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TECHNICAL REPORTS

Organic Compounds in the Environment

Distribution and Persistence of Pyrethroids in Runoff Sediments

^a Department of Environmental Sciences, University of California, Riverside, CA 92521
^b Department of Environmental Sciences, Zhejiang University, Hangzhou, China, 310029
^c University of California Corporative Extension-Orange County, Irvine, CA 92626

^* Corresponding author (jgan{at}ucr.edu)

Received for publication June 23, 2004.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Pyrethroids are commonly used insecticides in both agricultural and urban environments. Recent studies showed that surface runoff facilitated transport of pyrethroids to surface streams, probably by sediment movement. Sediment contamination by pyrethroids is of concern due to their wide-spectrum aquatic toxicity. In this study, we characterized the spatial distribution and persistence of bifenthrin [BF; (2-methyl(1,1'-biphenyl)-3-yl)methyl 3-(2-chloro-3,3,3-trifluoro-1-propenyl)-2,2-dimethylcyclopropanecarboxylate] and permethrin [PM; 3-(2,2-dichloroethenyl)-2,2-dimethylcyclopropanecarboxylic acid (3-phenoxyphenyl)methyl ester] in the sediment along a 260-m runoff path. Residues of BF and PM were significantly enriched in the eroded sediment, and the magnitude of enrichment was proportional to the downstream distance. At 145 m from the sedimentation pond, BF was enriched by >25 times, while PM isomers were enriched by >3.5 times. Pesticide enrichment along the runoff path coincided with enrichment of organic carbon and clay fractions in the sediment, as well as increases in adsorption coefficient K_d, suggesting that the runoff flow caused selective transport of organic matter and chemical-rich fine particles. Long persistence was observed for BF under both aerobic and anaerobic conditions, and the half-life ranged from 8 to 17 mo at 20°C. The long persistence was probably caused by the strong pesticide adsorption to the solid phase. The significant enrichment, along with the prolonged persistence, suggests that movement of pyrethroids to the surface water may be caused predominantly by the chemically rich fine particles. It is therefore important to understand the fate of sediment-borne pyrethroids and devise mitigation strategies to reduce offsite movement of fine sediment.

Abbreviations: BF, bifenthrin • ER, enrichment ratio • GC, gas chromatography • OC, organic carbon content • PM, permethrin

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
RUNOFF OF AGROCHEMICALS during storm and irrigation events is of significant concern from the standpoint of surface water quality. Delivery of pesticides into the surface water via runoff may contribute to acute or chronic ecotoxicological effects (Gilliom, 2001). Earlier studies show that transport of agrochemicals in runoff to the surface stream is facilitated primarily by sediment movement, and sediment-bound nutrients may account for up to 90% of the total amount transported in runoff (Schuman et al., 1973a, 1973b). In recent studies, residues of pyrethroids were found to move with runoff into surface streams (Werner et al., 2002; Weston et al., 2004). Pyrethroids are widely used insecticides in both agricultural and urban settings, and their use may further increase as the use of organophosphate insecticides is restricted. Pyrethroids usually have high toxicity to a wide range of water column and benthic aquatic organisms (Hill, 1989). Their strong affinity for the solid phase suggests that off-site transport of pyrethroids is probably mediated by sediment movement. Therefore, to understand the ecotoxicological significance of runoff-borne pyrethroids, it is essential to characterize their downstream distribution and persistence in the sediment phase.

Enrichment is an important phenomenon that occurs in the process of sorbed chemical movement by sediment in overland flow. It was observed that concentrations of phosphorus and nitrogen were richer in the eroded sediment than the source soil (Sharpley, 1980, 1985; Avnimelech and McHenry, 1984; Wan and El-Swaify, 1998). Enrichment is considered to be a result of selective or preferential erosion of organic matter and chemically rich fine particles during runoff (Sharpley, 1980, 1985). The magnitude of enrichment is quantified by the enrichment ratio (ER) that is the ratio of concentration in eroded sediment to that of the source matrix. Chemical enrichment in sediment is of great significance, as it not only determines the load of a pollutant entering a downstream water body, but also the form and hence the bioavailability of the runoff-borne pollutant. So far nutrients have been the subject of most studies on sediment enrichment, and few studies addressed pesticide enrichment during runoff (Ghadiri and Rose, 1991).

