Journal of Environmental Quality 30:71-77 (2001)
© 2001 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
TECHNICAL REPORT
GROUND WATER QUALITY
Fate of Atrazine in Sandy Soil Cropped with Sorghum
O.S. Mbuyaa,
P. Nkedi-Kizzab and
K.J. Bootec
a Center for Water Quality, Florida A&M Univ., Tallahassee, FL 32307-4100
b Dep. of Soil and Water Science, Univ. of Florida, Gainesville, FL 32611-0290
c Dep. of Agronomy, Univ. of Florida, Gainesville, FL 32611-0500
Corresponding author (Odemari.Mbuya{at}famu.edu)
Received for publication October 6, 1998.
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ABSTRACT
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A field study was conducted to determine the fate of atrazine (6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine) within the root zone (0 to 90 cm) of a sandy soil cropped with sorghum [Sorghum bicolor (L.) Moench] in Gainesville, Florida. Atrazine was uniformly applied at a rate of 1.12 kg a.i. ha-1 to a sorghum crop under moderate irrigation, optimum irrigation, and no irrigation (rainfed), 2 d after crop emergence. Bromide as a tracer for water movement was applied to the soil as NaBr at a rate of 45 kg Br- ha-1, 3 d before atrazine application. Soil water content, atrazine, and Br- concentrations were determined as a function of time using soil samples taken from the root zone. Atrazine sorption coefficients and degradation rates were determined by depth for the entire root zone in the laboratory. Atrazine was strongly adsorbed within the upper 30 cm of soil and most of the atrazine recovered from the soil during the growing season was in that depth. The estimated half-life for atrazine was 32 d in topsoil to 83 d in subsoil. Atrazine concentration within the root zone decreased from 0.44 kg a.i. ha-1 2 days after application (DAA) to 0.1 kg a.i. ha-1 26 DAA. Negligible amounts of atrazine (
5 µg kg-1) were detected below the 60-cm soil depth by 64 DAA. Most of the decrease in atrazine concentration in the root zone over time was attributed to degradation. In contrast, all applied bromide had leached past the 60-cm soil depth during the same time interval.
Abbreviations: DAA, days after application • ET, evapotranspiration • OC, organic carbon
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INTRODUCTION
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LARGE amounts of pesticides are used in modern production agriculture. A major challenge today is developing satisfactory techniques that combine optimum production with environmental protection. Contamination of ground water by agrichemicals is a serious environmental issue in the United States (USDA, 1991; USEPA, 1990a, b; Weil et al., 1990). It could be caused through leaching and/or surface runoff under normal agricultural use, excessive use, accidental spills, or storage tank leakage. It is therefore prudent to determine the fate of pesticides applied to the soil surface.
Atrazine is one of the most extensively used herbicides for weed control during maize (Zea mays L.) and sorghum production in the United States (Wollenhaupt and Springman, 1990). It is also widely used in many developed and developing countries where tropical cereal production is predominant. With increasing awareness and concern for environmental quality, it is important that we consider not only the effectiveness of a pesticide, but also its persistence and mobility in soil (Bergstrom et al., 1991). A number of processes that affect pesticide fate mainly occur within the root zone. They include water flow and subsequent pesticide displacement, plant extraction of water and pesticide, adsorption to soils, and degradation. The fate of a particular pesticide under a unique combination of environmental circumstances (e.g., soil type, rainfall, irrigation, and application rate) can best be estimated by simultaneously considering all important processes and integrating them through a modeling approach (Wagenet and Hutson, 1986; Wagenet and Rao, 1985; Gaber et al., 1995).
Reports of increases in levels of pesticides and other toxic organic pollutants in ground water have prompted a number of laboratory and field experiments to understand the processes and environmental factors that influence pesticide behavior in soils (Cohen et al., 1984; Pye et al., 1983; Rao et al., 1983). In a National Pesticide Survey (NPS), atrazine was one of the most commonly detected pesticides in ground water throughout the United States (USEPA, 1990a; Wollenhaupt and Springman, 1990). The federal allowable maximum contamination level (MCL) for atrazine in U.S. drinking water is 3 µg L-1 (USEPA, 1976).
