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Published online 9 August 2006
Published in J Environ Qual 35:1795-1802 (2006)
DOI: 10.2134/jeq2006.0123
© 2006 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
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TECHNICAL REPORTS

Organic Compounds in the Environment

Sorption and Degradation of Pesticides in Nursery Recycling Ponds

Jianhang Lua,*, Laosheng Wua, Julie Newmanb, Ben Faberb, Donald J. Merhautc and Jianying Gana

a Department of Environmental Sciences, University of California-Riverside, Riverside, CA 92521
b University of California Cooperative Extension–Ventura County, Ventura, CA 93003
c Department of Botany and Plant Sciences, University of California-Riverside, Riverside, CA 92521

* Corresponding author (Jianhang{at}ucr.edu)

Received for publication June 2, 2006.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Knowledge of pesticide distribution and persistence in nursery recycling pond water and sediment is critical for preventing phytotoxicity of pesticides during water reuse and to assess their impacts to the environment. In this study, sorption and degradation of four commonly used pesticides (diazinon, chlorpyrifos, chlorothalonil, and pendimethalin) in sediments from two nursery recycling ponds was investigated. Results showed that diazinon and chlorothalonil were moderately sorbed [KOC (soil organic carbon distribution coefficient) from 732 to 2.45 x 103 mL g–1] to the sediments, and their sorption was mainly attributable to organic matter content, whereas chlorpyrifos and pendimethalin were strongly sorbed (KOC ≥ 7.43 x 103 mL g–1) to the sediments, and their sorption was related to both organic matter content and sediment texture. The persistence of diazinon and chlorpyrifos was moderate under aerobic conditions (half-lives = 8 to 32 d), and increased under anaerobic conditions (half-lives = 12 to 53 d). In contrast, chlorothalonil and pendimethalin were quickly degraded under aerobic conditions with half-lives < 2.8 d, and their degradation was further enhanced under anaerobic conditions (half-lives < 1.9 d). The strong sorption of chlorpyrifos and pendimethalin by the sediments suggests that the practice of recycling nursery runoff would effectively retain these compounds in the recycling pond, minimizing their offsite movement. The prolonged persistence of diazinon and chlorpyrifos, however, implies that incidental spills, such as overflows caused by storm events, may contribute significant loads of such pesticides into downstream surface water bodies.

Abbreviations: EC, electrical conductivity • ECD, electron capture detector • GC, gas chromatography


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
CONTAINER NURSERY PRODUCTION is an intensive agricultural setting that requires frequent application of pesticides and fertilizers throughout the year. The plants are commonly watered by an overhead irrigation system, where as much as 70 to 75% of the irrigation water may run off the packed gravel beds or impervious surfaces that the container plants rest on (Beeson and Know, 1991). During overhead irrigation, pesticides and nutrients are washed away from plants and/or potting mixes, and entrained in the runoff water (Briggs et al., 1998; Mahnken et al., 1999). To recycle fertilizers and irrigation water and to reduce runoff to downstream water bodies, recycling or collection ponds are commonly used in outdoor nurseries to capture irrigation runoff, especially in large-scale nurseries. In practice, recycling ponds are often mentioned as the primary best management method for eliminating potential problems that arise from container nursery runoff (Fain et al., 2000).

To sustain healthy plant growth and ensure aesthetic quality, a great array of pesticides, including insecticides, nematicides, fungicides, algaecides, and herbicides are often used. All of these pesticides may reach and accumulate in recycling ponds, which can cause phytotoxicity of pesticides during water reuse and pose threats to the water quality of downstream water bodies if discharges occur, e.g., during pond cleaning or storm events. Whether or not a pesticide enters the reused water or runoff will closely depend on its distribution between the water and sediment phases, and its persistence in the recycling pond.

Sorption and degradation are two governing processes that determine the distribution and persistence of pesticides. These processes have been extensively studied for environments such as agricultural fields or surface aquatic systems. However, the physical, chemical, and biological conditions of recycling ponds are often very different from these environments, as featured by high concentrations of nutrients (N and P) and other applied chemicals, as well as elevated salinity and turbidity. Pesticide behaviors in recycling ponds are expected to be different from natural aquatic systems. For instance, our previous studies showed that the persistence of diazinon and chlorpyrifos appeared to be prolonged in recycling pond waters as compared to surface streamwaters (Lu et al., 2006). As for pesticide behaviors in nursery pond sediments, no studies have been reported so far.

