Analysis of Atrazine and Four Degradation Products in the Pore Water

These results can be compared with those of Adams and Thurman (23), who measured ATR, DEA, and DIA in the pore water and soil cores of a Eudora silt l...
0 downloads 0 Views 189KB Size
Download PDF
Environ. Sci. Technol. 2000, 34, 2131-2137

Analysis of Atrazine and Four Degradation Products in the Pore Water of the Vadose Zone, Central Indiana SANDRA Y. PANSHIN, DONNA S. CARTER,† AND E. RANDALL BAYLESS* U.S. Geological Survey, 5957 Lakeside Boulevard, Indianapolis, Indiana 46278

A new method is described for the analysis of atrazine and four of its degradation products (desethylatrazine, deisopropylatrazine, didealkylatrazine, and hydroxyatrazine) in water. This method uses solid-phase extraction on a graphitized carbon black cartridge, derivatization of the eluate with N-methyl-N-(tert-butyldimethylsilyl)trifluoroacetamide (MTBSTFA), and analysis by gas chromatography/ mass spectrometry (GC/MS). This method was used to analyze lysimeter samples collected from a field in central Indiana in 1994 and 1995. Atrazine and its degradation products were transported rapidly through the vadose zone. Maximum values of atrazine ranged from 2.61 to 8.44 µg/L and occurred from 15 to 57 days after application. Maximum concentrations of the degradation products occurred from 11 to 140 days after atrazine application. The degradation products were more persistent than atrazine in pore water. Desethylatrazine was the dominant degradation product detected in the first year, and didealkylatrazine was the dominant degradation product detected in the second year. Concentrations of atrazine and the degradation products sorbed onto soil were estimated; maximum concentrations ranged from 7.3 to 24 µg/kg for atrazine and were less than 5 µg/kg for all degradation products. Degradation of atrazine and transport of all five compounds were simulated by the vadose zone flow model LEACHM. LEACHM was run as a Darcian-flow model and as a non-Darcian-flow model.

Introduction Atrazine (ATR) is the most heavily applied herbicide in the United States. During 1990-1993, an average of 28 million kg of ATR was applied annually to crops in the continental United States (1). In Indiana, 3 million kg of ATR was applied in 1993 (2), virtually all of that amount as a pre-emergent herbicide on corn. Once in the environment, ATR can remain chemically intact, or it can degrade. Four of the major degradation products are shown in Figure 1. These are desethylatrazine (DEA), deisopropylatrazine (DIA), didealkylatrazine (DDA), and hydroxyatrazine (HYA). For a complete understanding of the effect of ATR application on the environment, it is important to have an analytical method * Corresponding author telephone: (317)290-3333; fax: (317)2903313, e-mail: [email protected]. † Present address: Crop Protection Research and Development, Uniroyal Chemical Company, Inc., Middlebury, CT 06749. 10.1021/es990772z Not subject to U.S. Copyright. Publ. 2000 Am. Chem. Soc. Published on Web 05/02/2000

FIGURE 1. Chemical structures of atrazine (ATR) and its major degradation products, desethylatrazine (DEA), deisopropylatrazine (DIA), didealkylatrazine (DDA), and hydroxyatrazine (HYA). capable of quantifying all of these compounds. ATR in the environment is of interest to many people because of its potential to contaminate drinking water sources. The Federal drinking water standard of 3 µg/L (3-5) is for the parent ATR only; the state of Wisconsin, however, has recommended that its water quality standard for ATR be applied to total atrazine residues, including DEA and DIA (6). ATR degradation products may have toxicological effects (7-9). DIA and DEA are not as phytotoxic as the parent ATR, but they do retain some herbicidal activity (10, 11). In addition to degradation, ATR is subject to transport. Once applied to the field, ATR can be carried with runoff to surface water, percolate to groundwater, or be retained in the soil column. Degradation products are subject to these same processes. Crawford (12) found that in the White River Basin, IN, less than 5% of applied ATR is transported to surface water. Analysis of DEA, DIA, DDA, and HYA in surface water found that these compounds also are not efficiently carried by runoff to surface water (13). In addition, ATR and DEA are detected infrequently in groundwater in central Indiana. In the White River Basin, 1 million kg of ATR was applied in 1993 (14), but ATR was detected in only 8.5% of wells (maximum concentration 0.13 µg/L) and DEA was detected in only 15% of wells (maximum concentration 0.09 µg/L) (15). Most groundwater samples (15) were not analyzed for DIA, DDA, or HYA. Behavior of ATR in the vadose zone has been studied by several researchers (16-23). Two factors are important in determining which compounds are likely to contaminate groundwater: (i) which compounds are formed in the soil and (ii) which are capable of being transported through the vadose zone. DEA and HYA were found to be the most prevalent degradation products in bulk soil (16), although this is dependent on the depth of the soil and the incubation period (17). HYA is the least mobile degradation product (18-22); DEA and DIA are expected to be more mobile than the other compounds (18). Many analytical methods have been developed for ATR in water (24-26). Little work has been done, however, on developing a method for the simultaneous determination of ATR and all its major degradation products. Such a method would facilitate studies on the degradation and environVOL. 34, NO. 11, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

