and control plots were instrumented with suction lysimeters (for sampling soil-pore water) installed at varying depths within the vadose zone, and ground water monitoring wells screened at varying depths within the aquifer.
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Monitoring wells were installed to collect ground water samples and were arranged in clusters within each test plot. For the Wisconsin and Ohio sites, 10 monitoring wells were installed at each location. One monitoring well was installed in each control plot, and three clusters of three monitoring wells were installed in each test plot. The clusters consisted of one shallow, one deep, and one extra deep monitoring well.
For the six remaining sites, 17 monitoring wells were installed at each location. One monitoring well was installed in each control plot, and eight clusters of two monitoring wells were installed in each test plot (Fig. 1). The clusters consisted of one shallow and one deep monitoring well.
Screens for the shallow monitoring wells were 3 m in length, to enable ground water samples to be collected in the event the depth to ground water increased after monitoring well installation. Monitoring well clusters were installed in a linear arrangement, with a 3-m distance between each monitoring well within a cluster. Monitoring wells were instrumented with permanent dedicated bladder or electric pumps with sampling tubes drawn to well-head manifolds contained within the protective casing of each monitoring well.
Soil-suction cup lysimeters were installed in clusters, with each cluster consisting of four lysimeters (Fig. 1). In each cluster, lysimeters were installed at 0.9 m (shallow), 1.8 m (medium), 2.7 m (deep), and 3.3 to 4.6 m (extra deep) below ground surface. The installation depth of the extra deep lysimeters was dependent on the depth to ground water encountered at each site. For the Wisconsin and Ohio sites, 32 lysimeters were installed at each location. The lysimeters were arranged in eight clusters; six clusters in each test plot, and two clusters in each control plot. For the six remaining studies, 40 lysimeters were installed at each location. Eight clusters were installed in each test plot, and two clusters in each control plot.
Each lysimeter consisted of a threaded porous ceramic cup, a PVC body in which the soil-pore water was collected, and a top plug with two tube fittings. The tubing was 6-mm-o.d. high-density polyethylene. The tubing was connected to an aboveground assembly where the sample was transferred into sample containers. Lysimeters within a cluster were installed approximately 3 m apart, along unplanted instrument rows. Boreholes for lysimeter installation were advanced using hand held or drilling rig mounted augers.
The lysimeter ceramic cup was installed into a silica flour slurry, and typically, 8 to 10 cm of native soil was placed above the silica slurry. The remaining borehole was filled with alternating layers of clean sand or native soil and bentonite. Lysimeters were installed vertically and centrally in the borehole. Lysimeter tubing was routed through a 45-cm-deep trench to a 10-cm PVC stickup.
A weather station was installed at each site to monitor on-site precipitation, air and soil temperature, soil moisture, solar irradiance, water-level variations in a selected piezometer, wind speed and direction, and relative humidity. An irrigation well and a linear two-span irrigation unit were installed to provide supplemental irrigation to the test and control plots.
Agronomy
The test and control plots were prepared according to conventional local corn-growing practices, which consisted of tilling, cultivating, and fertilizer application. Acetochlor was applied at the maximum-labeled rate permitted for the Natural Resource Conservation Service (NRCS) reported soil series at the study location (Table 3). Following the acetochlor application, a single broadcast application of potassium bromide tracer was made to each test plot. Following a USEPA request, applications of acetochlor and potassium bromide tracer were made to the Delaware site for a second year in the spring of 1999. To try and distinguish between the two applications of potassium bromide tracer, the second tracer application was applied only to one half of the test plot.
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The first growing season water input targets began on the date of acetochlor application and continued through October of the same year. The water-input target for each growing season was set at 110% of the 30-yr local historical monthly rainfall average (with the exception of Nebraska). The water-input target ceased on 31 October because of possible freeze damage to irrigation equipment. Subsequent growing season water input targets began on 1 May or the date the rotational crop was planted.
The growing season water input target for the Nebraska study was designed to simulate local "irrigated corn" practices, with water input targets for the months of July, August, and September increased to approximately 165, 175, and 190%, respectively, of the historical rainfall average for each month. The water-input targets for May, June, and October were set at 110% of the monthly historical rainfall average.
Supplemental irrigation at the Wisconsin, Ohio, Minnesota, Iowa, and Indiana studies was terminated after the first and second growing season targets were met. Following consultation with USEPA, supplemental irrigation at the remaining three study locations (Delaware, Nebraska, and Pennsylvania) was reinstated beyond the second growing season, and increased to promote the movement of the potassium bromide tracer and any acetochlor degradates into ground water.
