Envlron. Sci. Technol. 1994, 28, 73-79
Reduction of Nonpoint Source Contamination of Surface Water and Groundwater by Starch Encapsulation of Herbicides Margaret S. Mills' and E. Michael Thurman Water Resources Divlsion, U.S.Geological Survey, 482 1 Quail Crest Place, Lawrence, Kansas 66049
The loss of the preemergent herbicide atrazine in surface runoff from experimental field plots growing corn (Zea mays L.) was significantly reduced using a starchencapsulated formulation versus a conventional powdered formulation. Field edge losses of starch-encapsulated atrazine were described as following a Rayleigh distribution totaling 1.87% of applied herbicide compared to exponential powdered atrazine losses of 2.9 % applied-a40% decrease. This has important implications for the reduction of nonpoint source contamination of surface water by agricultural chemicals. Unsaturated zone release of starchencapsulated atrazine was gradual, but comparable weed control was maintained. Deethylatrazine was a major dealkylated metabolite from each formulation, and deisopropylatrazine was a minor metabolite. The determination of soil partition coefficients for deethylatrazine and deisopropylatrazine (0.4 and 0.3, respectively), aqueous solubilities (3200 and 670 mg/L, respectively), and melting points (133 and 177 "C, respectively) confirmed that the dealkylated metabolites should move more rapidly through the soil profile to groundwater than atrazine.
transport, which is thought to be a source for herbicides in rainfall (29-31). This study is one of the first to investigate the environmental behavior of starch-encapsulatedherbicides in the field. Specific objectives of this study were to compare starch-encapsulated atrazine to conventional wettable-powder atrazine in (1) the edge-of-field losses for the reduction of nonpoint source contamination of surface water, (2) the degradation potential of atrazine to dealkylate to deethylatrazineand deisopropylatrazine, and (3) the unsaturated zone leachingpotential of both atrazine and dealkylated metabolites for the mitigation of nonpoint source contamination of groundwater. Experimental Procedures
Laboratory Procedures. Methanol (Burdick and Jackson, Muskegon, MI), ethyl acetate, and isooctane (Fisher Scientific, Springfield, NJ) were pesticide-grade solvents used during the analyses of both soil and water samples. Deionized water was charcoal-filtered and glassdistilled prior to use. The atrazine standard was obtained from Supelco (Bellefonte, PA), and the triazine metabolites, deethylatrazine and deisopropylatrazine, were from Introduction Ciba Geigy (Greensboro, NC). The (2-18 solid-phase Atrazine (2-chloro-4-ethylamino-6-isopropylamino-s- extraction cartridges (Sep-Pak from Waters, Milford, MA) contained 360 mg of 40-pm C-18 bonded silica. Standard triazine) is a moderately soluble and relatively persistent solutions were prepared in methanol, and phenanthrenepreemergent herbicide. Extensive application of the dlo (New England Nuclear, Boston, MA) was used as an herbicide takes place annually in the midwestern United internal GC/MS quantitation standard. Deuterated atraStates during a l-2-month period of spring planting (1). zine (Cambridge Scientific Isotopes) was used as a Coincident with herbicide application are spring rains that surrogate standard to determine recovery in soil extracflush 2-5 % of applied active ingredient from field surfaces tions. Starch-encapsulated atrazine (1:9 amy1ose:amyto nearby streams and rivers (2-6). Percolation of water lopectin, 10% active ingredient, 20-40 mesh size)was made through the unsaturated zone also can transport dissolved and supplied by the US. Department of Agriculture active chemicals to groundwater (7, 8 ) . Consequently, (Peoria, IL). Commercial wettable-powder atrazine was widespread surface water and groundwater contamination obtained from commercialretailers (Co-op,Lawrence,KS). in the Midwest has been reported, and a significant Soil Extraction. Soils and suspended sediments were nonpoint source of contamination of water is recognized extracted according to the method described by Mills and (5-15). Thurman (32). Briefly, 20 g of soil (or its wet weight The controlled release of pesticides is a concept that equivalent) was spiked with the surrogate standard has developed since the 19709,in an attempt to extend the (deuterated atrazine), equilibrated for 1 h, and then life of herbicides and to reduce the amount required for extracted with a methano1:water mixture (15:5, v:v) at 75 efficacy. Active ingredients are either chemicallyattached "C for 30 min. For suspended sediment samples, only 1-3 or physically entrapped by a slowly degradable matrix g of soil was used and extracted with a 6:2 (v:v) methanol: (16). Over the past decade, starch matrices have proved water mixture. The sample was mixed for 24 h to ensure effective in the laboratory in the slow release of herbicides the release of the active ingredient from any starch granules and insecticides (17-28). The starch matrix used consists present. The sample was then centrifuged, and the clear of amylose (linear) and amylopectin (branched) starches supernatant was poured directly into a 40-mL tube for in a ratio of 1:9, the amylopectin disrupting the tightly evaporation. The extraction procedure was repeated on packed amylose chains and