The objectives of this study were to investigate the sorption and distribution behaviors of pesticides between the sediment and water column in nursery recycling ponds, and to evaluate the degradation rate of four heavily-used pesticides in recycling pond sediment under aerobic and anaerobic conditions. The four pesticides are diazinon (insecticide), chlorpyrifos (insecticide and nematicide), chlorothalonil (fungicide), and pendimethalin (herbicide). They were selected because they are widely used pesticides for outdoor container nursery production (PANNA, 2005) and they are highly toxic to aquatic organisms. For instance, the 96-h median lethal concentration against Ceriodaphnia dubia is 0.4 µg L–1 for diazinon and 0.08 µg L–1 for chlorpyrifos (Bailey et al., 1996).


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Chemicals
Standards of diazinon {O,O-diethyl O-[6-methyl-2-(1-methylethyl)-4-pyrimidinyl]-ester, 98% purity}, chlorpyrifos [O-O-diethyl O-(3,5,6-trichloro-2-pyridyl) phosphorothioate, 98% purity], chlorothalonil (2,5,6-trichloroisophthalonitrile, 98% purity), and pendimethalin [N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine, 99.2% purity] were purchased from Chem Service (West Chester, PA). Florisil used for cleanup of the pesticide extracts was purchased from Aldrich (Milwaukee, WI). Organic solvents (methylene chloride, acetone, and hexane) used for extraction were of pesticide grade, and for other uses were of gas chromatography (GC) resolution grade (Fisher Scientific, Pittsburgh, PA). Sodium sulfate used in pesticide analysis was of pesticide grade.

Sediments
Sediment samples from recycling ponds were collected from two representative commercial nurseries in southern California. The first sample was collected from the recycling pond of a nursery (Nursery A) located in Irvine, CA, and the other sample from the recycling pond of a nursery (Nursery B) located in Ventura, CA. Nursery A is a large-scale nursery with 100 ha of plant production area, and its recycling pond has a capacity of about 7500 m3. Nursery B is a small-scale nursery with 16 ha of plant production area, and a recycling pond of about 300 m3. The sediments were collected using a home-made grab sampler, which collected the top 10-cm of sediment. After collection, the sediment samples were drained of free water by gravity, mixed thoroughly, and stored at 4°C until use. The moisture content was kept the same as the gravity-drained samples to preserve the original microbial activity. Textural and chemical properties of the two sediments were analyzed and listed in Table 1. After storage at 4°C for 2 wk, the sediments were analyzed for background concentration of the four pesticides, and no significant residues were found.


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  		Table 1. Textural and chemical properties of the two nursery recycling pond sediments used in this study.

 
Sorption Experiments
Sorption isotherms of pesticides were measured using the conventional batch equilibration technique. Briefly, 5 g of dry weight equivalent sediments was equilibrated with pesticide solution at a sediment/solution ratio of 1:20. For Nursery A sediment, aliquots of 14.29 g were weighed into in a series of 120-mL glass centrifuge bottles, and 90.71 mL of a CaCl2 solution with an electrical conductivity (EC) of 3.3 mS cm–1 were added. For Nursery B sediment, aliquots of 24.15 g were weighed, and 80.85 mL of a CaCl2 solution with an EC of 1.2 mS cm–1 were added. The EC of CaCl2 solutions was made to be the same as the gravity-drained water for these sediments. For sorption of chlorothalonil, 5 mg of sodium azide was added to the bottles to suppress microbial activity because rapid microbial degradation of chlorothalonil in the sediments was observed in preliminary experiments. The bottles were then fortified with 2 to 80 µL of 1000 mg L–1 pesticide solution in acetone, capped with Teflon-lined caps, and shaken reciprocally on a mechanical shaker for 6 h. Preliminary experiments showed that sorption of the pesticides reached equilibrium (>95% sorption occurred) in this time period. After shaking, the bottles were centrifuged for 20 min at 1500 x g, and the pesticide concentration in the supernatant was analyzed using a procedure described by Lu et al. (2006). The amount of pesticide adsorbed was calculated from the difference between the initial and final solution concentration. All the tests were done in triplicate.

