Environ. Sci. Technol. 1998, 32, 1462-1465
Dissipation and Distribution of Herbicides in a Fluventic Hapludoll Soil STEPHEN R. WORKMAN* AND SUE E. NOKES Department of Biosystems and Agricultural Engineering, 105 Agricultural Engineering Building, University of Kentucky, Lexington, Kentucky 40546-0276 LOUIS M. MCDONALD, JR. Division of Plant and Soil Sciences, West Virginia University, P.O. Box 6108, Morgantown, West Virginia 26506-6108
Determination of dissipation rates and/or accumulation of herbicides from long-term field studies can increase our understanding of agricultural chemical distribution within the environment. Unfortunately, the cost of sample collection and analysis has limited the development of significant, long-term data sets describing chemical dissipation under natural climatic conditions. In this study, 15 dissipation curves for atrazine [2-chloro-4-(ethylamino)-6isopropylamino-1,3,5-triazine] and 25 dissipation curves for alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide] were developed by using data collected over a 5-year period in a Fluventic Hapludoll. Three agricultural management systems that included differences in tillage and crop rotations were imposed on the surface. Statistically significant differences in atrazine dissipation curves occurred among treatments in the fall and winter of the first year of the experiment. Only two of the 39 sampling events showed a statistically significant difference in alachlor dissipation over the 5-year period. Overall, there was not a measurable difference in dissipation of either herbicide that could be attributed to management system. Dissipation of atrazine was modeled well with a firstorder exponential decay rate constant of 0.02 d-1. Accumulation of atrazine in the soil profile did not occur. Alachlor was initially modeled well with a rate constant of 0.04 d-1. Alachlor behavior in the soil could be described by firstorder dissipation for the two months following application; zero-order dissipation controlled by desorption for fall, winter, and spring; and the accumulation of 20 µg kg-1 alachlor per year in a desorption resistant soil fraction.
Introduction The Ohio Management Systems Evaluation Area (OMSEA) is a large multi-agency, interdisciplinary research project funded as part of the 1989 Presidential Water Quality Initiative (1). The site, located in Pike County, OH, overlies the Scioto River Alluvial Valley Aquifer that was formed when fluvial and glacial-fluvial materials were deposited to a depth of 25 m in the preglacial valley of the Teays River (2). Fluventic Hapludolls with silt loam surface horizons are the predomi* Corresponding author telephone: 606-257-3000, ext. 105; fax: 606-257-5671; e-mail:
[email protected]. 1462
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nant soils that formed in the valley and are typical of soils formed in agriculturally productive valleys in the Midwestern United States. These soils extend to a depth of 1-3 m and overlie sand and gravel that make up the alluvial valley aquifer. A primary focus of the Water Quality Initiative was the evaluation of agricultural chemical persistence and movement through the soil profile in hydrogeologic settings within the North-Central Region of the United States. The target herbicides for the Initiative included atrazine and alachlor. Sorption (reversible associations of chemicals with the solid phase) affects the chemical’s spatial distribution and availability to microorganisms in the soil and is highly correlated to organic carbon content and clay percentages in the soil profile (3-5). Recent laboratory research on adsorption-desorption of herbicides has highlighted the formation of “slowly reversible or restricted” sites within the soil system (3, 6-8). These slowly reversible or restricted sites limit the availability of the chemical for biological degradation; especially in cases where biodegradation occurs in the solution phase (4). A recent review of pesticide transport studies in field soils found that most studies relate pesticide dissipation to measurements of pesticide concentrations in drainage water, groundwater, or water collected in suction lysimeters (9). Few long-term studies of year to year variations in measured pesticide concentrations in the soil profile have occurred primarily because of the expense of sample collection and analysis. A 3-year study of dissipation computed from measurements on soil cores in Iowa has indicated that atrazine and alachlor remain in the near-surface soil layers with small amounts of leaching (10). Approximately three soil sampling events occurred per year (preplant, during the growing season, and postharvest) in the Iowa study and indicated that no significant differences in dissipation could be attributed to tillage or cropping system. No long-term field studies have been found that demonstrate the effects of slow desorption and the presence of slowly reversible or restricted sites on the accumulation and dissipation of atrazine and alachlor in soil. The purpose of this paper is to present and interpret field data on the persistence and dissipation of atrazine and alachlor in the soils at the OMSEA over the 5-year period 1991-1995.
