Vadose Zone Journal 2:231-238 (2003)
© 2003 Soil Science Society of America
Endosulfan Losses through Runoff and Leaching from Calcareous Gravelly or Marl Soils
M. Zhoua,
Y. C. Li*,a,
P. Nkedi-Kizzab and
S. K. O'Haira
a Department of Soil and Water Science and Department of Horticultural Sciences, University of Florida's Institute of Food and Agricultural Sciences, at the Tropical Research and Education Center, Homestead, FL 33031
b Soil and Science Department, University of Florida's Institute of Food and Agricultural Sciences, Gainesville, FL 33611
* Corresponding author (yunli{at}ifas.ufl.edu)
Received 12 March 2002.
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ABSTRACT
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The non-point loss of a pesticide through runoff and leaching from agricultural fields depends not only on its chemical properties but also on environmental factors such as soil type, rainfall intensity, and water table level. Two major agricultural calcareous soils in South Florida, Krome (loamy-skeletal, carbonatic, hyperthermic Lithic Udorthents) very gravelly loam (Krome) and Biscayne (loamy, carbonatic, hyperthermic, shallow Typic Fluvaquents) marl (marl), were packed in tilted stainless-steel runoff–leaching chambers for measuring the transport of endosulfan (6,7,8,9,10,10-hexachloro-1,5,5a,6,9,9a-hexahydro-6,9-methano-2,4,3-benzadioxathiepin 3-oxide) isomers in leachates and surface runoff. Endosulfan loss was the greatest in runoff sediment, followed by loss in runoff water, and finally in the leachate. At a rainfall intensity of 75 mm h-1, the infiltration rate in the Krome soil was higher than in the marl soil. Endosulfan movement with infiltration water in Krome soil is a nonequilibrium process. Increasing rainfall intensity to 150 mm h-1 significantly decreased endosulfan leaching and increased runoff. High rainfall intensity and/or water table dramatically increased the endosulfan loss through the sediments in runoff. Marl soil has a higher endosulfan runoff potential than Krome soil. Endosulfan 
had a lower average experimental distribution coefficient, KD, than endosulfan ß (265 vs. 472). Loss of endosulfan dissolved in runoff water and through leaching was higher than that of endosulfan ß, while loss of endosulfan from runoff sediments was lower than that of endosulfan ß. Endosulfan concentrations in runoff water decreased exponentially with cumulative rainfall and runoff sediment.
Abbreviations: ENP, Everglades National Park • WTD, water table depth
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INTRODUCTION
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SINCE THE SUBTROPICAL CLIMATE of South Florida facilitates pests outbreaks in crops, many pesticides are applied during the growing season to control a variety of arthropods, plant pathogens, and weeds. The intensive agriculture in South Florida is practiced above the surficial Biscayne aquifer and adjacent to the fragile natural ecosystems in the Everglades and marine bays. Endosulfan, a low cost chlorinated hydrocarbon insecticide, applied in South Florida on winter-grown vegetables accounts for 34% of the total use in the USA, and its concentrations in surface water often exceed the water quality criteria of the Florida Department of Environmental Protection (0.056 µg L-1 for surface water and 0.35 µg L-1 for groundwater) (Miles and Pfeuffer, 1997; Pfeuffer and Matson, 2001). A field study conducted during 1993 to 1997 by Scott et al. (2002) showed that endosulfan was detected in water samples at 100% of 12 monitoring sites in South Florida. Eight of these monitoring sites within agricultural areas were in canals that drained into Florida Bay. Other monitoring sites were in eastern Florida Bay. Endosulfan caused more costal fish kills than any other pesticide during 1980 through 1989 (Lowe et al., 1991).