Sorption and transformation are major processes affecting the mass of solute available in solution for advective–dispersive transport through a soil profile. Many pesticides are nonpolar, and their adsorption on soils is predominantly due to the organic carbon (OC) content (Nkedi-Kizza et al., 1983), whereas pesticide degradation in the vadose zone involves both abiotic and biotic processes (Rao and Alley, 1993). Understanding sorption, degradation, and transport of pesticides and other organic contaminants in soils and aquifers is essential in predicting their fate and transport in the environment. Compiled degradation and sorption data for atrazine in soils can be found in Rao and Davidson, 1980; Rao et al., 1983; Nash, 1988; Scow, 1990; and Wauchope et al., 1991. The objectives of this study were to determine (i) adsorption, (ii) degradation, and (iii) leaching of atrazine within the root zone (0–90 cm) in a sandy soil cropped with sorghum.
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MATERIALS AND METHODS
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Field Experiment
A field trial was conducted in Gainesville at the University of Florida Irrigation Research and Education Park (29°38' N, 82°22' W) during the April through July 1991 growing season. Soil type at the experimental site was classified by Calhoun et al. (1974) as Millhopper fine sand (loamy, siliceous, hyperthermic Grossarenic Paleudult). Important soil properties are presented in Table 1. The experiment was developed as a two by three factorial, with two levels of N and three water management regimes, replicated four times on plots 14 by 14 m in size. The two levels of N were 100 and 300 kg ha-1, applied as KNO3. The water management regimes were optimum irrigation, moderate irrigation, and no irrigation. Optimum and moderate irrigations were based on visual signs of crop wilting in the afternoon (1200–1400 h). Dates and amount of irrigation water applied to experimental plots are presented in Table 2.
Sorghum (cv. Savanna 5) was sown on 22 April at an inter- and intrarow spacing of 90 and 5.6 cm, respectively (20 plants m-2). Sodium bromide (NaBr) was applied at a rate of 45 kg Br- ha-1 on 26 April, the date of crop emergence, as a tracer for water movement. Atrazine was applied at a rate of 1.12 kg a.i. ha-1 as a postemergence herbicide on 29 April, 3 d after crop emergence.
We collected 480 soil samples at random from all treatment plots at the 0- to 15-, 15- to 30-, 30- to 45-, 45- to 60-, and 60- to 90-cm depth increments over time using an auger 2.8 cm in diameter. Soil samples were taken on 1 May, 24 May, 6 June, and 1 July (i.e., 2, 23, 39, and 69 d after atrazine application, respectively). These samples were immediately placed in Ziploc plastic bags and frozen until analyses for atrazine, bromide, and water content were performed. Weather parameters (daily maximum and minimum air temperature, rainfall, total solar radiation, photosynthetic photon flux density, and mean soil temperature at the 60-cm depth) at the experimental site were electronically recorded throughout the experimental period (April–July) using a LI-COR (Lincoln, NE) LI-1200 weather station.
Extraction of Soil Atrazine and Bromide and Analytical Methods
Atrazine and Br- were extracted from the soil samples using 20 mL of a 1:1 solution of methanol and distilled water and 10 g of soil in 100 mL glass bottles. The mixture was shaken for 2 h using a reciprocating shaker and then allowed to settle overnight in a refrigerator. After settling, 0.5 mL of clear supernatant solution was taken from each sample and further diluted to 50:1 using deionized distilled water for atrazine analysis. Atrazine concentrations in solution were quantified using an immunoassay method developed by Ohmicron Corporation (1993). Bromide in the supernatant solution was analyzed using a Dionex (Sunnyvale, CA) ion chromatograph. The eluent used was a mixture of 1.8 mM Na2CO3 and 1.7 mM NaHCO3, with 25 mM H2SO4 as a regenerant (Nkedi-Kizza and Owusu-Yaw, 1989).