Experimental Section Site Description. The research site included three large plots (10 ha) used for the evaluation of three agricultural management systems on groundwater quality (11). Adjacent to the large plots (Figure 1) were 18 small plots (0.4 ha) consisting of three replicates of each phase of the three management systems (corn-soybean-wheat (C-S-W, S-W-C, or W-CS), corn-soybean (C-S or S-C), and continuous corn (CC)). The small plots were assigned one of the six phases randomly within each of the three blocks of replicates (1). A complete description of the herbicide, nutrient, and tillage inputs to the three management systems has been compiled by Nokes et al. (12). Most of the soils at the research site can be classified as Fluventic Hapludolls. Since they were formed from alluvial materials deposited during overbank flow of the Scioto River, these soils contain large amounts of organic carbon throughout the soil profile (Table 1). The presence of organic carbon within the profile has been shown to be a primary influence on the sorption and degradation of atrazine and alachlor (13, 14). S0013-936X(97)00926-7 CCC: $15.00
1998 American Chemical Society Published on Web 04/10/1998
FIGURE 1. Schematic used to describe the three large plots (10 ha each) and 18 small plots (0.4 ha each). Drawing is not to scale. Each phase of the three treatments were replicated three times in the small plots. Three locations within each plot were sampled during each sampling event. A total of 39 sampling events occurred over the 5-year period.
TABLE 1. Soil Horizon, Clay, Silt, Sand, and Organic Carbon Data for the Huntington Series (Fluventic Hapludoll) at the OMSEA depth (cm)
horizon
clay (kg/kg)
0-30 30-41 41-53 53-76 76-104 104-122 122-140 140-155 155-203
Ap1 Ap2 Bw1 Bw2 Bw3 Bw4 Bw5 C1 C2
0.24 0.27 0.25 0.25 0.21 0.18 0.13 0.08 0.02
silt (kg/kg)
sand (kg/kg)
org. C (mg/kg)
0.55 0.56 0.56 0.54 0.50 0.40 0.26 0.13 0.08
0.21 0.17 0.19 0.22 0.29 0.42 0.61 0.79 0.90
1580 1370 920 670 390 300 260 250 90
Soil Sampling and Analysis. Soil cores were taken from all plots approximately biweekly during the growing season and monthly during the nongrowing season from under the row for the period from 1991 to 1993. Soil cores were taken approximately monthly during the growing season and bimonthly during the nongrowing season in 1994 and 1995. To facilitate the selection of sampling locations, a grid was superimposed over the plots. A 30-m grid was used in the 10-ha plots, and a 7.6-m grid was used in the 0.4-ha replicated plots. Every grid point was identified with a plot and point designation (approximately 1400 uniquely numbered grid points). Within each plot, grid points were numbered in a serpentine pattern. Three subsamples were removed from each plot during each sampling event. In order for the three sampling locations to be evenly distributed over the field, a cluster sampling algorithm was used. The total number of grid points (55 within each of the replicate plots) was divided by the number of subsamples (three) to determine the number of grid points needed between each sample in a cluster (18 points in the replicate plots). The first sampling point was randomly selected, and then the other two sampling points for a given sampling event were obtained by advancing along the serpentine pattern a predetermined number of points (18 and 36 for the replicate plots). The clusters were sampled without replacement in each year. The large plots were sampled similarly, except different clustering algorithms were required. Over 9000 soil samples were collected and analyzed from 39 sampling events, which represent 15 site-years of
dissipation for atrazine (3 phases × 5 years) and 25 site-years of dissipation for alachlor (5 phases × 5 years) under natural field conditions. The 22 mm diameter soil cores obtained for soil chemical analysis extended to a depth of 0.9 m and were encased in acetate liners. The liners (23 mm diameter) were driven into the soil with a hammering device on a hand probe. The samples were stored in a freezer until processing. The cores were sectioned into intervals of 0-0.15, 0.15-0.30, 0.450.60, and 0.75-0.90 m, frozen, and shipped via overnight mail to the National Soil Tilth Laboratory (NSTL) in Ames, IA. A robotics system was used at the NSTL to perform the extraction procedure (10, 15). The chemical concentrations were reported in micrograms of chemical per kilogram of soil (µg kg-1). The detection limit was 5 µg/kg for atrazine and alachlor. Statistical Analysis. All phases of the crop management systems were present each year of the experiment. This resulted in three treatments with corn, two treatments of soybean, and one treatment of wheat in each year. Atrazine was applied to each treatment that included corn (three dissipation curves per year). Alachlor was applied to treatments that included either corn or soybean (five dissipation curves per year). An analysis of variance (ANOVA) was used to determine if the measured soil atrazine concentrations differed between the treatments at each sampling event (39 sampling events over the 5-year period).
