Field and Laboratory Dissipation of the Herbicide Fomesafen in the

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Field and laboratory dissipation of the herbicide fomesafen in the southern Atlantic Coastal Plain (USA) Thomas L Potter, David D Bosch, and Timothy C Strickland J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.6b01649 • Publication Date (Web): 07 Jun 2016 Downloaded from http://pubs.acs.org on June 15, 2016

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Journal of Agricultural and Food Chemistry

Field and laboratory dissipation of the herbicide fomesafen in the southern Atlantic Coastal Plain (USA)

Thomas L. Potter,* David D. Bosch, Timothy C. Strickland

________________________________________________________________________ T.L. Potter*, D.D. Bosch, and T.C. Strickland, USDA-ARS, Southeast Watershed Research Laboratory, P.O. Box 748, Tifton, GA 31793. *Corresponding author ([email protected]).

Mention of trade names or commercial products in this article is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture.

Keywords: pesticide transport, conservation tillage, strip tillage, conventional tillage, leaching, metabolites

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ACS Paragon Plus Environment


ABSTRACT 1

To control weeds with evolved resistance to glyphosate, Southeastern (USA) cotton farmers have

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increased fomesafen (5-(2-chloro-a,a,a-trifluoro-p-tolyloxy)-N-mesyl-2-nitrobenzamide) use. To

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refine fomesafen risk assessments, data are needed that describe its dissipation following

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application to farm fields. In our field studies relatively low runoff rates and transport by lateral

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subsurface flow, 3 years after application. Findings suggest

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low potential for fomesafen movement from treated fields however fate of fomesafen that

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accumulated in subsoil and the identity of degradates are uncertain. Soil and water samples were

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screened for degradates however none were detected.

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INTRODUCTION

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Use on crops with engineered resistance has made glyphosate the world’s most widely used

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herbicide.1 This practice also appears to have contributed to evolved resistance in many weeds

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and has created complex and costly problems for production of numerous crops.2,3 Highly

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glyphosate resistant Palmer amaranth (Amaranthus palmeri) is particularly troublesome to cotton

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growers in the Southeastern USA.4 Glyphosate continues to be used during cotton production in

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the region but older more selective herbicides with alternate modes of action are often required

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for Palmer amaranth control.5 This includes the protoporphyrinogen oxidase inhibitor, fomesafen

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(5-(2-chloro-a,a,a-trifluoro-p-tolyloxy)-N-mesyl-2-nitrobenzamide).6 When applied

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preemergence at recommended rates fomesafen did not negatively impact cotton yields and it

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was highly effective in Palmer amaranth control.7,8

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Fomesafen has been in use in the USA for nearly thirty years, primarily on soybean. It was

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labeled, i.e. licensed for use, on cotton in 2006.9 This coincided with the first published report of

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a glyphosate resistant Palmer amaranth biotype in central Georgia (USA).10 Since this time,

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resistant biotypes have been identified throughout the region prompting a rapid increase in

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fomesafen use.4,6 Between 2006 and 2012, there was an estimated 9-fold increase in Georgia and

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a 4-fold increase nationwide.11,12 In Georgia, grower surveys indicated that >80% of all cotton

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fields receive annual preemergence treatment with herbicides containing fomesafen.6 At the

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recommended label rate this translates to application of 170 metric tons of fomesafen annually

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on the approximately 0.5 million ha in cotton production.

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Risk assessments that lead to fomesafen approval for cotton and more recently on several

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vegetable crops concluded that human and ecological risks of exposure were within acceptable

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margins.13-15 However, these assessments indicated that the compound’s environmental fate

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properties make it mobile and persistent in terrestrial and aquatic environments suggesting that

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the rapid increase in fomesafen use may have adverse impacts. Published water solubility, soil

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Koc, and soil aerobic half-life (t1/2) are 50 mg L-1, 50 mL g-1, and 86 days, respectively.16

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Human health concerns were linked to potential for contamination of water supplies. For

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example, the New York Department of Environmental Conservation (NYDEC) determined that

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risks of fomesafen application in an area with sandy soils underlain by a vulnerable sole-source

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aquifer were unacceptable due to potential for the compound to leach to shallow groundwater.17

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The decision concluded that 0.017 mg L-1 in drinking water was a screening level of concern.

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The most sensitive surrogate species used in ecotoxicological assessments was freshwater green

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algae. The no observable adverse effect concentration (NOAEC) was 0.010 mg L-1.18 High

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sensitivity of green algae and fomesafen’s potential for persistence in aquatic environments were

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confirmed in outdoor microcosm studies.19 Potential for drift during application and or runoff to

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contact endangered plants at field margins have also contributed to proposals for relatively wide

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buffer areas, 100 to 350 m, around treated fields.20

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Several published investigations support conclusions regarding fomesafen’s relatively high

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potential for runoff, leaching, and persistence. Rainfall simulations conducted on a silt-loam soil

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in Georgia’s Atlantic Coastal Plain region showed that up to 5% of the herbicide applied may be

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lost in surface runoff during a single storm event one day after fomesafen application at the label

