Assessment of the Effects of Farming and Conservation Programs onPesticide Deposition in High Plains WetlandsJason B. Belden,†,* Brittany Rae Hanson,† Scott T. McMurry,† Loren M. Smith,†

and David A. Haukos‡

†Department of Zoology, Oklahoma State University, Stillwater, Oklahoma, United States‡Department of Natural Resources management, Texas Tech University, Lubbock, Texas, United States

*S Supporting Information

ABSTRACT: We examined pesticide contamination in sediments fromdepressional playa wetlands embedded in the three dominant land-usetypes in the western High Plains and Rainwater Basin of the UnitedStates including cropland, perennial grassland enrolled in conservationprograms (e.g., Conservation Reserve Program [CRP]), and nativegrassland or reference condition. Two hundred and sixty four playas,selected from the three land-use types, were sampled from Nebraska andColorado in the north to Texas and New Mexico in the south. Sedimentswere examined for most of the commonly used agricultural pesticides.Atrazine, acetochlor, metolachlor, and trifluralin were the mostcommonly detected pesticides in the northern High Plains and RainwaterBasin. Atrazine, metolachlor, trifluralin, and pendimethalin were the mostcommonly detected pesticides in the southern High Plains. The top 5−10% of playas contained herbicide concentrations that are high enough to pose a hazard for plants. However, insecticides andfungicides were rarely detected. Pesticide occurrence and concentrations were higher in wetlands surrounded by cropland ascompared to native grassland and CRP perennial grasses. The CRP, which is the largest conservation program in the U.S., wasprotective and had lower pesticide concentrations compared to cropland.

■ INTRODUCTIONThe United States High Plains grasslands and wetlands havebeen extensively altered over the past century.1,2 Cultivationagriculture has converted over 15 million ha of grassland tocropland and caused widespread unsustainable sedimentationof the dominant depressional playa wetlands.3−5 This hasdrastically impacted the natural goods and services provided bythese ecosystems to society.6 Despite our knowledge of theimpacts of row crop agriculture on ecology of depressionalwetlands, few studies have investigated the level of pesticidecontamination in these wetlands or the potential impacts ofpesticide residues on resident biota.Conservation programs have been implemented in the High

Plains, primarily by the U.S. Department of Agriculture (USDA),to reduce negative agricultural impacts. The dominant USDAprogram in the High Plains is the Conservation Reserve Pro-gram (CRP). The original goal of the program was replace-ment of highly erodible cropland to perennial cover. Thehighest density of CRP land in the nation is in the High Plainswith over 3 million ha planted to perennial grasses, primarilyexotics but also some native mid and tall grass planted inhistorical short-grass prairie zones.6 Indeed, over $97 millionis spent annually on CRP payments in the High Plains.7

Additionally, a second USDA program, the Wetland ReserveProgram (WRP), is common in some areas such as theRainwater Basin in central Nebraska and is focused on preserving

and restoring hydrological function of wetlands. Typicaltreatments include sediment removal and placement of aneasement of perennial grasses around the wetland.8

As depressional wetlands, most runoff, including water, asso-ciated sediments, and potentially contaminants such as pesti-cides, are deposited in playa basins. However, the existingresidue levels and spatial distribution of chemicals in playas ofthe High Plains is unknown. As primary sites of biodiversityprovisioning and recharge of the nation’s largest aquifer, thedeposition of contaminants in playas has important direct andindirect societal impacts.Therefore, we measured pesticide levels in playa sediments

embedded in the dominant land-use types in the western HighPlains and the Rainwater Basin in south central Nebraska(cropland, CRP or WRP, and native grassland or referencecondition). Two hundred and sixty four playas, randomlyselected from the three land-use types, were sampled fromNebraska and Colorado in the north to Texas and New Mexicoin the south. Sediments were examined for most of thecommonly used agricultural pesticides applied throughout theregion. Our approach was to analyze sediment samples as theyrepresent a primary sink for much of the pesticide load in

Received: July 15, 2011Accepted: February 22, 2012Published: February 22, 2012

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depressional wetlands, many of which are dry for the majorityof the year. We report pesticide concentrations found in playasand compare those values to relevant and sensitive toxicologicalend points. In addition, we examined the influence of surround-ing land-use and USDA conservation programs on the concen-tration of pesticides in playa sediments.