Results and Discussion Distribution of Herbicide in the Soil Profile. Figure 2 shows the distribution of herbicide detections in the surface meter of soil. Atrazine was detected in 85% and alachlor was detected in 83% of the 2348 samples collected from the 0-0.15-m depth in the profile. Approximately 59% of the samples contained atrazine and 23% of the samples contained alachlor in the 0.15-0.3-m depth increment. In addition to the reduced number of detections with depth, the mass of chemical was less with depth. The mean concentration of atrazine was greater than 69 µg/kg in the 0-0.15-m depth and 9 µg/kg in the 0.15-0.3-m depth. Similarly, the mean concentration of alachlor was reduced from 91 µg/kg in the 0-0.15-m depth to 4 µg/kg in the 0.15-0.3-m depth. As an indication of the slow movement of atrazine and alachlor deeper than 0.3 m in the profile, less than 8% of the 4700 samples taken from below 0.3 m contained alachlor and 15% of the samples contained atrazine. Atrazine Dissipation. Consistent dissipation of atrazine has occurred among treatments at the OMSEA over the 5-year study period. Figure 3 shows the atrazine dissipation curves for samples taken from the 0-0.15-m depth increment in the soil profile. As described earlier, statistical tests were performed at each sampling date to determine if the mean atrazine concentration in the 0-0.15-m depth increment differed between treatments. An asterisk (*) highlights events where a statistical difference occurred among at least one of the dissipation curves. Although the intensive sampling at the OMSEA helped to reduce the variability in observed herbicide concentrations, there was a statistical difference at the 5% level (*) in only nine of the 39 sampling events between the concentrations observed under the row in the treatments. One-half of the statistically significant differences occurred during the late summer and fall periods in 1991. A large residual concentration of atrazine (approximately 100 µg/kg) remained in the continuous corn treatment over the winter of 1991-1992 that was not present in the other treatments. Atrazine was not applied to the OMSEA site during the growing season of 1989 and 1990. The labeled rate of atrazine used in the continuous corn plots was twice the rate used in the corn-soybean and corn-soybean-wheat plots in 1991 and 1992. Label restrictions imposed in 1993 VOL. 32, NO. 10, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. Mean atrazine dissipation curve and first-order model of dissipation at the OMSEA.
TABLE 2. Slope and r 2 of the Regression Line between Observed Dissipation and Dissipation Predicted by a First-Order Model atrazine
FIGURE 2. Chart showing the percent detection of atrazine and alachlor with depth in the six agricultural management systems over the 5-year period.