Surface runoff and leaching of a pesticide depends on its properties and those of the soil. Endosulfan exists as two isomers, and ß, which have different physical properties and behaviors in soils. The ratio of and ß isomer in field sediment is much less than that in the endosulfan originally applied. This may be because the distribution coefficient, KD, between soil and water of ß isomer is higher than that of isomer (Peterson and Batley, 1993), and also because the ß isomer is relatively more persistent in soils than the isomer (Antonious and Byers, 1997). The endosulfan KD in soil or sediment tends to increase as soil organic C content increases (Peterson and Batley, 1993; Parkpian et al., 1998). The organic C–normalized sorption coefficient (Koc) of endosulfan is about 12 400 (Wauchope et al., 1992), with considerable variation among different soils. The large value of Koc indicates a hydrophobic compound that is strongly adsorbed to soils and sediments.
Pesticide runoff and leaching are also affected by rainfall intensity and water table elevation. Runoff increases linearly as rainfall intensity increases on high clay soils (Reichert et al., 1994), while sediment yield increases nonlinearly as runoff (or rainfall intensity) increases (Reichert et al., 1994; Huang, 1998). Surface runoff is positively related to the water table depth (Capece et al., 1987). Sediment concentration in surface runoff under high water table conditions is much higher than that under free drainage (Huang and Laflen, 1996). High-intensity rainfall, often observed during the rainy season, dramatically increases the water table in South Florida soils (Crane et al., 1997). Recently, to reinstate the historical natural hydroperiods in Everglades National Park (ENP), the U.S. Army Corps of Engineers has proposed to elevate the water table in the ENP about 45 cm higher than the water table in the agricultural area to the east of the ENP (U.S. Army Corps of Engineers, 2001). The effect of water table on runoff and leaching of more water soluble pesticide such as atrazine [6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine] from the soils without rock fragments has been well documented (Davis-Carter and Burgoa, 1993; Basta et al., 1997; Zhang et al., 1997; Mersie et al., 1999; Williams et al., 1999). The effects of very shallow water table depth and rainfall intensity on endosulfan transport over and through calcareous soils, especially the soil with a high limestone fraction in South Florida, need to be studied. Little is known about endosulfan losses through runoff and leaching in those calcareous gravelly or marl soils. The objective of this study was to determine the effect of soil type, water table depth, and rainfall intensity on endosulfan runoff and leaching.
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MATERIALS AND METHODS
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Krome very gravelly loam and Biscayne marl soils from the commercial vegetable fields in Miami-Dade County were used in this study. Krome very gravelly loam is a soil developed from rock-plowing. The native soils in the area had very shallow depths over bedrock. Rock-plowing crushes the natural oolitic limestone bedrock into a growth medium, and increases the soil depth for crop production. Soil properties are presented in Table 1. Endosulfan was not detected in these soils. Stainless-steel runoff–leaching chambers (45 by 20 cm) were packed with soil to a depth of 14 cm with the bulk densities of 1.40 and 1.34 g cm-3 for the Krome and marl soils, respectively. Soil depth used in this experiment is in the range of soil depths (12–25 cm) found in commercial vegetable fields in the area. The slope of each chamber was set at 3°.
The soil in each chamber was saturated with water from the bottom up and then drained for 3 h. A 10-mL aqueous suspension of endosulfan (Thiodan, FMC Corporation, Philadelphia, PA) (equivalent to 1225 g a.i.ha-1, which is similar to the commercial application rate) was uniformly applied to the soil surface 6 to 8 h before rainfall application. The endosulfan had 35% active ingredient with a 2.3:1 ratio of endosulfan /endosulfan ß.
Water tables in South Florida agricultural area are frequently <30 cm from the soil surface during the rainy season (USDA, 1996b). Therefore, three water table depths (WTDs), 4.5, 9.5, and >14 cm (free drainage) from soil surface at the middle of the slope were controlled by adjusting the outlet of the drainage tube for the Krome soil, and one WTD (>14 cm) was tested for the marl soil. The WTDs were kept the same level during the rain simulation. Water from a well with no detectable endosulfan (<0.05 µg L-1) was used to simulate rainfall. Two rainfall intensities of 75 and 150 mm h-1 were simulated for 2 and 1 h, respectively, and delivered a total of 150 mm of rain using a rainfall simulator with Vee Jet 80-25 nozzles (Chapin, Batavia, NY).