Earlier compounds from the pyrethroid class were known to be relatively unstable due to their susceptibility to photodegradation. The newer pyrethroids, such as bifenthrin, however, generally have enhanced environmental stability. The persistence of pyrethroids has been well studied in soil, but is poorly characterized in sediment. Knowledge on pesticide persistence in the sediment along a runoff path is critical for predicting the likelihood and also magnitude of pesticide export to a downstream water body.

In this study, spatial distribution and enrichment of bifenthrin (BF) and permethrin (PM) were determined in the sediment phase along a 260-m runoff path, and pesticide persistence in sediment was evaluated under both aerobic and anaerobic conditions. According to the California Department of Pesticide Regulation, PM was the most used pyrethroid in California in 2002, with a total annual consumption of 1.74 x 10⁵ kg (active ingredient) (California Department of Pesticide Regulation, 2004). The use of BF is mandatory for fire ant control in the nursery production in southern California. Both PM and BF have acute toxicity to Ceriodaphnia dubia and Daphnia magna, two freshwater invertebrates commonly used in toxicity tests, at concentrations of <1 µg L^–1 (Mokry and Hoagland, 1990). Findings from this study will be useful for understanding the behavior of these pyrethroids and other strongly adsorbing pesticides in surface runoff and the potential for their transport to the surface water.

	MATERIALS AND METHODS

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Chemicals and Sediments
Standards of Z-(cis)-bifenthrin (>98%) and permethrin (20% cis and 78% trans) were purchased from Chem Service (West Chester, PA). Organic solvents used in the study were of analytical or pesticide residue grade. Sediment samples were taken from different locations along a concrete-lined 260-m drainage channel receiving runoff water from a large nursery located near Irvine, California. Sediments were collected from the 0- to 5-cm surface layer into glass jars and immediately transported to the laboratory. After processing, the sediment samples were analyzed for textural and chemical properties by the University of California's Division of Agricultural and Natural Resources Analytical Laboratory in Davis, California (Table 1).

Incubation Experiments
Degradation of BF and PM was determined using the previously contaminated sediments from three locations (104, 145, and 210 m) along the runoff path. Incubation was conducted under simulated aerobic or anaerobic conditions, and at 20 or 4°C. The fresh sediments were drained off of free water and thoroughly mixed with a spatula. After analysis of water content, the wet sediments were used directly without air drying to preserve some of the original microbial activity. For the aerobic incubation experiment, 5.0 g (dry weight equivalent) of wet sediment was placed in a 20-mL glass vial (Wheaton, Millville, NJ), and 5 mL of deionized water was added to immerse the sediment. The sediment layer was about 2 cm deep, and the overlaying water layer was about 1 cm deep. The samples were loosely covered with aluminum foil, and incubated at 4 ± 1°C in a refrigerator, or at 20 ± 1°C in an incubator. The incubation experiment continued for 365 d, during which time water loss from the sample vials was periodically checked and compensated by addition of deionized water. Given the small sample size and that the samples were constantly exposed to the ambient air, it was assumed that aerobic conditions were maintained during the experiment. Duplicate samples were removed after different times of incubation, and stored in a freezer (–21°C) before analysis. For extraction, the sediment was quantitatively transferred to a 250-mL glass centrifuge bottle using 50 mL of hexane–acetone (1:1, v/v) as the rinse solution. The sample bottles were mixed on a mechanical shaker at high speed for 1 h, and then centrifuged at 1300 x g for 20 min. The same extraction procedure was repeated for one additional time, and the solvent extracts were combined. The solvent phase was passed through a layer of anhydrous sodium sulfate (about 60 g) in a funnel to remove the residual water, and the final extract was adjusted to 100 mL for GC analysis. Preliminary experiments showed that the above extraction method gave >90% recovery for BF and PM in the sediment.