9

2131

mental fate of ATR. Graphitized carbon black solid-phase extraction (SPE) cartridges can be used for the determination of polar pesticides in water (27, 28) and can adsorb polar compounds such as DEA and DIA (29). Although not suited to the direct determination of many polar compounds, GC can effectively separate ATR and the degradation products after they have been subjected to chemical derivatization. The thermally stable, nonpolar derivatives produced by this reaction can be analyzed by MS, which allows definitive identification and quantitation of trace constituents in complex environmental matrixes. This paper describes and demonstrates the efficacy of a new method for the analysis of ATR and four degradation compounds in water. The method uses extraction on carbon black SPE cartridges, derivatization with N-methyl-N-(tertbutyldimethylsilyl)trifluoroacetamide (MTBSTFA), and analysis with GC/MS. The described method is used to investigate transport of ATR and its four major degradation products in the pore water of the vadose zone. Measured concentration values are used to calibrate the vadose zone flow model LEACHM to better understand the processes affecting transport of ATR and its degradation products. The rate of transport, persistence, and importance of the different compounds over time are investigated as well as their impact on groundwater.

Chemical Methods Extraction. The described method requires a 100-175-mL sample. Terbuthylazine and deethylterbuthylazine are added to the sample for use as surrogate standards. The SPE cartridge is an ENVI-carb (Supelco, Bellefonte, PA) cartridge with 0.25 g of graphitized carbon black. The cartridge is cleaned and conditioned with 6 mL each of dichloromethane, dichloromethane/methanol (7:3), methanol, and reagent water. These solvents are allowed to pass through the cartridge with gravity drainage. Then, the sample is pumped through the SPE cartridge at a rate of 2-3 mL/min, and interstitial water is removed from the cartridge with a vacuum aspirator. The cartridge immediately is eluted with two different solvents. Fraction 1 is 3 mL of ethyl acetate; fraction 2 is 8 mL of dichloromethane/methanol (7:3). Water is removed from fraction 1 by passing it through a 1-g column of sodium sulfate. Fractions 1 and 2 are combined, and phenanthrene-d10 then is added to the sample as an internal standard. The sample is solvent-exchanged into acetonitrile, and final volume is brought to approximately 100 µL. All traces of methanol must be removed because methanol interferes with the following derivatization. More specific details of the extraction and analysis are given in Carter (30). Derivatization. Sample derivatization is achieved by adding 80 µL of MTBSTFA to the sample extract and heating the mixture in a sealed glass reaction vial at 65 °C for 45 min. MTBSTFA reacts with the hydroxy and primary amine groups of the ATR degradation products according to the following reaction:

where Y ) NH or O. A calibration solution containing 2 mg/L of each of the analytes, surrogates, and internal standards is solventexchanged and derivatized at the same time and in the same manner as each batch of samples. Analysis of this solution 2132

9

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 11, 2000

TABLE 1. Quantitation and Confirmation Ions, Method Detection Limits (MDL), and Koc Valuesa compound atrazine (ATR) desethylatrazine (DEA) deisopropylatrazine (DIA) hydroxyatrazine (HYA) didealkylatrazine (DDA) phenanthrene-d10 (INT) terbuthylazine (SUR1) deethylterbuthylazine (SUR2)

quant ion confirm ion MDL (amu) (amu) (µg/L) 200 244 230 254 316 188 214 258

215 246 232 255 99 184 216 202

0.07 0.04 0.03 0.04 0.03

Koc (L/kg) 140b 80b 128b 609b 1c

a INT, internal standard; SUR, surrogate compound; amu, atomic mass units. b Data from Seybold and Mersie (39). c Data from Thurman and Fallon (18).