Degradation Experiments
Degradation of the pesticides in sediments was determined by incubating pesticide-fortified sediment samples at room temperature (22 ± 2°C) or 10°C under either aerobic or anaerobic conditions. The detailed procedure is described as follows. A 4-g aliquot (dry weight equivalent) of the drained sediment was weighed into a 50-mL glass vial and 4 mL of deionized water was added to immerse the sediment. For the aerobic degradation, the sediment samples were fortified with 8 µL of acetone solution containing 8 µg of each of the four pesticides. The initial pesticide concentration in sediment was thus 2 mg kg–1. The treated vials were loosely covered with aluminum foil and kept at room temperature (22 ± 2°C), or in an incubator at 10°C. The moisture level was maintained during incubation by weighing the glass vials and by adding deionized water when necessary. For the aerobic treatments, samples were vortexed every week at a slow speed for 10 s to mix sediment and supernatant to ensure aerobic conditions. For the anaerobic treatments, sample vials were transferred into a gas-tight plastic inflatable glove chamber (Cole Parmer, Vernon Hills, IL), inflated with N2, and incubated at ambient temperature (22 ± 2°C) for 7 d to induce a reductive environment in the sediment. The sample vials were then sealed with aluminum caps and Teflon-lined butyl rubber septa while still remaining inside the plastic film chamber. The vials were removed from the chamber, and spiked with 8 µL of acetone solution containing 8 µg of each of the four pesticides with a microsyringe. The treated sample vials were returned into the N2–filled plastic chamber, and incubated at room temperature (22 ± 2°C). The anaerobic conditions inside the plastic chamber were maintained by adding N2 into the bag when noticeable deflation occurred. For all the treatments, three replicate samples were removed 0, 0.25, 0.5, 1, 3, 7, 14, 28, 56, and 90 d after the spiking of pesticides, and the samples were transferred immediately to a freezer (–20°C) to stop degradation. Due to significant variation in degradation rates of the pesticides, the samples were selectively analyzed for the four pesticides. Specifically, samples removed after 0, 3, 7, 14, 28, 56, and 90 d were analyzed for diazinon and chlorpyrifos, 0, 0.25, 0.5, 1, 2, 3, and 7 d for chlorothalonil, and 0, 0.5, 1, 2, 3, 7, and 14 d for pendimethalin.

Pesticide Analysis
Pesticide analysis in nursery recycling pond sediments was difficult because the sediments were clayey and of high organic matter content (60.8 g kg–1 in Nursery A and 177.9 g kg–1 in Nursery B). A procedure modified from the USEPA Method 8081 was developed and used. The procedure included organic solvent extraction, Florisil column cleanup, and GC-ECD/NPD determination, and is briefly described below.

Pesticide Extraction
The frozen sediment samples were thawed, and centrifuged at 1500 x g. The supernatants were decanted into 50-mL separatory funnels and extracted with 20 mL of methylene chloride, which was later combined into the extracts of the sediment pellets. For the pellets, 8 g of glass beads (3-mm in diameter, pre-cleaned with methylene chloride) and 30 mL of acetone/methylene chloride mixture (1:1, v/v) were added, and mixed thoroughly on a vortex mixer. Addition of the glass beads facilitated the mixing of the clayey sediments and the organic solvent. The sediment-solvent mixture was then shaken at high speed for 1 h on a reciprocal shaker, and centrifuged at 1500 x g for 20 min. After that, the organic solvent supernatant was collected, and the remaining sediment was extracted two more times with 30 mL of fresh acetone/methylene chloride mixture. The solvent phases were combined, dehydrated with 45 g of anhydrous sodium sulfate, and then evaporated to dryness on a vacuumed rotary evaporator at 38°C. The extract was then re-dissolved in 2 mL of hexane for cleanup.

Extract Cleanup
Three and a half grams of Florisil (activated by heating at 145°C overnight) was packed into a polypropylene filtration tube (1.3-cm diam.; 6-cm length), with polyethylene frits both at the bottom and at the top (Supelco, Bellefonte, PA). Eight mL of hexane was added to pre-elute the column. The re-dissolved extracts were then transferred onto the top of the Florisil column and eluted with a solvent mixture of acetone and hexane (20:80, v/v). Two fractions of the leachate were collected. The first fraction was collected after passing the first 6.5mL of the eluent, and the second fraction was collected after passing another 8 mL of the eluent. The first fraction was concentrated to 1.0 mL for determination of diazinon, chlorpyrifos, chlorothalonil, and pendimethalin, and the second fraction was diluted to 10.0 mL by adding hexane and an aliquot of 1 mL was withdrawn for determination of chlorothalonil. Preliminary experiments showed that >96% of diazinon, chlorpyrifos, and pendimethalin was eluted in the first fraction. About 12% of chlorothalonil was eluted in the first fraction, and 88% was eluted in the second fraction. The amount of diazinon, chlorpyrifos, and pendimethalin in the sediment sample was calculated based on the amount detected in the first fraction, whereas the amount of chlorothalonil in the sample was calculated based on the sum of the amounts detected in both fractions. Using the two-fraction technique was necessary because high amounts of organic matter from the sediments were co-extracted, and it was difficult to separate chlorothalonil from the co-extracted organic matter on the Florisil column due to their similar polarity. The high sensitivity of chlorothalonil on the micro-electron capture detector (µ-ECD) warranted the dilution of the second fraction, which significantly reduced the interference from organic matter.