FIGURE 3. Dissipation curves for atrazine applied to the corn phase of the continuous corn (CC), corn-soybean (CS), and cornsoybean-wheat (CSW) treatments. The asterisk (*) represents instances where at least one of the dissipation curves is statistically different from the others at that sampling date. resulted in similar atrazine observations measured between treatments during 1993-1995. Similarities between the dissipation curves from each treatment indicated that atrazine dissipation was not affected by cropping or tillage system. Because of these similarities, a mean atrazine dissipation curve was developed (Figure 4). A rate constant of 0.02 d-1 was found in a laboratory study on atrazine mineralization for surface sediments at the OMSEA (16). A dissipation curve is shown in Figure 4 that is described by first-order kinetics and the rate constant of 0.02 d-1. The best estimate of the initial concentration, Co, used in the calculation was assumed to be the measured concentration in the soil profile at the first sampling event after application. Dissipation of atrazine in the 0-0.15-m depth increment was modeled extremely well (Table 2) with the first-order kinetic equation. A limitation of the model was the inability to predict the slower dissipation that occurred approximately 100 days after application in 1991 and 1992 (Figure 4). In subsequent years 1993-1995, the dissipation was modeled extremely well from the time of chemical application to nearly complete dissipation (Figure 4 and Table 2). The amount of residual atrazine remaining in the system over the winter diminished each year of the experiment. 1464
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alachlor
year
slope
r2
slope
r2
1991 1992 1993 1994 1995
0.93 0.91 1.01 1.10 1.02
0.88 0.96 0.91 0.93 0.91
0.99 0.75 0.73 0.87 0.67
0.98 0.80 0.71 0.89 0.54
A linear regression with a forced zero intercept was fit to the observed dissipation data and the dissipation predicted by the first-order equation (Table 2). A slope of 1.0 and r 2 approaching 1.0 are indications of similarities between observed and predicted values. The lack of fit between the predicted and observed dissipation during the late summer and fall periods in 1991 and 1992 reduced the predicted slope to 0.93 and 0.91 in those years, respectively. The slope of the comparison line in 1993 and 1995 was nearly 1.0, indicating an excellent fit between predicted and observed dissipation. The higher slope of 1.10 in 1994 indicated that observed dissipation occurred faster than predicted dissipation. An inspection of the next soil layer (0.15-0.3-m depth increment, data not shown) indicated that leaching from the surface layer did not occur because there was not an increase in atrazine concentration consistent with a leaching event. The dissipation of alachlor in 1994 was also much quicker than the two previous years (Table 2). Alachlor Dissipation. Only two of the 39 sampling events indicated a difference in alachlor concentration among the treatments within the surface increment (Figure 5). As with atrazine, similarities between dissipation curves indicated that tillage and crop rotation did not affect alachlor dissipation at the OMSEA. A dissipation curve was constructed by computing the mean alachlor concentration measured at the 0-0.15-m depth increment (Figure 6). A 17-day half-life (rate constant of 0.04 d-1) was computed from the dissipation curve in 1991 and used to compute the reference first-order dissipation curve for 1992-1995. As with the atrazine analysis, the initial concentration, Co, was assumed to be the measured concentration during the first sampling event after application. Unlike the atrazine data, measured alachlor concentrations were not modeled well with the first-order equation after 1991. In fact, the fit to the first-order equation became worse each year with the exception of 1994 (Table 2). Records of herbicide applications at the site show that alachlor had not been applied from 1985 to 1991. Records before 1985 were not available. Sediment samples taken from above and below the water table during the installation of the groundwater monitoring
chemical that has moved to a sorption site that is unavailable for microbial activity (3, 6). For illustrative purposes, a dashed line with a slope of 20 µg kg-1 year-1 has been drawn in Figure 6, indicating the accumulation in alachlor measured each year. Although a January sampling event did not occur each year, inspection of the dissipation curves in Figures 5 and 6 indicate a trend of slower dissipation and slight accumulation with time.
Acknowledgments
FIGURE 5. Dissipation curves for alachlor applied to the corn phase of the continuous corn (CC), corn-soybean (CS), and cornsoybean-wheat (CSW) treatments and soybean phase of the cornsoybean (SC) and corn-soybean-wheat (SWC) treatments. The asterisk (*) represents instances where at least one of the dissipation curves is statistically different from the others at that sampling date.
The investigation reported in this paper (No. 97-05-23) is part of a project of the Kentucky Agricultural Experiment Station and is published with approval of the Director. Data were collected at the Ohio Management Systems Evaluation Area (OMSEA) project, which is a cooperative research and educational effort of the USDA-Agricultural Research Service, the USDA-Cooperative State Research Service, the Ohio Agricultural Research and Development Center at The Ohio State University, the U.S. Geological Survey, the USDAExtension Service, the U.S. Environmental Protection Agency, and other state and federal agencies.