The anaerobic incubation experiment was performed concurrently using a similar protocol, but under anaerobic conditions. Briefly, 5.0 g (dry weight equivalent) of wet sediment was placed in a 20-mL glass vial and immersed with 5 mL of deionized water that was previously purged with nitrogen. Sample vials in racks were transferred into a collapsible film chamber (Instruments for Research and Industry, Cheltenham, PA), which was inflated with nitrogen. After 7 d of acclimatization, all sample vials were sealed with aluminum caps and Teflon-faced butyl rubber septa while remaining inside the anaerobic chamber. The samples in the chamber were kept at 4 ± 1°C in a refrigerator, or at 20 ± 1°C in an incubator. Duplicate samples were removed after different time intervals and stored in a freezer before analysis. The same procedure as described for the aerobic experiment was used for extraction and analysis of BF and PM in the sediment.

Chemical Analysis
A Model 6890N GC system equipped with an electron capture detector (ECD) (Agilent Technologies, Palo Alto, CA) was used for the detection and quantification of BF and PM. An HP-5 column (30 m x 0.32 mm x 0.25 µm) was used with helium as the carrier gas at 2.1 mL min^–1 (Hewlett-Packard, Palo Alto, CA). The other GC parameters were as follows: inlet temperature, 250°C; detector temperature, 300°C; oven temperature, initially 150°C for 1.0 min, ramped to 280°C at 15°C min^–1, and kept at 280°C for 5.0 min; and injection volume, 1.0 µL. Samples were introduced in the splitless mode.

	RESULTS AND DISCUSSION

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Pesticide Enrichment in Sediment
The sediment samples were taken from a concrete-lined channel receiving surface runoff from a large commercial nursery, where products of BF and PM had been continuously used for ant control in plant containers. The pesticides were typically incorporated into the potting mix before seeding or transplanting. The intensive irrigation as required for nursery production resulted in widespread surface runoff, which carried ingredients of the potting mix (i.e., sand, silt, and organic matter) and pesticides, first to a sedimentation pond and then through the drainage channel before the runoff water was discharged. It was observed that as the runoff water moved through the sedimentation pond and the drainage channel, suspended solids settled out under gravity and gradually formed a sediment layer along the runoff path.

In this study, distribution of BF and PM in the sediment bed was examined along the drainage channel, using the sedimentation pond as the source point. Distribution of BF and PM isomers in the sediment as a function of sediment location is summarized in Table 2. Pesticide concentrations generally increased with increasing distance downstream from the source. For instance, while BF concentration was 0.33 mg kg^–1 in the sedimentation pond, it increased to 2.27 mg kg^–1 at 104 m downstream from the source, and further to 8.47 to 10.64 mg kg^–1 at 145 m and beyond (Table 2). Similar distribution patterns were also observed for cis and trans isomers of PM. Using the concentration for the sedimentation pond as the reference value, the relative enrichment ratio (ER) was calculated for the different locations. Assuming ER was 1.0 for the sedimentation pond, ER for BF increased to 6.9 at 104 m, and further to 25.7 to 32.2 after 145 m. The ER for cis and trans isomers of PM also increased with increasing distance from the source, although at rates smaller than those for BF (Table 2).