Runoff water was channeled through a tube into a glass bottle and leachates were collected through a stainless-steel funnel attached on the bottom of the chamber and then drained into 1-L bottles at 6- and 3-min intervals for the 75 and 150 mm h-1 rainfall intensities. The runoff water and sediment were separated using centrifugation after collection. All the samples were extracted with solvents within 24 h and stored in the refrigerator at 4°C before analysis.
Endosulfan was extracted from sediment using 1:1 ratio of analytical grade hexane and acetone, and analyzed using a Perkin Elmer (Wellesley, MA) GC-ECD auto system with a DB-5 capillary column (30 m by 0.32 mm, 1-µm film thickness). Ultra-high purity He was used as a carrier gas at an inlet pressure of 99 kPa. Injector, detector, and oven temperatures were 270, 300, and 270°C, respectively. Retention times of endosulfan and ß were 6.2 and 7.8 min, respectively. Endosulfan concentration in runoff water and leachate was analyzed using EPA method 505 (USEPA, 1991). The endosulfan ( and ß) standards were 99% pure (Ultra Scientific, North Kingstown, RI). Quality control included use of blanks, sample spikes, recalibration standards, and duplicate sample analyses. Spikes consisted of adding known amounts of the target analysts, dissolved in acetone, directly to the samples. Spike recoveries were within 100 ± 10%. The correlation coefficients of standard calibration curves were 
0.999. The method detection limit of endosulfan in water and sediment was 0.05 and 1 µg L-1, respectively.
Soil pH was measured in deionized water at a soil/solution ratio of 1:2.5 after 2 h of equilibration. The equivalent carbonate contents in soils were analyzed using a titrimetric method (USDA, 1996a). Soil organic C was determined using the Walkley and Black method (Walkley and Black, 1934). Soil particle-size distribution without the removal of carbonate was measured using the pipette method after removal of organic C using HClO3 with 0.2 mol L-1 NaCO3 and using NaCO3 as the dispersion agent (Miller and Baharuddin, 1987).
Modeling Endosulfan Runoff
Zhang et al. (1997) used a uniform mixed model to predict atrazine and metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] loss in runoff. Since atrazine and metolachlor have relatively low distribution coefficients, KD, the pesticide loss through sediment runoff was relatively low and was not included in their model. However, since endosulfan and ß have a much higher KD than atrazine or metolachlor, the sediment-bound endosulfan lost in runoff is to be an important part of the model. When the sediment loss is assumed to be small when compared with the total soil mass in the mixing zone, and the reduction in the mixing zone depth due to erosion loss is considered minimal, the endosulfan runoff data can be simulated using the following equation:
 | [1] |
where 
is the saturated volumetric water content (cm3 cm-3), z is the average mixing zone depth (cm), 
b is the soil bulk density(g cm-3); R(t) is the rainfall intensity (cm h-1), S(t) is the runoff sediment rate (g h-1), A is the soil surface area (cm2), KD is the endosulfan distribution coefficient (cm3 g-1), and Cw (µg mL -1) and Cs (µg g-1) are the endosulfan concentrations in the water and sediment phases, respectively.
The solution of Eq. [1] is
 | [2] |
where Co is the initial endosulfan concentration in soil solution (µg cm-3), D(t) is the cumulative rainfall depth (cm), and M(t) is the cumulative runoff sediment weight (g). M(t)Kd/A is the cumulative apparent runoff sediment depth (cm).