GC Determination
The cleaned samples were then analyzed on an Agilent 6890N GC equipped with a DB-5 capillary column (30 m x 0.25 mm x 0.25 µm, Agilent Technologies, Wilmington, DE). The GC operation conditions were: the inlet at 270°C, operated on a pulsed splitless mode with an injection volume of 1 µL; oven initially set at 50°C with a hold of 1.4 min, then climbed to 280°C at 25°C min–1 with a final hold of 2 min; µ-ECD (for chlorothalonil, chlorpyrifos, and pendimethalin) at 290°C; and nitrogen phosphorous detector (NPD, for diazinon) at 290°C. Under these conditions, the retention time was 10.08 min for diazinon, 10.13 min for chlorothalonil, 10.91 min for chlorpyrifos, and 11.18 min for pendimethalin. The method detection limit calculated as signal/noise ratio of 10:1 was 30 µg kg–1 for diazinon, 5 µg kg–1 for chlorothalonil, and 10 µg kg–1 for chlorpyrifos and pendimethalin. The method recoveries were 79 ± 5.6% for diazinon, 75 ± 8.2% for chlorothalonil, 81 ± 3.8% for chlorpyrifos, and 86 ± 3.2% for pendimethalin.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Compared to natural stream sediments or soils, nursery recycling pond sediments have two unique characteristics: high organic matter content and high salinity. The sediment from Nursery A had an organic matter content of 60.8 g kg–1 and its gravity-drained water had an EC of 3.3 mS cm–1; the sediment from Nursery B had an organic matter content of 177.9 g kg–1 and an EC of 1.2 mS cm–1 (Table 1). Organic matter in recycling ponds is derived from plant residues and potting mixes that are washed away during nursery operation, and the salts mainly originate from runoff fertilizers. Due to continuous reuses, sediment and water in recycling ponds usually contain high concentrations of nutrients (nitrate, phosphorus, and potassium) and are constantly exposed to various pesticides, which could induce unique microbial environments that are different from those in natural sediments. The unique physical, chemical, and biological conditions in recycling ponds are expected to affect pesticide sorption and degradation behaviors.

Sorption of Pesticides
The sorption isotherms of the four pesticides in the two nursery sediments are shown in Fig. 1 . These isotherms are generally linear over the range of equilibrium concentrations observed in this study, which were up to 40 µg L–1 for pendimethalin and chlorpyrifos and 120 µ gL–1 for chlorothalonil and diazinon. Sorption of these compounds in soils were usually well described by the Freundlich equation when the equilibrium concentrations were higher (e.g., in the range of several to several tens of mg L–1) (Felsot and Dahm, 1979; Nemeth-Konda et al., 2002). Because of the linearity of the isotherms (all r2 > 0.95 for linear regression, n = 7), the distribution tendency of the pesticides between sediment and water column can be adequately described by the sorption coefficient Kd (sediment/water distribution coefficient, Table 2), which is defined as the ratio of the concentration of pesticide sorbed by the sediment (µg kg–1) to the equilibrium solution concentration (µg L–1). For better comparison between sediments and soil samples, Kd (µg L–1) was also normalized on the basis of organic carbon content to obtain KOC (Table 2) by using the equation KOC = Kd/fOC, where fOC is the fraction of organic carbon in sediment.


Figure 1
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  		Fig. 1. Sorption isotherms of the four pesticides in nursery recycling pond sediments.

 

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  		Table 2. Adsorption coefficients of selected pesticides in the two nursery recycling pond sediments.