Enrichment of pyrethroids during runoff may have several important implications. First, although the concentration of a pyrethroid compound in the source may be low, the significant enrichment potential may result in high pesticide levels entering the surface water during irrigation or storm induced runoff. In a recent study, Weston et al. (2004) detected residues of several pyrethroids in the sediment from waterways in the agriculture-dominated Central Valley of California. This finding appears to contradict the assumption that pyrethroids are generally immobile in the environment due to their strong adsorption to soil. Formation and transport of chemically enriched fine particles during runoff as observed in this study may offer an explanation to the recent pesticide detections. It must be noted that although pyrethroids were enriched in the sediment phase over distance, it was observed that sediment accumulation in terms of mass quickly decreased along the channel. Therefore, the net export of sediment-borne pyrethroids to a downstream receiving waterbody may be limited unless under extreme conditions (e.g., after a high-intensity rain storm). On the other hand, bioavailability of a chemical is known to correlate inversely with its adsorption in the environmental media. The significant increases in K_d over a short distance as observed in this study suggest that although the total pesticide concentration is enhanced due to sediment enrichment, the bioavailability may simultaneously decrease. In Maund et al. (2002), the bioaccumulation potential and toxicity of cypermethrin for Daphnia magna, Hyallela azteca, and Chironomus tentans decreased with increasing sediment OC content. Therefore, pesticide enrichment during sediment movement must be evaluated along with pesticide bioavailability. As evident from this study, sediment is probably the predominant carrier for compounds such as BF and PM. Therefore, sedimentation-based mitigation practices may be valuable for reducing surface water contamination by pyrethroids through runoff. As demonstrated by Kabashima et al. (2003), such mitigation practices may include sedimentation traps, sedimentation ponds, vegetative filters, and use of flocculants to cause settling out of suspended solids. Because sedimentation ponds or similar practices are effective only at retaining large particles, practices aiming at reducing transport of fine particles may be essential for preventing pyrethroids from entering surface water streams.

Pesticide Persistence in the Sediment Phase
Persistence of BF, cis-PM, and trans-PM was measured in previously contaminated runoff sediments under different oxidation (aerobic and anaerobic) and temperature (4 and 20°C) conditions. The dissipation of pesticide residues over time was fitted to a first-order decay model to estimate the first-order rate constant k (d^–1) and half-life (t_1/2) (Table 4). The fit was generally good for the aerobic incubation treatments at 20°C and anaerobic incubation treatments at both 4 and 20°C, as shown for BF in Fig. 1 and PM isomers in Fig. 2 , but poor for some of the aerobic treatments at 4°C, probably due to the very slow degradation. Under aerobic conditions, noticeable differences in persistence were observed between the different pesticides, and at different incubation temperatures. At 20°C, BF exhibited similar persistence in the different sediments (Fig. 1), with t_1/2 ranging from 428 to 483 d, or 12 to 16 mo. Degradation of PM isomers under the same conditions was significantly faster than that of BF (Fig. 2). The t_1/2 of cis-PM in the sediments was 98 to 142 d (or 3 to 4.7 mo), and that of trans-PM was 60 to 312 d (or 2 to 10 mo). At 4°C, persistence of both BF and PM was significantly prolonged. The t_1/2 of BF increased to 764 to 1950 d (or 25 to 65 mo), which represented an increase of 1.6 to 4.5 times as compared with the same treatments at 20°C. The t_1/2 of both PM isomers also increased significantly, with t_1/2 ranging from 152 to 297 d (or 5 to 10 mo) for cis-PM, and 490 to 2150 d (or 16 to 72 mo) for trans-PM. Therefore, under aerobic conditions, while PM showed moderate persistence in the sediments at 20°C, BF was highly persistent. The persistence of both BF and PM was greatly prolonged at 4°C.

View larger version (19K): [in this window] [in a new window]   Fig. 1. Degradation of bifenthrin in nursery runoff sediments under 20°C and aerobic conditions at various distances from the inlet of the channel. Vertical bars are standard deviations of two replicates.

View larger version (21K): [in this window] [in a new window]   Fig. 2. Degradation of (a) cis-permethrin and (b) trans-permethrin in nursery runoff sediments under 20°C and aerobic conditions at various distances from the inlet of the channel. Vertical bars are standard deviations of two replicates.