Sampling
Soil, soil-pore water, and ground water samples were collected for residue analysis at predetermined sampling intervals. After collection, all samples were stored in coolers in the field (on blue or dry ice), until shipped by overnight courier to the ARP designated analytical facility. Field data were recorded to Good Laboratory Practice standards on custom designed field documentation forms. Samples were shipped under frozen (dry ice) or chilled conditions via overnight courier to the ARP analytical laboratory, where they were inventoried and held under either chilled or frozen conditions before analysis.
Deposition trays (approximate dimensions 380 x 250 x 30 mm) were used to collect acetochlor residues deposited during application. Trays were filled with dry sieved soil and randomly positioned in each test plot away from tractor wheel paths. Samples were collected immediately after the application of acetochlor was complete. To facilitate the collection of soil cores, each test and control plot was subdivided into three subplots (A, B, and C). Soil samples were taken along marked transects in each subplot, with no transect being used more than once throughout the study. Five positions were sampled along each transect using a zero contamination soil-coring device, with soil cores collected at two discrete depths: 0- to 15-cm and 15- to 30-cm horizons.
After sampling, boreholes were refilled with bentonite granules and then hydrated with water to retard the preferential migration of surface material into the soil profile. Soil was sampled until residue levels of acetochlor declined to below the limit of quantitation (LOQ) of the analytical method.
Soil-suction lysimeters were placed under approximately 0.05 to 0.07 MPa vacuum 12 to 24 h before sampling. To collect the sample accumulated in the lysimeter, the polyethylene vacuum tube of the lysimeter was connected to a battery-operated vacuum pump. The lysimeter was then pressurized to force the accumulated soil-pore water into the collection bottle. All monitoring wells were purged before sampling. The approximate well volume of each well was calculated by taking water-level measurements from the piezometers located at the corners of the test plot. An average ground water elevation was calculated from these water-level measurements and this was used to estimate the water-column volumes in the control and test plot-monitoring wells. During purging, water from the monitoring wells was routed through a flow-through chamber that allowed continuous measurements of water parameters. After one well volume had been purged, water temperature, pH, and conductivity were measured and recorded. If temperature, pH, and conductivity values were stable for three consecutive well volumes purged, a ground water sample was collected. To avoid cross-contamination during sampling, lysimeters and monitoring wells in the control plots were sampled before those in the test plots.
Analytical Methodology
Three laboratory facilities were used to analyze samples: (i) Zeneca Agrochemicals, Jealott's Hill Research Centre, Bracknell, United Kingdom; (ii) Covance Laboratories, Harrogate, United Kingdom; and (iii) Monsanto Company, St. Louis, MO, USA. Limits of detection (LOD) and LOQ varied slightly among the methods used at the various laboratories and are briefly summarized below.
Bromide residues in water were determined using ion chromatography with conductivity detection. The LOQ of the analytical method was 100 µg L–1 and the LOD approximately 30 µg L–1. Acetochlor residues in soil were determined by gas chromatography with mass selective detection (GC–MSD). The LOQ of the analytical method was 0.01 mg kg–1 and the LOD was approximately 0.005 mg kg–1. For the application rate verification analysis, where acetochlor residues were significantly higher, the LOD was calculated as 0.02 mg kg–1.
Acetochlor residues in soil-pore water and ground water were determined by GC–MSD. The LOQ of the analytical method was 0.05 µg L–1 and the LOD varied between 0.01 and 0.03 µg L–1. Acetochlor OXA residues in soil-pore water and ground water were initially determined by GC–MSD after derivatization. The LOQ of the analytical method was 1.0 µg L–1 and the LOD was approximately 0.02 to 0.05 µg L–1. Residues of acetochlor ESA were initially determined by high performance liquid chromatography (HPLC) with ultraviolet detection. The LOQ of the analytical method was 1.0 µg L–1 and the LOD was approximately 0.2 µg L–1. Analysis of soil-pore water and ground water for acetochlor OXA and acetochlor ESA degradates continued via these independent analytical methods until the availability of more selective analytical separation and detection equipment enabled a combined HPLC–mass spectrometry (MS)–MS analytical method to be developed with an LOQ of 0.5 µg L–1 for both degradates. With this improved method, the LOD for acetochlor ESA and acetochlor OXA was 0.2 and 0.1 µg L–1, respectively. Further description of the water methods for acetochlor and its degradates are contained in the companion paper (Hackett et al., 2005).