allows the determination of the efficiency of the derivatization reaction and the response of the MS to each compound. A response factor is calculated to quantify the analytes and surrogates relative to the internal standard. The response factors calculated from the calibration solution then are used to calculate the analyte concentrations in the samples. All samples are analyzed immediately following derivatization. Analysis. Samples are analyzed on a Hewlett-Packard 5890 Series II Capillary gas chromatograph (GC) coupled to a 5971A mass spectrometer (MS). GC conditions are as follows: splitless injection; injection port temperature, 250 °C; GC/ MS interface temperature, 280 °C; DB-5 column (J & W Scientific, Folsom, CA), 30 m × 0.25 mm i.d. × 0.25 µm film thickness. The temperature of the GC oven is held at 50 °C for 1 min and then increased to 280 °C at a rate of 6 °C/min. The mass spectrometer scans from 40 to 500 amu every 0.625 s. Table 1 shows the quantitation ion and confirmation ion for each analyte, internal standard, and surrogate. Quality Assurance Data. Three types of quality assurance samples were analyzed to test this method: blanks, reagent spikes, and matrix spikes. Blanks consisted of reagent water rinses of cleaned sampling equipment; two 100-mL blanks were analyzed. Blanks are used to assess contamination that may occur during sample processing. ATR and the four degradation products were not detected in the blanks. Reagent spikes consisted of reagent water spiked with ATR and the four degradation compounds at two different concentration levels. Low-level spikes had concentrations between 0.74 and 0.82 µg/L for each of the five analytes. High-level spikes had concentrations between 7.4 and 8.2 µg/L for each analyte. Two 100-mL replicates at each concentration level were analyzed. The mean recoveries of all analytes and surrogates at the low and high concentration levels were 94% and 96%, respectively. Complete recovery results are listed in Table 2. Matrix spikes were created by adding known concentrations of analytes to water collected from lysimeters installed in a field near New Palestine, IN. Samples of this soil pore water were spiked at two different concentration levels, as described for the reagent spikes. Two 100-mL replicates at each concentration level were analyzed, and one 100-mL sample was left unspiked and analyzed to allow for correction for analytes present in the matrix before fortification. The mean recoveries of all analytes and surrogates at the low and high concentration levels were 98% and 97%, respectively. The log percent difference of the spiked samples is a measure of relative change (precision) between two replicate analyses, x and y, and is defined as 100 ln(y/x) (31). The mean log percent difference of all analytes and surrogates determined in reagent water were 10 for the low concentration level and 6.2 for the high concentration level. For soil pore water, these values were 12 (low concentration) and 4.4

TABLE 2. Results of Matrix Spike Experimentsa compd

matrix

concn spiked (µg/L)

ATR ATR ATR ATR DEA DEA DEA DEA DIA DIA DIA DIA HYA HYA HYA HYA DDA DDA DDA DDA SUR1 SUR1 SUR1 SUR1 SUR2 SUR2 SUR2 SUR2

reagent water reagent water soil pore water soil pore water reagent water reagent water soil pore water soil pore water reagent water reagent water soil pore water soil pore water reagent water reagent water soil pore water soil pore water reagent water reagent water soil pore water soil pore water reagent water reagent water soil pore water soil pore water reagent water reagent water soil pore water soil pore water

0.80 8.0 0.80 8.0 0.82 8.2 0.82 8.2 0.80 8.0 0.80 8.0 0.80 8.0 0.80 8.0 0.80 8.0 0.80 8.0 0.80 8.0 0.80 8.0 0.74 7.4 0.74 7.4

a

concn measured (µg/L) replicate 1 replicate 2 0.68 7.12 0.74 8.08 0.74 8.36 0.94 7.95 0.79 7.52 0.78 7.92 0.66 7.68 0.62 7.52 0.84 7.52 0.93 7.92 0.74 6.96 0.77 8.40 0.75 7.25 0.63 7.40

