Mitigation of Permethrin in Irrigation Runoff by Vegetated Agricultural

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Chapter 19

Mitigation of Permethrin in Irrigation Runoff by Vegetated Agricultural Drainage Ditches in California 1

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D. L. Denton , M. T. Moore , C. M. Cooper , J. Wrysinski , W. M. Williams , J. L. Miller , K. Reece , D. Crane , and P. Robins 4

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Environmental Protection Agency, Region IX, Sacramento, C A 95814 National Sedimentation Laboratory, U.S. Department of Agriculture, Agricultural Research Service, Oxford, MS 38655 Yolo County Resource Conservation District, 221 W Court Street, Suite 1, Woodland, C A 95695 Waterborne Environmental Inc., 897-B Harrison Street, S E , Leesburg, V A 20175 AQUA-Science, 17 Arboretum Terrace, Davis, C A 95616 Aquatic Toxicology Laboratory, University of California at Davis, Davis, C A 95616 California Department of Fish and Game, Rancho Cordova, CA 95670 2

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As organophosphate use has decreased in California, a concomitant increase in their replacement insecticides (pyrethroids) has occurred. Although the probability of off-site movement of pyrethroids is less than their predecessors (organophosphates), transport of pyrethroids to aquatic receiving systems is still a potential threat. To mitigate possible harm, several in-field and edge-of-field management practices have been proposed, including conservation tillage, stiff grass hedges, riparian buffers, and constructed wetlands. By incorporating several individual components of these management practices, vegetated agricultural drainage ditches ( V A D D ) have been proposed as a potential economical and environmentally efficient management practice to mitigate effects of pesticides in irrigation and storm runoff. A field trial was held in Yolo County, California, where three ditches (Ushaped vegetated; V-shaped vegetated; and V-shaped © 2008 American Chemical Society Gan et al.; Synthetic Pyrethroids ACS Symposium Series; American Chemical Society: Washington, DC, 2008.

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unvegetated) were constructed and amended for 8 h each with a mixture of permethrin and suspended sediment simulating an irrigation runoff event. Spatial and temporal collections of water, sediment, and plant samples were analyzed for cis and trans permethrin concentrations. Because the cis- isomer of permethrin is considered more toxic than the trans- isomer, only cis-permethrin results are reported herein. Cis-permethrin half-lives in water were similar between ditches ranging from 2.4-4.1 h. The differences between half-distances (distance required to reduce initial pesticide concentration by 50%) among the V-shaped vegetated and unvegetated ditches were two times more efficient with vegetation, indicating impor tance of vegetation in mitigation. Cis-permethrin half-distances ranged from 22 m (V-vegetated) to 50 m (V-unvegetated). These studies are being used to validate a computer simulation model that is being developed to design V A D D for sitespecific implementation. Utilizing features already present in the agricultural landscape, such as drainage ditches, will provide farmers with an economical alternative that still is protective of the receiving aquatic environment.

Introduction Pyrethroid use in California has increased since 1992 (1,2). Approximately 241,570 kg (active ingredient) of the pyrethroid permethrin was applied to 64 crops in 2005 compared to 171,790 kg on 50 crops in 1992 (2). Although the probability of off-site movement of pyrethroids is less than their predecessors (organophosphates), transport of pyrethroids to aquatic receiving systems is still a potential threat. Recently, sediment toxicity to pyrethroids has been documented in urban waterways and agriculturally dominated waterways (3,4). While pesticide efficacy has greatly improved over the last several decades, there is still a void in research on management practices to decrease the likelihood of non-point source pollution. Lee and Jones-Lee (5) urged the need for quantitative information on best management practice (BMP) efficiency for agricultural runoff, particularly within California's Central Valley. Since the early to mid 1990s, there has been increased emphasis on the 303 (d) provision of the Clean Water Act, focusing on the total maximum daily load ( T M D L ) process. In California, pesticides are the leading cause of impairments to waterbodies (6). In 2002, U S E P A published their "Twenty Needs Report" on how research can enhance the T M D L process (7). Current research described here addresses many of those needs, including "Improve information on BMPs, restorations or other management practice effectiveness, and the related

Gan et al.; Synthetic Pyrethroids ACS Symposium Series; American Chemical Society: Washington, DC, 2008.