■ MATERIALS AND METHODSMaterials. Neat standards (>98%) of all analytes were

purchased from Sigma-Aldrich (St. Louis, MO). All solventsand other reagents were pesticide or GC/MS grade (Burdickand Jackson, Muskegon, MI).Site Selection. A total of 264 playas was selected and

sampled across five states. Based on pesticide usage patterns,cropping patterns, and climatic differences, playas were splitamong three regions including southern playas (westernOklahoma, eastern New Mexico, and western Texas), northernplayas (western Kansas, western Nebraska, and northeasternColorado), and the Rainwater Basin (south-central Nebraska).Figures 1−3 in the Supporting Information (SI) illustrate thegeographical range of the playas. Playas were classifiedaccording to the dominant land-use in their immediate vicinity(500 m zone). For cropland playas, the presence of each croptype within the 500 m buffer zone was determined for each siteusing appropriate data layers from Cropscape (United StatesDepartment of Agriculture; http://nassgeodata.gmu.edu/CropScape/; accessed December 2011). In the south, 156 playaswere sampled and proximal land-use determined (63 cropland,48 CRP, and 45 native grassland). Cotton was the mostcommon crop occurring at 64% of cropland sites followed bywinter wheat (54%), field corn (13%), and grain sorghum(10%). In the north, 66 playas were sampled (23 cropland, 21CRP, and 22 native grassland). Winter wheat was the mostcommon crop occurring at 83% of cropland sites followed byfield corn (35%), grain sorghum (17%), and millet (4%). In theRainwater Basin 42 playas were sampled (15 croplandagriculture, 15 WRP, and 12 reference wetlands). Referencewetlands are the wetlands remaining in this intensively farmedregion that retain most of their natural function as determinedthrough the Hydrogeomorphic Method. These were classifiedas such by Nebraska Game and Parks Commission personnel.Field corn and soybean were the dominant crops occurring at93 and 53% of cropland sites, respectively.Sediment Collection. Sediments were collected as a

composite sample by sampling from the top 6 cm in threerandom locations within each wetland (∼500 g total sample).Samples were kept on ice in chemically clean glass jars untilarrival at the laboratory where they were frozen until analysis.Most wetlands were dry at the time of sampling. Texas, NewMexico, and Oklahoma samples were collected in June and Julyof 2008, while Kansas, Colorado, and Nebraska samples werecollected in June and July of 2009.Analyte List. Pesticides were chosen as analytes based on

use within the region, feasibility for analysis using the availabletechniques, and potential toxicity. SI Table 1 lists the analytesalong with their chemical abstract numbers and selected toxicityend points. Insecticides were emphasized based on theirpotential toxicity. In addition, several fungicides were added forthe northern playas due to increased trends in usage andpotential toxicity.9 All of the compounds on this list areextractable with organic solvent and can be directly analyzedusing gas chromatography coupled with mass spectrometryallowing a single measurement technique.

Analysis of Samples. Twenty grams of each compositesample was ground to dryness with 40−60 g of anhydroussodium sulfate and extracted using dichloromethane as asolvent in a Soxhlet apparatus (6 h, greater than 24 rinses).Following extraction, extracts were evaporated to less than 10mL and solvent exchanged to hexane. The reduced extract waseluted through a commercially prepared florisil column (0.5 gSigma Aldrich, Atlanta GA) followed by 10 mL of hexane:ethylether 1:3. This combined eluate contained the pesticides,leaving many interfering compounds on the column. Thevolume of the clean extract was reduced to 1 mL of hexane.A second aliquot of each sample was weighed and heated at105 °C for 24 h to determine the percent solid of each sample.All concentrations are reported on a dry-mass basis.Separation of the analytes by gas chromatography was

performed on an Agilent 6850 GC (Agilent, Palo Alto, CA)with a 30 m × 0.25 mm HP-5 column (Agilent) and splitlessinlet. The oven was programmed to start at 70 °C, hold for 1.0min, ramp at 10 °C/min to 160 °C, ramp at 3.5 °C/min to255 °C, ramp at 3.0 °C/min to 290 °C and hold for 2.0 min.The inlet temperature was 240 °C and the transfer line was290 °C. The column flow was 1.0 mL/min (37 cm/sec averagevelocity).Detection and quantitation of analytes were conducted by

mass spectrometry on an Agilent 5975c inert source instrument(Agilent, Palo Alto, CA). Electron ionization was used (70 eV)as the ionization source. Temperature of the source was 230 °Cand the quadropoles were at 150 °C. Detection was based on3-ion selected ion monitoring (SIM; SI Table 2). Calibrationwas conducted using internal standards (decachlorobiphenyl,atrazine D-5, and tributylphospate). Quantitation limits (QL)are based on the lowest calibration standard. Each QL was atleast 3× the method detection limits calculated by measuringlow-level concentrations of analytes in extracts of clean soil.QLs are reported instead of method detection limits to ensurethat low level hits in samples are quantitatively accurate andreduce qualitative uncertainties by ensuring that all quantitativeand qualitative ions are measurable (SI Table 2).Quality control was performed throughout sampling and

analysis. Field/travel blanks were performed at a 5% frequencyof samples as were laboratory blanks. Field blanks weregenerated by adding reference sediment mixture to a jar in thefield, while laboratory blanks were generated by extractingreference sediment at the same time samples were extracted.Laboratory matrix spikes and matrix spike duplicates (usingclean reference sediment) were also performed at 5% frequencyand at a concentration of 10 μg/kg. Surrogates (triphenylphos-phate, trichloromethylxylene, and p-terphenyl) were added toeach sample to monitor accuracy of each extraction/analysis(10 μg/kg spike level).