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RESULTS AND DISCUSSION
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Runoff Sediment, Runoff, and Infiltration Waters
The amounts of runoff sediment were strongly affected by the rainfall intensity, soil type, and water table depth (Table 2). Sediment load doubled when rainfall intensity was doubled on the marl soil. At the lower rainfall intensity (75 mm h-1), no runoff occurred from the Krome soil, while there was a fairly high amount of sediment in runoff from the marl soil. At the higher rainfall intensity (150 mm h-1) and with the water table at 14 cm, the amounts of sediment in runoff were 6.4 times higher from the marl soil than from the Krome soil (Table 2). This may be because the soil properties play an important role in sediment runoff. The Krome soil is dominated by rock fragments and sand, while silt and clay are the predominant fractions in the marl soil (Table 1). It is thus anticipated that the marl soil particles will have more surface sediment transport than the Krome soil. The higher volume of runoff water from the marl soil compared to the Krome soil (Table 2) is another reason for the increase in runoff sediments from the marl soil. The rainfall intensity did not significantly affect volumes of runoff water and leachates from the marl soil, possibly because of the saturation of soil with water before rainfall application.
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Table 2. Total water table depths (WTD), runoff water and infiltration water volumes at different rainfall intensities, and runoff sediment weight for the two soils studied.
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Water table depth also affected the amount of sediments and water in runoff and water infiltration. The Krome soil has a high gravel fraction (high porosity); thus, leaching was an important water pathway when WTD > 14 cm. As the water table depth decreased in the Krome soil, the amount of sediment in runoff increased, the runoff water volume slightly increased, and subsequently infiltration water decreased (Table 2). Capece et al. (1987) also reported a similar relationship between water table depth and surface water runoff from sandy soil. However, the effects of water table depth on sediment runoff from very gravelly soils (rock fragment = 638 g kg-1) have never been reported. Thus, managing water table depth or avoiding application of agrochemicals during high water table events is necessary not only for fine-textured soils but also for very gravelly soils.
In all cases sediment concentration began high and decreased with cumulative rainfall until an apparent equilibrium sediment concentration was achieved (Fig. 1–3). The sediment concentrations in runoff from marl soil at 150 mm h-1 rainfall were much higher than those from marl soil at 75 mm h-1 rainfall and from Krome soil at 150 mm h-1 rainfall. Meanwhile, the runoff sediment concentration from marl soil at 150 mm h-1 rainfall decreased at a faster rate with rainfall depth than at the lower rainfall intensity, or from Krome soil at the same rainfall intensity. As WTD decreased, the sediment concentration in runoff increased. However, the change in sediment concentration in runoff with cumulative rainfall was similar for all three WTDs in the Krome soil (Fig. 2).
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Fig. 1. The effect of rainfall intensity on sediment concentrations in runoff and cumulative runoff sediment mass as a function of cumulative rainfall for the Biscayne marl soil.
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Fig. 3. The effect of soil type on sediment concentrations in runoff and cumulative runoff sediment as a function of cumulative rainfall.
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Fig. 2. The effect of water table depth (WTD) on sediment concentrations in runoff and cumulative runoff sediment mass as a function of cumulative rainfall for the Krome soil.
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Endosulfan in Runoff Sediment and Water, and in Leachate
The pesticide distribution coefficient, KD, relates to the equilibrium pesticide concentration between the aqueous phase and the sediment phase. Endosulfan distribution coefficients (KD) between runoff sediment and water can be calculated as:
 | [3] |
where Cs is the endosulfan concentration in the runoff sediment (µg g-1) and Cw is the endosulfan concentration in the runoff water (µg mL-1).
