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TECHNICAL REPORTS

Surface Water Quality

Occurrence and Bioavailability of Pyrethroids in a Mixed Land Use Watershed

^a Dep. of Environmental Sciences, Univ. of California, Riverside, CA 92521
^b Univ. of California Cooperative Extension, Orange County, Costa Mesa, CA 92626

^* Corresponding author (rbudd001{at}ucr.edu)

Received for publication June 28, 2006.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
The shift in land use patterns within many urban areas has the potential to influence the magnitude and nature of nonpoint-source pollution. The presence of pyrethroid insecticides in urban surface streams is of particular concern due to the broad spectrum toxicity of pyrethroids to aquatic organisms and the widespread use of pyrethroid products for agricultural and urban pest control. Sediment samples were collected throughout a mixed land use watershed in southern California during two sampling periods and analyzed for a suite of pyrethroids. Bifenthrin and fenpropathrin were found most frequently in the sediment samples, with the highest concentrations associated with sites adjacent to large commercial nurseries. Sediments from residential areas or residential-commercial mixed areas had fewer detections and significantly lower concentrations than the nursery runoff sediments. No apparent difference was found between wet and dry season concentrations, which may be attributed to the fact that the lack of flow under dry weather conditions rendered pyrethroid residues immobile. Organic carbon-normalized sediment concentrations were poorly correlated with the freely dissolved pore water concentrations measured by solid phase microextraction (SPME), suggesting factors other than sediment organic carbon content should be considered when relating concentrations to potential toxicities.

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
WITH the increasing encroachment of urban sprawl, there has been a dramatic shift in land use patterns within many watersheds. For instance, land use designated for agriculture within the San Diego Creek-Newport Bay Watershed in Orange County, CA, decreased from 22 to 12% between 1983 and 1993, while residential and commercial areas increased from 48 to 64% during that same period (USEPA, 2006). Similar to other regions in the United States, such transitions have significantly accelerated over the last decade. There is a concern that this shift in land use will result in an increase of nonpoint-source pollution; in particular pesticides and nutrients originating from residential pest control and lawn care. In recent studies, residues of pyrethroid insecticides were frequently detected in the sediment from a number of urban streams in northern California (Bacey et al., 2005; Weston et al., 2004, 2005; Amweg et al., 2006). Although chemistry data were unavailable, a sediment toxicity survey also suggested a potential association between the observed toxicity in sediments collected within the Upper Newport Bay in southern California and pyrethroid residues (Bay et al., 2004).

Pyrethroids are a class of insecticides used for controlling a wide range of pests in both agricultural and urban environments. In Orange County, the total amount of pyrethroids reported by licensed applicators for structural pest control increased from 2500 lbs in 1990 to 72 500 lbs in 2004 (California Department of Pesticide Regulation, 2006). A potentially much more significant, but poorly regulated or recorded, use of pyrethroids is by homeowners, as evidenced by the sale of numerous pyrethroid-containing products at retail stores and garden centers (Amweg et al., 2006). On the other hand, although traditional agriculture has greatly diminished, hundreds of commercial nurseries ranging from <1 to 200 ha in size are distributed throughout Orange County. These nurseries are sites of concentrated pyrethroid use (Kabashima et al., 2003; Gan et al., 2005). Therefore, the San Diego Creek-Newport Bay Watershed represents an urban watershed with mixed land uses. To identify sources and successfully implement regulatory measures such as the Total Maximum Daily Load (TMDL) program, it is important to delineate the role of land use types in nonpoint-source pollution.

Temporal variance in sediment pesticide concentrations is influenced by environmental conditions. Contaminant transport and loadings into receiving water bodies have been shown to be greatest after storm events (Kimbrough and Litke, 1996; Schroeter, 1997; Werner et al., 2004). Due to the strong affinity of pyrethroids to solid surfaces, large storms may be the most significant contributor to offsite movement of pyrethroids. Further downstream, transport may be a result of movement of finer particles and dissolved organic matter (DOM) (Gan et al., 2005). However, at present the seasonality of pyrethroid runoff is poorly understood.