Previous studies on persistence of pyrethroids were mostly limited to soil. The soil t_1/2 reported by the manufacturer was 30 d for PM under aerobic conditions, 108 d for PM under anaerobic conditions, and 95 d for BF under aerobic conditions (Wauchope et al., 1992). Many studies show that as the contact time increases, certain organic compounds become increasingly recalcitrant to biodegradation in the environment (e.g., Steinberg et al., 1987). Therefore, the fact that the sediment samples used in the incubation experiment contained aged pesticide residues from previous contamination may have contributed to the generally long t_1/2 values observed in this study. Soil microorganisms have been commonly found to be involved in degradation of pyrethroids in soil, and bacterial degraders have been isolated from various soil sources (Kaufman and Haynes, 1977; Khan et al., 1988; Maloney et al., 1988; Grant et al., 2002). For instance, bacteria strains capable of degrading PM, deltamethrin, and cypermethrin have been isolated, and t_1/2 values from a few days to a few weeks have been observed when the degraders were used to transform these compounds in solution media (Kaufman and Haynes, 1977; Khan et al., 1988; Maloney et al., 1988; Grant et al., 2002). In a recent study (Lee et al., 2004), a large number of bacteria strains capable of degrading BF and PM were enriched from the same sediments as used in this study. In solution media, selected bacteria strains were shown to effectively degrade both compounds, with t_1/2 ranging from 1.3 to 5.5 d for BF, and from 1.5 to 3.3 d for PM isomers (Lee et al., 2004). However, it was also observed that in the presence of sediment, the ability of the same bacteria for degrading BF or PM was greatly inhibited, and the inhibition was attributed to the strong adsorption of these compounds to the sediment phase. Many other studies also show that microbial degradation of hydrophobic compounds is generally reduced by strong adsorption (Weber and Coble, 1968; Subba-Rao and Alexander, 1982). Therefore, even though microbial degraders may be widespread in the sediment, persistence of pyrethroids in the sediment can be prolonged due to their strong affinity for the solid phase. The relatively long persistence of these compounds may have contributed to their detections in the sediment in waterways receiving runoff (Weston et al., 2004).

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Pyrethroids are commonly used insecticides, and their importance may further increase as they replace some of the organophosphate insecticides. The high toxicity of pyrethroids to aquatic organisms and their recent detections in sediments from surface streams dictate that their fate and distribution in the sediment phase be better understood. In this study, BF and PM were found to be enriched during runoff transport, resulting in progressively higher pesticide levels in the sediment downstream from the source. The enrichment was attributed to size selection during settling of runoff-borne sediments, and probably also to disaggregation of large sediment particles. The enrichment of BF and PM was found to coincide with enrichment in sediment OC and clay fractions. This finding suggests that the chemically enriched fine sediment may have the greatest potential for surface water contamination, and that its role in ecotoxicological effects should be further evaluated. Both BF and PM exhibited relatively long persistence in sediments under aerobic or anaerobic conditions. The limited degradation was probably a result of the strong adsorption of these compounds on the sediment phase. The long persistence implies that although pyrethroids are known for their immobility in soil, surface erosion and runoff may ultimately lead to significant off-site pesticide movement to surface streams over a sufficiently long time scale. As many other pyrethroids are also in wide use, it is important to evaluate their behavior during runoff and sediment movement. In addition, it is expected that practices reducing sediment export should be also effective in mitigating off-site pyrethroid movement through runoff. Such mitigation strategies should be considered and evaluated.

	ACKNOWLEDGMENTS

 
This study was supported by the California Department of Food and Agriculture and the University of California's Water Resources Center.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Related articles in JEQ Similar articles in this journal Similar articles in ISI Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (19) Citing Articles via Google Scholar Google Scholar Articles by Gan, J. Articles by Kabashima, J. N. Search for Related Content PubMed PubMed Citation Articles by Gan, J. Articles by Kabashima, J. N. GeoRef GeoRef Citation Agricola Articles by Gan, J. Articles by Kabashima, J. N. Related Collections Pesticides Bioremediation and Biodegradation Water Pollution Ground Water Quality