419 processes of system recovery." Several BMPs currently promoted by the U S D A ' s Natural Resource Conservation Service (USDA-NRCS) include, but are not limited to, buffer and filter strips, riparian buffers, grassed waterways, and constructed wetlands. Each of these practices requires farmers to remove acreage from production landscape to meet physical B M P requirements. A n economical alternative is needed that will allow production acreage to remain intact, but still accomplish the necessary environmental tasks for water quality improvements. Moore et al. (8) demonstrated the usefulness of vegetated agricultural drainage ditches as one such alternative to traditional BMPs. Drainage ditches are a common part of the agricultural landscape, but are often considered of little value other than for movement of excess water from the field. These unique ditch ecosystems provide a host of potential services other than water conveyance, including sediment trapping and mitigation of nutrients and pesticides (8). The current study involved a field trial to determine efficiency of recentlyconstructed vegetated drainage ditches for mitigation of permethrin-associated runoff from tomato fields. The concept was based on earlier studies that resulted in substantial sorption of pyrethroid insecticides (lambda-cyhalothrin, bifenthrin, and esfenvalerate) by ditch vegetation from agricultural fields in Mississippi (8,9,10,11). For example, three hours following initiation of simulated storm events, 97% of lambda-cyhalothrin was associated with plant material. O f the measured bifenthrin and esfenvalerate, 52% and 66%, respectively, were associated with vegetation. Three main objectives involved in the current study were to 1) evaluate mitigation efficiency of two types of ditch design—U (typical in Mississippi Delta) versus V (typical in California)—with the pyrethroid permethrin; 2) evaluate the benefit of vegetation in a typical California V ditch by comparing its permethrin mitigation efficiency to an unvegetated V-ditch as a control; and 3) determine permethrin mass distribution within the water, sediment, and (if applicable) plants located in the U-vegetated, V-vegetated, and V-unvegetated ditches to estimate permethrin half-lives, half-distances, while providing data for modeling efforts.

Materials and Methods Ditch Design Three ditches, each 116 m in length, were constructed on a farm in Yolo County, California. Two different ditch designs ("U" and " V " ) were employed in this research. Although the " V " ditch design is most common throughout Yolo County, researchers also wanted to compare the broader " U " shape design for potential improved permethrin mitigation efficiency. One U-shaped ditch was



420 constructed with a 3 m top bank width, and a maximum holding capacity water depth of 0.37 m. Two, V-shaped ditches were identically constructed with top bank widths of 1.8 m and a maximum holding capacity water height width and water depth of 0.6 m and 0.24 m, respectively. Vegetation in U - and V-ditches was similar in density and distribution through a cross-section of both vegetated ditches. Because of sandy field soil conditions, no significant outflow occurred from either U - or V-ditches. One V-ditch remained unvegetated to serve as a control ditch. The other V-ditch and single U-ditch were planted with Hordeum vulgare (barley) and Lolium multiflorum (annual ryegrass). Lamb's quarter (Chenopodium album) was an invasive prevalent weed within the vegetated ditches, and it served as an unexpected source of organic material within the ditch systems. Identical sampling sites were established within all three ditches at the simulated runoff inlet (0 m) (site 1), 42 m (site 2), 51 m (site 3), 88 m (site 4), and 108 m (site 5), sampling sites were delineated to reflect different types of vegetation within the ditch. Prior to the initiation of the simulated irrigation runoff event (24 h), ditch vegetative cover and dominant plant species were determined by sampling three 0.23 m quadrants at each sampling site. The study was designed such that any runoff leaving the ditch was routed into a vegetated sump pond to prevent direct release into the aquatic receiving system; however, the small volume of water entering the sump filtered through the soil column within 16 hours of entry. 2

Simulated Irrigation Runoff Event In July 2005, a simulated irrigation runoff event was delivered into each of the three constructed drainage ditches. A mixture of permethrin (Pounce® 3.2 EC), and 45 kg of dry soil was added to a 3800 L steel water tank filled with ground water and kept in suspension using a small submersible pump. The concentration of permethrin in simulated runoff (0.02 mg/L) was based on the recommended Pounce® 3.2 E C application rate (0.37 L/ha) for a 32-ha contributing area of tomatoes and an assumed 0.09% permethrin runoff with a targeted discharge of 7 L/s into experimental ditches (12). Using an Atwood™ 450 submersible pump (1703 L/h maximum flow) and 1.9 cm tubing, simulated runoff was pumped from the tank into calibrated values entering ditch inflows. Irrigation pipe (30-cm diameter) carried dilution water (approximately 198,000 L per ditch) from a nearby pump to the ditches.