Given the strong hydrophobicity of pyrethroids, the toxicity of pyrethroid residues in sediment will likely depend on their phase distribution, and specifically, the freely dissolved concentration in the sediment pore water that can only be measured using selective methods such as solid phase microextraction (SPME) (Yang et al., 2006a, 2000b). Numerous studies show that the freely dissolved concentration in the sediment pore water is the most indicative of bioaccumulation potential or toxicity effects of hydrophobic contaminants. However, monitoring for pyrethroids has so far only employed conventional methods based on exhaustive solvent extraction. The correlation between the measured total chemical concentrations and toxicity was therefore indirect and generalized.

The objectives of this study were to understand the relationships of sediment contamination by pyrethroids and land use patterns in a mixed land use setting, to evaluate seasonal effects on pyrethroid occurrence in the sediment, and to examine the potential bioavailability of sediment-associated pyrethroid residues.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Study Sites
The San Diego Creek Watershed encompasses 112 square miles within Orange County, CA (Fig. 1 ). The San Diego Creek provides over 90% drainage to the Upper Newport Bay that serves as a protected marine estuary and a habitat for hundreds of species of birds, fish, mammals, and native plants. Many migratory bird species also depend on the aquatic system during their winter routes.

View larger version (47K): [in this window] [in a new window]   Fig. 1. Site locations for sediment collection within the San Diego Creek Watershed in Orange County, CA.

Sediment Sampling
Sediment samples were collected at locations distributed throughout the San Diego Creek-Newport Bay Watershed during each sampling event (Fig. 1). The samples were collected during two time periods to capture storm runoff deposits (April–May 2005) and dry season conditions (August 2005). Nineteen samples were collected during the dry season and 18 samples were collected during the wet season at the same sites. Sediment was collected using a hand scoop into a 500-mL glass jar. The top layer of sediment (about 3 cm) was collected to represent the most recent sediment depositions. The sampling at each site was biased toward areas with finer sediments. All samples were transported to the laboratory within 4 h from the time of sampling and were kept at 4°C in the dark before analysis. Aliquots of the homogenized sediment samples were air-dried, sieved through a 1-mm sieve, and analyzed for total organic carbon (TOC) content at the University of California, Division of Agricultural and Natural Resources Analytical Laboratory in Davis, CA using a high-temperature combustion procedure.

Chemicals
Standards of bifenthrin (98.8%), permethrin (97.0%), and cypermethrin (95.1%) were obtained from FMC (Princeton, PA). Cyfluthrin (92%) and deltamethrin (99.4%) were obtained from Bayer CropScience (Stilwell, KS). Esfenvalerate (98%), lambda-cyhalothrin (98.7%), and fenpropathrin (99.7%) were obtained from Chem Service (West Chester, PA), Syngenta (Bracknell, Berks, UK), and Valent (Walnut Creek, CA), respectively. All solvents and other chemicals used in this study were analytical grade or GC grade. Florisil (60–100 mesh) was purchased from Supelco (Bellefonte, PA). All glassware and sodium sulfate were baked at 400°C for 4 h before use to prevent carryover contamination of pyrethroid residues.