0.89 7.76 0.84 7.52 0.74 7.87 0.81 7.71 0.72 7.84 0.86 7.84 0.58 7.20 0.61 7.44 0.79 7.60 0.78 7.76 0.85 8.16 0.81 7.60 0.73 7.18 0.78 6.96

log % difference between replicates

mean recovery (%)

27 8.6 13 7.2 0 6.0 15 3.1 9.3 4.2 9.8 1.0 13 6.5 1.6 1.1 6.1 1.1 18 2.0 14 16 5.1 10 2.7 1.0 21 6.1

98 93 99 98 90 99 107 95 94 96 103 99 78 93 77 94 102 95 107 98 99 95 99 100 100 98 95 97

SUR1, terbuthylazine; SUR2, deethylterbuthylazine.

(high concentration). The method precision is concentration dependent; in general, method precision decreases with analyte concentration. Estimated method detection limits (MDLs) are shown in Table 1. MDLs were set at concentrations where the analyte signal was three times higher than background noise in soil pore water matrix spikes. MDLs vary according to the analyte, sample volume and matrix, and instrument conditions. Site Description and Sampling Methods. The analytical method described above was used to study ATR degradation and transport in a field near New Palestine, IN, 25 km east of Indianapolis (Figure 2). The soils in this field are somewhat poorly drained to poorly drained, nearly level with slow permeability (32), and have an organic carbon content of 1-3%. Soil pH is in the range of 5.1-7.3. The field has tile drains installed at a depth of approximately 1.2-2.7 m to improve drainage. This field is part of a working farm and was cropped in corn in 1992 and 1994; in 1993 and 1995, it was cropped in soybeans. Surrounding fields were planted in soybeans in 1992 and 1994 and in corn in 1993 and 1995. ATR is applied to corn but not to soybeans. On April 28, 1994, ATR was applied at a rate of 1.7 kg/ha (active ingredient) to the study field. It was sprayed onto the surface of the field along with fertilizer and immediately cultivated into the top 5-8 cm of soil. Simazine, which also can degrade to DIA and DDA, has not been applied to this field. Lysimeters were installed at depths of 0.9 and 1.5 m at each of three sites (A, B, and C) within the field. Water levels were measured in a well screened 8.5-10.0 m below land surface at site C. These measurements indicated that the water levels fluctuated between 0.8 and 2.7 m below ground level. Because the aquifer tapped by this well is potentially confined, the depth to water may be greater than indicated by this well. The water level was almost always below the level of the lysimeters. Site A has Crosby silt loam soil with 32% clay, 51% silt, and 17% sand in the top 1.4 m of the soil horizon. Site B has Brookston silty clay loam soil with 23%

FIGURE 2. Map of study area showing White River Basin and location of lysimeters. clay, 46% silt, and 31% sand in the top 1.3 m of soil. Site C has Crosby silt loam soil with 26% clay, 46% silt, and 28% VOL. 34, NO. 11, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

9

2133

sand in the top 1.6 m of the soil horizon. Sites A and B are situated on nearly flat ground, and site C is on a 12% northsouth slope. Lysimeter samples were collected by expelling all water that had seeped into the lysimeter between sampling periods, applying a 80-cbar vacuum to the lysimeter for 24 h, and expelling this sample water into a precleaned amber glass bottle. Samples were collected periodically from May 1994 to December 1995. A total of 72 samples were analyzed. On several occasions, sample collection was attempted, but soil moisture was insufficient to generate an adequate lysimeter sample. Samples were collected in 1994 to examine the behavior of ATR immediately after it was applied. Collection continued in 1995 to investigate the transport and persistence of ATR and its degradation products during a year when no new ATR was applied. Solute Transport Models. Numerical models were constructed to examine the processes affecting distribution of ATR and its degradation compounds at the field site. The computer program LEACHM (33) was used because it emphasizes evaluation of physiochemical processes rather than crop-management systems. In addition, LEACHM includes modules to simulate pesticide transformation to multiple degradation compounds. Two physical representations of the flow system are built into the LEACHM code, and both were examined during the simulations. One representation is a Darcian-flow based model. A second representation is based on work by Addiscott (34), Addiscott et al. (35), and Nicholls et al. (36). The Addiscott representation is a non-Darcian-flow model that was explored because of obvious semblance to a macropore-dominated flow system. In this study, the period April 1, 1994-December 30, 1995, was simulated because it bracketed the dates of ATR application and of data collection and included a wide variety of soil moisture and chemical conditions. Data collected at the field site were used to calibrate the numerical models. Model accuracy was judged on the basis of minimized bias between computed and measured soil moisture, concentrations of ATR, and concentrations of degradation compounds. Bayless (37) provides a more thorough description of this model. A table listing several parameters used in this model is given in the Supporting Information.