During the program, 772 soil, 13404 lysimeter, and 6112 ground water samples were collected. Available sample volume (particularly for lysimeter samples) dictated the number of different analytical determinations that could be performed on each sample collected. If limited sample volume was available, priority was placed on conducting bromide tracer and acetochlor residue determinations. Approximately 52700 successful analytical determinations were performed during the course of the entire PGW program.
Associated Studies
Several laboratory and field-based studies were conducted in association with the acetochlor PGW program. Laboratory adsorption and degradation studies were conducted on relatively undisturbed soil samples collected from select PGW sites. The objective of these studies was to quantify the extent of acetochlor adsorption and degradation in soil (both surface and subsoil) collected from various PGW study locations.
Field incubation studies were conducted on relatively undisturbed soil samples collected from two sites (Wisconsin and Iowa). The objective of these studies was to determine the potential for surface and subsurface soils to degrade acetochlor under field conditions (Mills et al., 2001). Additionally, an incubation study was conducted at the Iowa PGW site, to investigate the potential for relatively undisturbed subsoil to degrade acetochlor when treated in situ and incubated horizontally under field conditions (Mills et al., 2001).
Leaching Model Predictions
Detailed model predictions were not undertaken as part of the acetochlor PGW program, however the Tier I Screening Concentration in Ground Water (SCI-GROW) model was used to predict a ground water exposure concentration for acetochlor.
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RESULTS |
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Acetochlor Residues in Soil
The results of acetochlor application rate verification, zero-day recovered soil residues, and 50 and 90% disappearance times (DT50 and DT90) of acetochlor in surface soil (0–15 cm) are displayed in Table 5. The acetochlor application verification samples (deposition trays) gave very good accountability of day of application acetochlor residues (77–130%). Slightly lower recoveries (53–96%) were observed when regular zero-contamination soil coring equipment was used to collect surface soil samples. The improved accountability of acetochlor residues using deposition trays can be attributed to the rapid collection time (1 h after acetochlor application), the larger surface area of the collection devices, and no requirement for sample preparation before analysis (other than thorough mixing to ensure homogeneity).
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The results from the acetochlor adsorption studies indicated that acetochlor adsorption to surface soils was low (Koc < 350 L kg–1). The incubation studies demonstrated that acetochlor degradation in both field- and laboratory-incubated subsoils was rapid. Field and laboratory DT50 values ranged from 2 to 88 d in subsoil and 1 to 18 d in surface soils. The estimated DT50 of acetochlor obtained from the in situ field study was comparable with that from laboratory incubations of the same soils, confirming the validity of performing laboratory-based degradation studies to determine the DT50 values in subsoils.
Bromide Residues in Soil-Pore Water and Ground Water
Soil-pore water and ground water samples were analyzed to monitor the downward progression of the potassium bromide tracer through the vadose zone and into ground water in the unconfined aquifer. Bromide data for the Iowa, Indiana, and Pennsylvania PGW sites are summarized in the lower panels of Fig. 2 through 4
. To simplify the graphical representation of the bromide data, an average bromide concentration (µg L–1) was determined for each suction lysimeter and monitoring well depth for each sampling event. An arbitrary concentration of one half of the method LOQ (50 µg L–1) was assigned to water samples with residues less than the method LOQ (100 µg L–1), or with no detectable residues of bromide ion. Breaks in the graphed data denote sampling events where no samples were collected (i.e., no average bromide concentration could be determined).
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No residues of acetochlor were detected in any ground water samples, with the exception of seven very low detections (maximum residue of 0.06 µg L–1) observed at the Iowa study location. Six of these detections were below the LOQ (0.05 µg L–1) of the analytical method. These detections were all observed in ground water samples collected at approximately 30 d after acetochlor application. No further residues of acetochlor were detected in any ground water samples analyzed after this sampling event.