Sediment Analysis
Sediment samples were analyzed using modified EPA methods 3550B, 3620, 3660, 8081, and the method published by You et al. (2004). A 100-g aliquot of sediment was weighed into a 250-mL high density polypropylene centrifuge bottle and centrifuged at 7000 rpm for 25 min. After the overlying water was decanted, a 20-g subsample (wet weight) of the homogenized sediment was weighed into a 250-mL beaker and 100 µL of 500 µg L^–1 of decachlorobiphenyl solution was added as the surrogate. Anhydrous sodium sulfate (Na₂SO₄) was mixed into the sediment sample to remove excess water, followed by addition of 75 mL acetone-methylene chloride mixture (1:1, v/v). The sediment sample was placed in a Sonic 550 high-intensity ultrasonic processor (Fisher) for 5 min with a 3 s (on) and 1 s (off) pulse cycle. The solvent extract was decanted through a Whatman No. 41 filter paper (Whatman, Maidstone, UK) filled with 2 g of anhydrous Na₂SO₄ into a round bottom flask. The sonication procedure was repeated two additional times with fresh solvents. The sonicator probe was rinsed with acetone in between samples. The solvent extracts were combined and concentrated on a vacuumed rotary evaporator at 35°C to approximately 5 mL. The sample extract was transferred to a 10-mL tube and was concentrated to dryness under a gentle stream of nitrogen. The sample was reconstituted in 1.0 mL hexane and subjected to the following Florisil cleanup procedure.

For sample cleanup, a 32 by 1.2 cm glass column was packed with 10 g of deactivated Florisil and 1 g of anhydrous Na₂SO₄. Before use, the Florisil was precleaned with methanol, methylene chloride, and hexane in a sonication bath, activated by drying at 140°C overnight and deactivated with 6% (by volume) deionized water. The Florisil column was first rinsed with 40 mL of hexane and then the sample was loaded onto the column. The sample was eluted through the column with 50 mL of ethyl ether-hexane mixture (30:70, v/v) into a 250-mL pear-shaped flask, and the eluent was reduced to about 5 mL. To remove sulfur residues in the sample, a small amount (<1.5 g) of activated copper was added into the sample extract and mixed on a vortex for 20 s. Before use, the copper was activated with 20% nitric acid, neutralized with deionized water, washed with acetone and then dried under nitrogen. The sample extract was then passed through anhydrous Na₂SO₄ into a concentrator tube and was further condensed to 0.5 mL under a stream of nitrogen. The final sample was transferred to a 2-mL autosampler vial and reconstituted to 1.0 mL with n-hexane. An aliquot was used for analysis by gas chromatography (GC) coupled with an electron capture detector (ECD). The mean method recovery was 109% for bifenthrin, 89% for fenpropathrin, 91% for lambda-cyhalothrin, 111% for cis-permethrin, 98% for trans-permethrin, 98% for cyfluthrin, 95% for cypermethrin, 99% for esfenvalerate, and 85% for deltamethrin.

Analysis of Freely Dissolved Pore Water Concentrations
The solution associated with the wet sediments was physically separated from the sediment phase and operationally defined as "pore water" in this study. The sediments were packed into a cylindrical funnel plugged with glass wool and the solution was collected into in a 20-mL glass centrifuge tube under vacuum. Dry sediment samples were excluded from this analysis. A 2-mL aliquot of the pore water sample was diluted to 20 mL with organic free deionized water and analyzed for dissolved organic carbon (DOC) on an Apollo 9000 Carbon Analyzer (Teledyne Instruments, Mason, OH). A previously developed SPME method was used to determine the freely dissolved concentration in the sediment pore water (Liu et al., 2004). Briefly, a 10-mL aliquot of pore water was transferred to a 20-mL glass scintillation vial. A Supelco SPME sampling assembly with polydimethylsiloxane (PDMS, 30 µm)-coated fiber was used for the SPME sampling. The fiber exposure time was 15 min, and the fiber immersion depth was fixed at 2 cm from the surface. During sampling, a small Teflon-coated magnetic bar was used to stir the sample solution at 600 rpm. After sampling, the fiber was directly injected into a 6890 Agilent GC-ECD (Agilent, Palo Alto, CA) and kept in the inlet for 3.0 min to thermally desorb the enriched pesticides into the capillary column to be eluted under the following conditions.