 
Sorption affinity of the pesticides in the two recycling pond sediments generally followed the order of pendimethalin > chlorpyrifos > chlorothalonil > diazinon (Fig. 1). Sorption of pesticides on Nursery B sediment was consistently greater than that on Nursery A sediment for the same compound. The Kd values of pendimethalin, chlorpyrifos, chlorothalonil, and diazinon for Nursery B sediment were approximately 3.1-, 3.2-, 1.4-, and 1.3-fold, respectively, of that for Nursery A sediment for the same compound (Table 2). For diazinon and chlorothalonil, the stronger sorption in Nursery B sediment may be mainly attributable to its higher organic matter content (Table 1), as the KOC values for the two sediments were similar (789 mL g–1 for Nursery B sediment and 732 mL g–1 for Nursery A sediment for diazinon, and 2.45 x 103 mL g–1 for Nursery B sediment and 2.27 x 103 mL g–1 for Nursery A sediment for chlorothalonil) (Table 2). For pendimethalin and chlorpyrifos, factors other than organic matter content appeared to have influenced sorption, since their KOC values for Nursery A sediment were almost twice those for Nursery B sediment (Table 2). One of these factors might be the difference in organic carbon structure between the two sediments. Substantial variations in KOC values have been observed for sorption of pesticide and other chemicals in soils with organic matter of varying aromaticity and/or polarity (Ahmad et al., 2001; Wang and Xing, 2005). In addition, the texture of the sediments might also contribute to the difference in KOC values between the two sediments. Sorption of pendimethalin and chlorpyrifos was previously found to correlate positively with the clay content of soil (Valverde Garcia et al., 1992; Zheng and Cooper, 1996; Rai et al., 2000a). The much finer texture of Nursery A sediment may therefore have resulted in higher KOC values for the two compounds in this sediment, as compared to those obtained for Nursery B sediment.

The obtained KOC values of pendimethalin, chlorpyrifos, and chlorothalonil were in the range of the values previously reported for these compounds in soil [e.g., 7750 to 16 264 mL g–1 for pendimethalin (Zheng and Cooper, 1996), 995 to 31 000 mL g–1 for chlorpyrifos (Felsot and Dahm, 1979; Racke, 1993), and 2330 to 7336 mL g–1 for chlorothalonil (Caceres et al., 2002)]. However, the KOC values of diazinon were lower than the previously reported values [e.g., 1000 to 1598 mL g–1 in soils (Wauchope et al., 1992; Nemeth-Konda et al., 2002), and 1320 to 1430 mL g–1 in urban stream sediments (Bondarenko and Gan, 2004).

The high KOC values of pendimethalin and chlorpyrifos indicate that these pesticides were preferentially distributed into the sediment phase in the recycling pond. Therefore, in a nursery recycling pond, the majority of these pesticides may be expected to reside in the bulk sediment or with suspended sediment. Therefore, persistence of pesticides in the sediment phase will determine predominantly, the overall persistence of pesticides in the pond environment. The strong sorption of these pesticides suggests that the practice of runoff recycling using sedimentation ponds may effectively retain the pesticides on the property and minimize the potential runoff input into downstream surface water bodies. Likewise, the strong sorption also implies that other mechanisms to remove suspended solids, such as constructed wetlands (Moore et al., 2002; Stearman et al., 2003), sedimentation basins, and the use of flocculants (e.g., polyacrylamide) may result in substantial reduction of the pesticide loads in nursery runoff.

Degradation of Pesticide
Pesticide concentrations measured after different time intervals of incubation are plotted in Fig. 2 for Nursery A sediment and in Fig. 3 for Nursery B sediment. The disappearance of pesticides was fitted to a first-order kinetics plot to estimate the half-life t1/2 (d) (Table 3). As shown in Table 3, most of the correlation coefficients (r2) are ≥ 0.97, suggesting degradation of pesticides in the nursery pond sediments followed first-order kinetics, which has also been observed for these pesticides in soils and surface stream sediments (Bondarenko and Gan, 2004). Relative poor fit occurred for anaerobic degradation of chlorothalonil and pendimethalin, where rapid degradation (t1/2 = 0.19 to 1.9 d) was observed (Table 3).


Figure 2
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  		Fig. 2. Dissipation of diazinon, chlorothalonil, chlorpyrifos, and pendimethalin in sediment from the recycling pond of Nursery A. Error bars are standard deviations of three replicates.

 

Figure 3
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  		Fig. 3. Dissipation of diazinon, chlorothalonil, chlorpyrifos, and pendimethalin in sediment from the recycling pond of Nursery B. Error bars are standard deviations of three replicates.