providing space for active the soil sample, and the second supernatant was combined ingredient (20, 21). The starch granules swell in the with the first. The combined extract was evaporated using presence of water, and the solubilized active ingredient a turbovap (Zymark, Palo Alto, CA) until only 10 mL of diffuses out of the matrix (25, 27). In a field situation, water remained. This was transferred to a test tube for encapsulated herbicides should maintain a low mass of automated solid-phase extraction. released active chemical in soil and potentially decrease losses by surface runoff, unsaturated zone leaching (24, Solid-Phase Extraction GC/MS Analysis. Solid261, volatilization (17, 22, 23, 28), and dust particle phase extraction of the analytes was accomplished by the Thle artlcle not subJectto
US. Copyrlght. Published 1993 by the American Chemical Society
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procedure previously described (32,33). Briefly, a Waters Millilab workstation (Milford, MA) was used for solidphase extraction of the analytes using C-18 Sep-Pak cartridges (Waters, Milford, MA). Samples were spiked with a surrogate recovery standard, terbuthylazine (2.4 ng/pL, 100 pL), and pumped through the preconditioned (3-18 cartridge at a rate of 20 mL/min. Analytes were eluted with ethyl acetate and spiked robotically with phenanthrene-dlo (0.2ng/pL, 500 pL). Soil-extract eluates were passed through an anion-exchange cartridge (QMA Accell resin, Millipore water, preconditioned sequentially with 2 mL each of water, methanol, and ethyl acetate) to remove co-eluted humic substances. Finally, the extract was evaporated automatically by a turbovap (Zymark,Palo Alto, CA) at 45 "C under a nitrogen stream to 50 /*La Automated GC/MS analyses with selected ion monitoring (SIM) of the eluates were performed on a Hewlett Packard Model 5890 GC (PaloAlto, CA) and a 5970A mass selective detector (MSD), with operating conditions identical to those described previously (32, 33). Determination of Partition Coefficients and Solubility Measurements. Partition coefficients for each compound were determined at room temperature by standard sorption isotherm batch techniques (34). A known mass of each analyte was spiked into 10 g of dry soil and equilibrated for 24 h with 6 mL of distilled water. The mass of herbicide in the water phase only was quantified by centrifuging the slurry and taking an exact weighed aliquot of supernatant to determine the exact mass present by GC/MS analysis. The mass of analyte adsorbed to the soil was inferred from the difference of that spiked into the soil and that present in the water phase, which should yield accurate results due to the low water:soilratioused (35). The solubility of each compound was determined at 22 "C by dissolving the analyte in an exact, weighed volume of distilled water until a saturated solution was obtained. The solution was centrifuged, an aliquot was removed, and the mass of analyte present in the aliquot was determined by GC/MS (36). Field Dissipation Plots. A field dissipation study was conducted at the Kansas River Valley Experimental Field, near Topeka, KS. Two 65-m2Eudora silt-loam plots (81, each with a slope of less than 1% ,were isolated from one another by perimeter berms and flashing. Encapsulated atrazine [20-40 meshsize granules, (approximately 1mm3), irregularly shaped] was applied at a rate of 2 kg/ha (active ingredient) to one plot with a calibrated fertilizer distributor, and wettable-powder atrazine was applied at the same rate to the adjacent plot by spraying on May 22, 1990. Herbicides were incorporated to approximately 5 cm with a rotor tiller, followed by planting of corn. Bromide was applied at a rate of 80 g/m2usinga calibrated spray cannister over a 50-m2 region of lysimeters and incorporated into the soil to approximately 5 cm using a hand hoe. Natural precipitation maintained crop growth and was supplemented with sprinkler-applied irrigation in May and July. A total of 42 cm of precipitation and irrigation was recorded throughout the season, with precipitation taking place approximately every 14 days. Surface runoff was collected downslope in a bucket level with the field surface and then pumped into a380-L storage tank using a sump pump. The volume of surface runoff collected was recorded, and a 4-L sample of water and suspended sediment was taken for filtration and analysis. Runoff samples were filtered using buchner filtration 74
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apparatus, and suspended sediments were scraped from the filter paper, weighed, and frozen until extraction. The summer plots were replicated in duplicate in the fall of 1990 on four additional 45-m2 experimental plots, on Eudora silt-loam,to verify surface runoff results from the spring planting. Runoff was initiated using sprinkler irrigation which was applied intensely (3.5 cm/h) every 2 weeks for a period of 8 weeks. No corn was grown on the replicate experimental plots due to the onset of fall and colder conditions not suitable for growing corn. Suction cup lysimeters were installed in duplicate on both experimental plots, at nested depths of 30, 60, 90, 120, and 150 cm for unsaturated zone transport investigations. Background atrazine and metabolite concentrations were recorded prior to herbicide application (