Analytical Conditions
A 6890 Agilent GC-ECD system coupled with a DB-5MS column (30 m x 0.25 mm x 0.32 µm) was employed for separation and analysis of all pyrethroid compounds. The inlet temperature was 260°C, and the detector temperature was 320°C. The column temperature program was as follows: initial temperature 160°C held for 1 min, ramped to 290°C at 3°C min^–1, and held 20 min at 290°C. The column flow rate was 1.5 mL min^–1 (helium). The sample was injected in a pulsed splitless mode with the pressure set at 25 psi for 0.5 min. The total splitless time was 1.0 min. For compounds with multiple peaks, the sum of all peak areas was used for calibration and quantification. Confirmation of pyrethroids were verified by running the same samples on a DB-1701 column (30 m x 0.25 mm x 0.25 µm) under the same conditions as described above. Calibration standards of pyrethroids for SPME analysis were prepared daily at five different concentrations (0.5, 0.2, 0.04, 0.02, and 0.01 µg L^–1) in 100 mL volumetric flasks by consecutive dilution and used for quantifying the freely dissolved concentration in pore water (C_free) with a unit of µg L^–1. Quality control was evaluated by running duplicate samples for every five samples and a blank for each set of samples. The 3 standard deviation for the recovery of four sediments spiked with 5 µg kg^–1 pyrethroid standards was used as the method detection limit (MDL) for sediments. The respective MDLs were 0.03 µg kg^–1. for bifenthrin and cyhalothrin, 0.08 µg kg^–1 for permethrin and deltamethrin, 0.09 µg kg^–1 for cyfluthrin, and 0.11 µg kg^–1 for fenpropathrin. The MDLs were estimated at 1 to 5 ng L^–1 for SPME analysis of pore water samples.

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Sediment Concentrations
The total concentration of pyrethroids found in the sediments ranged from <0.02 to 560 µg kg^–1 in the dry season sediments and from <0.02 to 142 µg kg^–1 in the wet season sediments (Fig. 2 and 3 ). Bifenthrin and fenpropathrin were detected most frequently in both sets of samples, while other pyrethroids were detected less frequently. Bifenthrin was detected in 95 to 100% of the samples, with the maximum concentration of 542 µg kg^–1 for the dry season samples and 122 µg kg^–1 for the wet season samples. Fenpropathrin was the second most frequently detected pyrethroid, found in 68% of the dry season samples and 90% of the wet season samples. Concentrations for fenpropathrin ranged from 14.9 µg kg^–1 during the dry season to 19.35 µg kg^–1 during the wet season. All other pyrethroids were detected less frequently and often at lower concentrations, with the highest concentrations at <4 µg kg^–1 for cyhalothrin (2.51 µg kg^–1), permethrin (3.09 µg kg^–1), cyfluthrin (2.48 µg kg^–1), and deltamethrin (1.73 µg kg^–1). Esfenvalerate was detected in one sample at 10.45 µg kg^–1.

View larger version (21K): [in this window] [in a new window]   Fig. 2. Concentrations of pyrethroid insecticides in dry season sediments. (A) Study sample sites excluding nursery outlets; and (B) nursery outlet sites.

View larger version (27K): [in this window] [in a new window]   Fig. 3. Concentrations of pyrethroid insecticides in wet season sediments. (A) Study sample sites excluding nursery outlets; and (B) nursery outlet sites. Note: Samples were not collected at Site 16 during this sampling period.