For the Krome soil, the different water table depths had little effect on KD value (Table 3). The average KD of endosulfan and ß was 265 and 472, respectively. However, different rainfall intensities strongly affected the values of KD for the marl soil. The KD value was much higher in runoff sediment produced by low-intensity rainfall than that produced by high-intensity rainfall for the marl soil. This may be attributed to low-intensity rainfall causing preferential transport of the finer particles (Farmer, 1973), which may have result in a higher pesticide distribution coefficient than coarse particles because of a higher accumulation of organic C (Nkedi-Kizza et al., 1983). The higher intensity rainfall might have preferentially transported silt particles (Miller and Baharuddin, 1987) that contain less organic C. The KD values for endosulfan ß were about 1.6 and 1.3 times higher than that of endosulfan for the runoff sediments from Krome and marl soils, respectively. Thus, a higher percentage of endosulfan ß is lost through sediment runoff than endosulfan . This also explains why a higher percentage of endosulfan is lost through runoff water and leachate when compared with endosulfan ß (Table 4). The endosulfan and ß losses in runoff were higher in sediments than in the dissolved water phase, except for endosulfan loss from Krome soil with a large water table depth.
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Table 3. Endosulfan distribution coefficient (KD) between runoff sediments and runoff water for different soil types, rainfall intensities, and water table depths (WTD).
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Table 4. Losses of endosulfan through runoff and leaching for two types of soils and two rainfall intensities, and with different water table depths (WTD).
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The sediment-bound endosulfan in runoff was the main contributor to endosulfan loss, followed by runoff of endosulfan in dissolved water (Table 4). Loss of endosulfan in leachate was the least. This was mainly because endosulfan has a high distribution coefficient (Table 3), which means that a large proportion of endosulfan is adsorbed to soil particles in runoff, and also soil particles would also retard endosulfan leaching below the soil surface. Low solubility (<1.0 mg L-1) of endosulfan in water contributed to less loss of it in leachate.
Loss of endosulfan from soil was strongly affected by the rainfall intensity, soil type, and water table depth (Table 4). At the 75 mm h-1 of rainfall, endosulfan leaching through infiltration water was the important loss mechanism for the Krome soil, while the endosulfan in the runoff sediment was the major loss path from marl soil (Table 4). The soil properties such as rock fraction, soil particle-size distribution, porosity, and organic C content play an important role in the endosulfan runoff and leaching process. Krome soil had a large fraction of gravelly particles, while in the marl soil, fine particles were predominant (Table 1). Because of the high distribution coefficient of endosulfan between water and soil, KD (Table 3), approximately 2% of applied endosulfan was lost due to leaching in Krome soil at a rainfall intensity of 75 mm h-1. The early breakthrough and long tailing represent the nonequilibrium transport process in endosulfan leaching (Fig. 4). The nonequilibrium may be due to both sorption and physical kinetics. This could result from sorption on organic matter and also from endosulfan in infiltrating water undergoing preferential flow through noncapillary structure (Beven, 1991). The nonequilibrium can enhance the endosulfan loss through the soil, since endosulfan will have a short resident time to adsorb on the soil particles. When rainfall intensity increased, runoff became the major endosulfan loss mechanism for the Krome soil (Table 4). The major soil particles in marl soil were silt and clay, which have a lower infiltration rate than a soil dominated by a gravelly particle fraction that often contributes to the big soil pore size. Leaching contributed to <0.05% loss of endosulfan applied to marl soil, which was much less than the endosulfan leached in Krome soil. At the same rainfall intensity, endosulfan runoff from marl soil was much higher than from Krome soil. When the rainfall intensity increased or the water table depth decreased, the endosulfan in runoff increased (Table 4). Since higher rainfall intensity or water table produced higher amounts of sediment in runoff (Table 2), the runoff sediment accounted for the majority of the endosulfan loss from the soils (Table 4). Thus, both the effects of rainfall intensity and high water table combined to produce larger endosulfan losses (Table 4).
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Fig. 4. The concentrations of endosulfan (filled symbols) and ß (open symbols) in leachates as a function of cumulative rainfall mass for the Krome soil at 150 mm h-1 rainfall intensity.