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rate.21 High frequency of detection in river water samples in a region in Central Canada where

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fomesafen was used on soybean was another indication of runoff potential.22 Leaching was

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indicated in a groundwater investigation in the North Carolina 23 and in the aforementioned

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rainfall simulations. Before simulations 12.5 mm of irrigation was applied to selected plots

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immediately after herbicide application. This reduced fomesafen runoff during subsequent

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simulations by more than 2-fold. It was concluded that runoff availability was reduced by

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movement of fomesafen into the soil with infiltrating irrigation water.21 Indications of

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persistence were provided in field dissipation investigations. Days to 50% dissipation (DT50)

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averaged 37, 47, and 50 days in agricultural fields in Brazil, Tennessee, and New York

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respectively.24-26

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While studies have demonstrated that fomesafen may persist in soil in farm fields and that runoff

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and leaching may contribute to adverse water quality impacts, investigations were limited in

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scale, scope, and region. Of particular interest to our research, is the lack of field scale

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investigations in the Atlantic Coastal Plain region of Georgia. As noted, fomesafen use in the

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area has increased rapidly in the past decade. The objective of the current study was to obtain

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data from field studies needed to refine risk assessments and devise mitigation measures as

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necessary.

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MATERIALS AND METHODS

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Site description and hydrologic monitoring. A topographic map that included a field plot

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layout was recently published. 27,28 Plots were located in Tift County Georgia, USA

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(N31o26’13”, W83o35’17”) within a gently sloping 1.2-ha field subdivided into two 0.6-ha fields

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running up and down the slope. Soil series identified in the fields, Tifton loamy sand (fine-

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loamy, kaolinitic, thermic Plinthic Kandiudult), Fuquay loamy sand (loamy, kaolinitic, thermic

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Arenic Plinthic Kandiudult), and Carnegie sandy loam (fine, kaolinitic, thermic Plinthic

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Kandiudult) have closely related properties including permeable surface horizons with >85%

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sand and dense argillic horizons at about 50 cm with plinthite at depth that impedes internal

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drainage and promotes lateral subsurface flow.27 This flow was captured with 15-cm diameter

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perforated tile drain pipes installed to a depth of 1.2 m below grade at the base of the slope and

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along the northern boundaries of each field. Flow from each drain tile was conducted to 0.24-m

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metal HS flumes through non-perforated 15-cm diameter drain pipe. Each 0.6-ha field was

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further divided into three 0.2-ha plots running across the slope. Earthen berms that were

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constructed around each plot directed surface runoff to 0.46-m metal H flumes located at NW

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corners. Pressure transducers (Druck Inc., New Fairfield, CT, USA) linked to Campbell

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Scientific data-loggers (Campbell Scientific, Inc., Logan, Utah, USA) measured depth of flow in

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all flumes at 1 min intervals. Rainfall was measured with a tipping bucket rain gage (Texas

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Electronics Inc., Dallas, TX) located 10-m from the NW field boundary.

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Plot management. University of Georgia extension service recommendations guided

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management. All crops planted in May and harvested in September-October (Table 1). After

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harvest, rye (Secale cearale) was planted as a cover crop and maintained until termination by

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glyphosate application. Prior to planting cotton in 2009, the block of plots on the NE side of the

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field were conventionally tilled (CT) by inversion plowing to a depth of 20 cm, disking, and

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ripping followed by seed bed formation. Cotton in the SW block was planted into 15-cm strips

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tilled (ST) into the desiccated cover crop residue mulch. CT and ST practices had been

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continuous use on these plots since 1999.27,28 The single fomesafen application in the study was

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made to this cotton crop 1 h after planting at the label maximum, 0.42 kg ha-1, using the

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commercial formulation, Reflex®(Syngenta, Greensboro, NC, USA). Application was with a

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tractor-mounted boom sprayer. The herbicide was incorporated within 4 h with 12.5 mm

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irrigation delivered by a solid set system. Other irrigation applied to meet crop water needs or

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facilitate tillage ranged from 82 to 230 mm yr-1 (Table 2). All plots were converted to no-till for

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the millet and sorghum crops (Table 1). The rye winter cover crops were terminated with

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glyphosate, allowed to dry, and rolled prior to directly drilling seed using a Sukup no-till planter

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(Sukup Manufacturing Co., Sheffield, Iowa, USA).

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Water sample collection. For surface runoff, ISCO® (Lincoln, NE, USA) autosamplers were

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programmed to collect 0.05 L into 9-L glass jars for every 1040 L that passed flumes during each

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event.28 The minimum sample volume required for analysis (0.10 L) was equivalent to about 0.5-

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mm runoff. No events exceeded the upper bound on the sample collection system, i.e. events

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which would overfill the glass jars. Over the 4 years of runoff sample collection sampling

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efficiency defined as the percent of total runoff for which samples were collected divided by

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total flow was 88%. For lateral subsurface flow, refrigerated ISCO® automated samplers were

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programmed to draw 0.05 L into 9-L glass collection jars at 0.5-h intervals during periods of

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flow. Samples were retrieved at 2 to 4 day intervals. Sample collection efficiency, defined as the

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percent of days when samples were collected compared to total days that flow occurred was

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99%.