Data Analysis. Data were heavily “left-censored” due to ahigh frequency of values (>50% in most cases) reported at thequantitation limit. Thus, standard descriptive statistics includingmean and standard error would be biased.10 Frequency abovethe quantitation limit (QL) was calculated and comparisonswere made among land-use groups using contingency tablesand χ2 analysis (the probability of making a Type I error (α) =0.05). Percentile ranks at 75th, 90th, and 95th percentiles werecalculated using rankings and not assuming any standarddistribution (percentile function, Excel, Redmond, CA). Forexample, the 50th percentile is the median. Since the data wereheavily censored, nonparametric statistical approaches wereused for comparisons.10 Statistical comparisons among land-use

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groups were performed using a Kruskal−Wallis test (α = 0.05).If significant differences existed (p < 0.05), Tukey-Krameranalysis was performed on ranked data for pairwise comparisonof land-use categories (using Statview 5, SAS Institute, Cary,NC). Statistical tests were only conducted for pesticides thatoccurred at an overall frequency of >10% and had maximumvalues >3× the QL. Additionally, the median and maximum ofthe concentrations above the QL are reported. This is themedian of values above the QL and not of the complete dataset. Percentiles provide a more reliable measure of the expectedconcentration for a randomly selected sample.Toxicological end points are provided as a comparison for

adequacy of QL and hazards posed by reported values (such aspercentiles). Values will be referred to as level of concern(LOC) and do not represent a specified risk value; they simplyprovide a benchmark for assessing hazard and directing futurestudy. In order to obtain LOC values for most pesticides on theanalysis list, we derived values using three approaches based onavailable data and most sensitive nontarget species. For mostinsecticides, median lethal toxicity values (LC50) for toxicity toamphipods (typically Hyalella azteca) were available. Acutetoxicity to amphipods through sediment exposure is a sensitiveand widely used end point.11 For other insecticides, mostly thewater-soluble organophosphates, sediment toxicity values werenot available. Thus, for these pesticides we used a secondaryapproach and derived sediment toxicity values from availableaquatic toxicity values (48 h Daphnia magna LC50).Equilibrium partitioning was assumed and a value was obtainedby multiplying the soil/water partitioning coefficient (Kd) bythe acute, 48 h Daphnia magna median lethal concentration(LC50). A previous study demonstrated that this approachprovided a reasonable estimate for cypermethrin sedimenttoxicity.12 Finally, for herbicides, the toxicity to nontarget plantsthrough sediment/soil exposure was greater than the toxicity to

aquatic organisms (based on values in the EPA ECOTOX).13

Therefore, for herbicides we used a third approach and basedLOC values on terrestrial toxicity to nontarget plants.). Playasare often dry much of the year and plant productivity is animportant ecosystem service. SI Table 1 lists these values andthe method used to obtain each value for each pesticide.

■ RESULTS

Quality Control and Method Development. Pesticideconcentrations in all field and laboratory blanks were belowQLs (recovery for matrix spikes (n = 46) are in SI Table 2).Mean recovery for most compounds was between 70 and 120%.The primary exception was for a few organophosphate insec-ticides that ranged between 58 and 70%. These compoundshad >70% recoveries in initial method development; however,interferences within the matrix spikes resulted in lower recov-eries in field collected samples. Although these compoundswere rarely detected, recovery was high enough to suggest thatall compounds would have been detected if present. Aldicarband acetamiprid could only be detected at relatively highconcentration and thus the technique should be onlyconsidered qualitative. However, no values were detectedabove a conservative quantitation limit. Quantitation limitswere below LOC values for all compounds (SI Table 1) withthe exception of acetochlor, where the QL and LOC wereequal.

Pesticides in Sediments from Southern Playas.Atrazine, metolachlor, pendimethalin, and trifluralin were themost frequently detected pesticides in the southern playasediments (Table 1). In addition to these herbicides, threeinsecticides, chlorpyrifos (3.9 μg/kg), malathion (2.1 μg/kg),and cypermethrin (6.1 μg/kg) were found above the QL once.All three insecticide occurrences were in wetlands nearcroplands. No other pesticides occurred above QLs.