Results and Discussion Concentrations of ATR, DEA, DIA, DDA, and HYA versus time in the 1.5-m-depth lysimeters at sites B and C are shown in Figure 3. This figure shows that ATR and its degradation products move rapidly through the soil. ATR is detected at most of the lysimeters within 5 days of application. This ATR is unlikely to be residue from 1992 because the data show that ATR is not detected at most of these lysimeters past the end of the growing season during which it is applied. The rapid transport of ATR in this system is quicker than that reported by Kalkhoff et al. (38), who determined that it took 1 month for ATR to reach a 0.9-m-deep lysimeter and almost 5 months to reach a 1.5-m-deep lysimeter installed in Iowa River Alluvium. Maximum ATR concentrations occurred at all of the sites between 15 and 57 days after application and ranged from 2.61 to 8.44 µg/L. Maximum concentrations of the degradation products were measured 11-140 days after ATR application. Ranges of maximum concentrations for the degradation products are as follows: 0.76-1.48 µg/L for DEA, 0.11-0.78 µg/L for DIA, 0.20-1.25 µg/L for DDA, and 0.080.37 µg/L for HYA. These results can be compared with those of Adams and Thurman (23), who measured ATR, DEA, and DIA in the pore water and soil cores of a Eudora silt loam and a Kimo silty clay loam. At depths of 0.9 and 1.2 m in the clay loam plot, maximum ATR concentrations in pore water 2134

9

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 11, 2000

FIGURE 3. Concentrations of ATR, DEA, DIA, DDA, and HYA vs time for pore water from lysimeter at site B (1.5-m-depth) and site C (1.5-m-depth). were approximately 4 and 5 µg/L, respectively. Maximum DEA concentrations at these depths in the same plot were approximately 2 and 3 µg/L, respectively. In the silt loam plot, ATR concentrations at 0.9- and 1.2-m-depths were low ( DIA > HYA. During the second growing season, however, the order of concentration changes to DDA > DEA > DIA > HYA. This prominence of DDA during the second growing season is logical. As DEA and DIA degrade during the winter and the second growing season, more DDA is generated and it becomes the dominant degradation product in pore water. In addition, DDA is the degradation product with the lowest adsorption coefficient (Table 1); the greater mobility of DDA could contribute to its increased pore water concentrations in the second year, as the degradation products reach equilibrium and most of them are sorbed. Adams and Thurman (23) found that the concentrations of DEA were always at least five times higher than the concentrations of DIA in pore water samples from clay-loam and silt-loam plots; they did not measure DDA. Finally, these data show that, in this environment, the degradation products of ATR persist longer in pore water than does the ATR. By the start of the second growing season, ATR is detected in only one lysimeter; DEA, DIA, and DDA are detected in most of the lysimeters during the second year. Also, the concentration of ATR rises and falls relatively rapidly during the first year, but the degradation products are detected at similar levels during the first and second years. Some of the lysimeters show a secondary maximum in the concentrations of some degradation products during the second year. The continued presence of the degradation products likely indicates a source of DEA, DIA, and DDA that was established in the past and can continue to supply these compounds for an extended period of time. This source could be a reservoir of ATR sequestered in the top several centimeters of the soil column that is slowly degrading and 2136

9

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 11, 2000

This project was supported by the National Water-Quality Assessment Program of the U.S. Geological Survey. We thank Ciba-Geigy for providing standard reference material. We also acknowledge C. G. Laird for extracting and analyzing the samples.The use of trade, product, or firm names in this paper is for descriptive purposes only and does not constitute endorsement by the U.S. Government.

Supporting Information Available One table showing the parameter values used in the numerical models (2 pages). This material is available free of charge via the Internet at http://pubs.acs.org.