Collection of Water, Sediment, and Plant Samples Velocity (m/s), temperature (°C), pH, dissolved oxygen (mg/L) and electrical conductivity (|uS/cm) were measured with calibrated hand-held field meters at inflow (site 1) and near the outflow (site 5) of each of the three



421 constructed ditches at times 0, 0.5 h, 1 h, 4 h, 8 h, and 16 h. Grab samples of water were collected in pre-cleaned, certified 1 L amber Boston round, narrow mouth glass bottles with Teflon® lined closures at 0 h, 0.5 h, 1 h, 4 h, 8 h, and 16 h, post-application from each site. Sediment samples were collected in 120-mL wide mouth glass bottles with Teflon® lined closures at times identical to water collection (including 24 h, 48 h and 120 h samples). Plant samples were also collected along the same time schedule as sediments. Sediment samples were obtained from the top 1 cm using solvent-rinsed stainless steel spatulas, while plant materials were collected with solvent-rinsed scissors. Only plant material exposed in the water column (between sediment-water surface) was collected for analysis. Plant samples were wrapped in aluminum foil and placed in pre-labeled 3 Lfreezerbags. All samples were preserved on wet icefromcollection through transport to the Aquatic Toxicology Laboratory (ATL) at the University of California, Davis. Water samples were kept in the dark at 4°C prior to transport to the California Department of Fish and Game Water Pollution Control Laboratory (DFG-WPCL). Sediment samples (also transported to DFG-WPCL) were frozen and kept in the dark until transport. Plant material was frozen immediately upon receipt at the ATL and shipped overnight to the USDA Agricultural Research Service National Sedimentation Laboratory (NSL) for sample preparation. Upon arrival at the NSL, plant samples were dried and ground using a Thomas-Wiley Model 4 laboratory mill. After preparation, samples were placed in glass vials and shipped to DFG for permethrin analyses.

Permethrin Extraction - Water Water samples were extracted within 7 days, according to USEPA Method 3510C - Separatory Funnel Liquid-Liquid Extraction. One-liter water samples were fortified with triphenyl phosphate and dibromooctafluorobiphenyl to monitor extraction proficiency and extracted twice with dichloromethane (DCM) using a mechanical rotating extractor. Extracts were dried using sodium sulfate, concentrated, and solvent exchanged with petroleum ether (PE) using KudernaDanish (K-D) evaporative glassware equipped with a 3-ball Snyder column followed with a micro-Snyder apparatus and adjusted to a final volume of 2 mL in iso-octane. Concentrations of cis and trans isomers of permethrin were generated separately. Since cis-permethrin is generally considered more toxic than trans-permethrin, results discussed herein will only focus on the cis- isomer.

Permethrin Extraction and Cleanup - Sediment and Vegetation Sediment and vegetation sample extraction followed USEPA Method 3545A - Pressurized Fluid Extraction. Homogenized sediment (10 g) and dried vegetation (2.5 g) samples were mixed with pre-extracted Hydromatrix® (7 g,


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Varian Corporation) and fortified with triphenyl phosphate, dibromooctafluorobiphenyl and dibutylchlorendate. Samples were extracted twice with acetone/DCM (50/50, v/v) using a Dionex® Accelerated Solvent Extractor (ASA 200, 100°C, 1500 psi). Extracts were dried using sodium sulfate, concentrated and solvent exchanged with PE using K - D evaporative glassware equipped with a 3-ball Snyder column followed with a micro-Snyder apparatus and adjusted to final volume of 2 mL in iso-octane. Clean up of sulfiir, chlorophyll and other matrix interferences followed U S E P A Method 3600C guidelines, as needed.

Instrument Analysis Water, sediment, and vegetation sample final extracts were analyzed for permethrin using U S E P A 808IB guidelines for permethrin analysis. Permethrin was analyzed using dual column high resolution gas chromatography equipped with electron capture detectors. The aqueous reporting limit for cis-permethrin was 0.005 |ig/L, while The sediment reporting limit (dry weight) was 4.00 ng/g. Vegetation reporting limit (fresh weight) was 5.00 ng/g for cis-permethrin.