Table 1. Summary Data for Pesticide in Southern Playa Sediments (TX, OK, NM)a

number ofsamples QL1 freq. >QL, %

max. value, μg/kg

median > QL, μg/kg percentiles for all samples, μg/kg2

totalaboveQL 75 90 95

atrazine all land uses 158 21 <2 13 37 3.3 <2 2.2 4.7cropland 63 10 <2 16 37 9.3 <2 3.3 9.9CRP 48 4 <2 8.3 2.3 2.2 <2 <2 2.10grassland 45 7 <2 16 25 2.8 <2 2.4 4.4

S-metolachlor all land uses 158 4 <2 2.6 134 11 <2 <2 <2cropland 63 3 <2 4.8 134 12 <2 <2 <2CRP 48 0 <2 0 <2 <2 <2 <2 <2grassland 45 1 <2 2.2 10 10 <2 <2 <2

pendimethalin all land uses 158 9 <1 5.8 900 8.8 <1 <1 3.10cropland 63 7 <1 11 900 8.8 <1 2.6 8.80CRP 48 2 <1 4.2 11 7.4 <1 <1 <1grassland 45 0 <1 0 <1 <1 <1 <1 <1

trifluralin all land uses 158 37 <0.5 24 90 3.7 <0.5 4.6 7.9cropland 63 20 <0.5 32 90 6.3 2.5 8.4 12CRP 48 12 <0.5 25 7.9 2.4 <0.5 2.6 3.4grassland 45 5 <0.5 11 4.8 1.8 <0.5 0.5 1.7

aA few other pesticides occurred infrequently in samples, please see text. 1QL, quantitation limit of the analytical method. 2Percentile values indicatethe pesticide concentration which would be greater than the listed percentage (75, 90, 95) of all samples. Thus, only 5% of samples would exceed the95%ile concentration.

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Land-use significantly influenced the occurrence of pendi-methalin and trifluralin with the highest level of occurrencefound in cropland playas, followed by CRP playas, and thennative grassland playas (p < 0.05, Figure 1A). Cropland playas

had significantly higher occurrence of pendimethalin andtrifluralin compared to grassland playas, but no differenceswere found between cropland and CRP or CRP andgrassland playas for either herbicide. Land-use did not affectatrazine occurrence. Similarly, land-use influenced the con-centration of pendimethalin and trifluralin (p = 0.04, and p =0.01), but not atrazine (Figure 1B). For both pendimethalinand trifluralin, concentrations were higher in cropland playas,which had concentrations at the 95%ile that were at leastthree times higher than CRP and four times higher thangrassland playas. Because metolachlor occurred in <5% ofsamples, statistical comparison among land-use groups was notperformed.Nearly all pesticide values were below LOC values (listed

in SI Table 1). Atrazine was the exception; the top 10% ofsamples near row crop agriculture (3.3 μg/kg) had atrazinelevels near or above the LOC (3.5 μg/kg) and the top 5%of grassland samples were above that level (9.9 μg/kg;Table 1).Pesticides in Sediments from Northern Playas.

Acetochlor, atrazine, metolachlor, and trifluralin were themost frequently detected pesticides in the sediment fromnorthern playas (Table 2). In addition to these herbicides,chlorpyrifos (6.7 μg/kg), propiconazole (3.1 μg/kg), andpendimethalin (3.5 μg/kg) were all found above the QL once

in samples from cropland playas and bifenthrin (1.1 μg/kg) andmetribuzin (2.6 μg/kg) were found above the QL once in agrassland playa.Land-use influenced the occurrence of atrazine (p < 0.05)

with the level of occurrence greater in cropland playas thaneither CRP or grassland playas (Figure 2A). Land-use alsoinfluenced occurrence of metolachlor with less frequent occur-rence in CRP playas than cropland or grassland playas(p < 0.05). Additionally, concentrations of atrazine andmetolachlor were influenced by land-use (p = 0.02 and 0.05,respectively). Atrazine and metolachlor concentrations at the95%ile were 25× and 50× greater in cropland playas ascompared to concentrations found in CRP or grassland playas(Table 2). Concentrations in cropland playas were significantlydifferent from CRP and grassland playas, which were notdifferent from each other. Because acetochlor occurred in lessthan 5% of samples, statistical comparisons among land-usegroups were not performed. Similarly, trifluralin was foundcommonly, but never at concentrations greater than twice theQL and in most cases very near the QL. This increases thecensoring effect and therefore further statistical comparisonswere not made for trifluralin.Nearly all pesticide values were below the LOC (Table 1; SI

Table 1) with the exception of atrazine and metolachlor. Forgrassland and CRP playas, the top 5% of samples had atrazineconcentrations near or above the level of concern, but valuesfor cropland playas exceeded the LOC (109 versus 3.5 μg/kg).For metolachlor, concentrations in grassland and CRP playaswere below LOC values; however, cropland samples greatlyexceeded the LOC (72 versus 10 μg/kg).