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Modeling of Endosulfan Runoff
The plot of endosulfan concentration in runoff water (Cw) vs. sum of cumulative rainfall depth, D(t), and the apparent runoff sediment depth, M(t)Kd/A, was fitted to Eq. [2] (Fig. 5–7). The model predicted endosulfan loss reasonably well except for the front data points where the runoff sediment concentrations were very high. According to the model, the endosulfan in sediment and in the dissolved water phase both contributed to loss of endosulfan and ß in surface runoff from the calcareous soils in South Florida. The endosulfan concentrations in runoff were also strongly affected by the concentration of sediment content in runoff water.
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Fig. 5. Measured and calculated endosulfan concentrations in runoff water vs. the sum of cumulative rainfall depth and apparent runoff sediment depths for Krome soil at all water table depths.
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Fig. 6. Measured and calculated endosulfan ß concentrations in runoff water vs. the sum of cumulative rainfall depth and apparent runoff sediment depth for Krome soil at all water table depths.
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CONCLUSIONS
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Endosulfan and ß losses through runoff sediments constitute the major loss mechanism, and leaching is the least significant for the soils studied. Management practices that can prevent surface runoff, specifically of sediment, will minimize the loss of endosulfan from calcareous gravelly or marl soils. The fine particle–dominated marl soil has a higher runoff potential than the gravelly Krome soil. A high rainfall intensity and a high water table both significantly increase the runoff of endosulfan and ß.
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ACKNOWLEDGMENTS
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This research was supported in part by a grant from the Center of Natural Resources, University of Florida. The authors thank Dr. L.T. Ou for technical support in the endosulfan analyses and Dr. Waldemar Klassen and Renuka Rao for review of the manuscript.
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NOTES
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Florida Agricultural Experiment Station Journal Series No. R-07431.
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REFERENCES
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- Antonious, F.G., and M.E. Byers. 1997. Fate and movement of endosulfan under field conditions. Environ. Toxicol. Chem. 16:644–649.
- Basta, N.T., R.L. Huhnke, and J.H. Stiegler. 1997. Atrazine runoff from conservation tillage systems: A simulated rainfall study. J. Soil Water Conserv. 52:44–48.
- Beven, K. 1991. Modeling preferential flow: An uncertain future? p. 1–11. In T.J. Gish and A. Shirmohammadi (ed.) Preferential flow. ASAE, St. Joseph, MI.
- Capece, J.C., K.L Campbell, L.B. Baldwin, and K.D. Konyha. 1987. Estimating runoff volumes from flat, high-water-table watersheds. Trans. ASAE 30:1397–1402.
- Crane, J.H., C.F. Balerdi, M. Lamberts, D. Hull, and T. Olczyk. 1997. Flood damage assessment of agricultural crops in South Dade County as a result of tropical storm Gordon. Proc. Fla. State Hort. Soc. 110:152–155.
- Davis-Carter, J.G., and B. Burgoa. 1993. Atrazine runoff and leaching losses from soil in tilted beds as influenced by three rates of lagoon effluent. J. Environ. Sci. Health, B. 28:1–18.
- Farmer, E.E. 1973. Relative detachability of soil particles by simulated rainfall. Soil Sci. Soc. Am. Proc. 37:629–633.
- Huang, C.H. 1998. Sediment regimes under different slope and surface hydrologic conditions. Soil Sci. Soc. Am. J. 62:423–430.[Abstract/Free Full Text]
- Huang, C.H., and J.M. Laflen. 1996. Seepage and soil erosion for a clay loam soil. Soil Sci. Soc. Am. J. 60:408–416.[Abstract/Free Full Text]
- Lowe, J., D. Farrow, A. Pait, S. Arenstam, and E. Lavan. 1991. Fish kills in costal waters, 1980–1989. Strategic Environmental Assessment Division, ORCA/NOS/NOAA, Rockville, MD.