Data Analysis Ordinary least-squares linear regression analyses (13) were used to fit curves to log-transformed permethrin water concentrations (y) versus the log of the distance down ditch from the inlet (x). Mass balances were performed using data on water, plant and sediments collected along transects of the ditch length for each sample time point (0.5 h, 1 h, 4 h, 8 h, 16 h, 24 h, 48 h, and 120 h). Ditch chemical depuration rate constants (k ) were determined for water in each of the three ditches. This was accomplished by plotting the In (total concentration) as a function of time and, through linear regression analysis, determining the slope. Pesticide half lives (t ) in water were estimated using the equation ln(2) / k . Using the same premise, ditch half-distances were determined by plotting the In (total concentration) as a function of ditch sample distance, determining the slope, and using the ln(2) / k equation. 2

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Results Although all ditch delivery systems were calibrated and re-checked prior to the simulated irrigation event, variability of inflow concentrations of cispermethrin still occurred. Samples collected from the inflow pipe at time 0 (test initiation), indicated cis-permethrin concentrations of 27.0 |ig/L, 225 |ug/L, and 117 jig/L for the U-ditch, V-vegetated, and V-unvegetated ditches, respectively. At V-vegetated ditch site 5 (108 m down-ditch), final cis-permethrin


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concentration decreased 80% from the 1 h sampling to the 8 h sampling (Table I). Due to changes in pump pressure, constant flows were difficult to maintain. Average inflow measurements for the U-ditch, V-vegetated, and V-unvegetated ditches were 3.59±0.56 m/s, 3.95±0.53 m/s, and 3.11±0.27 m/s, respectively. Flow measurements were also taken at site 5 (outflow), approximately 5 m from the actual slotted board riser drain pipe. Average values for the U-ditch, V vegetated, and V-unvegetated ditches were 0.45±0.27 m/s, 0.56±0.38 m/s, and 0.49±0.37 m/s, respectively.

Table I. Selected aqueous pesticide concentrations (pg/L) in inflow and outflow (site 5) of three experimental drainages ditches following a simulated irrigation event in Yolo County, CA.

Inflow (0 h) Site 5 (1 h) Site 5 (4 h) Site 5 (8 h) Site 5 (16 h)

U-vegetated

V-vegetated

V-unvegetated

27.0 18.2 6.80 1.08 0.981

225 9.63 1.37 1.89 *

117 13.5 1.35 1.17 *

* indicates no water available for sampling

Even though initial inflow water concentrations of cis-permethrin differed between ditches, by converting concentration to mass, ditches can be compared to one another. A mass balance shift for cis-permethrin in water occurred from the 1 h sample compared to the 8 h sample. In the U-ditch, 26±11% of measured cis-permethrin mass was located in the water at 1 h; however, only 4±1% of the mass was in the water column at the 8 h sample. Similar trends were evident for the same time periods in the V-vegetated (31±8% and 9±3%) and V-unvegetated (32±7% and 17±3%) ditches. Examination of each ditch indicated 14±6%, 16±8%, and 20±6% of measured cis-permethrin mass during the 8 h dose was located in water of the U-ditch, V-vegetated, and V-unvegetated ditches, respectively. Using all time and distance sediment measurements, percent mean measured mass (± SE) of cis-permethrin in sediment was 64(5), 52(2), and 80(6), respectively, for the U and, V-vegetated, and V-unvegetated ditches. Cispermethrin mean percent masses (± SE) in plants were 23(7) and 33(5) for the U and V-vegetated ditches, respectively. .Total cis-permethrin masses measured during the 8 h exposure and additional samples collected at 16 h, 24 h, 48 h, and 5 d post-exposure (including water, sediment, and plants) ranged from 225-1901 mg for the U ditch, 192-843 mg for the V-vegetated ditch, and 206-2149 mg for the V-unvegetated ditch. Mass estimates are that 65%, 56%, and 47% of cispermethrin applied were accounted for in the U - , V-vegetated and V-unvegetated ditches.



424 Several sites in the drainage ditches were dry before the 16 h sampling. As a result, sediment-permethrin masses shifted. When examining the overall experiment, cis-permethrin mass percentage in sediment for the U-ditch, V vegetated ditch, and V-unvegetated ditch was 70±3%, 58±6%, and 86±6% respectively. By analyzing data where no water was present, the cis-permethrin sediment mass percentages change to 75±3%, 72±3%, and 100±0% respectively, for the U-ditch, V-vegetated, and V-unvegetated ditches. Cis-permethrin halflives in ditch water were 4.1 h (U ditch), 2.4 h (V-vegetated) and 3.5 h (Vunvegetated). Half-distances for cis-permethrin were 169 m (U ditch), 22 m (Vvegetated) and 50 m (V-unvegetated).