Pesticides in Sediment from the Rainwater Basin.Acetochlor, atrazine, metolachlor, and trifluralin were also themost frequently detected pesticides in sediment from theRainwater Basin (Table 3). In addition to these herbicides,metribuzin (2.2 μg/kg), pyraclostrobin (3.0 μg/kg), andpendimethalin (6.2 μg/kg) were all found above the QL oncein samples from WRP wetlands and bifenthrin (17 μg/kg) andmetribuzin (2.8 μg/kg) were found above the QL once inreference wetlands.Land-use and conservation program did not influence

occurrence of any pesticide in the Rainwater Basin samples(p > 0.05, Figure 3) despite trends for cropland playas to havemore pesticides than WRP and reference wetlands. Statisticalanalysis had little discriminatory power due to the limitednumber of detections coupled with the high degree of censoreddata (Table 3). Despite the lack of statistical power, the 95%ileconcentrations were much higher for cropland playas withgreater than 60× the atrazine concentration and 10× themetolachlor concentration for WRP and reference wetlands. Aswith the northern playas, trifluralin was found commonly, butnever at concentrations greater than twice the QL and in mostcases very near the QL. This increases the censoring effect andtherefore, further statistical comparisons were not made fortrifluralin.Nearly all pesticide values were below the LOC (Table 1)

with the exception of atrazine, acetochlor, and metolachlor. Foratrazine, sediment concentrations at the 95%ile concentra-tion in all three land-use categories exceeded the LOC value(3.5 μg/kg) with concentrations of 380, 6.1, and 22 forcropland, WRP, and reference wetlands, respectively. Aceto-chlor concentrations in sediment samples from cropland andWRP wetlands also exceeded the LOC at the 95%ile level (5.6 and17 μg/kg, respectively, as compared to 1 μg/kg). Acetochlor

Figure 1. Percent of southern playa sediments samples (TX, OK, NM)with pesticide concentrations above the quantitation limits (A) and the95%ile concentration for each pesticide (B). Both figures are separatedby land use and a different letter indicates values were not the sameamong land use groups. Comparisons were conducted for eachpesticide independently (p < 0.05).

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was not detected at reference sites. Metolachlor concentrationsin reference and WRP playas were below LOC values at the95%ile level; however, cropland playas exceeded the LOC atthe 95%ile level (72 as compared to 10 μg/kg).

■ DISCUSSION

Presence and Occurrence of Pesticides. Throughout thesouthern and northern playas and the Rainwater Basin, pesti-cides were commonly detected in sediment samples, which wasnot unexpected given the intensity of agriculture in the HighPlains and the relatively high sensitivity of the analyticaltechnique. The most commonly found pesticides in this studywere moderately persistent herbicides (atrazine, metolachlor,acetochlor, trifluralin, and pendimethalin), which have some ofthe highest use frequencies and rates (2005 data; USDA-NASS).14 In 2007, all five herbicides were ranked in the top 17most used based on mass in the agricultural market sector withatrazine, metolachlor and acetochlor being ranked among thetop five.15 Additionally, each of these pesticides can berelatively persistent as they have dissipation half-lives in soilthat can exceed 50 days depending on environmentalconditions (EPA Pesticide Fate Database).16

Occurrence patterns of each pesticide within playas paralleledapplication patterns based on USDA reports for 2005 (USDA-NASS).14 Atrazine and metolachlor are commonly applied tomajor crops (e.g., corn) in all regions studied and were alsocommonly detected in all regions studied. For example, atrazinewas applied to 55 and 77% of corn grown in 2005 in Texasand Nebraska, respectively. Atrazine was detected in 16% ofsouthern cropland playas and in over 40% of northern croplandplayas and within cropland wetlands within the RainwaterBasin. Similarly metolachlor was applied to 10 and 26% ofTexas and Nebraska corn, respectively, and detected in 4.8% ofsouthern playa cropland sediment and greater than 25% ofnorthern cropland playas and within cropland wetlands withinthe Rainwater Basin. Acetochlor had limited use in Texas, butwas widely used in Nebraska on corn (17% and accordingly wascommonly detected in Nebraska (4−6% of playas) and not inTexas. Trifluralin and pendimethalin were the predominate

Table 2. Summary Data for Pesticides in Northern Playa Sediments (KS, NE, CO)a

number ofsamples QL1

freq. of samples>QL, %

max. value,μg/kg

median of values aboveQL, μg/kg percentiles for all samples, μg/kg2

totalaboveQL 75 90 95

acetochlor all land uses 66 3 <1 4.5 3.3 2.5 <1 <1 <1cropland 23 1 <1 4.3 2.6 2.6 <1 <1 <1CRP 21 0 <1 0 <1 <1 <1 <1 <1grassland 22 2 <1 9.1 3.3 2.5 <1 <1 1.6

atrazine all land uses 66 17 <1 26 300 4.3 1.6 4.8 31cropland 23 10 <1 43 300 12 7.7 51 109CRP 21 4 <1 19 5.2 3.4 <1 2.8 4grassland 22 3 <1 14 4.3 2.7 <1 2.1 2.70