Literature Cited (1) Pesticide Use in U.S. Crop Production, National Data Report; National Center for Food and Agricultural Policy: Washington, DC, 1995. (2) Agricultural Chemical Usage, 1993 Field Crops Summary; U.S. Department of Agriculture-National Agricultural Statistics Service: Washington, DC, 1994. (3) Drinking water standards. Fed. Regist. 1991, 56 (20), 3526-3597. (4) U.S. EPA. Integrated Risk Information System. http://www. epa.gov/ngispgm3/iris. (5) Drinking water regulations and health advisories, May 1993; U.S. Environmental Protection Agency, Office of Water, U.S. Government Printing Office: Washington, DC, 1993. (6) Belluck, D. A.; Benjamin, S. L.; Dawson, T. In Pesticide transformation products: Fate and significance in the environment; Somasundaram, L., Coats, J. R., Eds.; ACS Symposium Series 459; American Chemical Society: Washington, DC, 1991; pp 254-273. (7) Day, K. E. In Pesticide transformation products: Fate and significance in the environment; Somasundaram, L., Coats, J. R., Eds.; ACS Symposium Series 459; American Chemical Society: Washington, DC, 1991; pp 217-241. (8) Stratton, G. W. Arch. Environ. Contam. Toxicol. 1984, 13, 3542. (9) Arru, G.; Congiu, A. M.; Burdino, E.; Ugazio, G. G. Ital Med. Lav. Ergon. 1997, 19, 17-19. (10) Ebert, E.; Dumford, S. W. Residue Rev. 1976, 65, 1-103. (11) Papilloud, S.; Haerdi, W.; Chiron, S.; Barcelo, D. Environ. Sci. Technol. 1996, 30, 1822-1826.

(12) Crawford, C. G. Occurrence of Pesticides in the White River, Indiana, 1991-95; U.S. Geological Survey Fact Sheet 233-95; U.S. Geological Survey: Indianapolis, IN, 1995. (13) Crawford, C. G. Written communication. (14) Anderson, J. E.; Gianessi, L. P. Pesticide use in the White River Basin; National Center for Food and Agricultural Policy: Washington, DC., 1995. (15) Fenelon, J. M.; Moore, R. C. Occurrence of pesticides in ground water in the White River Basin, Indiana, 1994-95; U.S. Geological Survey Fact Sheet 084-96; U.S. Geological Survey: Indianapolis, IN, 1996. (16) Koskinen, W. C.; Conn, J. S.; Sorenson, B. A. In Herbicide Metabolites in Surface Water and Groundwater; Meyer, M. T., Thurman, E. M., Eds.; ACS Symposium Series 630; American Chemical Society: Washington, DC, 1996; pp 125-139. (17) Kruger, E. L.; Rice, P. J.; Anhalt, J. C.; Anderson, T. A.; Coats, J. R. J. Environ. Qual. 1997, 26, 95-101. (18) Thurman, E. M.; Fallon, J. D. Int. J. Environ. Anal. Chem. 1996, 65, 203-214. (19) Schiavon, M. Ecotoxicol. Environ. Saf. 1988, 15, 55-61. (20) Kruger, E. L.; Somasundaram, L.; Kanwar, R. S.; Coats, J. R. Environ. Toxicol. Chem. 1993, 12, 1959-1967. (21) Kruger, E. L.; Zhu, B.; Coats, J. R. Environ. Toxicol. Chem. 1996, 15, 691-695. (22) Lerch, R. N.; Blanchard, P. E.; Thurman, E. M. Environ. Sci. Technol. 1998, 32, 40-48. (23) Adams, C. D.; Thurman, E. M. J. Environ. Qual. 1991, 20, 540547. (24) Vermeulen, N. M. J.; Apostolides, Z.; Potgieter, D. J. J. J. Chromatogr. 1982 , 240, 247-253. (25) Thurman, E. M.; Meyer, M.; Pomes, M.; Perry, C. A.; Schwab, P. A. Anal. Chem. 1990, 62, 2043-2048. (26) Rubio, F. M.; Itak, J. A.; Scutellaro, A. M.; Selisker, M. Y.; Herzog, D. P. Food Agric. Immunol. 1991, 3, 113-125. (27) Di Corcia, A.; Marchetti, M. Anal. Chem. 1991, 63, 580-585. (28) Di Corcia, A.; Marchetti, M. Environ. Sci. Technol. 1992, 26, 66-74. (29) Di Corcia, A.; Samperi, R.; Marcomini, A.; Stelluto, S. Anal. Chem. 1993, 65, 907-912. (30) Carter, D. S. Determination of atrazine and its major degradation products in soil pore water by solid-phase extraction, chemical derivatization, and gas chromatography/mass spectrometry; Open-File Report 96-459; U.S. Geological Survey: Indianapolis, IN, 1996; 12 pp. (31) Tornqvist, L.; Vartia, P.; Vartia, Y. O. Am. Stat. 1985, 39, 43-46. (32) Ruesch, D. R. Soil Survey of Hancock County, Indiana; U.S. Department of Agriculture, Soil Conservation Service: 1978; 60 pp. (33) Hutson, J. L.; Wagenet, R. J. LEACHM Leaching Estimation and Chemistry Model-A process-based model of water and solute movement, transformations, plant uptake and chemical reactions in the unsaturated zone, Version 3; New York State College of