Atrazine Adsorption Isotherms
Air-dry soil samples collected from the 0- to 15-, 15- to 30-, 30- to 45-, 45- to 60-, and 60- to 90-cm depths were used to measure atrazine adsorption isotherms. Solutions containing 1, 5, 10, and 15 mg L-1 of analytical grade (98.7%) 12C atrazine (Ciba-Geigy Co., Greensboro, NC) were prepared in a background solution of 0.005 M CaBr2 using deionized distilled water. The 12C atrazine solutions were then spiked with 0.05 mL of radioactive ring-labeled 14C atrazine solution, to give 1.66 x 105 Bq L-1. Duplicates of 10 mL of the different concentrations of spiked atrazine solutions were added to 10 g of soil in centrifuge tubes, including blanks containing no soil or containing soil without 14C atrazine. The centrifuge tubes were shaken for 24 h using a reciprocating shaker and then centrifuged at 10000 rpm for 15 min. A 0.5-mL aliquot of the supernatant solution from each centrifuge tube was added to 10 mL of Scintiverse II scintillation cocktail and analyzed for 14C radioactivity using a liquid scintillation counter (Beckman [Fullerton, CA] LS-100c liquid scintillation system). The difference between atrazine solution concentration before adding soil samples and of supernatant solution concentration after 24 h was attributed to adsorption. Atrazine adsorption was described by the Freundlich linear equation,
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where S is the adsorbed concentration of atrazine at equilibrium (mg kg-1 of soil), C is the concentration of atrazine in solution at equilibrium (mg L-1), Kd is the adsorption coefficient (L kg-1) and n is an exponent (assumed to be equal to 1).
Degradation of Atrazine in Soil
A degradation study was conducted in duplicate to determine the rate of disappearance of atrazine from the soil under controlled laboratory conditions. Soil samples taken from the experimental site were divided into two categories, topsoil (0–30 cm) and subsoil (30–60 cm). Atrazine (equivalent of 10 mg kg-1 of soil at field capacity) was dissolved in methanol and plated on the bottom of 250-mL Erlenmeyer flasks. The Erlenmeyer flasks were then placed under a hood to allow the methanol to evaporate. Soil samples containing an equivalent of 150 g oven-dry soil were added to the flasks and water was added to bring the soil to 10% moisture content by weight. To ensure homogeneous distribution of the pesticide in the soil, the samples in the flasks were thoroughly stirred with a stainless steel spatula for 5 min. After stirring, 10 g of soil were immediately sampled and analyzed for initial atrazine content (M0). Subsequent subsamples were taken at 1, 2, 4, and 7 d, and weekly thereafter, for 60 d. Extraction of atrazine from soil samples was done using an 80:20 methanol to 1 M monochloroacetic acid buffer (USEPA, 1989). Concentrations of atrazine in the extracts were determined by high pressure liquid chromatography (HPLC). The flow rate was 1 mL min-1 using a mobile phase of a mixture of methanol, acetonitrile, and water at 60:10:30, with the detector set at a wavelength of 252 nm.
The degradation of atrazine was assumed to follow the first-order rate equation:
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where M = mass of pesticide per unit weight of soil (mg kg-1) at a given time (t); M0 = initial mass of pesticide in the soil (mg kg-1) at time zero (t0), k = degradation rate coefficient (d-1), and t = time (d).
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RESULTS AND DISCUSSION
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Weather Conditions
The experimental site received a cumulative rainfall of 629 mm between 1 April and 30 July, with monthly values of 153, 138, 135, and 203 mm for April, May, June, and July, respectively (Fig. 1)
. Cumulative irrigation amounts of 73.3 and 35.3 mm were supplemented to the treatments under optimum irrigation and moderate irrigation, respectively. The season was wetter than normal and the precipitation was fairly evenly distributed. In general, both rainfall and temperature were favorable for crop growth and for atrazine degradation and leaching.
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Fig. 1. Daily rainfall and application and sampling dates of atrazine and bromide at the Irrigation Research and Education Park during the 1991 growing season
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Atrazine Adsorption Isotherms
Atrazine adsorption isotherms were described by the Freundlich linear equation (Fig. 2)
. Data from blank centrifuge tubes indicated no atrazine sorption to container walls. The value of the adsorption coefficient (Kd) decreased with soil depth from 0.65 L kg-1 for the top 30 cm of the soil profile to 0.16 L kg-1 for the 60- to 90-cm soil depth (Fig. 2). The decrease in adsorption paralleled the decrease in OC content with depth (Table 1). The average Koc was 101 g kg-1, equal to that reported in the literature (Nkedi-Kizza et al., 1985). Soil OC content in the upper horizons accounted for the higher adsorption coefficients observed. As a result of sorption processes, atrazine moved much more slowly through the soil profile than the water transporting it or other nonsorbing solutes. The topsoil (0–30 cm) had a retardation factor (R) of 7.5 for atrazine at a gravimetric soil water content (
w) of 10% compared with 2.5 for the subsoil (using the equation R = 1 + Kd/w). Adsorption was an important process in reducing the movement of atrazine in the soil.