S-metolachlor all land uses 66 11 <1 17 150 1.3 <1 1.3 1.60cropland 23 6 <1 26 150 1.5 <1 1.6 72.00CRP 21 0 <1 0 <1 <1 <1 <1 <1grassland 22 5 <1 23 1.3 4.3 <1 1.3 1.30

trifluralin all land uses 66 7 <0.5 11 1.4 0.9 <0.5 0.5 0.9cropland 23 2 <0.5 8.7 1.4 1.2 <0.5 <0.5 0.8CRP 21 1 <0.5 4.8 1.2 1.2 <0.5 <0.5 <0.5grassland 22 4 <0.5 18 1.2 0.9 <0.5 0.9 0.9

aA few other pesticides occurred infrequently in samples, please see text. 1QL, quantitation limit of the analytical method. 2Percentile values indicatethe pesticide concentration which would be greater than the listed percentage (75, 90, 95) of all samples. Thus, only 5% of samples would exceed the95%ile concentration.

Figure 2. Percent of northern playa sediments samples (KS, NE, CO)with pesticide concentrations above the quantitation limits (A) and the95%ile concentration for each pesticide (B). Both figures are separatedby land use and a different letter indicates values were not the sameamong land use groups. Comparisons were conducted for eachpesticide independently (p < 0.05).

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pesticides found in southern playas where they were applied to58 and 10% of cotton acreage and they were the mostcommonly detected at 32 and 11%, respectively. Althoughtrifluralin was reportedly used on 5% of soybean acreage inNebraska in 2005, it was commonly found in the northernplayas and Rainwater Basin at trace levels (greater than 20%

was ∼1 μg/kg). Historically, trifluralin was used extensively onsoybeans. However, use has been limited recently due toreplacement by glyphosate and genetically modified soybeans.Trifluralin can have an extensive half-life in soil (>300 days)following aging processes.17 Thus, although trifluralin maydissipate from fields relatively rapidly during the first few half-lives, the remaining fractions could remain detectable in soilfor years as the parent compound. Trifluralin is applied atconcentrations greater than 1000 μg/kg in fields and ourquantitation limit was 1 μg/kg. Under potentially similarcircumstances, legacy organochlorine pesticides have also beenreported to occur in playas at trace levels.18

Few direct comparisons exist to other studies as pesticidecontamination in depressional wetlands in the High Plains hasrarely been studied, especially in regard to sediment residues.Pesticide levels were measured in water from playas in a fourcounty area in western Texas that all received direct runofffrom cotton and corn.19 Their findings were similar to ourstudy in that the primary reported pesticides were moderatelypersistent herbicides. Only one insecticide was detected out ofthe 32 playas. Overall, pesticides were found more frequentlyby the previous study19 than in our study, as atrazine was foundin 70% of samples as compared to 16% of our samples collectedfrom southern cropland playas. However, this may be due inpart to analyzing sediment rather than water. Detection limitswere 50× lower than in the previous study as compared to ourstudy (mass/mass basis), as is common for water compared tosediment samples.19 Also, herbicide use patterns have changedsince 1997 when the Thurman et al. study was conducted.19

For example, from 2001 to 2007 the U.S. EPA reports thatglyphosate use in the agricultural market has increased 2-fold,whereas pendimethalin and trifluralin usage has decreased2-fold.15

The design for this study allowed for pesticide measurementin wetlands across a broad geographical region and allowed forcomparisons among land-use categories, but did not target

Table 3. Summary Data for Pesticide in Sediments from the Rainwater Basina

number of samples percentiles for all samples, μg/kg2

total above QL 75 90 95

acetochlor all land uses 42 5 <1 12 46 4.3 <1 1.3 4.2cropland 15 1 <1 6.7 19 19 <1 <1 5.6WRP 15 4 <1 27 46 3.4 <1 3.6 17reference 12 0 <1 0 <1 <1 <1 <1 <1

atrazine all land uses 42 16 <1 38 1200 6.9 3.9 10 26cropland 15 7 <1 47 1210 10.6 9.4 26 380WRP 15 5 <1 33 7.9 3.6 2.7 4.7 6.1reference 12 4 <1 33 40 5.3 3 6.9 22

S-metolachlor all land uses 42 16 <1 38 22 1.7 1.4 2.4 10cropland 15 6 <1 40 22 7.5 3.2 12 16WRP 15 5 <1 33 1.9 1.2 1 1.3 1.5reference 12 5 <1 42 2.5 1.5 1.5 1.5 2

trifluralin all land uses 42 10 <0.5 24 1.1 1 <0.5 1 1cropland 15 3 <0.5 20 1 0.9 <0.5 0.9 0.9WRP 15 2 <0.5 13 1 1 <0.5 0.5 0.9reference 12 5 <0.5 42 1.1 1 0.9 1 1.1

aA few other pesticides occurred infrequently in samples, please see text. 1QL, quantitation limit of the analytical method. 2Percentile values indicatethe pesticide concentration which would be greater than the listed percentage (75, 90, 95) of samples. Thus, only 5% of samples would exceed the95%ile concentration.