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Soil sample collection. About 1 h after applying fomesafen and prior to irrigation incorporation,

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composite soil samples to a depth of 2 cm were collected from each plot. Samples were

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combined by tillage group, CT or ST. Soil samples was collected similarly in 2010 prior to

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planting the millet and application of preemergence herbicides used for this crop. Four 7.5-cm

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diameter cores were collected from each of the plots to a depth of 1.2 m after sorghum harvest in

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2012 using a tractor mounted Giddings hydraulic coring device (Giddens Machine Company,

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Windsor, CO, USA). Polycarbonate sleeves were used. For sampling, plots were divided into

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four quadrants with cores collected at quadrant centers. Cores were sectioned in 15 cm intervals

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and combined by depth increment to yield a single set of 8 composite samples for each plot.

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Soil incubations. After sieving though a 2-mm stainless steel screen, 24 field-moist 50-g

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subsamples from the ST and CT treatments collected after fomesafen application, were placed in

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250-mL French-square glass bottles. Soil water content in all bottles was adjusted to field

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capacity (12% v/v) with deionized water. Fifty mL methanol was then added to three bottles

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from each treatment groups. These bottles were sealed with Teflon®-lined caps and stored at -

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20oC. All remaining bottles were similarly capped, shaken, and placed in a dark 25oC laboratory

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incubator. Three, 7, 14, 28, 49, 77, and 100 days later all bottles were shaken and 50 mL

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methanol was added to 3 bottles from each treatment group. After recapping they were also

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stored at -20oC. The surface soil sample collected prior to planting millet in 2010 was handled

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similarly with the exception that after placing soil subsamples in bottles, analytical grade

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fomesafen coated on silica sand passing a 0.25-mm sieve was added to each. The target soil

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fomesafen concentration was 1.4 µg g-1, was equivalent to the maximum label rate. Bottles were

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then capped and shaken.

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Soil and water sample handling and preparation. One L aliquots were used reserved for

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analysis from all lateral subsurface flow samples. When runoff sample volume was 95% when analyzed by HPLC-MSMS

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as described below. The surfactant and zinc powder were purchased from Sigma-Aldrich (St.

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Louis, MO, USA).

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Instrumental analysis. Extracts were brought to room temperature, fortified with 0.2 mL 0.1%

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formic acid, and analyzed by high-performance liquid chromatography (HPLC)-mass

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spectrometry with a ThermoFinnigan TSQ® Quantum Mass Spectrometer system

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(ThermoFisher, San Jose, CA, USA). The HPLC column was a 4.6 mm x 100 mm, 3.5 µm,

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Zorbax Eclipse Plus C18 (Agilent Technologies, Santa Clara, CA, USA). Initial conditions of the

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methanol (B) 0.1% formic acid (A) gradient elution were 90% A: 10% B. After injection, mobile

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phase composition was changed linearly to 10% A: 90% B in 4.0 min and held isocratic for 7

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min. Mobile phase was then increased 100% B in 0.5min and held isocratic for 2.5 min followed

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by return to initial conditions in 1 min. The combined flow rate was 0.5 mL min-1. Data were

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collected by selected reaction monitoring (SRM) using electrospray ionization (ESI). Negatively

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charged parent and product ions were 437→286 (fomesafen) and 407→329 (fomesafen amine).

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Quantification was based on the product ions. The method detection limit (MDL) for both

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compounds in water samples was 0.01 to 0.03 µg L-1 (depending on volume of sample extracted

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and in soil samples, 0.002 to 0.004 µg g-1 (depending on mass of soil extracted). To screen for

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other degradation products, all extracts were also analyzed using the same HPLC conditions with

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the mass spectrometer sequentially scanned from m/z = 100 to 500 in positive and negative ion

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modes. Prior to these analyses the mass spectrometer response to m/z = 437 or 439 was

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optimized while infusing a 10 µg mL-1 fomesafen solution in methanol.

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Quality control. Target analytes were not detected in laboratory blanks prepared with HPLC

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grade water. Calibration standards of the two target compounds were run with each sample set

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(50 to 100 samples). Recovery of the analytes by SPE from water was evaluated by preparing

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replicate (n=4) solutions of the compounds at 0.1 µg L-1 in HPLC grade water. The average

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(standard error) of recoveries were fomesafen 96 (4.0) %, and fomesafen amine, 47 (6.0) %.

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Spikes at 0.1 ug g-1 of Tifton surface soil collected in an untreated area yielded recoveries of 97

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(3.9)% and 52 (3.8)% for fomesafen and fomesafen amine respectively.

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Data analysis. When surface runoff occurred on consecutive days and or on weekends and

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holidays, samples were composites of the runoff that occurred during these intervals. Measured

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concentrations were assigned to each of the contributing days. No assignments were made for

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days when runoff was recorded but samples were not collected due to sampler malfunction or

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runoff

Field and Laboratory Dissipation of the Herbicide Fomesafen in the

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