Effect of Land Use
Land use has been shown to play an important role in contaminant loadings to surrounding waterways. Agriculture fields are a well documented source of pyrethroids into sediment beds. Weston et al. (2004) detected pyrethroids in 75% of sediment samples collected within the agriculture dominated Central Valley of California (Weston et al., 2004). In the current study, the highest concentrations of pyrethroids within the San Diego Creek-Newport Bay Watershed were detected in areas in close proximity to agricultural land use areas. In particular, those sites located near the outlets of commercial nurseries (sites 11, 12, and 13) contained the highest concentrations of bifenthrin during both sampling events (Fig. 2B and Fig. 3B). Concentrations were also the highest for fenpropathrin (20 µg kg^–1) and esfenvalerate (10 µg kg^–1) at site 13. Cyhalothrin, cyfluthrin, and deltamethrin were detected sporadically near these agricultural sites at concentrations <3 µg kg^–1. This observation is in agreement with findings from other studies in which the highest concentrations were found near storm outlets (Weston et al., 2005). Use of bifenthrin by commercial nurseries in Orange County is mandatory as part of the quarantine requirements for red and imported fire ant control (Kabashima et al., 2003). Products of bifenthrin such as the granular formulation Talstar are directly incorporated into potting mixes before seeding or transplanting. In previous studies, high concentrations of bifenthrin and permethrin were found in irrigation-induced runoff water and sediments inside a large nursery (Kabashima et al., 2003; Gan et al., 2005). However, because many nurseries in this area capture and recycle irrigation runoff (Kabashima et al., 2003), the actual export under dry weather conditions should be small. In contrast, storm water runoff is essentially uncontrolled, and storm water runoff from the nursery sites can contribute significantly to loadings of pyrethroids into the San Diego Creek and potentially to the downstream Newport Bay.

Concentrations of pyrethroids were generally lower in the residential areas. The lower observed concentrations surrounding residential areas could be a result of use patterns (i.e., much lower application rates and scattered uses) and environmental conditions. Vegetative ground cover has been shown to be very effective in preventing pyrethroid transport during storm water runoff events (Werner et al., 2004). The sites surrounding residential areas were generally well vegetated with grasses and shrubs, which could be an important reason why pyrethroid residue concentrations were low at these sites.

Fenpropathrin was detected at several sites designated as having primarily residential inputs. However, fenpropathrin is currently only registered for agriculture or nursery use in California. Therefore, fenpropathrin detected within the residential areas most likely originated from agricultural/nursery sources through unknown pathways. To explore this finding further, we examined the relationship between bifenthrin and fenpropathrin and found a fairly high correlation between the concentrations of these two compounds (r = 0.76). This could be indicative of similar source inputs for these compounds. In 2004, over 3600 lbs of bifenthrin was used in nonagriculture structural control, while only 300 lbs were registered for agricultural use. If bifenthrin and fenpropathrin indeed had the same source inputs (i.e., agricultural use), this might suggest that residential use was a less important contributor than agricultural use in mixed land use environments. The difference may be due to the diffused use pattern and the specific application methods and surface conditions (e.g., grassed lawns) in residential areas. This observation might also help explain the low number of permethrin detections in this study, despite the fact that 66 400 lbs of permethrin were applied for structural pest control in Orange County in 2004. However, this is fairly speculative in nature and more direct sampling of residential inputs is necessary to draw more concrete conclusions.

Although concentrations in samples located near residential or commercial areas were generally lower than the agricultural outlets, there was evidence for broad pyrethroid usage. Sites 4, 6, 7, 16, 17, and 18 were located in areas for which residential runoff was dominant. Pyrethroids were detected at each of these sites, with concentrations ranging from 0.78 to 16.29 µg kg^–1 in the wet season sediments, and 0.24 to 17.21 µg kg^–1 in the dry season sediments. Three of the sites (sites 6, 8, and 19) running adjacent to residential or mixed land use contained all of the analyzed pyrethroids. While permethrin was not detected in any of the agricultural samples, it was detected from 0.8 to 5.01 µg kg^–1 at the three sites located within residential areas. This observation could be indicative of the recent shift in land use to the use of more diverse products by homeowners, with permethrin being the most common active ingredient in retail products for residential use (Amweg et al., 2006).

While agricultural and commercial applications of pesticides in California are tracked and reported in California Department of Pesticide Regulation's Pesticide Use Reporting (PUR) databases, homeowner retail sales are excluded, making it difficult to estimate the total pyrethroid usage in residential areas. However, residential usage was attributed as the primary contributor of pyrethroids found in two separate monitoring studies of selected northern California urban streams (Amweg et al., 2005, 2006). Although samples near residential and mixed land use areas were detected with lower concentrations than those at agricultural sites in this study, our findings reaffirm the contribution of urban land use areas to total pesticide loadings in urban surface water bodies.