Figure 3. Percent of Rainwater Basin sediments samples (central NE)with pesticide concentrations above the quantitation limits (A) and the95%ile concentration for each pesticide (B). Both figures are separatedby land use and a different letter indicates values were not the sameamong land use groups. Comparisons were conducted for eachpesticide independently (p < 0.05).

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temporal trends or focus on runoff events. Thus, there is a highpotential that some pesticides with shorter half-lives werepresent in the wetlands immediately following runoff events,but degraded prior to sample collection and analysis. The con-centrations reported should not be considered “peak” concen-trations. In addition, due to the broad sampling range andnumber of sites, pesticide application records were not avai-lable. Thus, pesticide contamination cannot be tracked back toindividual application rates or agricultural practices. Instead, thedata provides a measure of the degree of contamination thatoccurs in natural settings across a variety of land-uses.Importance of Land-Use. Land-use and conservation

programs had a strong influence on pesticide levels in playas.Cropland wetlands had the highest frequency of detection andhighest concentrations. USDA conservation programs (CRPand WRP) reduced pesticide contamination in sediment tosimilar levels found in grassland or reference playas and thusproved to be effective at mitigating pesticide contamination.This could be active filtering (CRP filters runoff), asdemonstrated by previous studies wherein grassland bufferareas reduced pesticide runoff and increased dissipation rates ofpesticides.20,21 Pesticide levels could also be lower due tosediment removal, a common WRP practice in RWB playas.Additionally, there were potentially differences among playas inregard to duration of wet and dry periods. Cropland playas arelikely to receive more runoff, but also more sediment and maybe shallower. CRP and WRP practices frequently employ non-native grasses that reduce sediment transport, but may alsoallow less water to reach the playa. Native grassland typicallyallow water to flow to the playa while removing sediment.2,22

These difference likely influence both the movement and thefate of pesticides within these systems. Finally, lower pesticidelevels may be a passive response as pesticides are notcommonly applied to grassland and CRP or other restoredland.Land use had the greatest influence on pesticides that

occurred frequently (trifluralin and pendimethalin in thesouthern playas, atrazine and metolachlor in the northernplayas and the Rainwater Basin). In the cases where either thefrequency of detection was relatively low, or the number ofsamples with concentrations above the QL was limited (<7 perland-use; i.e., Rainwater Basin), or all reported concentrationswere near the QL (i.e., trifluralin in the northern playas), land-use did not necessarily have a statistical effect. Statistically, it ismore difficult to evaluate factors in which data are heavilycensored, especially when a high percentage of data arecensored or when reported values are very near the censoredvalue (the QL).10 In addition, the land-use classifications werederived based on the major use within a 500 m radius. Thisdefinition is useful from a conservation viewpoint. However,some playas that were not defined as cropland likely had someamount of row crop agriculture within the greater watershed.Therefore, it is not surprising that more polar pesticides, suchas atrazine, were transported through grassed areas that wereproximal to the playa resulting in some degree of contam-ination.Toxicological Interpretation of Sediment Concentra-

tions. The majority of pesticide studies target water con-tamination. Sediment contamination is usually only evaluatedwhen compounds are highly hydrophobic and persistent suchas legacy organochlorine insecticides, PCBs, and pyrethroidinsecticides. However, depressional wetlands such as playasrepresent a unique situation as they experience unpredictable

inundation events and hydroperiod lengths, cycle through wet/dry phases, and have no outflow. Pesticides are more likely toremain in sediments as water subsides, and dry playas are oftenthe norm. Thus, potential for exposure exists not only duringthe wet phase but also during the dry phase. Exposure duringthe dry phase may be particularly relevant for plants, as atrazine,not commonly measured in sediment or considered a“sediment contaminant” due to low Kd values, along withmetolachlor and acetochlor, were frequently detected insediments and all three are quite toxic to some developingterrestrial plants (SI Table 1). For most herbicides, this ispotentially the most toxicologically sensitive end point asuptake from water to invertebrates and aquatic plants arelimited, resulting in limited toxicity in most cases. Thus, theLOC values for the five pesticides most commonly found(acetochlor, atrazine, metolachlor, pendimethalin, and triflura-lin) were all based on toxicity values for plant growth ofsensitive plants. Toxicity to terrestrial plants is a particularconcern as playa basins are primary site of plant biodiversity inthe region and the main production site for over >450 plantspecies that rely on a wet/dry cycle for germination.23