Literature Cited
FIGURE 6. Mean alachlor dissipation curve and first-order model of dissipation at the OMSEA. Dashed line represents an accumulation of 20 µg/kg of alachlor in the soil per year. network at the OMSEA lacked both atrazine and alachlor degraders (17). The shallowest sediment samples were collected from a depth of 1.5 m. Biological degradation of atrazine and alachlor was very long (half-life >150 days) in the 11 samples that exhibited degradation. Seventy-two samples did not exhibit any degradation. Subsequent studies have identified atrazine degraders nearer the soil surface, but no studies of alachlor degraders have been completed (16, 18). The degradation of alachlor is primarily a biological process and requires the chemical to be available to the microorganisms (13, 19). Certainly, the dissipation of alachlor fit the first-order model well during 1991 (Table 2) with a slope of 0.99 and r 2 of 0.98 for a regression line between observed and predicted dissipation. After 1991, something occurred to slow the dissipation, as indicated by a reduction in the slope and correlation of the regression line between observed and predicted concentrations. First-order dissipation occurred from chemical application until July (approximately 2 months). After July, zero-order dissipation occurred as a result of slow desorption of alachlor from sorption sites within the soil matrix. Linear kinetics can occur under conditions where biodegradation is limited by diffusion and desorption to locations where it can be degraded (4). A residual concentration of approximately 20 µg/kg alachlor resided in the soil profile through the fall and winter period of 1991. The residual concentration increased each year after 1991 and seems to represent a portion of the
(1) Ward, A. D.; Hatfield, J. L.; Lamb, J. A.; Alberts, E. E.; Logan, T. J.; Anderson, J. L. Soil Tillage Res. 1994, 30, 49-74. (2) Jagucki, M. L.; Finton, C. D.; Springer, A. E.; Bair, E. S. Hydrogeology and water quality at the management systems evaluation area near Piketon, Ohio; U.S. Geological Survey Water-Resources Investigations Report 95-4139; USGS: Denver, 1995; p 117. (3) Locke, M. A. J. Environ. Qual. 1992, 21, 558-566. (4) Skow, K. M. Effect of sorption-desorption and diffusion processes on the kinetics of biodegradation of organic chemicals in soil. In Sorption and Degradation of Pesticides and Organic Chemicals in Soil; Linn, D. M., Carski, T. H., Eds.; Special Publication 32; SSSA: Madison, WI, 1993; pp 73-114. (5) Barriuso, E.; Koskinen, W. C. Soil Sci. Soc. Am. J. 1996, 60, 150157. (6) Clay, S. A.; Koskinen, W. C. Weed Sci. 1990, 38, 74-80. (7) Pignatello, J. J.; Huang, L. Q. J. Environ. Qual. 1991, 20, 222228. (8) Xue, S. K.; Selim, H. M. J. Environ. Qual. 1995, 24, 896-903. (9) Flury, M. J. Environ. Qual. 1996, 25, 25-45. (10) Weed, A. A. J.; Kanwar, R. S.; Stoltenberg, D. E.; Pfeiffer, R. L. J. Environ. Qual. 1995, 24, 68-79. (11) Workman, S. R.; Nokes, S. E.; Ward, A. D.; Fausey, N. R. Overview of the Ohio Management Systems Evaluation Area. Proceedings of ASCE Irrigation and Drainage Conference, July 22-26, Honolulu, HI; Ritter, W. F., Ed.; ASCE: New York, 1991; pp 725731. (12) Nokes, S. E.; Fausey, N. R.; Subler, S.; Blair, J. M. Soil Tillage Res. 1997, 44, 95-108. (13) Clay, S. A.; Moorman, T.B.; Clay, D. E.; Scholes, K. A. J. Environ. Qual. 1997, 26, 1348-1353. (14) Novak, J. M.; Moorman, T. B.; Cambardella, C. A. J. Environ. Qual. 1997, 26, 1271-1277. (15) Koskinen, W. C.; Jarvis, L. J.; Dowdy, R. H.; Wyse, D. L.; Buhler, D. L. Soil Sci. Soc. Am. J. 1991, 55, 561-562. (16) Ostrofsky, E. B.; Traina, S. J.; Tuovinen, O. H. J. Environ. Qual. 1997, 26, 647-657. (17) Radosevich, M.; Crawford, J. J.; Traina, S. J.; Oh, K. H.; Tuovinen O. H. Biodegradation of atrazine and alachlor in subsurface sediments. In Sorption and Degradation of Pesticides and Organic Chemicals in Soil; Linn, D. M., Carski, T. H., Eds.; Special Publication 32; SSSA: Madison, WI, 1993; pp 33-41. (18) Radosevich, M.; Traina, S. J.; Tuovinen, O. H. Biodegradation 1996, 7, 137-149. (19) Beestman, G. B.; Deming, J. M. Agron J. 1974, 66, 308-311.
Received for review October 21, 1997. Revised manuscript received February 25, 1998. Accepted March 2, 1998. ES970926F
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