Acetochlor Ethanesulfonic Acid and Acetochlor Oxanilic Acid Residues in Soil-Pore Water and Ground Water
No detectable residues of acetochlor ESA and acetochlor OXA were determined in soil-pore water and ground water samples collected from each PGW control plot. A total of 2147 (16%) of the 13505 soil-pore water and ground water samples analyzed from the treated PGW plots for acetochlor ESA contained residues equal to or greater than 1.0 µg L–1. Soil pore water detections were widespread and were observed at all sites, with a maximum soil-pore water residue of 66 µg L–1, observed at the Nebraska study location in a 0.9-m suction lysimeter (Table 6). Acetochlor ESA residues (>1.0 µg L–1) were detected in ground water at five sites with a maximum ground water residue of 13 µg L–1 observed at the Indiana study location in a shallow monitoring well (Table 6). Acetochlor ESA data for the Iowa, Indiana, and Pennsylvania PGW sites are summarized in the upper panels of Fig. 2 through 4. To simplify the graphical representation of the acetochlor ESA data, an average acetochlor ESA concentration (µg L–1) was determined for each suction lysimeter and monitoring well depth for each sampling event. An arbitrary concentration of one half of the method LOQ (0.50 µg L–1) was assigned to water samples with residues less than the method LOQ (1.0 µg L–1), or with no detectable residues of acetochlor ESA. Breaks in the graphed data denote sampling events where no samples were collected (i.e., no average acetochlor ESA concentration could be determined).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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The three sites that displayed soil pore water or ground water residues of acetochlor greater than 0.05 µg L–1 were composed of fine-textured surface soils (Table 1), not expected to be prone to classical pesticide leaching. Closer examination of site-specific soil characteristics, microtopography, and rainfall patterns within the first 14 d following acetochlor application provides possible explanations for the detection of acetochlor at these locations. A general trend is that these three sites were rather wet in the two weeks following applications, while the other sites started with near-normal precipitation.
Iowa
The Iowa study location consisted of a Marshall silty clay loam with a small lobe of Minden silty clay loam (see Table 1 for soil series descriptions). These soils are moderately permeable and were formed in loess on uplands and high stream benches. The entire soil profile was extremely wet at the time of study initiation (199 mm of rainfall was recorded on-site in the 32 d before acetochlor application). It is likely this high antecedent soil moisture led to a condition in which only the largest of the soil pores were unsaturated and open at the time of acetochlor application. When heavy rainfall (77 mm over 3 d) was recorded between 12 and 14 d after acetochlor application, these larger pores could have provided a mechanism for rapid preferential transport of acetochlor through the vadose zone and into shallow ground water. The on-site depth to ground water (as measured by a pressure transducer installed in a test plot piezometer) decreased by approximately 286 mm during this 72-h period, indicating recharge of the unconfined aquifer, probably the result of rapid infiltration of rainfall water.
Further evidence of macropore flow is demonstrated by the pattern of bromide, acetochlor, and acetochlor ESA detections observed during the first 30 d after acetochlor application. During this time period, no bromide, acetochlor, or acetochlor ESA detections were observed in any of the 0.9- and 1.8-m lysimeters. All detections were confined to the 2.7- and 4.5-m lysimeters, and monitoring wells indicating the rapid "by-pass" movement to these deeper sampling zones. Significant spatial variability in the soil-pore water and ground water acetochlor detections was observed. Acetochlor residues were observed in four of the eight test plot lysimeter and monitoring well clusters. Detections were observed in monitoring wells located at the northeastern and southeastern portions of the test plot, but not in suction lysimeter clusters located in these areas. Bromide and acetochlor ESA data for the Iowa PGW site are displayed graphically in Fig. 2.
Indiana
The Indiana study location consisted of Door and Lydick loams, which are very deep, well drained soils formed in loamy glacial outwash, somewhat more permeable than the loess soils of the Iowa site. This site was also subject to very wet conditions at the time of study initiation, receiving 108 mm of rainfall in the first 8 d after application of acetochlor. This rainfall resulted in shallow ponding for several days over a topographic low spot in the test plot where one of the lysimeter clusters was positioned. The shallow lysimeter in this single cluster, installed at a depth of 0.9 m, was found to contain 0.33 µg L–1 of acetochlor at 1.5 mo after application. This residue declined to less than 0.05 µg L–1 over the next 10 mo, and no further downward movement of acetochlor within this lysimeter cluster was observed. Because no lysimeters in any other clusters at any depths were found to contain acetochlor, it appears that the localized ponding of water around this lysimeter cluster provided sufficient hydraulic head to facilitate preferential movement (increased infiltration of water) within this topographically low area of the test plot. A similar pattern of early movement of bromide ion and acetochlor ESA in the same lysimeter cluster supports this observation. Bromide and acetochlor ESA data for the Indiana PGW site are displayed graphically in Fig. 3.