(34) (35) (36) (37) (38) (39) (40) (41) (42)

(43) (44) (45) (46) (47)

(48) (49) (50)

(51)

Agriculture and Life Sciences, Cornell University: Ithaca, NY, 1992. Addiscott, T. M. J. Soil Sci. 1977, 28, 554-563. Addiscott, T. M.; Rose, D. A.; Bolton, J. J. Soil Sci. 1978, 29, 305-314. Nicholls, P. H.; Walker, A.; Baker, R. J. Pestic. Sci. 1982, 13, 484494. Bayless, E. R. Written communication. Kalkhoff, S. J.; Detroy, M. G.; Cherryholmes, K. L.; Kuzniar, R. L. Water Res. Bull. 1992, 28, 1001-1011. Seybold, C.A.; Mersie, W. J. Environ. Qual. 1996, 25, 11791185. Mills, M. S.; Thurman, E. M. Environ. Sci. Technol. 1994, 28, 600-605. Goolsby, D. A.; Thurman, E. M.; Pomes, M. L.; Meyer, M. T.; Battaglin, W. A. Environ. Sci. Technol. 1997, 31, 1325-1333. Zaugg, S. D.; Sandstrom, M. W.; Smith, S. G.; Fehlberg, K. M. Methods of analysis by the U.S. Geological Survey National Water Quality LaboratorysDetermination of pesticides in water by C-18 solid-phase extraction and capillary-column gas chromatography/mass spectrometry with selected-ion monitoring; Open-File Report 95-181; U.S. Geological Survey: Denver, CO, 1995; 49 pp. Sichani, S. A.; Engel, G. A.; Monke, E. J.; Eigel, J. D.; Kladivko, E. J. Trans. Am. Soc. Agric. Eng. 1991, 34, 1732-1737. Ciba-Geigy. Written communication Xing, B.; Pignatello, J. J.; Gigliotti, B. Environ. Sci. Technol. 1996, 30, 2432-2440. Cohen, S. Ground Water Monitor. Rev. 1992, Summer, 87-88. Goolsby, D. A.; Boyer, L. L.; Mallard, G. E. Selected papers on agricultural chemicals in water resources of the midcontinental United States; Open-File Report 93-418; U.S. Geological Survey: Denver, CO 1993, 89 pp. Hurl, K.; Kibler, E. Weeds 1976, 2, 627-633. Pereira, W. E.; Rostad, C. E. Environ. Sci. Technol. 1990, 24, 1400-1406. Perry, C. A. Observed and simulated distribution of selected herbicides in silty loam, sandy loam, and clay soil profiles near Topeka, Kansas, 1986-1988; Water-Resources Investigative Report 91-4017; U.S. Geological Survey: Lawrence, KS, 1991; 61 pp. Rutledge, A. T.; Helgesen, J. O. Characterization of ground water flow and chemical transport beneath two irrigated fields in southcentral Kansas, 1988; Water-Resources Investigative Report 904065; U.S. Geological Survey: Lawrence, KS, 1990; 37 pp.

Received for review July 9, 1999. Revised manuscript received March 10, 2000. Accepted March 14, 2000. ES990772Z

VOL. 34, NO. 11, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

9

2137