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Fig. 2. Linear adsorption isotherms of atrazine for soils from different depths at the Irrigation and Education Park in Gainesville, Florida
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Degradation of Atrazine in Soil
Degradation data for atrazine could not be described adequately by a single equation. There were two degradation rate coefficients, described via initial and secondary phase. Two first-order equations similar to our data were found for metribuzin and dinitroaniline degradation in soils (Hyzak and Zimdahl, 1974; Zimdahl and Gwynn, 1977). The degradation rate of atrazine for the first 7 d (initial phase) was fast, followed by a slow rate (secondary phase) thereafter (Fig. 3a,b)
. Atrazine had initial degradation rate coefficients (k) of 0.1095 and 0.1075 d-1 for the topsoil and subsoil, respectively. The secondary phase produced degradation rate coefficients of 0.0165 and 0.0060 d-1 for topsoil and subsoil, respectively. Estimated half-life (t1/2) varied from 6.3 to 32 d for topsoil and 6.4 to 83 d in subsoil, depending on the degradation phase used. Short and Enfield (1988) found atrazine half-life values between 12 and 238 d in three different soils, while Wauchope et al. (1991) reported atrazine half-lives ranging from 12 to 120 d. Degradation was an important process that reduced the amount of atrazine in the soil, thus minimizing leaching.
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Fig. 3. Degradation of atrazine in Millihopper topsoil (A) and subsoil (B) from the Irrigation and Education Park under controlled laboratory conditions
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Depth of Water within the Root Zone (0–90 cm)
Depth of water within the root zone was computed by multiplying volumetric soil water content (v) by the depth of soil. Observed depths of water within the 0- to 90-cm soil profile between 13 May and 15 July never exceeded 10.5 cm (Fig. 4)
. Decrease in depth of water was due to evapotranspiration (ET) and/or drainage, while increase in depth of water was due to rainfall and/or irrigation. Based on rainfall and irrigation data (Fig. 1 and Table 2) between 13 May and 15 July, there was a cumulative input water depth of 467 mm. The root zone retained an average of 72 and 87 mm of water on 13 May and 15 July, respectively. Using an estimated average daily potential ET of 5 mm of water (weather data at the experimental site), about 320 mm of water were lost via ET during this time. About 132 mm of water were therefore leached beyond the 90-cm depth between 13 May and 15 July. It is expected that a nonadsorbed chemical (e.g., Br-) or a chemical solution would move with any water that leached out of the root zone.
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Fig. 4. Water stored within the 0- to 90-cm soil depth in a sorghum field under three water management regimes between 13 May and 15 July 1991
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Bromide
Since Br- is biologically stable and does not undergo gaseous losses, it is reasonable to assume that Br- not recovered from soil and plants had leached beyond the root zone. In Fig. 5
, the average concentration of Br- in the root zone over time is presented. Five days after application, most of the Br- was within the top 15 cm of the soil profile. This was due to the fact that there was no rainfall and/or irrigation between day of application and first soil sampling (5 DAA). The sorghum crop was also very young to have absorbed a substantial amount of Br-. Crop Br- uptake was not measured in this study. For the second soil sampling date, 30 DAA, less than one-half of the applied bromide was left within the root zone (0–90 cm). Such downward movement of Br- was achieved with cumulative rainfall of 65 mm, spread over 17 d (Fig. 1). Plant uptake of Br- during the first 17 DAA was negligible because the sorghum plants were quite small and Br- had been leached below their established root zone. No Br- was found within the 0- to 90-cm profile (root zone) by 67 DAA. High rainfall and rapid water infiltration and percolation rates of the sandy soil caused the high leaching potential for Br-. The Br- leaching pattern is consistent with the high amount of water estimated to have leached below the root zone.