Effect of Seasonality
Pyrethroids adsorb strongly to solid surfaces and have been shown to move with loose sediment particles in runoff (Gan et al., 2005). Therefore, it was hypothesized that sediments collected immediately after the storm season would have higher concentrations than during the dry season. A quantitative and qualitative analysis was conducted to assess differences in seasonal sediment concentrations of pyrethroids. The wet season sediments had a more diverse profile of detections than the dry season sediments (Fig. 1 and Fig. 2). The frequency of sediments with a positive detection was consistently higher for the wet season sediments than the dry season sediments for fenpropathrin (89 vs. 68%), cyhalothrin (56 vs. 16%), permethrin (17 vs. 0%), cyfluthrin (33 vs. 5%), esfenvalerate (44 vs. 11%), and deltamethrin (28 vs. 11%). However, a signed pair test ( = 0.05) found that individual and total pyrethroid residue concentrations were not statistically different between the wet season and dry season samples. A possible explanation for similar concentrations in both sampling periods could be due to the regional environmental conditions. The months (January–April) before the first sampling round had a cumulative precipitation of 380 mm, with several heavy rainfalls. Most of the agricultural sampling sites received only storm runoff, and were dry throughout the dry season lasting from April through October. Degradation and aerial drift were most likely the only factors that would cause changes in pesticide concentrations between sampling periods at these sites. Higher pyrethroid concentrations found during the dry season at the agriculture sites indicate high persistence of pyrethroids in the sediments, and the increases could be a result of spatial variability in sampling. However, although runoff and downstream transport of pyrethroids from the agricultural sites was likely negligible under dry weather conditions, sediments within these agricultural drainage paths represent temporary storage areas of sediments with high concentrations of pyrethroids that could be transported downstream during subsequent heavy storm events.

Bioavailability of Sediment-borne Pyrethroids
With the exception of bifenthrin, most of the pyrethroids were detected infrequently by the SPME method in this study (Table 1). The freely dissolved concentrations measured by SPME (C_free) and OC-normalized sediment concentrations for bifenthrin are plotted on logarithmic scales in Fig. 4 . Only those samples with SPME analysis were considered, and MDL was used for the non-detects. The OC-based sediment concentrations of bifenthrin showed poor correlation with the C_free values, with R² = 0.25 for the linear regression for all samples (Fig. 4).

View larger version (9K): [in this window] [in a new window]   Fig. 4. Bifenthrin organic carbon-normalized sediment concentration vs. the freely dissolved pore water concentration (Cfree) measured by solid phase microextraction (SPME) for dry and wet season sediments. Triangles are dry season samples, and diamonds are wet season samples.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Pyrethroids were detected at various concentrations and frequencies in bed sediments within the San Diego Creek-Newport Bay Watershed in southern California. Bifenthrin and fenpropathrin were among the most frequently detected pyrethroids in sediments taken following the wet season and during the dry season. While agriculture land use was determined to have the greatest potential to contribute high concentrations of pyrethroids in the runoff sediments, residential land use was noted to be a potential source for diffused loadings. The distribution patterns of pyrethroid residues were similar between the wet and dry seasons, suggesting that the lack of flow in the dry season rendered pyrethroid residues immobile in parts of the watershed. Poor correlation was found between OC-based sediment concentrations and the freely dissolved pore water concentrations measured by SPME, implying that other factors, such as characteristics of sediment organic matter and aging, may significantly influence the bioavailability of sediment-borne pyrethroids. Selective sampling methods such as SPME are therefore more appropriate for predicting sediment toxicity from pyrethroid contamination. Further studies are needed to understand homeowners' use of pyrethroid products, persistence and offsite transport behaviors of pyrethroids in residential landscapes, and conditions that are conducive to movement of pyrethroids during surface runoff.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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