- Mersie, W., C.A. Seybold, and C. McNamee. 1999. Effectiveness of switchgrass filter strips in removing dissolved atrazine and metolachlor from runoff. J. Environ. Qual. 28:816–821.[Abstract/Free Full Text]
- Miles, C.J., and R.J. Pfeuffer. 1997. Pesticides in canals of South Florida. Arch. Environ. Contam. Toxicol. 32:337–345.[Medline]
- Miller, W.P., and M.K. Baharuddin. 1987. Particle size of interrill-eroded sediments from highly weathered soils. Soil Sci. Soc. Am. J. 51:1610–1615.[Abstract/Free Full Text]
- Nkedi-Kizza, P., P.S.C. Rao, and J.W. Johnson. 1983. Adsorption of diuron and 2,4,5-t on soil particle-size separates. J. Environ. Qual. 12:195–197.[Abstract/Free Full Text]
- Parkpian, P., P. Anurakpongsatorn, P. Pakkong, and W.H. Patrick, Jr. 1998. Adsorption, desorption and degradation of -endosulfan in tropical soil of Thailand. J. Environ. Sci. Health, B. 33:211–233.
- Peterson, S.M., and G.E. Batley. 1993. The fate of endosulfan in aquatic ecosystems. Env. Pollut. 82:143–152.
- Pfeuffer, R.J., and F. Matson. 2001. Pesticide surface water and sediment quality report. South Florida Water Management District, West Palm Beach, FL.
- Reichert, J.M., L.D. Norton, and C.H. Huang. 1994. Sealing, amendment, and rain intensity effects on erosion of high-clay soils. Soil Sci. Soc. Am. J. 58:1199–1205.[Abstract/Free Full Text]
- Scott, G.I., M.H. Filton, E.F. Wirth, G.T. Chandler, P.B. Key, J.W. Daugomah, D. Bearden, K.W. Chung, E.D. Strozier, M. Delorenzo, S. Sivertsen, A. Dias, M. Sanders, J.M. Macauley, L.R. Goodman, M.W. Lacroix, G.W. Thayer, and J. Kucklick. 2002. Toxicological studies in tropical ecosystems: An ecotoxicological risk assessment of pesticide runoff in south Florida estuarine ecosystems. J. Agric. Food Chem. 50:4400–4408.[Medline]
- U.S. Army Corps of Engineers. 2001. Central and Southern Florida Project comprehensive review study. USACOE, Jacksonville District, Jacksonville, FL.
- USDA. 1996a. Soil survey laboratory methods manual. Soil Survey Investigations Report No. 42. Version 3. USDA-NRCS, National Soil Survey Center, Lincoln, NE.
- USDA. 1996b. Soil survey of Dade County area, Florida. Available online at http://soils.usda.gov/soil_survey/surveys/fl_dade/main.htm (verified 7 Mar. 2003). USDA-NRCS.
- USEPA. 1991. Methods for the determination of organic compounds in drinking water. EPA/600/4–88/039. U.S. EPA Environmental Monitoring Systems Laboratory, Cincinnati, OH.
- Walkley, A., and I.A. Black. 1934. An examination of the degtjareff method for proposed modification of the chromic acid titration method. Soil Sci. 37:29–38.
- Wauchope, R.D., T.M. Buttler, A.G. Hornsby, P.W.M. Augustijn-Beckers, and J.P. Burt. 1992. The SCS/ARS/CES Pesticide Properties Database for Environmental Decision-Making. In W. Georage (ed.) Rev. Environ. Contam. Toxicol. 123:1–164
- Williams, C.F., S.D. Nelson, and T.J. Gish. 1999. Release rate and leaching of starch-encapsulated atrazine in a calcareous soil. Soil Sci. Soc. Am. J. 63:426–432.
- Zhang, X.C., L.D. Norton, and M. Hickman. 1997. Rain pattern and soil moisture content effects on atrazine and metolachlor losses in runoff. J. Environ. Qual. 26:1539–1547.[Abstract/Free Full Text]
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ABSTRACT
INTRODUCTION