Pennsylvania
The Pennsylvania study location consisted of Clarksburg and Duffield silt loams, which are very deep, moderately well drained and permeable soils formed in colluvium, glacial till, or residuum from limestone, calcareous and noncalcareous shale, and sandstone. This site received 88 mm of rainfall in the first 14 d following application of acetochlor, including two storm events of 47 and 35 mm of rainfall at 5 and 13 d after application, respectively. These storm events (and associated infiltration of water) resulted in detections of acetochlor above 0.05 µg L–1 in a number of lysimeter units across the test plot at depths of up to 2.7 m. These detections, however, were at low concentrations (maximum of 0.16 µg L–1 at 1 mo after application at a depth of 0.9 m). Residues of acetochlor dissipated rapidly with residues declining to less than 0.05 µg L–1 by 5.5 mo after application. Bromide and acetochlor ESA data for the Pennsylvania PGW site are displayed graphically in Fig. 4.
Acetochlor Ethanesulfonic Acid and Acetochlor Oxanilic Acid Detections
Soil degradates have been the focus of many recent publications, which suggest that degradates may be more mobile (Barrett, 1996) and more frequently detected (Kalkhoff et al., 1998) than the parent analytes. This study gave similar results. Acetochlor was not detected in ground water, except at the Iowa site, and then only at very low levels (<0.1 µg L–1). By contrast, acetochlor ESA was detected in ground water (>1.0 µg L–1) at four sites and at higher concentrations, with a maximum concentration of 13 µg L–1 observed at the Indiana site (Table 6). The acetochlor OXA degradate was detected in ground water at only one site, Minnesota, at a maximum concentration of 1.4 µg L–1 (Table 6). These detection trends are entirely consistent with the pattern observed in the companion 175-site acetochlor ground water monitoring program (De Guzman et al., 2005), in which acetochlor ESA was also detected more frequently than acetochlor OXA (24 vs. 6% of all samples analyzed had detections) and at higher concentrations (90th percentile annualized means were 0.87 to 1.28 µg L–1 for acetochlor ESA vs. <0.1 µg L–1 for acetochlor OXA).
Comparison with SCI-GROW Model Predictions
The SCI-GROW model determined an acetochlor ground water concentration of 0.1 µg L–1, which was based on input parameters of 3.5 kg a.i. ha–1, 162 L kg–1, and 10 d, for maximum single application rate, Koc (soil organic carbon partition coefficient), and aerobic soil metabolic half-life, respectively. No acetochlor detections were observed with the exception of the Iowa study location (maximum concentration of 0.06 µg L–1). At this site, preferential macropore flow likely assisted the transport of acetochlor to ground water. It is not surprising that the acetochlor PGW program results do not agree with the SCI-GROW predicted concentration, as SCI-GROW predicts conservative or high-end exposure values. Also, the SCI-GROW model is based on data derived from PGW monitoring studies conducted at highly vulnerable sites not eligible for application of acetochlor due to its soil use restriction (i.e., coarse-textured permeable soils of low organic matter overlaying shallow ground water).
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Acetochlor dissipated rapidly in the surface soil and only 0.25% of the water samples analyzed contained residues of acetochlor greater than the method LOQ. The sporadic detections were confined to the Iowa, Indiana, and Pennsylvania sites. The soil degradate acetochlor ESA was detected more frequently and at higher concentrations, with 16% of the water samples analyzed containing residues greater than or equal to 1.0 µg L–1. Soil pore water detections of acetochlor ESA were observed at all eight sites, and leached ground water (71.0 µg L–1) at five sites. In contrast, only 0.15% of the water samples analyzed contained acetochlor OXA residues greater than or equal to 1.0 µg L–1. Soil-pore water detections of acetochlor OXA were confined to four sites, and reached ground water at only one site (Minnesota).
The acetochlor PGW program demonstrates that the parent molecule dissipates from surface soil rapidly, and that it lacks the potential to leach to ground water at detectable concentrations. Acetochlor's impact on ground water quality is primarily limited to the potential for a degradate, acetochlor ESA, to reach shallow aquifers, generally at concentrations of 1 to 5 µg L–1. A second degradate of acetochlor, acetochlor OXA, has a considerably lesser potential to reach ground water, and in those rare instances when it is observed, only at trace levels less than 1.0 µg L–1.
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