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Fig. 5. Bromide concentrations within the 0- to 90-cm soil depth over time in a sorghum field under optimum irrigation in Gainesville, Florida
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Atrazine Concentration in Soil
Analyses for atrazine in the soil profile were performed 2, 26, 39, and 63 DAA. In Fig. 6
, the amount of atrazine recovered from different soil depths over time is presented. An average of 121, 25, 14, 8, and 6 µg kg-1 of atrazine were recovered from the 0- to 15-, 15- to 30-, 30- to 45-, 45- to 60-, and 60- to 90-cm depths, respectively, 2 DAA. There was no rainfall or irrigation between the day of atrazine application and the first soil sampling date (2 DAA), therefore, it is unlikely that any of the applied atrazine could have leached beyond the top 15 cm of soil. Any atrazine recovered from soil below the top 15 cm must have been residual from previous applications. There was a substantial reduction in atrazine concentration within the top 15 cm of soil for the first 26 DAA, decreasing from 125 to 16 µg kg-1 in plots under optimum irrigation. Such a decrease could be attributed to degradation (t1/2 6.3 d for the first week to 32 d for the rest of the period) and leaching. The initial high rate of degradation observed during the first 7 DAA supports the rapid disappearance of atrazine from the topsoil.
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Fig. 6. Atrazine concentrations at different soil depths under three water management regimes at the Irrigation and Education Park in Gainesville, Florida
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An average of 0.44 kg a.i. atrazine ha-1 was found within the 0- to 90-cm depth 2 DAA (Fig. 7)
, which was much less than what was calculated as the application rate of 1.12 kg a.i. atrazine ha-1. The difference between applied atrazine and amount recovered in the soil profile 2 DAA was attributed to absorption by weeds, since atrazine is a postemergence herbicide. As there was no rain or irrigation during the 2 DAA, the atrazine concentration of 0.44 kg a.i. ha-1 in the soil profile was taken as the initial concentration.
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Fig. 7. Atrazine concentrations within the 0- to 90-cm soil depth in a sorghum field under three water management regimes in Gainesville, Florida
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Twenty-four days later, 0.10 kg a.i. atrazine ha-1 were found within the 0- to 90-cm depth and levels remained fairly constant thereafter (Fig. 7). Overall, the topsoil (0–30 cm) had the highest concentration of atrazine in all sampling dates, implying that the leaching of atrazine was minimal. Atrazine concentrations below the 60-cm depth were less than 5 µg kg-1. Using the water content, bulk density, and sorption data for the 0- to 30-cm depth, atrazine average velocity in this layer was at least 15 times slower than that of Br- or water. Water management regimes did not have a significant (
0.05) differential effect on atrazine movement, because of the abundant rainfall that masked the intended irrigation treatments.
Despite the fact that a considerable amount of atrazine can be adsorbed to soils having high organic matter, leaching could still be a problem at high application rates and/or frequent applications to sandy soils with low organic matter content and low water holding capacity. Water management is an important factor since excessive irrigation could increase the potential of leaching. Smith et al. (1990) reported that 350 µg L-1 of atrazine were detected 19 DAA in soil water at a depth of 0.6 m when atrazine was applied at a rate of 4.5 kg a.i ha-1. In the same study, 90 µg L-1 of atrazine were measured in shallow ground water 6 mo after application. In a different study for a sandy soil in California, Troiano et al. (1990) found atrazine leaching as deep as 3.0 m.
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SUMMARY AND CONCLUSIONS
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Atrazine concentrations within the 0- to 90-cm soil profile decreased with increasing depth throughout the experimental period. This trend is supported by the higher atrazine adsoption coefficient (Kd) for topsoil compared with subsoil. Soil OC content decreased with increasing depth, suggesting that it had a direct influence on atrazine adsorption and subsequent leaching of atrazine. Degradation rates of atrazine were higher in topsoil compared with subsoil. Higher degradation rates of atrazine were probably a function of higher microbial activity in topsoil. Concentrations of atrazine measured below the 60-cm soil depth were less than 5 µg kg-1, suggesting that leaching beyond the root zone was minimal. Adsorption and degradation, therefore, reduced atrazine leaching below the root zone when compared with Br-, which does not adsorb to or degrade in the soil used for this study. Crop management practices that improve soil OC content such as incorporation of green manure and crop residue in the soil are likely to reduce atrazine leaching by enhancing microbial activity, which accelerates atrazine degradation and increases atrazine adsorption.
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ACKNOWLEDGMENTS
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The authors are grateful to SADCC/ICRISAT/INTSORMIL for financial support of this research. We also thank Mr. Kafui Awuma, Ms. Dana Brown, Mr. Mark Ou, Mr. Andrew Kweh, and Dr. Vincent Tanya for their assistance in field and laboratory work.
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ABSTRACT
INTRODUCTION