 

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  		Table 3. First-order half-lives (d) of selected pesticides in nursery recycling pond sediments. Values in parentheses are correlation coefficient r2 (n = 7).

 
Persistence of pesticides in sediment, in general, is influenced by the chemical properties of the pesticide, sediment characteristics, temperature, intensity of sunlight, and redox conditions. Photolysis of pesticides was suppressed in this study by the experimental conditions. Chemical structure of a pesticide is usually the dominant factor for its persistence since it determines the chemical stability for abiotic transformation and the accessibility for microbial metabolism. Under the different temperature and redox conditions, the persistence of the four pesticides in the recycling pond sediments generally followed the order of chlorpyrifos > diazinon > chlorothalonil > pendimethalin (Table 3). In addition to the type of pesticide, the degradation rate in this study also varied with sediment characteristics, redox conditions (availability of oxygen), and temperature (Table 3). The obtained t1/2 values are evaluated below with respect to these factors.

Sediment Characteristics
The two organophosphate pesticides, diazinon and chlorpyrifos, showed moderate persistence in recycling pond sediments. Under 22°C and aerobic conditions, t1/2 of diazinon was 8.6 d in Nursery A sediment, and 10.2 d in Nursery B sediment, whereas t1/2 of chlorpyrifos was 27.3 d in Nursery A sediment, and 31.5 d in Nursery B sediment (Table 3). Organophosphate compounds are known to undergo both biotic and abiotic transformations in soil and water, and the rate of degradation is affected by pesticide application rate and many environmental variables, such as temperature, pH, moisture, and redox potential (Wolfe et al., 1990; Racke, 1993). As a result, there is a great variability in the observed degradation rate of diazinon and chlorpyrifos in soil or sediment (Wauchope et al., 1992; Racke, 1993). The t1/2 of diazinon under aerobic conditions has been reported to be in the range of 3 to 40 d in soils (Wauchope et al., 1992; McLaughlin et al., 1993) and 14 to 21 d in surface stream sediments (Bondarenko and Gan, 2004). The t1/2 of aerobic degradation of chlorpyrifos has been reported to be in the range of 11 to 141 d in soils (Getzin, 1985; Racke, 1993) and 20 to 24 d in surface stream sediments (Bondarenko and Gan, 2004). The t1/2 obtained in this study for the two organophosphate compounds occurred in the range of those previously reported values.

In contrast, rapid degradation was observed for chlorothalonil and pendimethalin in the same sediments. The t1/2 of chlorothalonil was 0.32 d in Nursery A sediment and 0.45 d in Nursery B sediment under 22°C and aerobic conditions (Table 3), which was significantly shorter than in soil and aquatic environments that were determined to be 30 to 90 d (Davies, 1988). Pendimethalin also showed accelerated degradation in the recycling pond sediments, with a t1/2 of 2.8 d in both sediments. The t1/2 of pendimethalin in soil was reported to be around 50 to 90 d (Walker and Bond, 1977; Zimdahl et al., 1984; Kulshrestha and Singh, 1992). Both chlorothalonil and pendimethalin are considered moderately persistent in soil. There may be several possible reasons for the enhanced degradation of chlorothalonil and pendimethalin in the recycling pond sediments. First, the high organic matter content may provide an ample carbon source for microorganisms responsible for the degradation of the two pesticides, and thus enhanced their degradation. Microbial transformation is the primary degradation mechanism for chlorothalonil (Sato and Tanaka, 1987; Rouchaud et al., 1988; Katayama et al., 1992) and pendimethalin (Kulshrestha and Singh, 1992; Scheunert et al., 1993). Accelerated microbial degradation has been reported for chlorothalonil in soils amended with farmyard manure (Mori et al., 1996). The other factor for the enhanced degradation may be the high moisture. High moisture has been shown to accelerate the degradation of chlorothalonil and pendimethalin (Walker and Bond, 1977; Savage, 1978; Kulshrestha and Singh, 1992) in soil. Since the moisture content (65% for Nursery A sediment, and 79.3% for Nursery B sediment, Table 1) in the recycling pond sediments was much higher than the field capacity of most soils, it may have provided a favorable condition for the degradation of these two pesticides. In addition, it is likely that microbial adaptation may have occurred for these compounds. Although these compounds were not found originally in the sediments used in this study, the recycling ponds received runoff year-round, and thus the sediments were constantly exposed to the two pesticides used at the sites. This repetitive exposure may contribute to induction of microbial strains capable of metabolizing these compounds.