Toxicity of most pesticides to terrestrial plants that typicallyoccur in wetlands has not been well established. Fieldapplication for target plants typically results in soil concen-trations above 600 μg/kg for all five herbicides (acetochlor,atrazine, metolachlor, pendimethalin, and trifluralin) a levelgreater than all but the maximum concentrations found in thisstudy. However, field application is designed to eliminate manyspecies of target plants, primarily in agricultural settings, andmany herbicides can influence the development of sensitiveplants at very low concentrations (SI Table 1). In the southernplayas, atrazine concentration in 5−10% of playas is in a rangethat successful development of some plants may be impaired. Inthe northern playas and Rainwater Basin, atrazine reachedlevels that would be up to 10% of field application levels andthe top 25% of samples exceeded LOC values in croplandplayas. In addition acetochlor and metolachlor were also aboveLOC values. Although atrazine concentrations were lower innative grassland, CRP, WRP, and reference playas thancropland playas, concentrations were still above LOC valuesfor 5−10% of wetlands. Thus, plants were still at risk frompesticide contamination in these playas.Further work with the toxicity of these herbicides to plants

that would occur in wetlands is needed. The end points that theLOC values were based upon are based on sensitive plant assaywith model plants (e.g., Lactuca sativa, 21d, development,EC25). Toxicity tests are rarely conducted on nontarget plantsthat are not of agronomic concern. However, the limitedstudies that have been done suggest that plant toxicity tests arenot necessarily biased to more or less toxicity.24 Extrapolationof lab experiments, such as those cited, to field experiments canbe problematic.24 Herbicide concentrations found in the studysuggest that further work is needed to address how herbicidesin wetlands, especially near row crop agriculture, can influenceplant diversity and productivity.Concentrations of the five herbicides that were broadly

detected in this study were below that expected to cause acutetoxicity to aquatic animals. Dinitroanaline herbicides likependimethalin and trifluralin have low toxicity toward aquaticinvertebrates with LC50 values above 50,000 μg/kg.25 Sedi-ment toxicity values for atrazine, metolachlor, and acetochlorare not available; however, aqueous acute toxicity to inver-tebrates is 500 ppb or higher (EPA Reregistration decisions,

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http://www.epa.gov/pesticides/reregistration/, accessed 02/30/09). Thus, it is unlikely that acute effects would occur asvalues were generally below 500 μg/kg. However, the potentialfor chronic effects is mostly unknown. For example, exposure ofamphibians to atrazine may occur after playa inundation.Atrazine and metabolites have desorption partitioning coeffi-cients (Kd) less than 4.26 Given atrazine concentrationsexceeding 100 μg/kg in the top 5% of samples in the northernplayas and the Rainwater Basin, it could be estimated that ashallow filling of the wetland with uncontaminated water,providing equal amounts of water to sediment (i.e., 10 cm ofwater interacting with 10 cm of sediment), would result indissolved water concentration exceeding 20 μg/L. These waterconcentrations would exceed values reported to result inintersex frogs (<2 μg/L).27 In the top five percent of sample inthe Rainwater Basin water concentration would exceed the39.5 ppb threshold set by the EPA for surface water.28

A final concern for herbicide contamination of playas is thepotential for groundwater contamination. Depressional wet-lands are a major source of groundwater recharge and thuscontamination of sediment may be indicative of groundwatercontamination. Atrazine, metolachlor, and acetochlor are water-soluble and readily move into groundwater. They are amongthe most frequently detected contaminants in streams andgroundwater in areas where corn is grown.29 Thus, theirpresence in sediment is problematic as they are likely carriedinto groundwater through playas as they have low adsorption tosediment (Kd values less than 20). Atrazine and metolachlorhave been previously reported in groundwater from theRainwater Basin.30

Conservation Benefits. The contamination levels formoderately persistent herbicides reported in this study providea clear perspective on the potential for pesticide contaminationamong the land-uses studied. The trends in differences inconcentration among land-use categories indicate pesticidemovement from agricultural sites to playas. There is potentialfor movement of pesticides into playas and the movement ismuch greater in watersheds dominated by cropland. CRP andWRP appear to be protective and have decreased pesticideconcentrations to that observed for grassland or reference sites.This trend is especially strong for the herbicides that are foundat the highest concentrations, pendimethalin and trifluralin inthe south and atrazine and metolachlor in the north. If moretoxic pesticides are used in the future or specialized eventsresult in greater transport, the protection provided by abuffered area such as CRP or native grassland may be especiallyimportant for wetland protection. These data can be combinedwith other ecosystem service data to simultaneously evaluatethe effects of conservation programs and land-use changes onsustainable provisioning of services to society.31

■ ASSOCIATED CONTENT*S Supporting InformationAdditional figures and tables. This material is available free ofcharge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*Phone: 405-744-1718; fax: 405-744-7828; e-mail: jbelden@okstate.edu.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

Funding for this project was provided by CEAP WetlandsProgram, U.S. Department of AgricultureNatural ResourceConservation Service. We thank April Bagwill, JessicaO’Connell, Benjamin Beas, Amie Hankins, and LacreciaJohnson for assistance in collecting samples.

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