Discussion Pesticide entry into receiving waters following storm or irrigation events depends on several factors, such as pesticide chemistry, rainfall intensity, time of application, and surrounding soil properties. In efforts to reduce the possibility of this occurring, management practices have been suggested to mitigate pesticide runoff. Vegetation plays a significant role in many suggested BMPs. Stiff grass hedges, grassed waterways, and riparian filter strips are just three examples of incorporating vegetation into runoff mitigation strategies. Vegetation has been documented to assist in mitigation of permethrin. Filter strips containing trees, shrubs, and grasses at widths of 7.5 m and 15 m reduced permethrin-associated contaminants 27-83% (14). Vegetated drainage ditches are becoming increasingly popular among farmers and landowners with little available production acreage to set aside for potential mitigation purposes. A mathematical model is being developed as part of this study as both a design tool to determine the ideal properties (e.g., length and width) for a particular farm system, and as an analysis tool to evaluate the efficacy that might be obtained with an under sized ditch. The tool will be able to estimate ditch performance for different chemicals, soils, plant species, and climatologic conditions. The Vegetated Filter Ditch Model ( V F D M ) simulates pesticide fate and transport from agricultural fields through a vegetative filter ditch based on water, sediment, and pesticide mass balance. Water mass balance accounts for inflow, precipitation, evaporation, seepage, and outflow. Sediment mass balance accounts for settlement and resuspension. Pesticide mass balance can accommodate dilution; volatilization; partitioning between water, sediment, and foliage; decay in water, sediment, and foliage; uptake by plants; resuspension from sediment and foliage; and outflow from overflow or drainage. Model input includes boundary condition loadings in terms of time series influx of water, sediment, and pesticide; ditch geometry including length, width, depth, and riser height; chemical properties including solubility, degradation rates (water, wet & dry sediment, foliage), adsorption coefficients (sediment, foliage), and uptake by plants; plant properties related to plant biomass and



425 growth; and sediment properties including field capacity, wilting point, porosity, bulk density, and initial soil moisture. Model output includes time series outflow of water, sediment, and pesticide; and chemical concentrations in water, sediment, foliage. A n existing pesticide fate model, RICEWQ version 1.7.2 (75) was the starting point for the generation of the V F D M . This model was selected because of its pesticide chemistry (degradation, partitioning, and ability to simulate metabolites) and water balance (variable inflow rates, water levels, and drainage) algorithms. Pesticide application routines were replaced by boundary condition inflow files for water, sediment, and pesticide. Geometry changes were made to allow channels of varying configuration. Mass balance algorithms were changed to track pesticide residues in sediment and foliar in multiple vertical compartments. Preliminary model predictions are encouraging but not conclusive because of the number of assumptions required to configure the model. The assumptions relate to uncertainty regarding the variability of pesticide and sediment dose over time and plant uptake and adsorption. Model validation will be assessed using information from additional field studies being conducted as part of this research study. Additional research is being conducted within the context of this study on the role of pesticide uptake and adsorption by plants. Additional field studies are being conducted that involve the implementation of V A D D on working farms in Yolo County receiving permethrin application to tomato and alfalfa fields.

Conclusions The use of vegetative ditches is effective for the mitigation of pesticides, and particularly pyrethroids, as demonstrated in this project and previous studies (8,9,10,11). Since pyrethroids have shorter environmental half-lives than organochlorines and many of the OP insecticides, there is less concern for pesticide accumulation in ditch water, sediment, and plants. Distances needed to reduce initial cis-permethrin concentrations by 50% were two times less in the V-vegetated ditch (i.e., more efficient) than the V-unvegetated ditch. When comparing the V-vegetated to U-vegetated ditch, cis-permethrin half-distances were eight times more in the U-vegetated ditch, thus making the V-vegetated the most efficient of the three ditches. Research into the significant differences reported between U - and V-vegetated ditches is one possible avenue for further study; however, it is beyond the time and financial resources available for the current study. Although an effective B M P , vegetated ditches should be considered one tool of many available options for mitigation of pesticides, including constructed wetlands, sediment retention ponds, grassed buffers, etc. Site specific needs routinely call for multiple BMPs in sequence to sufficiently address the non-point source problem.


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Acknowledgements This project was made possible by Proposition 13, Pesticide Research and Identification of Source Mitigation (PRISM) funds through the California State Water Resources Control Board. Special thanks to Blake Harlan for access and field site development. Without the field and laboratory assistance of Sam Testa, Kyle Wooldrige, Jennifer Drewitz, William Spong, Charissa Codina, Dan Riordan, Elizabeth Kelber, Kevin Goding, and Abdu Mekebri, the project could not have been completed.

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Mitigation of Permethrin in Irrigation Runoff by Vegetated Agricultural

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