Redox Condition
Influence of redox potential on pesticide degradation was examined by comparing their dissipation rates under aerobic and anaerobic conditions. Results showed that the effect of redox potential was dependent on the type of pesticide. Low redox potential under anaerobic conditions either prolonged or reduced the persistence of the pesticides (Table 3, Fig. 2 and 3). For the two organophosphate compounds, diazinon and chlorpyrifos, the persistence was consistently prolonged under anaerobic conditions. The t1/2 of diazinon in Nursery A sediment was 8.6 d under aerobic conditions, but increased to 12.2 d under anaerobic conditions (Table 3). Similarly, the t1/2 of chlorpyrifos increased from 27.3 d under aerobic conditions to 41.2 d under anaerobic conditions (Table 3). The inhibitory effect of anaerobic conditions on diazinon and chlorpyrifos degradation has been observed previously for soil (Racke, 1993) and surface stream sediment (Bondarenko and Gan, 2004). For example, t1/2 of chlorpyrifos was 20 to 24 d in surface stream sediments under aerobic conditions, but increased to 56 to 223 d under anaerobic conditions (Bondarenko and Gan, 2004).

On the contrary, the persistence of chlorothalonil and pendimethalin was further reduced under anaerobic conditions when compared to aerobic conditions. The t1/2 of aerobic degradation of chlorothalonil was 0.32 d in Nursery A sediment and 0.45 d in Nursery B sediment, which decreased to 0.19 and 0.39 d, respectively, under anaerobic conditions (Table 3). Similarly, t1/2 of pendimethalin was 2.8 d under aerobic conditions in both sediments, but decreased to 1.9 d under anaerobic conditions in Nursery A sediment and 0.78 d in Nursery B sediment (Table 3). Enhanced degradation of chlorothalonil and pendimethalin under anaerobic conditions in the sediments is in accordance with the degradation behavior of these two pesticides in soil. For instance, anaerobic conditions were conceivably the most suitable conditions for chlorothalonil degradation in soil (Sato and Tanaka, 1987). Degradation of pendimethalin in soil has also been observed to be faster under flooded, anaerobic conditions than under aerobic conditions (Kulshrestha and Singh, 1992; Rai et al., 2000b).

Temperature
Increased temperature is known to enhance degradation of pesticides by its acceleration of both abiotic chemical reactions and microbial activity. Significant effects of temperature have been observed for these four pesticides in many studies. For example, temperature has been shown to accelerate degradation of diazinon and chlorpyrifos in soil or surface stream sediment (Getzin, 1985; Racke, 1993; Bondarenko and Gan, 2004), and also that of chlorothalonil in soil (Zheng and Cooper, 1996). As expected, persistence of all four pesticides in the two recycling pond sediments was significantly longer at 10°C than at 22°C. The t1/2 of the pesticides increased by 0.8- to 2.3-fold after the 12°C temperature decrease (Table 3), with the degree of enhancement varying with the type of pesticide and characteristics of the sediment. The greatest increase in t1/2 was observed for chlorothalonil in Nursery A sediment, where t1/2 increased by 3.3-fold, from 0.32 d at 22°C to 1.37 d at 10°C (Table 3). Temperature dependence implies that the degradative capacity of the recycling ponds will increase in the summer months, eliminating the pesticides from the pond system at a faster rate. For the same reasons, the degradative capacity of the recycling ponds will decrease in cold seasons, where the risk of pesticide accumulation may increase.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Although results from laboratory experiments are not directly applicable to field conditions, results obtained in this study suggest that chlorothalonil and pendimethalin will not accumulate significantly in nursery recycling ponds, due to their rapid dissipation. In contrast, the two organophosphate pesticides, diazinon and chlorpyrifos, may have a tendency to be persistent in recycling ponds. The high KOC value of the pesticides, combined with the high organic matter content of the sediments, implies that the majority of the four compounds will reside in the sediment phase. Practices that could retain suspended particles, such as sedimentation in a recycling pond or constructed wetland, and use of flocculants, will effectively minimize runoff potential of the pesticides from nursery production areas and thus protect downstream surface water bodies.


   ACKNOWLEDGMENTS
 
This study was partially funded by the California State Water Resources Control Board through an agreement pursuant to the Costa-Machado Water Act of 2000 (Proposition 13).


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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